What determines the optimum frequency range for the transmission of signals?

Question: What determines the optimum frequency range for the transmission of signals in a system that's transmitting electromagnetic signals between an antenna on the surface of the earth and an orbiting satellite antenna.
Comment: Do you know the frequency the satellite is listening to?
Answer: Many factors, and since your question is so broad, a broad answer follows:
The signal bandwidth (amount of information per second) will determine a minimum carrier frequency. In practice, the modulated signal should have a bandwidth of not more than say 1% of the carrier frequency.
Then, at lower carrier frequencies, you need to consider antenna size (which can be too large for a satellite), and ionospheric effects (mostly in HF range).
Commercial satellites communicate in the microwave range (L band, all the way to the K bands and above).
At high carrier frequencies, atmospheric attenuation becomes a problem. Also clouds and rain provide attenuation in the K bands.
Small amateur radio satellites normally operate in UHF and VHF, but those frequencies allow for a very small bandwidth (1200 or 9600 bits per second).
Comment: Note that "bandwidth" has two distinct meanings which are both useful in this context: [signal processing bandwidth](https://en.wikipedia.org/wiki/Bandwidth_%28signal_processing%29) (maximum minus minimum frequency of the RF output of the transmitter), and [transmission bandwidth](https://en.wikipedia.org/wiki/Bit_rate) or bit rate. In analog radio we are most often concerned with how wide the signal is (RF bandwidth), whereas in digital communications one is most often concerned with how much data can be transmitted (bit rate).
Comment: I think the limitation to 9600 kb/s on amateur satellites is due more to the archaic technology employed than the frequency. AFSK has horrible efficiency and sensitivity by modern standards.
Comment: I've upvoted this answer because the information is good, considering how broad the subject is - and how concise the answer is. I do agree that "those frequencies allow for a very small bandwidth" is incorrect though - I would rather that it said that by convention amateur use of those frequencies is at excruciatingly slow bit-rates, that were actually slow 20 years ago. Otherwise, this is a great general answer to a broad question.


Finding the nearest neighbor of polygons

Question: I have a set of non-convex polygons and I want to merge them. I am able to find the connection between two polygons. If there are more than two polygons, one connection can intersect another polygon within the set. I do not want to check for a possible intersection. So I have to find the nearest neighbor of a single polygon. Is there an existing algorithm? I can't find anything in my literature.
Comment: The diagram suggests you are looking for the shortest distance between vertices of one polygon and another, which will not always be the same as the shortest distance between the polygons. For example, all of the connections in the diagram are longer than if you made them perpendicular to the edge to the right, rather than meeting a vertex on the right. Is your question specifically looking for vertex to vertex connections?
Comment: Yes you are right, I'm searching for vertex-vertex connections.
Answer: You're looking for a nearest neighbor search: https://en.wikipedia.org/wiki/Nearest_neighbor_search
We've solved similar problems in 3D space using spatial subdivision (kd tree). Depending on polygon count and uniformity, a linear search or hash grid would be options too. A linear search, while O(N^2) can still be faster than setting up extra data structures for small a small number of polygons/vertices. A hash grid should be faster to update than a kd tree if your scene is dynamic, and I would expect it to be more efficient when the scene is uniform in vertex density.
Comment: Searching for the nearest point is an easy task. There is another problem. What if the nearest point cause an intersection with another polygon. Or what if the connection intersects the first polygon itself. Please look at the second image.
Comment: Either I must hear overlooked the second image or you added after I typed my response. Either way, in 3D this is typically solved with Ray tracing and an appropriate spatial subdivision, and I would expect the same to work in 2D.
Answer: I have successful developed an algorithm with the help of a Kd-tree and a simple test for overlapping lines. Here is a result:
Comment: To make this into an answer, could you explain the algorithm?


PEAR Mail Queue Database Explaination

Question: I recently set up a mail queue server to help with an issue sending a newsletter to 600+ recipients, which though untested at that volume, it should do the job... hopefully.
However during testing I noticed that a recipient got stuck in the queue and wouldn't send, the problem seemed to be that the try_sent field in the database was set to 25, whereas the rest were 0 (I think this was after a while of letting it try to send during construction and with some coding errors, so probably didn't start out as 25 when initially inserted). When I changed it back to 0, it sent fine. What does try_sent actually mean in this context (is it an error code or an increment of how many times an attempt to send it has been made, or something else?).
The majority of the fields are relatively self explanatory, but I'd like to know what some relate to so I know what's going on.
The fields are:
time_to_send - same as create_time - can you setup a send delay?
sent_time - guess it'd show when sent if not deleted from queue on send
id_user - ? (set to -1 when added)
ip - seems to be IP of the server that ran the add script ie. my IP
try_sent - ¿que?

any light that could be shed on this would be great.
Answer: Look in Mail/Queue.php:

define('MAILQUEUE_TRY', 25);
  This constant tells Mail_Queue how many times should try to send mails again if was any errors before.

PEAR Mail Queue Database Explaination


HTML5 Audio m4a

Question: Is it possible to play m4a music files using html5 audio? If that is not possible do you know of any flash player that can play m4a outside JWPlayer, I need a free one.
Answer: Any Flash player should be able to play MP4 content - this capability is coded into Flash, not the player product built on top of it.
JPlayer is a nice free player that uses HTML5 where it can, and falls back on Flash when it can't.
Answer: You can use Soundmanager2 which can use Flash fallback if HTML5 does not natively support M4A.
http://www.schillmania.com/projects/soundmanager2/
M4A should be supported by the browsers from companies who support MPEG-4 video and are the licensees of MPEG4-LA patents (Microsoft, Apple).

HTML5 Audio m4a


Itunes "genres"

Question: I manually changed the genres on some of my songs in my Itunes library to minimize the number of genres. I have found that after I add songs to my library and then sync them to my ipod, Itunes will change the genres back to the original listings. It never used to do this. It just started recently. Any suggestions?
Answer: It has nothing to do with organizing. I've been hearing cases of this recently, especially if people have custom genres that they've created. For some reason, iTunes is switching these back. Is that your case, or are these the standard genres that are in iTunes by default?
Answer: I'm assuming that you're changing the genre of a song in iTunes and not on the iOS device.
If so then maybe having iTunes organise your library is the cause (it's an option buried somewhere in the preferences)

Itunes "genres"


connecting videos?

Question: I'm running Windows 7 and I surprisingly can't find a free video editor which will allow me to "merge" video files shot by a GoPro into 1 video file and slice it from the beginning and end. There seem to be several programs for Linux and Mac, but I can't find anything which will do the job on Windows.
I've tried Blender but its very slow for video editing (my video is 3 hours long).
Answer: You can use FFmpeg:

Free and open-source
Runs on Windows (and Linux/Mac)
Concatenate two files for most video formats.
Fast.
Is CLI.

connecting videos?


Is there a way to turn a photograph into a line-art sketch vector?

Question: Can anyone tell me how to get this kind of vector line drawing from a photo? I'm beginner and can't find any answer. Thanks in advance :)
Comment: I'd suggest tracing paper.
Comment: I doubt very much that you will ever get that effect from photograph in digital tools. It is a hand-drawn sketch, and from what I can see it is drawn in a vector software such as Illustrator or Inkscape.
Answer: The image that you're showing is actually a hand drawn sketch and not a filter effect so unfortunately there's no easy way around it.
You can create similar effects using filters on photo's but never that good.
You could also try using the Threshold tool Image>Adjustment>Threshold and adjust the settings until you get an image you're happy with:
Comment: You can mess with some of the other filters. Apply some posterization, crosshatching to draw out a bit more of the sketch look. They'd need to be very subtle though.
Answer: Faux-sketch is very demanding. The interest lies in the question of what is a sketch? Often it would be drawn on paper with a pencil and focuses on the outlines of objects and some shading/texturing, like what you submitted. I have made a few experiments(top right is source):

With Gimp, Edge Detect/Edge can be a useful filter here(then
invert the result) - resulting in the two images on the first row
With a program relying on an edge detector such as canny, you may have more power over the detection and it's possible to alter the rendition with randomness(for instance, 2nd row, variations in the shades of grey(1) or drawing two lines per edge(2)); trying to act human here(!), but achieving a balance between the information you want to keep(like the two front "windows") and this is quite a challenge. An image extracted could be brought back in Gimp, you could then select by color, then select to path. Then remove the image, and stroke the path using Brush Dynamics like I did here... The third item just shows the Artistic/Cartoon filter which can be useful in some contexts - at least it looks good if everything else fails!
A morphological gradient is the difference between diluting and eroding an image(see Filters/Generic). When you layer those and use dodge as mode you end up with a quick version of another solution(bottom left).
The final(and different image) serves as a reminder that the object, its texture and its color matter when you're trying to extract an outline.

One might as well learn how to sketch by hand!
Answer: Your sample image is hand drawn sketch.
For exactly correct result you need to sketch your photo by hand in similar style. No filters, no plugins.
You can use:

Wacom tablet + Adobe Illustrator for creating vector illustration
Wacom tablet + Adobe Photoshop for creating raster illustration and then place it in Adobe Illustrator and convert to vector. But result will be with a lot of dirty shapes and points.
And best way for you - go to odesk.com or guru.com and ask there for help. This is very simple task for professional so it will cost $5-20 to you.
Answer:
Try this trick, hope it helps...:

With the photo newly opened in Photoshop, press CTRL + J(Win) / COMMAND+J (Mac) to duplicate the Background layer
Press Shift+Ctrl+U (Win) / Shift+Command+U (Mac) to desaturate the
layer
Press CTRL + J (Win) / COMMAND+J (Mac) to duplicate the desaturated
layer
Press CTRL + I (Win) / COMMAND+I (Mac) to invert the layer
Press Shift+Alt+D (Win) / Shift+Option+D (Mac) to change the blend
mode to Color Dodge
Go to Filter > Other > Minimum. Leave the Radius value set to 1
pixel and click OK to close out of the filter's dialog box
Press Shift+Alt+Ctrl+E (Win) / Shift+Option+Command+E (Mac) to merge the layers onto a new layer above the others
Press Shift+Alt+M (Win) / Shift+Option+M (Mac) to change the blend mode of the merged layer to Multiply, which darkens the sketch
effect
Lower the layer Opacity value if the sketch now appears too dark
Click on the Background layer in the Layers panel to select it,
 then press CTRL + J (Win) / Command+J (Mac) to duplicate it
Press Shift+Ctrl+] (Win) / Shift+Command+] (Mac) to jump the Background copy layer to the top of the layer stack
Press Shift+Alt+C (Win) / Shift+Option+C (Mac) to change the blend mode to Color to colorize the sketch
Lower the Opacity value to reduce the intensity of the color if
 needed.
Answer: In Illustrator, open the Image Trace panel:
Window > Image Trace
It will give an option of 'Line Art' as a targeted style.
Different customisations are possible. However the result will depend on contrast and lines in the image.
Answer: Just my 2 cents. This is by no means perfect. I was just experimenting for like 5 minutes. But that's the goal here—you have too play around with filters to get it right. There is no step-by-step. Also in this case I lost a lot of detail because the image I tested with was only 250px.
Answer: The best way to create vector line drawing is using softwares like  adobe illustrator or CorelDRAW.
Now for your answer here is a site that might help to create vector line drawing.
http://design.tutsplus.com/tutorials/create-a-nautical-sketch-style-in-adobe-illustrator--cms-20379
i hope this might help to turn a photograph into a line-art sketch vector.
Comment: Could you add an excerpt or quick summation of the method used? That way, your answer is still valuable in case the link breaks at a later time. Thanks!


Continuation to Divisional

Question: An application is being filed as a continuation application. After filing that can it be changed to a Divisional applicaiton at a later stage?
Answer: Continuation and divisional applications originate under different conditions. Divisional application is typically carved out from the original application when the examiner issues a restriction requirement if the original application is shown to have more than one distinct inventions. Continuation application is filed when you would like to file additional claims for the invention disclosed in the parent - this you can initiate.
So you can have a divisional application carved out from that application if the examiner issues a restriction requirement and based on that requirement but you cannot voluntarily file or convert the continuation to divisional.

Continuation to Divisional


Is it more effective to insulate the walls, or ceiling of an unheated basement?

Question: We have a new house, well built, spray foam insulation, tight seals.  My daughter's room is over an unfinished area of the basement and is COLD this winter.  There are vents in the unfinished area, but we have them closed because we don't want to heat that area.  We probably wouldn't ever finish that room because it is our main storage space.  The whole unfinished space is below ground.  My husband and I are having a debate.  Do we install foam board insulation on the walls in the un finished area or do we spray foam or use other insulation in the ceiling of the unfinished area?  Or both?  Our HVAC is not the issue, we have checked that out.
Comment: What kind of insulation is in the floor of the bedroom?
Comment: The ceiling below the floor is uninsulated...The floor in the bedroom is hardwood
Comment: I don't remember what insulation we put in the floor.  Going back and putting insulation under the floor probably won't be a viable option right now.  The floor isn't just cold, the whole room is cold.  We don't care about chilly floors, we wear socks or slippers!
Answer: If the ceiling of the unfinished space is the floor of the bedroom and is uninsulated, the floor of the bedroom is going to be cold. It would help a lot if that floor was insulated. There is not any real downside to insulating that floor or the unheated space. Insulation on the walls of the unheated space would help a bit, but you will see much better results if you insulate the ceiling/floor first. Doing both would be great.
Comment: Thanks...we have been debating for a while about the first thing to do.
Comment: I was watching This Old House the other day and they had a segment about this.  They made a comment that whatever you do, if it is unheated below, you will never have a comfortable floor.  I believe you are fighting science:  Heat rises.  Since you have a hardwood floor it will never be warm, even with insulation.  They suggested heating elements of some sort.  I believe they installed a small hydronic system.  In your case, if you had a duct underneath the floor, you might just heat the basement room in the winter...
Comment: Thanks - I will check on the duct....I honestly have never paid attention.
Answer: There is no point in insulating a cold unheated room!
Concentrate instead on keeping the heat inside the warm bedroom.
The best way to insulate a cold floor is, to lay sheets of tightly butted polystyrene or similar closed cell insulation on the existing floor and to then cover them with glued t&g oriented strand board. This way you have a fully floating floor, there are no heat bridges to syphon away your heat
To have a really warm floor (no cold feet) have sheet polystyrene, OSB, underlay, carpet. This will give you a carpet surface temperature the same as the room air temperature at floor level. ie; with an air temperature of 22C you will have a carpet surface temperature of 22C.
Comment: The bedroom is over _unfinished_ basement space. What you describe is a lot of work in a finished bedroom, compared with simply insulating the bedroom floor from below. In my mind, the answer is simple. If the basement is to be an unheated space, the "ceiling" of the basement space, which is the space between the joists for the floor above, should be insulated.
Comment: Thanks everyone.  The house is 6 months old so I am not going to do what is suggested for the floor above.  Let me clarify - the floor isn't "cold", the entire temperature of the room is 5-10 degrees colder than the house.  No leaks in walls, windows, etc... spray foam insulation in all exterior walls.  We have wood floors to help with allergies so carpet is not an option.   Also -the finished space is pretty small so the cost of insulating isn't going to be huge.  Maybe a 10X10 or 10X12 space.


What is a reasonable depth and diameter for postholes dug in clay soil to support an 8' fence?

Question: What is a reasonable depth and diameter for postholes dug in clay soil where the frost line is no more than 12" and that will support an 8 foot tall solid wood fence that uses PostMaster metal posts?
I hope to have someone else install an 8 foot high PT wood fence.  There will be no light or air passing through or under the fence and it will be heavier than average, given the additional height and board weight.  One side will run for 135'.  The attached diagram provides more detail.
The PostMaster Installation Instructions describe a 30" deep, sloped, 10" diameter hole, filled with 6" of gravel plus 24" of concrete for a 6 foot high fence.  They caveat that the "exact diameter and depth will be determined by local weather and soil conditions."  Got it.  Huntsville, AL; very clay soil.  Maximum wind gusts of 65 mph.  (This is not in the normal tornado path and is not intended to withstand a tornado.)
In reviewing the StackExchange posts on fencing and post holes I came across the concept that the diameter of the pier may be increased to compensate for a shallower post hole.

I would like to know the recommended depth and diameter without relying on the 2 buttress posts, which are my small overbuild idea.  We will be able to walk under them.
26 APR 18 update: I just found a Master Halco youtube video specifying 6-8" diameter holes dug 36" deep for an 8' board-on-board fence, such as I would like to have.  https://www.youtube.com/watch?v=mgjryMn7jI4
- I also spoke to a fence builder experienced using PostMaster posts who opined that the 10" diameter and 30 deep holes were more than adequate, but that the gravel was of no value in our climate.  So, 30" x 10" dia inches of concrete vs. 36" x 8" dia of concrete.
Comment: Bit late, but the standard metric is you want about 1/4 to 1/3 of the post's entire length to be underground... more if it is a high-wind shear area. So for 8ft above ground, you want 3-4ft below ground. Of course, that's the rule of thumb for *wooden* fence posts, and I see below that you opted for metal fence posts.
Answer: I say the 30" depth of post hole is adequate for said fence with given construction method; no need for additional support.
Answer: Posts were installed this week in 9" diameter holes, with a minimum depth of 30".  They were pounded a bit into the clay before 30" of concrete was 'poured' into each one, with a few inches of clay on top.  They flex, so should handle any wind loads with ease.


It's been more than 10 years (since I designed hardware with RS-232), and we're still using RS-232?

Question: Are there any alternatives to this ancient yet still popular way of communication?
Answer: What do you mean - just 10 years.  RS-232 was defined in 1962 and so is coming up to its 48th birthday.
RS-232 uses a simple asynchronous serial data transfer method that requires very little silicon, is easy to code in software and can be used on micros with minute amounts of RAM or program memory.  It has also had the advantage in the past that all computers and dumb terminals had RS-232 interfaces so using it for debug output was a no-brainer.
If you needed higher speed or long distance transmission then you just replace the line drivers with RS-422 or RS-485 differential driver/receivers and no change to the software was needed.
Implementing USB on tiny micros is just not possible.  The processing/memory/silicon overhead is just too costly.
SPI or I2C are possible (and used in some places) but the interface cards for your pc are unusual and expensive.  I2C has a nasty lockup state that can make it die if the clock stops halfway through a slave data output cycle and the data bit is low.
Comment: Why do you say I2C "dies" in that scenario?  It's true that starting a transmission when the previous state of the bus is unknown may require sending up to nine clocks, but that can be handled by having the i2c_start routine check the state of the data line and output up to nine clock pulses if it's low (stop outputting pulses as soon as it goes high).  Even my 6502 I2C code does that.  Is there some other problem condition you're thinking of?
Comment: That is the scenario and cure that I was referring to.  If you are using an I2C or (Atmel)TWI integrated peripheral then you can have to remap the port pins in use and then manually toggle the clock line nine times to exit the lockup before reassigning the pin back to the I2C function.  All a bit messy.
Comment: That makes sense.  I generally bit-bang I2C masters, so I don't often have to deal with that issue but I can see that for people using a hardware-supported I2C master it could be a problem.  I was wondering whether you might be referring to a different "I2C lockup" problem I've observed when trying to run a full-speed (400KHz) I2C bus when one slave on the bus is a micro whose system clock is running in low-power (6MHz instead of 24MHz) mode.  Because I2C supports handshaking, the only limits for how fast a bus can safely "try" to go, if the master supports handshake, ...
Comment: ...should be related the differential rise times of SCK/SDA (if e.g. SCK were to rise faster than SDA, releasing SDA and then releasing SCK too soon thereafter could cause a false start condition).  It seems, however, that some slave I2C implementations won't handshake SCK pulses which aren't low for at a few of their internal clock cycles, and can get hosed if one tries to talk full-speed to other devices on the bus.  Grr...
Answer:
It's been more than 10 years, and we're still using RS-232?

It's just a communications standard, and very bare-bones at that.
Your statement is not much different than, "It's been more than 10 years, and we're still using wires?"  Despite optical and RF solutions being available, wire still has some very distinct advantages, just like RS-232.
The main advantages are

It's cheap
It's trivially easy to use
It uses fewer wires than parallel data transmission
It goes long distances (measured in hundreds of feet/tens of meters, rather than inches/cm for most TTL/CMOS signals)
It's widely used in many products
Compatibility issues are rare
It's standardized, and a UART is included on all but the most simple microcontrollers
Lots of converters are available to convert it to other standards (such as USB)
Uses trivial NRZ encoding, so it's easy to interpret even without a UART

Disadvantages include

Slow
Doesn't specify protocol or higher layers of the communications stack
Standard connectors are bulky
Doesn't supply power
No standard identification mechanism (ie, can't plug it in and have the host know what was plugged in and its capabilities)

Are there any alternatives to this ancient yet still popular way of communication?

There are certainly alternatives, but no competitors that have all the above advantages.
We could list off the thousands of communication standards one might use in place of RS-232, but the bottom line is that RS-232 hasn't been replaced or superceded.  USB is the closest thing to it, but is severely limited in terms of distance, cost, ease of use, and included silicon on most microcontrollers.
USB has other significant advantages that RS-232 doesn't have, though.  They aren't replacements for each other, and in fact happily coexist on many computers.
The key point is that RS-232 is a tool, and as such isn't intrinsically bad, or in need of replacement.  It's getting less use these days as people want more advanced features, or faster speeds, or have some other requirement which RS-232 doesn't fulfill.
But that doesn't mean that it should or will go away, because there are still a lot of new applications where RS-232 is the perfect fit.
Answer:
Very reliable.
Easy to implement.
Requires only 3 wires, most of the time.
Uses wide voltage swing.
It has many devices that have used it.

mice
printers I have at least one
modems
scanners
bar-code scanners
multi-meters
GPS units
various scientific equipment

Can be used to transfer data between two computers with only changing around a few wires.
( Just try that with USB )
Has been implemented in many different micro-controllers
Doesn't require expensive cable. Except in extreme circumstances, including high speeds.
You can easily make your own cable, without special tools.
Can be easily adapted for longer cable runs, by using RS-422 or RS-485 signalling.

Frankly with that record, I would be surprised if it ever goes away entirely.
Comment: The differential signalling is nonsense.
Comment: @community, @starblue just used a technique called "sarcasm". Sortof.
Answer: There are many alternatives.  USB is the one that seems most commonly used nowadays.  I2C is another.
RS-232 still exists for legacy reasons.  If you can put up with the mishmash of different data rates, stop bits, parity settings and wiring configurations, it is actually quite a simple, solid and reliable interface for low data rate applications.

I listened to a podcast recently that discussed the handling of audio at a seminar.  All of the audio pros still use XLR connectors and conventional microphone cables, a technology that's at least 50 years old, even though newer alternatives are available. They use it because it is stable, proven, reliable, and industry-standard.
Answer: Distinguish serial UART communication from the various voltage levels used to transmit it: RS232, RS485, LVDS.
Serial UART communication is reliable and simple.
Answer: A lot of systems will convert the RS-232 to USB using a converter IC.
The ones that seem to be the most popular are the FTDI devices.
FTDI sells a cable with a USB connector on one end and six pin header
on the other (FTDI TTL-232R-3V3). This converts from a USB to RS232 at TTL levels
FTDI also makes an IC to perform the same function -- FTDI FT232RL. This
is in an SMD package. You connect the data inputs to the USB connector and the
outputs to an RS232 at TTL Levels. The cable is in the $15-$20 range.
The IC is around $2.50 (100). The IC requires a half dozen passives. I
also like to add a TVS IC.
It you need to get true RS232 levels you would need a driver IC.
More of the hobbyist parts are now starting to integrate USB into the low
end controllers. RS232 will start to be used only where you need a large
number of simple UARTs.
Comment: In practice you will find those converters are not without issue - for example, they expose the reality that USB is actually *slower* than typical serial baud rates for certain kind of query/response traffic!  And no, "large numbers of UARTs" will not be the only justification - simplicity has far wider appeal than that.
Answer: I'm a fan of USB <--> Serial IC's, handles all the dirty work of implementing USB, and you can pump up the bitrates to extreme levels (official RS232 tops out at what, 115k? I've seen USB<-->serial do several mbit/sec before)
Comment: RS232 can go significantly faster than 115k. I have run RS232 at MBAUD before, it is just a distance thing and clock error.
Comment: Which devices do you use?
Answer: RS232 voltage-level signaling is becoming less common, as more and more devices are designed to plug into a USB port rather than an RS-232 port.  The common async 8-N-1 signaling protocol, however, allows data to be sent full duplex with predictable timing and no byte stuffing.  A byte can be started at any desired moment when the line is otherwise idle, and there is no danger of data underrun.  Hardware to support 8-N-1 signaling is common and cheap, and while the protocol isn't entirely optimal, many changes that would make it better for some applications would make it worse for others.  The only significant downside to 8-N-1 is that it requires that both sides of the communication have a stable clock which is a known (generally integer) multiple of the desired communications rate.
Incidentally, most async 8-N-1 hardware allows for at least one type of out-of-band signalling: the long break.  While there are certainly times it would be helpful to have a few more types of out-of-band signaling, it's useful to have at least one.


Which types of tomatoes are good for which dishes?

Question: I am just beginning to realize the difference between the tastes and quality of different tomatoes for different dishes. I need help from a tomato expert. Which tomatoes are best for the following dishes :
1. Lasagna
2. Penne
3. Sauce for  polenta
4. Pizza or focaccia
Thank you for your help!
Comment: Digityogi, welcome to Seasoned Advice! Let me point you to our [help], especially to the [*type of questions to avoid*](http://cooking.stackexchange.com/help/dont-ask): this is quite subjective, probably too subjective for a definitive answer and at risk of being closed.
Comment: related : http://cooking.stackexchange.com/q/66071/67
Comment: Thank you for the welcome! I think I can see why you say that. I just saw this link which shows how subjective it is:.http://homeguides.sfgate.com/top-10-tomato-plants-sauces-56577.html  I was told that San Marzano tomatoes were better than Roma for pizza and it was it was so true! I realized how little I knew about the tomatoes--their sweetness, thickness, etc. But I think @rackandboneman had a very good answer.
Comment: An addon that doesn't fit well anywhere else: San Marzanos are, in some places and at some times, sold green. They can and often will ripen on the counter.
Answer: The general consensus is that for occidental-style, cooked tomato sauces - all of the sauces you mention are - you want to use the more plum-shaped, thicker-skinned types like Roma or San Marzano, NOT globe tomatoes, especially not the greenhouse grown, large, watery varieties. The more ripe, the better.
Also, good quality canned product (whole peeled, or peeled strips) is usually considered superior to fresh, due to them being canned at optimal ripeness which would make transporting them fresh impractical. I found that the cheapest brands often are sloppily peeled and/or not ripe enough and/or too sour, it is usually worth going for the 90 cent can vs the 30 cent can.
Also, do experiment with what part of the tomato you use. You could:
-Use it whole except the stem, and puree or strain the sauce later (does not apply to canned peeled obviously :) )
-Use everything except the peel (will need looooong stewing to deal with the seeds)
-Use the flesh, and also the juice strained from the seeds
-Use only the flesh (lot of tomato needed, and you might lose some of the umami from the juices)
Also, mashing vs pureeing vs straining can make a lot of textural difference.
Comment: Interesting. There's a  lot to learn. Do you know the differences between Roma, San Marzano or others? I can see there would be different texture using the different parts of the tomato, but is the flavor different? Better for certain sauces?
Comment: Haven't tried ALL combinations :) The juice/pulp are usually considered umami-rich, however the seeds (which you would have to leave in to get the maximum of the juices) are considered bitter and hard to digest if they are not thoroughly cooked. Using only the flesh, or using the flesh and juices separately, allows you to saute the flesh (useful method eg in indian curries, which are commonly based on a tomato sauce that gets more sauteed than stewed in the early phases) and reap the benefits of sauteeing...
Answer: rackandboneman's answer is excellent, yes.  I would like to add: I feel that there are two ways to use tomatoes: raw, in salads, cooked, in sauces. In sauces you want ripe, sweet tomatoes with thin peels, because you are concentrating the flavour. Using acidic tomatoes would result in a very acidic sauce. I don't think there is a detailed answer to which cultivar for which sauce, although I would love to read the opinion of a tomato-obsessed person on this.
For salads (I know that is not your question, but you need to recognise this type too when choosing) you would use firmer, less sweet variaties with some more complex, maybe acidic flavours. Do not mix up the two.
And because it is generally impossible to buy good, ripe, sauce tomatoes,I would say: go with the advice to use good tinned once. Tomatoes are one area where the food industry really destroyed a magical food product into an easy transported and stored food-like substitute where taste did not enter the equation...
Comment: When you refer to another answer, it's best to be specific about it - there's no guarantee about what order future readers might read them in, and plenty more answers might get posted too.
Comment: Most plum tomatoes are rather thicker in peel than your typical salad water balloon... and even though they do have a slightly grainier texture, I still find Roma more pleasant in a salad than these ;)


My screen got broken, how to see on the computer what I'm doing in my device?

Question: My Android device screen got broken and I can't see what I'm doing. That's the only wrong thing with my device, the rest is apparently working OK. Is there any way to send my device screen to my computer?
I don't want to work blindly anymore!
Answer: You don't have to continue working blindly, you can use Droid@Screen: it shows the screen of a real or emulated Android device on a computer running Windows, Mac OS or Linux.

Droid@Screen is a stand-alone Java Swing GUI application that shows the screen of an Android device on a computer. Its author says that its typical usage is showing an app demo or during training, but it also fits your case.
In order to use it, you need the following to be installed in your computer:

Java 6 (at least)
Android SDK, installed and configured.
Environment variable ANDROID_HOME (or ANDROID_SDK_HOME) pointing to the Android SDK installation directory. (Optional)
USB driver for the target Android device installed.

Note: USB Debugging must be enabled on your device. (Thanks @JasonC for the heads up)
I have been using it with my Galaxy Nexus and I must say that despite a little lag between what happens in the phone and what is displayed in the computer screen (2-4 s), it does what it says. You don't even need to be rooted for that!
You can download it from the author's site, where you also can watch a complete video and text instructions about its installation and usage.
Comment: You have to enable USB debugging for it to work, which means its useless if debugging was disabled when you broke your screen, because the Android folks failed to make the connection that everything you'd want to do with a broken screen requires a working screen to change that one setting.
Comment: It is possible to enable developer mode and usb debugging via connecting a USB keyboard with adapter to the phone and sending a carefully planned set of keystrokes. Plan that set of keystrokes with an a phone of the same manufacturer and OS version. For example, down, down, ENTER will get you to the app chooser menu. Use arrow keys to navigate.
Answer: If your device has an HDMIport, you can connect that to a display with HDMI port. There are programs, like Droid@Screen, as Dan said, that can cast the device's video output to a PC over the USB connection.
Comment: I improved the wording of your answer a bit, Daniel. Hope you don't mind


Do servers hold one website only?

Question: As from what I understand the DNS link the domain name with the IP address of the server the website is stored on, does that mean each server can only hold one website? If they don't, how does calling the server's IP address know which website I want if there are many on the same server?
Comment: Wikipedia has a good introduction to [Shared Web Hosting](https://en.wikipedia.org/wiki/Shared_web_hosting_service). If you enter http:/// in your browser, the HTTP request won't have a domain within the `Host:` header. In case of shared hosting, the web server can be configured by the provider to handle this in different ways (e.g. have a default, redirect to provider etc.).
Comment: Great question. Every website used to need its own IP address (a server can have more than one IP address). The Host header in HTTP/1.1 was introduced to get around the exact problem you describe. See "Internet address conservation" in http://www8.org/w8-papers/5c-protocols/key/key.html
Comment: If http 1.1 didn't have the host header, ipv6 would be implemented by now ;-) :-(
Comment: In case you're looking far a way to run multiple applications on a single server-- say you have two apps MyApp and YourApp on ports 8001 and 8002 respectively. You can have two load balancers or application proxies at: myapp.com and yourapp.com. Have them receive requests at the default ports (80/443) and forward it to the actual server(s) at ports 8001 and 8002 respectively.
Comment: I have clicked links that break with messages like "this server has never/currently does not host the website you are looking for".
Comment: Not only do you get many websites per server, with some technologies (eg Apache vhost) an infinite number of websites can be called into existence at the moment someone asks for one.
Comment: OP, also keep in mind that each physical server can have multiple virtual servers running on it. Virtualization offers a lot of helpful IT admin abilities.
Answer: Basically: the browser includes the domain name in the HTTP request, so the webserver knows which domain was requested and can respond accordingly.

HTTP requests
Here's how your typical HTTP request happens:

The user provides a URL, in the form http://host:port/path.
The browser extracts the host (domain) part of the URL and translates it into an IP address if necessary, in a process known as name resolution. This translation can occur via DNS, but it does not have to (for example, the local hosts file on common OSes bypasses DNS).
The browser opens a TCP connection to the specified port, or defaults to port 80, on that IP address.
The browser sends an HTTP request. For HTTP/1.1, it looks like this:
GET /path HTTP/1.1
Host: example.com

(The Host header is standard and required in HTTP/1.1. It was not specified in the HTTP/1.0 spec, but some servers support it anyway.)

From here, the webserver has several pieces of information it can use to decide what the response should be. Note that it is possible for a single webserver to be bound to multiple IP addresses.

The requested IP address, from the TCP socket

The IP address of the client is also available, but this is rarely used - sometimes for blocking/filtering

The requested port, from the TCP socket
The requested hostname, as specified in the Host header by the browser in the HTTP request.
The requested path
Any other headers (cookies, etc.)

As you seem to have noticed, the most common shared hosting setup these days puts multiple websites on a single IP address:port combination, leaving just Host to differentiate between websites.
This is known as a Name-based Virtual Host in Apache-land, while Nginx calls them Server Names in Server Blocks and IIS prefers Virtual Server.

What about HTTPS?
HTTPS is a bit different. Everything is identical up to the establishment of the TCP connection, but after that an encrypted TLS tunnel must be established. The goal is to not leak any information about the request.
In order to verify that the server actually owns this domain, the server must send a certificate signed by a trusted third party. The browser will then compare this certificate with the domain it requested.
This presents a problem. How does the server know which host (website)'s certificate to send, if it needs to do this before the HTTP request is received?
Traditionally, this was solved by having a dedicated IP address (or port) for every website requiring HTTPS. Obviously, this becomes problematic as we start running out of IPv4 addresses.
Enter SNI (Server Name Indication). The browser now passes the hostname during the TLS negotiations, so the server has this info early enough to send the correct certificate. On the server side, configuration is very similar to how HTTP virtual hosts are configured.
The downside is the hostname is now passed as plain text before encryption, and is essentially leaked information. This is usually considered an acceptable tradeoff, considering the hostname is normally exposed in a DNS query anyway.

What if you request a site by IP address only?
What the server does when it does not know which specific host you requested depends on the server implementation and configuration. Typically, there is a "default", "catchall" or "fallback" site specified that will provide responses to all requests that do not explicitly specify a host.
This default site can be its own independent site (often showing an error message), or it could be any of the other sites on the server, depending on the preference of the server admin.
Comment: Also, one site may be split across many servers, as is the case with load balancers, like Heroku and Amazon uses.
Comment: @phyrfox Yea, I thought about adding that, but it's only tangentially related to the question and I didn't want to make the answer too long. Might still end up adding a section for it later.
Comment: Legend has it that subdomains point so specific computers inside a network. In theory
Comment: "Traditionally, this was solved by having a dedicated IP address (or port) for every website requiring HTTPS. Obviously, this becomes problematic as we start running out of IPv4 addresses.", So without that invension, we all would have ipv6 by now.
Answer: I have this explanation for non-tech people.
Jack, Jill and Joe live at a dormitory, and they don't have cellphones.
In the phonebook, they are all listed with the same number. (A-record)
You dial the number, and somebody picks up the phone; you say "I'd like to speak to Jill", and you get her on the line.
Instead of an A-record (A phonenumber/IP-adress) in the phonebook, it may just say "Dormitory X", then you must look further for the number for Dormitory X. This is a CNAME record.
If Jill is not available, you might get

404 Jill is not here
410 Jill is dead.
301 Jill is moved in with Peter
302 Jill is visiting Peter,
call him instead
400 I can't understand you.
401 Who are you? What is the password? or We don't allow male callers after 10pm
402 Payment Required (Are you sure Jill is her real name ;-) )
403 No, that is not the right password.
418 Jill is a teapot :-)
429 Jill can't take any more calls.
451 You are violating your restraining order.
500 Our phone system has broken down.
Comment: For the curious the RFC behind 418 is https://tools.ietf.org/html/rfc2324 and an interesting article https://sitesdoneright.com/blog/2013/03/what-is-418-im-a-teapot-status-code-error :)
Answer:
As from what I understand the DNS link the domain name with the IP address of the server the website is stored on, does that mean each server can only hold one website?

First, you need to understand that there are a number of distinct concepts here.

Web site, a group of web pages that form a coherent whole.
IP address, a numerical address (32-bit for IPv4, 128 bit for IPv6) used by the internet protocol as the source or destination for traffic.
Server, a machine whose job is to serve requests from clients.
Hostname, a name used to identify a machine in DNS (e.g. "www.example.com" or "en.wikipedia.org")

There is not a one-to-one relationship between any of these things. One server can have multiple IP addresses; multiple hostnames can point at one IP address; one host name can point at multiple IP addresses. Multiple websites can be under the same hostname. One website can be spread across multiple hostnames.

If they don't, how does calling the server's IP address know which website I want if there are many on the same server?

In the old days (HTTP 1.0 and before) each hostname that the server wanted to handle differently had to have its own IP address. This was rather wasteful.
HTTP 1.1 added the Host"header as a mandatory field in the HTTP request (IIRC some vendors had previously supported this as an extension). This told the server which hostname had been requested and hence allowed it to serve different content for different hostnames on the same IP address. Support for HTTP 1.1 in clients is now ubiquitous.
Unfortunately, SSL (later TLS) added a wrinkle. Establishing a SSL/TLS session requires the server to present a certificate to the client that covers the requested hostname, but the HTTP request doesn't arrive until after the SSL/TLS session is established.
It is possible to have one certificate cover multiple hostnames through the use of the SubjectAltName field or the use of wildcards in the CommonName field. However, this poses administrative challenges, especially if the hostnames involved are under domains with different ownership.
So TLS introduced the "server name indication" (SNI) extension. With this extension, the client sends the requested hostname to the server during the TLS handshake procedure. The server can then present the appropriate certificate. Unfortunately, while current versions of all major SSL/TLS implementations support SNI, it has taken a long time for older versions to fall out of use.
Comment: You forgot to mention that TCP can listen on many ports, running different servers on each...
Comment: Yes, but you don't have portnumbers in dns, and you can't expect joe.p.user to go to http://our.fabulous.site:81
Besides, some firewalls block outgoing access to non-standard portnumbers.
Answer: The answer is a bit more complicated than some of the answers have made it out to be. When you perform a DNS lookup you MUST get an IP address (A record for IPv4, AAAA for IPv6). You have to be able to open a socket over TCP/IP to communicate or the whole thing fails. That address may represent a server or it might represent a load balancer. It could even represent a proxy. If the host is behind CloudFlare, for instance, the address you get is of a CloudFlare server. The real server is somewhere else. This lets the host avoid problems like Denial-of-Service attacks.
Virtual hosting is what you're asking about (some of the other questions touched on this, but not in any detail). Virtual hosting takes the web request and looks at the host name (i.e. domain.com) to determine which website to serve up. So in the Apache HTTP web server you would have a configuration like this
<VirtualHost *:80>
    ServerName www.domain.com
    ServerAlias domain.com

    DocumentRoot /var/www/domain.com
</virtualHost>

This is simplified for example. So we're telling Apache to listen on port 80 of any IP (in modern virtual machine hosting your machine's IP may be different from its live IP). We then tell it that this is the domain.com website and what directory that website lives under. We can then repeat this block over and over to tell Apache to handle different websites. Every web server supports this type of system.
Another way to handle this would be to tell the web server to direct all web traffic to one single programming script (i.e. PHP, ASP.NET, etc) and then that single script will determine what website and page to display.
Answer: Using DNS you can assign as many names to an individual IP address as you want (in your hosts file you may simply separate each name with spaces, for example). Using a DNS server you may also assign multiple IP addresses to a single name. It is not limited to a one-to-one relationship.
A web server knows which site to serve by examining the requested URL. It looks at which domain was requested, the port which was requested and what what protocol was used. This has nothing to do with DNS and is handled by the HTTP protocol.
Answer: Web server has the concept of the host container (here is the documentation for Tomcat, for instance). Multiple host containers can be configured for the same box/ip address, servicing multiple domains. Containers have independent working directories, authentication realms, log directories and things the like.
The server finds the relevant container for the new request buy the name of the domain is part of this HTTP request.
Completely different web server instances can share the same IP address if they run on different ports. This is mostly used in various development and testing environments where domain names may not be available, as production server cannot run on arbitrary port.
Finally, even if the strictly unique IP address is required for a website, a server box often has multiple network adapters so is configured to use multiple IP addresses.
Answer: Your server IP address can hold many different domain names at the same time.
When you access the website your browser send the HTTP request with domain name in it, and the server can found which website data it should send you back then.
It's called the virtual hosts, so simple as that :)
Take a look here for more information about DNS and virtual hosts.

Do servers hold one website only?


If the North and South Poles' ice melted instantly, what would be the effect on humans and other animals?

Question: If the North and South Poles' ice melted instantly, what would be the effect on humans and other animals?  Would the sea creatures thrive?  Would a lot of land get covered by water?
Comment: Have fun ! http://www.floodmap.net/
Comment: The city I'm from gets flooded, but my house stays dry, if I recall correctly. :P Yay for hills.
Comment: If you're in syndney or on a beach...ya there's a chance.  I'm around 185 meters above the ocean, so as far as I know this would result in my relatively ugly view becoming desirable ocean-front.  heh
Comment: @Cragor Well im from australia so im not sure if my city gets flooded if this would happen
Comment: @Vincent Quite Usefull. It glitches out a bit. But its in beta so yeah.
Comment: Just how "instantly" are we talking here? Because the resulting waves would be interesting....
Comment: @Vincent thats an awesome site.
Answer: Well, the first effect is the temperature on the earth would drop significantly.  It takes a lot of energy to melt the ice on the planet, and if it comes from the atmosphere, you get a massive drop in temperatures.  (Find that here.)
Second effect...yes the water levels would rise.  The north pole is mostly sea ice and this would have little effect on ocean water as the ice displaces the same amount the melted water would.  The south pole melting would increase sea levels by around 200-300 feet (pending study you want to accept).  Most of humanity does in fact live close to the ocean...a country like Bangledesh goes 47% underwater (that's the highest population density country on the planet).  This is around 1/4 of the American population under water as well.
Sea creatures won't necessarily thrive.  Most of them have adapted to the salt water concentrations they are used to and a quick adjustment stands to harm them.  Otherwise, unless there is something fundamentally helpful (delicious?) for them to take on land, it wouldn't have much a difference.  As a more scary alternative, certain nuclear power generation sites could be submerged, potentially flooding the nearby water with radiation.
Your worst case scenario is if this sudden increase in fresh water alters the current (great oceanic conveyor belt if you will).  This is the cycle of heat throughout the ocean...certain areas (Europe primarily) remain temperate by warmer tropical currents cycling water to the northern regions.  If this cycle was to falter, England would likely freeze over.
Most of these points here are answers to much more specific versions of this question on Worldbuilding.
Comment: As for first effect, I should think that would be massive tidal waves around the globe as millions of square miles of multi-mile deep ice suddenly turns to water and rushes into the oceans.
Just a land slide from a single mountain face into water can cause a tidal wave that will travel hundreds of miles and cause some devastation. This would be like nothing that had been seen before.
Comment: Certain places in Europe would be completely flooded as well: the Dutch would have their conservation status turned from a LC into a CR (if not an EW) overnight.
Answer: The ice near the North Pole is floating, so no sea level rise would result from that. The South Pole melt would only have an effect if it reached the ocean.  If it was a sea near the South Pole, no effect either.
If you melt all the ice caps on Greenland and Antarctica, a significant sea rise would occur. Usually numbers like 200 feet are quoted.
Comment: No, it is already displacing the ocean as floating ice.  Only land based ice can raise sea levels.
Comment: There would be a sea level rise if the north polar cap melted. The water has to go *somewhere*.
Comment: Ah. Of course it is.
Answer:
If the North and South Poles' ice melted instantly, what would be the effect on humans and other animals?

Humans would be in a bit of trouble as a significant portion of the worlds arable land would be under water.  If this all happened at once with no warning that may not be terribly detrimental as the bulk of the world's population would be dead...so thats good.
Land animals would be similarly in trouble as entire habitats and species would be completely wiped out.

Would the sea creatures thrive?

Maybe in the long run, the rapid changes to temperature and ocean depth would wreak havoc on ocean environments.  Reefs would be in water 200-300 METERS deeper meaning they would have trouble surviving with their food and light sources no longer being available.  Similar to land based animals there would be a great upheaval and mass extinctions most likely.  In the long run marine life would bounce back, there would likely be a significant increase in algae and other simple water life with the reduction in the numbers of predators.  Ocean animals would definitely be the first to bounce back though.

Would a lot of land get covered by water?

See the map link Vincent provided, its an excellent resource

Some additional notes
Geopolitical Stuff

Every communist nation would be either A.  Underwater or B.  Decimated
Northern Europe is totally hosed...water joke for you there.
The US East coast is now an island
Russia is essentially gone, only the sparsely populated west would remain
Africa becomes the center of the world as it is the least impacted by the water level rise, its hard to predict how the weather pattern would end up but if you assume the continent gets more moisture in the north it would end up being the population center of the world.
The pacific rim...more of a lip now
Japan survives surprisingly well landmass wise

Eventually a new balance would be achieved of course, nature is skilled like that.
Comment: Your forgot Australia. They are probably doomed given that most of Australia's population is near the coasts.
Answer: Beyond sea level increase, also weather patters will drastically change. After melting Greenland's glaciers, less dense fresh water would stop Gulf Stream which transfers heat from equatorial Atlantic to Europe. Europe would go colder (decrease of agriculture) and hurricanes would get stronger.
Melting permafrost in northern tundra's of Canada, Alaska and Siberia will release huge amounts of methane, which is very potent greenhouse gas (more than CO2) which will increase greenhouse effect.
If things go really bad, like it did 55 MY ago in Paleocene–Eocene Thermal Maximum and it was tropical hell in Alaska - up to 74F warmer than present.
Salinity of water would not change significantly (amount of added fresh water is small).


Could ships in space use a Steam Engine?

Question: When in space, the amount of force required to move goes down drastically, due to the virtual lack of drag. In this environment, would be feasible for engines in space (more specifically belonging to generation ships) to use steam?
Comment: Technically speaking, modern rocket engines that use liquid hydrogen and liquid oxygen as the fuel *are* steam engines.  The combustion reaction between the oxygen and hydrogen flash boils the product (which is water) and ejects the steam from the nozzle at ludicrous speeds.  So, high-pressure steam is what provides thrust, the only difference is from where the steam gets its high temperature/pressure.
Comment: I would use a steam engine in space to make espresso.
Comment: Just to clarify, this system will not use the steam engine to climb out of atmosphere right?
Comment: What stream engine you imagined? Obviously not the one used to turn wheels, so what's your idea? What would you call a steam engine?
Comment: @MozerShmozer: technically speaking, a steam engine is specifically defined as an _external combustion_ engine, which a LH/LOx rocket engine is not. But you're right, it does make some sense to call such a rocket "steam engine".
Comment: @TrEs-2b do you ask about steam engines as a part of the ship's infrastructure (internal transporting, generating electricity, etc.), or are they supposed to propel the ship?
Comment: @CemKalyoncu right
Comment: I would assume the engine would boil, pressurize and release steam for thrust.
Comment: Your reactor is also probably going to be steam driven, most people don't realise that nuclear reactors (and coal and gas and oil) are all basically steam engines.
Comment: To be clear, is the question on using H2O for reactionary mass?
Comment: You know steam engines use a huge amount of water, right? Where would you get that water, and how would you justify using it in a steam engine rather than, say, letting hydrogen and oxygen combust and produce thrust directly? What's the *benefit* of using a steam engine? It's not like you can use wheels for propulsion in space :)
Comment: @MozerShmozer - I think that the *crucial* difference is that steam engines can [recycle](https://en.wikipedia.org/wiki/Steam_engine#Cold_sink) their water (they condense the steam back to liquid state for reuse), while there is some loss of water (which theoretically can be minimized for space age technology) - this is radically different from rocket engines, which basically "throw away" the gas to produce thrust - they lose a lot of mass, by design, and there's no way around that.
Answer: Using steam to propel yourself in space obviously requires that you:
1) Have a steady supply of water
2) Have energy to boil said water
You might be able to scavenge water from comets, etc., but this is still a very wasteful process, and water doesn't simply grow on trees ... in space.
Furthermore, water and large, complex systems don't typically play nice together. The last thing you want is pipes rusting out in your engine, not to mention that any time you shut the system down said water would start freezing, and burst pipes could really ruin your sojourn to another star system. Here on Earth we have the luxury of being able to constantly manufacture new pipes, and fix/maintain them, but on a generation ship it would be a lot more difficult.
All in all, there would be a lot of practical challenges to implementing that sort of propulsion system.
Comment: Burst pipes, tanks, corrosion and overheating are very real problems in all rocket engines. Some propellant is so cold you need to pre-chill the engine so it doesn't flash boil when used. The LM engine propellants did so much corrosive damage that the engine was effectivly a run-once device. Most engines would melt apart but for the fancy coolant systems in use. Soyuz can only be fueled for a short time before the fuel corrodes the tanks. A bit of rust on your plumbing seems pretty mild in comparison to real rocket engines.
Comment: @vsz but where, sir, will the electricity propel the ship?
Comment: *"Have a steady supply of water"* - not necessarily. You can have a supply of Hydrogen and Oxygen. Many modern rocket engines do it. Or, you can have a closed loop where the water circulates and turns a turbine, generating electricity.
Comment: Also you don't need constant supply of materials. A generation ship requires minimal amount of course correction. It will run during launch and descent.
Comment: @Innovine - I didn't know all that. All the more reason to come up with a system that _won't_ rust when on a space ship meant to last hundreds of years, don't you think?
Comment: @TheGreatDuck : Electricity can have many uses, even if it's not used to propel the ship. The question didn't specify that it must be used for propulsion.
Comment: @vsz RE having a steady supply of hydrogen and oxygen - I read using steam as boiling water to get pressure and using that pressure.  If you have hydrogen and oxygen you are not likely to first create water and then heat said water, it's easier and more effective to make a hydrogen rocket engine. - Of course said engine do expell steam, so you could make the case that the rocket is using a steam engine, but I don't think this is what OP meant.
Comment: @vsz the op is welcome to comment but I'm pretty sure they were intending the obvious meaning of "power a rocket". After all, powering things inside a rocket is no difference than powering things on Earth and in that case the question would be more like "can steam engine's be used to power modern machines?".
Comment: I remember reading something about 'steam engines' that use liquid other than water. I found one that worked using CO2 https://www.scientificamerican.com/article/can-carbon-dioxide-replace-steam-to-generate-power/ I don't know about the practicality of it but it would be quite fun as a use for CO2 on the generation ship.
Comment: Just so it's said plainly here.  Any spaceship, using any real physics, must loose mass to accelerate
Comment: @Andrey Must lose mass - or interact with preexisting outside mass.
Comment: Another practical obstacle is that steam engines generate **a lot** of [waste heat](https://en.wikipedia.org/wiki/Steam_engine#Cold_sink). In space, you can't radiate that heat to the environment, so you'll need some other form of cooling (which takes energy)...
Answer: Depends on what you mean by "steam engine".  A NERVA-type engine using a nuclear reactor with water as the reaction mass would be a steam engine, no?  Likewise if water was heated with a concentrating solar reflector.
Indeed, comets do change their orbits that way, as water (or other volatiles) is boiled by sunlight.
Comment: Nuclear reactors in general are steam engines. They turn water to steam and use it to spin turbines, doing work (most commonly, generating electricity).
Comment: @JAB: Good point, though turning a turbine doesn't seem to do much for spacecraft propulsion.  Though maybe you use a reactor to turn a turbine to generate electricity for your ion drive.  That's keeping with known science: I suppose something like a reactionless drive could be powered with steam-generated electricity.
Answer: A steam engine in space (assuming you mean a steam rocket engine) is not only feasible, but under some circumstances even desirable. The Neofuel site makes a strong case that using water directly from comets, asteroids etc. rather than breaking it down to hydrogen and oxygen, mostly because the extra mass of equipment needed overwhelms the advantages of higher ISP.
All you really need is a source of energy to heat the water into steam, and away you go. This can range from solar mirrors (a solar moth), beaming laser or microwave energy to the ship or using an on board nuclear rector. So long as the water is receiving enough heat energy, you have steam. Throttling the engine is as simple as adjusting the flow rate of water through the heat source.

The NEOfuel proposal for a steam rocket

Simplified schematic of the NTR steam rocket. The water bladder(s) would be far, far larger in real life
The down side of a steam rocket is truly terrible ISP of 195, compared to @450 for the ideal H2/O2 liquid fuelled engines or between 800-1200 for hydrogen NERVA rocket engines. You would be using enormous masses of water to get from point A to point B, and getting from Earth to the outer planets would take many years at best. The NEOfuel site calculate a water tanker carrying a payload of 10,000 tons would carry 325,000 tons of water as reaction mass.

Final Configuration: H2O NTR Option

10,000 tons Net payload
358,000 tons water propellant
104 tons water bladder
208 tons water bladder tank armouring
656 tons Nuclear Thermal Rocket engines
122 tons water extractor

By contrast, the LH2 option works out like this:

10,000 tons useful payload
(138,000 to 453,000 tons water for LH2) 15,400 tons to 50,300 tons LH2 propellant
154 tons to 503 tons armored LH2 tanks
472 to 1536 tons of LH2 NTR engines
48 - 156 tons to extract water
559 to 10248 tons for electricity for electrolysis
70 to 5027 tons for cryolizer hardware
372 to 10609 tons for electricity for cryolizer
So steam is a means of getting low cost bulk payloads around the solar system cheaply, but you will have to accept low ISP, massive amounts of water being used as reaction mass and long trip times.
Comment: If you read the NEOfuel site, the design is predicted on the availability of large quantities of water on the Moon's poles, as discovered by the Clementine mission in 1994. NEOfuel is based on the idea that there is water virtually *everywhere* in the Solar System.
Comment: A Nerva type rocket is heating and accelerating water, with a higher molecular mass than the LH2 of a conventional NERVA. A LH2/LOX rocket runs at a much higher pressure and temperature than the nuclear reactor of a NERVA. It is possible to run the NERVA at higher temperatures and pressures, but the increased stress could be detrimental to the reactor. Breaking water down exposes the reactor to molecular oxygen as well.
Comment: Read the Atomic Rockets link. Lower mass=higher ISP, all other factors being equal. Xenon is preferred for other technical reasons rather than molecular mass. The LH2 NERVA vs a LH2/LOX rocket should make that point clear (800-1200 ISP vs 450 ISP). And pushing the temperature to improve the ISP of a steam rocket will have very detrimental effects on the reactor.
Comment: I think OP said "generations ship" to point us more to the interstellar travel. Is there enough water (comets) outside the Oort cloud? Anyway could you edit to delete the weird picture, because predicting thousands of tons of ice available on Moon surface severely hurts the credibility of the idea (which seems suitable to me for kidnapping whole ice comets).
Comment: I'm not clear why a NERVA-type engine using water would be different than an H2/O2 rocket.  They're both ejecting steam out the back, no?  So assuming the nuclear engine can reach the same temperature & pressure as the chemical reaction, the result should be the same.  And you don't have to fool around with cryogenic liquids.
Comment: @Thucydides: I'd think a higher molecular mass would be good (all else being equal), as it means less space/mass for tankage.  Thus we see ion engines using xenon (the heaviest non-radioactive inert gas) rather than helium or neon.  And I'm assuming some technical development, so the exhaust velocity of a NERVA would be at least as high as an H2/O2 rocket...
Answer: Yes, you could boil water to build up lots of pressure, and vent the steam as a propellant, and the spaceship would move. But it would move very slowly, and then you'd be out of water.
It's not a very effective propulsion mechanism.
Comment: increase the pressure. the only important factor is how fast the exhaust is
Comment: Heating water to the point it becomes plasma will dramatically increase ISP, but at that point it no longer is a "steam rocket".
Comment: how slow only depends on the speed of the exhaust. it does not have to be that slow.
Comment: How would you suggest making it faster? Boiling more water? That'll make it heavier...
Comment: @njzk2 the mass of the vessel is also important. You need more water to make higher steam pressure. Tyranny of the rocket equation. Steam powered spaceship doesn't scale well, at all.
Comment: @Innovine they probably don't scale, indeed. The bulk and weight of a high pressure boiler is certainly very limiting
Comment: @njzk2 As is the weight of the extremely inefficient water fuel.
Comment: @Innovine the weight of water is irrelevant. density might, in terms of container size (and therefore weight)
Comment: Dont think its that simple many efficient high isp systems have a lot of heavy radiators which disperse heat .Using some of that mass for water and steam would seem more efficient
Answer: The question didn't state that the steam engine needs to be used to propel the ship.
So yes, if the ship is large enough and needs a lot of electricity, it is feasible to use a steam engine, or more specifically, a steam turbine. The water is boiled not by burning coal, but by nuclear fission. There are modern ships and submarines which do this. Of course, they also use it for propulsion, as electrical motors turn propellers. You obviously can't use propellers in space, but if your internal electrical needs are high enough, a nuclear reactor can be warranted on such a spaceship. And, nuclear reactors are actually a form of steam engine.
Comment: This is interesting. Makes me wonder about using other kinds of liquids to drive turbines in a closed loop system (no loss), but that don't require any additional heating (like water). For example a liquid that can be stored and kept cold in space and away from living quarters, but will start to boil and drive turbines as it is moved to areas that are being kept climate controlled for human comfort anyway. Perhaps the collection and compression there might negate any savings from not having to heat it. Anyway, nice lateral interpretation of the question.
Comment: @coblr, using the heat energy from the living quarters to heat up the liquid will cool down the living quarters—probably far too much.  Even if it is only a small amount of liquid you are heating, that energy has to come from *somewhere.*  On the other hand, you could limit the flow of liquid through the living areas and use it for free air conditioning.  ;)
Comment: @Wildcard: ya, damnit. I always forget the second part. I'd be curious to see the numbers but far to lazy (and ignorant) to try and ballpark it. Cooling for all those computers and bodies would be a great idea.
Answer: It depends on what do you call a steam engine. A steam locomotive obviously can't be used, since in space we have no external support, so we need a jet engine. But can a jet engine use steam to propel itself? A rocket engine might work using liquid hydrogen and liquid oxygen, producing hot water (=steam) in the process. But can we still call it "a steam engine" though?
Comment: This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - [From Review](/review/low-quality-posts/25098)
Comment: @Azuaron you're right, but the question wasn't clarified, unfortunately. It's still unclear, was OP asking about using engines to propel the ship, or are they just a part of the ship's infrastructure.
Comment: And that's what comments are for, not answers.
Comment: @Azuaron I agree with you. How can I improve my answer? The answer was "yes, you can use steam to propel the ship, but that will be a jet engine, not a steam engine".
Comment: Well, first, it seems like you have some questions you want clarified. If that's the case, you should put a comment on the question and wait for a response. After you get your clarification, you'll be able to write a better answer.
Comment: @Azuaron but the question wasn't clarified. You can say that about any answer, not only mine.
Comment: Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/46786/discussion-between-azuaron-and-enkryptor).
Answer: I haven't done the math on this but it seems to me a steam engine would be incredible dumb since the H and O that you would be mixing and then boiling just to be released into space would take more energy to produce similar or less thrust than just jettisoning oxygen/? like most RCS thrusters currently do. So possible, but not feasible.
Answer: The Neal Stephenson book Seveneves shows characters embedding a nuclear reactor into the center of the ice core of a comet, then melting the ice of the comet into steam to use for thrust, turning the entire comet into a spaceship and rocket engine.
So if you have a piece of ice 2km in diameter to use as a water source, it's certainly possible.
Comment: This is the example I was thinking of.  They didn't seem to have much in the way of water left when they reached their destination, but they started with so much that what they had left over was still central to the plot from there on.
Answer: No, not by any sensible definition of steam engine, at least not for propulsion (since you mentioned drag, I will focus on propulsion first).
A rocket works by throwing out propellant to the back. This provides forward thrust. Now, in order to gain more forward speed, you need to either throw out more propellant, or throw the propellant out faster.
In orbital mechanics, there is no drag, yes. But you are still subject to gravity. You are nit weightless, you are in free fall, which means objects and people inside the spaceship appear to be weightless. Being in orbit means actually falling sideways fast enough that you miss the ground and swing around.
In order to change your orbit, you need to change your velocity. This is called delta-v (which literally means "change in velocity").
You need to change velocities quite drastically. In order to get fom the ground to low earth orbit (LEO) you need to have ~9400m/s delta-v alone.
So, back to what I said before, you can either throw out more propelannt, or throw it out faster. In your spaceship, the supply of propellant is limited. Thus, maximizing the speed at which you throw propellant out is the key factor. This is called the exhaust velocity.
But, you can't boil water to arbitrary temperatures, so the exhaust velocity (which is achieved by expanding the exhaust in a de laval nozzle) is still limited. And even if you could boil it to arbitrary temperatures, you still need fuel to boil it in the first place, which is limited, too.
It turns out, you can build a steam rocket. but its horribly inefficient. According to this Wikipedia site, you can achieve an Isp of 195s with a steam rocket - well below the modern Hydrolox rocket which achieve Isp greater then 450s (Isp and exhaust velocity are closely related, ve = Isp * g0).
So your efficiency is horrible. This means the only way you can get anywhere is by increasing propellant mass. The concept of "mass fraction" is actually quite important for modern rockets. To give you an example, the STS external tank was 96% fuel and only 4% structure. A soda can has 94% soda and 6% structure (by weight). So the ET held more percentage of fuel then a soda can. Modern rockets like the Ariane V or upcoming Ariane 6 have even more propellant.
You can easily see that its not really feasible to miracuosly improve the fuel fraction by a great margin (or we would have already done it, while still using more efficient propellants).

This leaves you with using steam engines for electrical systems. You could - in theory - use a nuclear reactor, which boils water and then turns propellers.
Buts thats not as exciting and steampunky as using it for propulsion.
Answer: $F=ma$
Newton's first law (correct me if I am wrong).
Second law, for every action there is an equal and opposite reaction.
Basically, in order to move forward, you need to push a lot of something behind you at a low speed or very little of something at a high speed.
When we walk, we apply a small amount of force, acceleration, opposite to where we walk. It just is not noticeable, because the earth is very big and we are very very small.
Comment: Actually, those are [his second and third laws](https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion).
Comment: Yes but how does it work in space?
Comment: what you really want is the rocket equation, and figure the exhaust velocity as a function of the pressure you can mount (which in turn in a function of how hot you can get the steam)
Answer: Let's find out!
In space, you use conservation of momentum to propel yourself. Basically, you throw mass very quickly at the back to push yourself forward.
The most efficient systems use very small particles with very very high velocities. The more simple systems use combustion to create hot gas that creates pressure that creates exhaust velocity.
The thrust of such an engine is the exhaust speed multiplied by the exhaust rate (how much mass you send * how fast you send it).
How much mass can be variable on the size of the exhaust, how many engines you have... so let's focus on the exhaust speed:
Locomotives steam boilers have been know to hold up to 1,500 psi (10.34 MPa), or about 100 atm.
Unfortunately, I have no idea where to go from here. My understanding of that topic is too limited to make the computations.
I fear that would be disappointing, though, as methalox rocket engines have exhaust velocities of 5000m/s and more.
Comment: see https://en.wikipedia.org/wiki/Hypersonic_wind_tunnel
Answer: I suspect that lurking behind the question is a steampunk motivation.
In that case it is useful to recognize that what the 19th century called "steam power" was really a code word for coal powered via a steam turbine.
While the Earth bound practice with steam engines was to vent the steam and refuel with water along the way, you could have some internal functions of a space ship that were powered by coal and rather than venting the steam, sent it to a cooling area exposed to the chill of space and then recycled it.
This would be useless for propulsion purposes, and would take a lot of weight per energy density, and you probably would want to vent the particulate exhaust of the coal combustion into empty space, but it could run life support systems on board and the heat generated by the engine could also heat the ship.
For propulsion into orbit, from a 19th century steampunk kind of perspective, you might want to launch the space ship with coal powered internal systems from a huge cannon a la Jules Verne, or some sort of giant catapult, or a hydrogen gas explosion in a long glass tube, possibly from a high altitude attained via a balloon or an airship.
To return its occupants to Earth, the ship might break into two pieces like a one stage rocket that would spring apart from each other perhaps with a highly coiled metal spring that could be released when it was time to re-enter. Neither the launching method nor the return method would provide all that much thrust, so you'd need to limit space voyages to near Earth orbit (and if the landing capsule were pointed the wrong way when activated everyone would die, while if someone was not in the landing capsule when it was activated, that person would die, in either case in deep space after life support supplies eventually run out). After the return spring operation, an Apollo style parachute (or better yet a parachute modeled after a huge Victorian umbrella) could be used to protect the folks in the landing capsule.
While you couldn't make it to the Moon, let alone anywhere else that way, you could get a super high altitude view of the world (for purposes from spying to weather prediction) without having to have a working aircraft wing (something not developed until the early 20th century by the Wright Brothers), and you could also go up and then land elsewhere on Earth to go around the world in a matter of hours rather than Verne's 80 days by balloon. You could also use the vantage point of space for an early space-telescope for precision astronomy impossible from Earth at the time.
As long as you stayed inside the strong, airtight ship (perhaps lined with lead to prevent cosmic rays from doing too much harm), there would be no need for space suits. You could travel in a coat and tie.
Comment: There is no "chill of space". Instead you have a near-perfect isolation, which means it's *hard* to release any heat away from the ship. For real  space vessels the overheating is a major design problem.
Comment: A fair point. Still some amount of infrared radiation must escape the ship, somehow - perhaps that would have to be incorporated with a heat exchanger and the soot removal system.
Answer: You can use steam engine heated by a very high energy density source of heat to produce steam to turn turbines to produce electricity (such system are known as 'nuclear power plants' here on Earth). Recycle your steam back into the system otherwise you will need to hunt for comet ice to refill your tanks.
Use that electricity to power pure laser/microwave propulsion which doesn't require any additional propellant (well, almost, you'll be losing mass carried out by photons leaving the business end of your rocket). Your propellant is the light, which is very lightweight but flies away at the maximum possible velocity, can't get faster than a speed of light in this reality.
Since steam engines are kind of inefficient, significant portion of the energy will be escaping as heat and will heat up the ship and will eventually be radiated away from it, which may work for or against your desired acceleration vector (depends on which side of your ship heats up higher).
You may need to build a large array of radiators to get rid of that waste heat, for efficiency reasons keep them on the same side of your ship as the laser engine so the infrared photons from the waste heat also accelerate your ship.
Note that you may need a generation ship to get from Earth orbit to Moon orbit using this set up.
Answer: Yes you could
But that wouldn't be the best way, the question is how are you heating up your liquid and how exaclty do you wan't to probell, if you would use solar panels and then use the electricity to heat your liquid up, you could propell yourself forward, however it won't work on a "generation ship" since you can't use it mechanicly "propeller or something like it" and so your only way to use the energy would be to throw the liquid in gasform out, witch wouldn't last that long.
Also would it be an unneccesary step between, since you could use the electricity directly on photon engines or even simpler, light engines, which propell very slowly but in an environment where you have basicly infinite energy and nothing to slow you down these engines can archive far bigger speeds than rockets or similar.
Answer: Yes you can use steam effectively, whatever steam comes out will instantly freeze. This frozen water can be collected in a device like a solid parachute at the back and then recycled again and again.  Nuclear fission can be used for heating the water, and as space is already super  cold the  necessary cooling effect for the nuclear device can be regulated.  Solar energy can be stored for other things.
This way you have a cheap and endless supply of fuel to make you move in any direction.
Comment: Welcome to Worldbuilding! I'm not sure where you get the steam instantly freezing from - inside a generation ship, you need reasonable temperatures, so it's too warm inside. Meanwhile, outside the ship, the pressure's so low that it would probably just dissipate off. Similar for the nuclear fission cooling - if you want to cool something quickly, you'll probably need some way for the heat to dissipate as I suspect that radiation cooling won't be fast enough


Does a direct deposit arrangement also enable direct withdrawals?

Question: Suppose, for example, an employer in the United States deposits employees' paychecks directly into their bank accounts. If they determine (whether correctly or incorrectly) that a former employee was paid more than they should have been due to a payroll error, is there any (legal) mechanism by which they could take the alleged excess pay out of the former employee's bank account without the former employee's consent?
Comment: This is probably a question for law.SE not money.SE. There are plenty of mechanisms by which they could take money (anyone with access to ACH who knows your routing and account number has the mechanism to take money) so the interesting part of the question is whether it's legal.
Comment: @R.. The legality will largely depend on the agreement/contract that the OP signed to authorize the direct deposit. Thus, we'd need access to that agreement to be able to determine what has actually been agreed to.
Comment: FWIW I worked with someone who maintained two bank accounts, and transferred their paycheck from the Direct Deposit account to the other account immediately on payday, exactly because they didn't trust the company's ability to withdraw as granted by the Direct Deposit agreement.  This was a company that required Direct Deposit.
Comment: @R: Out of curiosity, what would those mechanisms include other than ACH?
Comment: For a (Sept. 2019) case of this actually happening, and getting screwed up to boot, see https://krebsonsecurity.com/2019/09/ny-payroll-company-vanishes-with-35-million/
Answer: Usually, when you sign up for direct deposit you have to sign a form giving your consent. This contains a clause which allows the company to make withdrawals up to the paycheck amount, in order to correct any errors.
In short, if you signed up for direct deposit then you have already given your consent for them to fix errors by withdrawing money.
If you do not consent (and therefore do not get direct deposit), they will usually require you to write a check to the company for the difference.
Comment: There is a small window of time (i believe a few days) for them to reverse a deposit.
Answer: Note that many large companies don't offer any other payment methods, so you are bound to use direct deposit (or work for free or get another job).
I have never signed anything, and from the people in HR I know they would never do that (taking money back). If you owe them a correction, they (snail-)mail you about it, and give you thirty days or more to correct it. The assumption is that the moment money gets into your account, you trusted it's yours and spent it immediately.
Update: after some googling, it seems that such forms typically/often(?) do authorize 'appropriate corrections/debits'. See for example this Intuit template that comes with Quicken: https://http-download.intuit.com/http.intuit/CMO/payroll/support/PDFs/Misc/DD_Form.pdf.
Second update: many states have laws that allow the employer to enforce direct deposit. See for example https://www.patriotsoftware.com/payroll/training/blog/can-employers-make-direct-deposit-mandatory/ for a map (scroll down one page). Overall, 82% of all employees use direct deposit.
Comment: @DoktorJ The US Federal Government [requires direct deposit](https://www.law.cornell.edu/uscode/text/31/3332) by law for both salary and Social Security payments. Many companies in the US, particularly those with large numbers of lower-wage employees (retail, construction) do require direct deposit or else they will issue a payroll card.
Comment: I would comment that this general rule about not taking money back may not apply in the case of very large mistakes. If your paycheck is normally $2500 and they deposit $80,000, you can pretty much be sure they will be taking it back pronto and not waiting for you to decide to pay it back over the next 30 days. And spending this money could be considered a crime by itself, since you would have had reasonable reason to believe it was in error.
Comment: HR wouldn't make this call, accounting would.  You better believe the accountants will want that money back.
Comment: Where do you live that large companies don't offer any other payment methods? In the US, while direct deposit is vastly preferred, companies *will* still cut paper paychecks for those not enrolled.
Comment: @user71659 Good call, most places are switching to EBT cards for those not on direct deposit. Regardless, while functionally similar, it's not actually the same as direct deposit (namely, they don't have access to your bank account details, which is salient to the question at hand).
Answer: It depends on the clause of the Direct Deposit agreement.
Some Direct Deposit agreements include a clause including permission to make withdrawals, the actionary phrase may be as simple as 'I permit {organisation} to deposit and withdraw on my account'.
If there is no withdrawal authorization provided, such things as correcting overpayments or incorrect deposits are handled through normal banking procedures.
Answer: Personal: A company I had worked for erroneously posted two checks to my direct deposit. There was also a debit in my account, for correction. So I can not say if it is a withdrawal per se, or just a reversal of the electronic money transfer.
Comment: @insipidlight - Edit much appreciated!
Answer: While the company may be able to take money back from your account, they still have to pay your salary. For example if the company decided to lower their salary, they have no right to do that and take their money back from your account.
In an extreme case, if the company was going bankrupt and decided to take money from your account to pay someone else, I believe law.stackexchange.com would tell you they are in trouble.
Answer: No, direct deposit doesn't permit withdrawal. But keep in mind that every company or person performing transactions can file for a withdrawal of same amount that they deposit. However their is a limited time period after which banks will revoke the ability to even reverse the transition. [ Won't apply if court or police is involved ].
Read this government article on 5 myths of direct deposit for additional information. https://www.tpsgc-pwgsc.gc.ca/recgen/dd/prom-1-eng.html


Start Xen domU automatically and connect console when dom0 boots

Question: Is anyone aware of a way to connect to the console (like using xm create -c) of a domU that's set to start automatically upon dom0 boot, such that the user only saw the login prompt of the domU when the machine finished booting?
Answer: Do you really want the physical console of the dom0 to only show a login console of the domU? How do you plan to administer the machine if you need physical access?
What about creating a user on the dom0 that has its shell set to connect to the console of the domU? That way you can give the users of the machine this account and then they can login to their domU.
Answer: Console is, what the names says, a 'console' link to the domU. The point of having a console is just to see the output of the boot process otherwise i suggest you to give raw ssh access (using keys)


What do Christianity and Judaism have in common?

Question: Most Christians relate to Judaism more than to Islam, Hinduism, or Buddhism, or any other religion. So, my question is, what do Christianity and Judaism have in common so as to make them so related to one another? Thanks.
Comment: Welcome to Christianity.SE. For a quick overview, please take the [Site Tour](http://christianity.stackexchange.com/tour). Your question is a big one--perhaps too big for this site. But we'll see what some of the others think. Meanwhile, for more on what this site is all about, please see: [How we are different than other sites](http://meta.christianity.stackexchange.com/questions/1808/how-we-are-different-than-other-sites) and: [What topics can I ask about here?](http://christianity.stackexchange.com/help/on-topic)
Comment: Christianity really grew out of Jewish ideas. Jesus was actually brought up Jewish.
Comment: Related: [How did the Christian movement distinguish itself from Judaism?](http://christianity.stackexchange.com/q/40714/21576)
Comment: @resident_heretic: I suppose that statement might make sense from a certain perspective of gross ignorance. But nobody truly familiar with Christianity, Judaism, or any of the various forms of paganism, would make such a rediculous statement.
Comment: A great question, but far, far too broad.
Answer: Jesus Christ is Jewish. Therefore we are connected to the Jewish faith because it is our faith with the addition of the teachings of Christ.
Maybe better stated, the Jewish faith, is the history of Christianity by way of Christ's Jewish lineage. His faith and physical ancestors was Jewish.
Comment: @Korvin that is not a traditional belief at all. In fact I've never heard it before, and don't see why anyone would think it was true.
Comment: Well only Christian perspectives are ontopic here, so yes...
Comment: @curiousdannii I suppose that's only from a Christian perspective.  The other matter is the nature of the deity, and if one accepts the Triune model, to where was and is become irrelevant completely.
Answer: What Christianity and Judaism share is a part of salvation history, as presented in the Old Testament.

Salvation History (in German Heilsgeschichte) seeks to understand the
  personal redemptive activity of God within human history to effect his
  eternal saving intentions. This approach to history is found in parts
  of the Old Testament written around the sixth century BC, such as
  Deutero-Isaiah and some of the Psalms.

What Christians teach (in the main) is that the Bible is a record of salvation history and the relationship between God and Mankind.  Jews and Christians agree that there is one God.
While the question is very broad, that in a nutshell is where common ground can be found.
The significant differences (books have been written) begin where Christians believe in how salvation history continues as presented in the New Testament.  (Among the varied Christian sects the disagreements in detail run rampant).  Those adhering to Judaism disagree, and follow a different understanding of the continuing relationship between God and Mankind since they do not hold Jesus as having been the promised Messsiah.
Per your request for amplification on 'the relationship:' the relationship between God and mankind is established in Genesis and continues on throughout the Old Testament.  It follows the pattern of God and mankind being close, then mankind turning away (the Fall) and then a series of turns towards God and turns away from God, among God's people, as time goes on.   C.S. Lewis wrote of that pattern in his summary of the Bible.  Further examples of the relationship are found in the covenants made with Abraham, Moses, and David.
Comment: Could you please elaborate on this relationship between God and Mankind you mention? Also, define salvation. Thanks.
Comment: So essentially salvation means entering paradise, if I understand this well. Thanks.