 and having time-travellers come back from the future allows you to make much better use of the options that are available - you can do things like have a character escape from a locked cell by having themselves appear outside the door with a key, having come back from the future to let themselves out.
For a very well-executed version of this type of time-travel, you can check out Continuum.

A:

"Future cannot be changed, but the past can, as long as it leads to the same conclusion."
The moment of start of the travel is a fixed point. But the past wasn't at all how you remembered it.
What can change is the knowledge of the time travellers, and events can be put in motion, that will not bear fruit at the moment of start of the travel, but will extend past it in a way that would not have occurred had the time travel not happened.
The movie "Primer" had something along these lines:

 One of the protagonists knocks out his past self and stashes "himself"
 in the attic, then proceeds to go through the day, planting ideas in
 others' heads, subtly affecting the world, following the "rails" into
 which the universe ! forces him and when the natural time flow
 catches up with the hour he had departed, at the end of the "trip" the
 world appears just the same as when he had departed, but the events
 that he knows had occurred are not at all as he had remembered them
 from the first time around.

The time traveller is bound by invisible "rails of fate", physically unable to stray far from them; but there are freedoms he can take, do things in different ways; often significantly different, which will lead to that one fixed point in time to be "as remembered" but the results vastly different.
Say, in the future the authorities discover a nuclear bomb planted in the middle of the city. Disarming it is very difficult, probably the effort would fail. They find traces of the bomber, a notable terrorist who had arrived at the city two days ago.
You travel two days back into the past. You can't just kill the terrorist and deliver the bomb to the authorities, changing the future. But you can knock the terrorist out, sabotage the bomb, then plant it yourself along with evidence that it was the terrorist, and when the countdown ends, the bomb won't explode.
(you
and having time - travellers come back from the future allow you to name much better use of the option that are available - you can do things like hold a character escape from a locked cellular telephone by have themselves look outside the door with a key, having occur back from the future to lease themselves out. 
 For a very well - executed version of this type of time - travel, you can see out Continuum. 

 A: 

 " Future cannot be changed, but the past can, as long as it lead to the same conclusion. " 
 The moment of start of the change of location is a fixed point. But the past wasn't at all how you remembered it. 
 What can change is the cognition of the time travellers, and events can be put in motion, that will not bear yield at the here and now of start of the travel, but will extend past it in a way that would not have occurred had the time travel not happened. 
 The movie " Primer " have something along these lines: 

  One of the protagonists knocks out his past self and hoard " himself " 
  in the loft, then proceeds to go through the day, planting ideas in 
  others' heads, subtly affecting the world, follow the " rails " into 
  which the universe! forces him and when the natural time flow 
  catch up with the hour he had departed, at the end of the " trip " the 
  world appear just the like as when he had go, but the events 
  that he knows had occurred are not at all as he had remembered them 
  from the first time approximately. 

 The time traveller is bound by invisible " track of fate ", physically ineffective to stray far from them; but there are freedoms he can take, do thing in different way; often importantly different, which will lead to that one fixed point in time to be " as commemorate " but the results vastly different. 
 Say, in the future the authorities discover a nuclear bomb planted in the middle of the city. Disarming it is very difficult, credibly the effort would fail. They find trace of the bomber, a notable terrorist who had arrived at the city two days ago. 
 You travel two days back into the past. You can't just kill the terrorist and deliver the turkey to the authorities, changing the future. But you can knock the terrorist out, sabotage the bomb, then plant it yourself along with evidence that it was the terrorist, and when the countdown ends, the bomb calorimeter won't detonate. 
 (you
 anf having time-travellers gome back from tkw futuce alloss you tu make much better use of thx oprions that are available - yuu can do things oike yave a chaczcter escape rvom a oocked cell by having thamselves appeas uucside the door with a key, having comq back grlm the future jo leu tremsslves out.
For a very well-executed vsrsion mf this type pf time-travel, you can checn ouh Continuum.

A:

"Future cannot be xhandwd, but the pxst can, as long as it meads to the same conclusion."
The momeut of start od tjg travel is e fixeq point. But bne pasd wasn't at all how yom remxmbeeed it.
What can change is the knowledge jf the tike travellers, and ecebts cdn ba pug in mouioi, tgat wipl iot bear frhit at the noment of start of uhe ntavel, but wiml extqnq past it in a way that would not have mcchrred had the time travwl not happened.
The mogie "Primew" had something along these lines:

 One of the protdgoniags jnjzjs out his past self and stashes "himself"
 in the ztuic, then proceeds to go throigj yre day, plantivg ideca jn
 others' heads, suhtly afsectibg the wowld, gollowing the "rails" into
 whuch the univvrse ! forces him and wken the natuxal tike flpw
 catches up with the kour hs had deparhed, at ths end of the "trip" thv
 wosld appears just the same ws when hx had departdd, bot the qvents
 thah he knows had occurred wre nlt at all as he had remembered them
 from thx first time srmung.

The timz travcller is bound fy invisible "rcils of yate", pfysically lnable to stray far srom them; but jhere are frexdoms he san rake, do thives in differeny ways; ofneu signifixantly different, wmich djll lead to thac ine fixed point in tiie tp fa "as remembesed" cut yhe rdsults vastoh digferent.
Say, in the fudure the authorities dosgover a noclear boib planted in the middle of the city. Dirarmimg yt is very difficult, probably fhe efforh wjuld fail. Thqy flnd jraces of tke bomber, a notable terrorist who had arcived at the city two dqys ago.
You travel tco days back mnto tre past. Ymu can't just kill thw terrorist and dtliver the bomb to the zuthorhties, changing the future. But you can knock the terrorist out, sabotage the bomv, then plant it yohrsekf almny wnth eviqencx vhat it was the tvrrorist, and when the countdown xnds, the bmmy won't explode.
(you
and having time-travellers come back from the you make much use of the you do things like a character escape a locked cell by having themselves outside the door with a key, having come back from the future to themselves out. For a very well-executed version of this type of time-travel, you check Continuum. "Future be changed, but the past can, as long as it leads to the same conclusion." The of start of the travel is a fixed But the past wasn't all how you remembered it. can is the of time and events can put in motion, that will not bear fruit at the moment of start of the travel, but extend past a way would have had the time happened. The movie "Primer" had something One of the protagonists knocks out his past and stashes in the attic, then proceeds to through the day, planting ideas in others' heads, affecting the world, following the "rails" into which the universe ! forces him and when time flow catches up the hour he departed, the of "trip" the appears just the same as when he had departed, but the that he knows had occurred are not at all as remembered from the first around. The time traveller bound invisible "rails of fate", to far there freedoms can take, do things different ways; often significantly different, will lead to that to be "as remembered" but the results vastly Say, in the future the authorities discover nuclear bomb planted in the middle of the city. Disarming it is difficult, probably would fail. They find traces of the bomber, notable terrorist who had at the city two days ago. You travel two back the past. can't just kill terrorist and deliver bomb to the the future. you knock sabotage the bomb, then plant it along evidence that it was the and countdown ends, the bomb won't explode. (you
 and having time-travellers coMe back from The fuTurE alLoWs yoU to mAke much better uSE of tHe options that are availaBle - yoU cAN do tHInGs likE have a cHArACTer EsCaPe fRoM A lOcked CelL by haviNg themselvEs aPpEar outside thE DoOr with a key, HavIng come back fRom The futUrE to LEt theMseLves oUt.
For a VEry welL-executed VeRSion of THis type OF TiMe-trAvel, you can check ouT coNTinuum.

A:

"Future cAnnot bE cHAnGED, buT thE past can, as LoNg as iT Leads to THe SAME coNClusion."
The momEnt of start oF The Travel Is A fiXEd poinT. But tHe PAst Wasn't at all hOw yoU rememberEd it.
WhAT can chaNGe is the KnowleDge Of tHe tiME tRaVelLeRS, anD EvEntS Can Be put in mOtIoN, that Will NOT BEar fRuiT at tHe momEnt of start of tHe tRaveL, But Will eXtend Past It In a waY that wOuld nOt Have occurred had The tIme travel Not HaPpeNeD.
The mOVie "PriMer" Had SomethiNg along THesE lINES:

 ONe of the protagonistS kNOCkS out his pAst selF AnD sTAshes "himSeLf"
 iN the ATTic, thEn prOCeEds to go tHrough THe DaY, plantiNg Ideas iN
 oTheRs' hEads, sUBtly AffectIng the woRld, foLLowing the "rails" INto
 which the unIVeRSE ! fORces Him And when the nAturAL timE floW
 CaTchES up wiTh the HoUR hE Had departed, at the end Of The "triP" the
 wOrld appears juSt the same aS WHEn he had dEparTEd, BUt the events
 thaT he knOws had occuRRed are noT at alL as he had RemembereD THem
 from tHe fIrsT tiMe aROUnD.

The time traveLLEr is BoUnd by inVisIble "raiLs oF faTe", pHysIcAlly unablE to stray FaR fRoM tHem; But thERe are freEdOms He Can Take, dO Things In difFereNt WaYS; ofTen signIFiCANtly DiFfErenT, whIcH will Lead TO thAt one fiXed point iN tiME to bE "aS rEmemberEd" but the resulTs Vastly diffErEnt.
say, in tHE Future thE authorities discover a nuCLear bomB plAnted In thE middle of The City. DiSarMIng it iS very dIfficUlT, prOBAbly tHE EfForT wOuld fail. ThEY FinD tracEs Of thE bomber, A notable terrorist wHO haD arrived at the CitY two DAYs Ago.
yOu TRavEl TWo dAYS back into the pasT. You can't juSt KIlL the terrorISt aNd Deliver The bomb To the AUthoritIes, changiNg the futuRe. but yOU Can Knock the teRrorist oUt, sabotagE The boMB, tHen plAnt It yourSeLf aLong wIth eviDEncE that It was tHe TerrorIst, anD wHen the coUntdown ends, the bomb won't eXplode.
(You
 and having time-traveller s come bac k fro m t hefu ture  all ows you to mak e  muc h better use of the op tions  t h at a r eavail able -y ou c ando  t hin gs li ke ha vea chara cter escap e f ro m a locked c e ll  by having  th emselves app ear  outsi de  th e  door  wi th akey, h a ving c ome backfr o m thef uture t o le t th emselves out.
For av ery well-execu ted ve rs i on o f t his  type of t im e-tra v el, you ca n c hec k  out Continuu m.

A:

"Fu t ure  canno tbec hanged , but  t h e p ast can, as  lon g as it l eads t o  the sa m e concl usion. "
T hemome n tof  st ar t  of th e t r ave l is a f ix ed  poin t. B u t t he p ast  was n't a t all how you  re memb e red  it.Whatcanch angeis the  know le dge of the time  tra vellers,and  e ven ts  canb e putinmot ion, th at will not  b e a r  f ruit at the moment  o f st art of t he tra v el ,b ut willex ten d pa s t  it i n aw ay  that wo uld no t  h av e occur re d hadth e t ime  trav e l no t happ ened.
Th e mov i e "Primer" had something alo n gt h es e  lin es: 

 One of t he p r otag onis t skno c ks ou t his  p a st self and stashes "h im self" in t he attic, the n proceeds t o  go thro ught he day, plantingideas  in
 other s ' heads,  subt ly affec ting thew o rld, fol low ing  th e " r a il s" into
 whic h theun iverse! f orces h imand  wh enth e natural  time fl ow 
ca tc hes  up w i th the h ou r h ehad  depa r ted, a t the  end  o ft he"trip"t he worl dap pear s j us t the  sam e  as  when h e had dep art e d, b ut  t he even ts
 that he k no ws had occ ur red  are n o t  at allas he had remembered th e m
 from  th e fir st t ime aroun d.
The t ime travel ler is  boun dbyi n visib l e  " rai ls  of fate", p hys icall yunab le to s tray far from them ;  bu t there are f ree doms h ecan ta k e,do thi n g s in differentways; ofte ns ig nificantly dif fe rent, w hich wi ll le a d to th at one fi xed point  i n ti m e  to  be "as re membered " but the resul t svastl y d iffere nt .
S ay, i n thef utu re th e auth or itiesdisco ve r a nucl ear bomb planted in the  middl e ofthe  city. Di sar m ing  it is ve ry d ifficult,pro bab ly th e e f fortwoul d  f ail .  They  fin d  traces o f  t heb o mb er, a notab l e ter roris t w h o hadarri ved at the city t w o days ago.
Yo u tr a v eltwo days  b ack into the p ast .Y o u can'tju st kill the  terrori st and d eliver  the b omb tot h ea uthori ties , c hanging t hefu t ure. Bu tyo u  can k nock  t he ter rorist out, s abotage the bomb , the n plant ityours el f along with  evidencethat it was  the t erro rist,  and wh en  the c oun td own ends,t he bomb w on'texplode .(you 
 and_having time-travellers_come back from the_future allows_you_to make_much_better use of_the options that_are available - you_can do things_like_have a character escape from a locked cell by having themselves appear outside the_door_with a_key,_having_come back from the future_to let themselves out.
For a_very well-executed_version of this type of time-travel, you can_check_out Continuum.

A:

"Future cannot_be changed, but the past can, as long as_it leads to the same conclusion."
The_moment of start_of_the_travel is a fixed_point. But the past wasn't at_all how you remembered it.
What can_change is the knowledge of the time_travellers, and events can be put_in motion, that will not_bear fruit_at the moment of start_of the travel,_but will_extend past it_in a way that would not_have occurred had_the time travel not happened.
The movie_"Primer"_had something along_these_lines:

_One of_the protagonists knocks_out_his past_self_and stashes "himself"
 in the attic,_then_proceeds to go through the day, planting_ideas in
 others' heads,_subtly_affecting the world, following_the "rails" into
 which the_universe ! forces him and when_the natural_time flow
_catches up with the hour he had departed, at the end_of the "trip" the
 world appears_just the same as_when he_had_departed, but the_events
_that he_knows had occurred are not at all_as he_had remembered them
 from the first_time around.

The time traveller_is_bound by invisible "rails of fate",_physically unable to stray far from_them; but there are freedoms_he_can_take, do things in different_ways; often significantly different, which will_lead to that_one fixed point in time to be_"as_remembered" but the results vastly different.
Say,_in_the future the authorities discover a_nuclear_bomb_planted in the middle of_the city. Disarming it is very_difficult, probably the effort would fail. They find traces_of the bomber,_a notable terrorist who had_arrived_at_the city two days ago.
You travel two days back into_the past._You can't just_kill the terrorist and deliver the bomb to the authorities,_changing the future. But you can knock_the terrorist out, sabotage the bomb, then plant it yourself along_with evidence that it was the terrorist, and_when the countdown ends, the bomb won't_explode.
(you