# Can we see elementary particles?

1. Sep 16, 2009

### EinsteinII

Can we see elementary particles using any device? I mean the way we can observe smallest of the viruses by a microscope and the farthest of the objects by a telescope?

2. Sep 16, 2009

### humanino

We certainly can produce meaningful plots for the distribution of electric charge inside a hadron for instance. We can even slice those plots with respect to the fraction of energy carried by the constituents. Generally speaking the equivalent of Wigner distributions for relativistic quarks and gluons inside hadrons has been defined. Apart from electric charge, other charge distributions can be computed and measured as well. This would also apply in principle to other bound states, like (say) muonium.

Outside bound states, I am not sure what kind of "imaging" we could construct. If there is no bound state, there is no stable structure to display since elementary particle are point-like. Of course, we can definitely measure the charge distribution in a beam of particle, which essentially is a coherent wave packet profile.

3. Oct 6, 2009

### Soveraign

It's a good question and hard to answer. When we "look" with our eyes at a regular object, what we actually see are the photons (light) reflecting off (or being emitted by) the object. This works well for even small objects, but there is a limit (primarily related to the wavelength of light). When we want to look at even smaller objects, we might use an electron microscope, which allows us to see even shorter wavelengths.

The point I would like to stress is that when we look at something, what we really are doing is observing how that object interacts with other objects. For example a cat interacting with light to for the image of a "cat". Also you have a virus interacting with electrons (which then interact with a machine) to form the image of a "virus".

So when you ask "What does it look like?" what you are really asking is "How does it interact?". And in this case, when you are dealing with fundamental particles, interactions start to become defined in terms of electromagnetic, weak, strong, etc... To get a feeling for what there interactions "look" like, we might, for example, collide particles together to see what happens. By colliding these particles with higher and higher energies, we start to probe the different ways the particles can interact.

Hope this helps a little, not sure how clear I am being.

-Brian

4. Oct 6, 2009

### humanino

We can also use virtual particles, with an arbitrarily short wavelength.
It's an interesting point. The above "Wigner functions" I was referring to are the relativistic equivalent of a wavefunction. We have so-called factorization theorems which ensure that, no matter which interaction takes place, the measured cross sections are always parameterized by those same non-perturbative quantities. So one can measure the Wigner functions in electromagnetic processes, and check that they are indeed the same in weak or strong processes. It's important because it gives us confidence we are really talking about universal functions describing the structures themselves, and not how we look at them.

5. Oct 6, 2009

### Bob_for_short

No, because they are not really elementary.

If a noble gas atom can be prepared in a more or less " free" state, the electron and other "elementary" charged particles are so sticky that can be observed only as some "parts" of something, not free. And the "image" obtained from such observations really depends on the state of the entire something.

6. Oct 6, 2009

### Soveraign

The case you are describing here seems like a situation where the wave functions combine to act like a single particle (like Bose-Einstein condensate).

Also, this might be just like saying free quarks cannot be seen, but always as part of something. Yet, a quark is generally considered more fundamental than a proton.

But this might all just be semantics since what really matters is how the "something" interacts with other "somethings".

7. Oct 7, 2009

### Bob_for_short

There is a contradiction in this statement: if we cannot deal with free elementary particles, why to say that they are point-like? Point-likeness is a certain image. We all know about Rutherford experiments "supporting" this "image". A more careful theoretical treatment shows, however, that Rutherford observed the inclusive rather than elastic picture. The elastic picture shows quantum mechanical smearing of charged particles in any state. The weaker binding of a charge in an atom, the wider the QM smearing. So in the "free" particle limit the charge is smeared over entire space, and not point-like http://www.springerlink.com/content/h3414375681x8635/?p=4bc24ecd5d4e44919c4106a0763782a6&pi=0".

Last edited by a moderator: Apr 24, 2017
8. Oct 7, 2009

### ZapperZ

Staff Emeritus
I'm just surprised at the responses in this thread because it is obvious that the OP is asking a rather elementary question. I think only Soveraign had actually addressed this in such a way that the OP might be able to understand AND might actually provide an answer on this type of a question, which is what exactly do we mean by "see". Would the OP realize that when we look at viruses using, say, SEM or x-ray crystallography, we aren't really seeing it "directly"?

As always, when faced with a seemingly "simple" question, often times, we need to clearly define and clarify the question before volunteering some answers. It is of little use if the answers just blow by over the top of the heads of people who asked.

Zz.

9. Oct 7, 2009

### humanino

I've read the same books for years, maybe more than a decade, before I got satisfied with simple questions.

10. Oct 7, 2009

### humanino

Because they are in conventional QFT. Some people prefer to believe those are strings, but they have reasons for that.

11. Oct 9, 2009

### EinsteinII

You are amazing. You made physics as smooth as silk!!!

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