We take it for granted that the past tense is fixed . chronicle always pass off the way we remember it come about . But how do we know for certain that that ’s the case ? If time traveling is potential , fourth dimension traveler could be zipping back and modify things around all the sentence . How would we even be able to enjoin if that was the case ?
We take a routine of experts on time travel — none of whom have actually jaunt in clip themselves . ( Or if they had , they were keeping it under their hats . )
Top image : DISENT on Deviant Art .
First of all , it ’s really difficult to know one way or the other , because “ presumptively , when a time traveler would commute our past , this would also in a flash exchange our memories of the past tense in purchase order to render them consistent with the ‘ newfangled ’ past , ” state Christian Wüthrich , a prof of Philosophy and Science Studies at UCSD . “ If the changes to our past take place in a flash and completely consistently , i.e. , call for update to all memories and book of the past tense , then we may not just not know it for sure . ”
But accord to the experts , here ’s how you’re able to evidence that someone is n’t interchange the yesteryear all the clock time .
meter travel is out of the question .
We heard this a lot , from many of the experts . AsJon Thaler , a physics professor at the University Illinois at Urbana - Champaign , tells io9 : “ The problem is that we do n’t acknowledge how to construct a possibility that permits time locomotion . Without a theory , it is unmanageable to cognize what phenomenon to look for . ” Thaler indite thetime travel section of the Usenet Physics FAQ , in which he explains that the Theory of Relativity might permit “ closed timelike curves ” that allow time travel — but the notable “ grandpa paradox ” ( in which you go back and stamp out your own grandpa as a child ) proves the whole thing is unacceptable . Thaler state io9 :
“ In a nutshell , it appears that “ closed timelike curves ” — jargon for the physical setup that permits time locomotion — are incompatible with quantum mechanics . ” This is basically how physicists interpret the “ grandad paradox , ” except that this glide path is more quantitative , “ and therefore the form of situation that physicists like to analyze . ”
Even if time travel was possible , you could n’t shift the past anyway .
We get wind this one a lot , too . Wüthrich says that many philosophers of skill sham that the past times must be ordered , to avoid those foul paradoxes . These “ consistency restraint ” think of that there ’s only one past , and it ’s fixed . Therefore , enunciate quantum physics expert Todd Brun with USC , “ even if you travel back to the past with the intention of changing story , events will conspire to push you instead to adapt to it ( and history would already include the front of time traveler ) . ”
This is why many philosopher of time like account such asRobert A. Heinlein ’s “ All You Zombies,”where it ’s totally consistent , says Nick Huggett , source ofEverywhere and Everywhen : risky venture in Physics and Philosophy . Huggett , a philosophy prof at University of Illinois at Chicago , adds :
It ’s now not at all clear what it could mean to change the past at all . To say something change is to say that it was one mode at one clip , and another way at another time , right-hand ? So would you have shift the past tense ? That seems to require that yesterday was first one way , and then another — first you did n’t get in by clock time locomotion on that day , and then you did ? But in our story you did go far by time travel , and so the first choice never happened , and so the yesteryear was not changed by your arriver — again because it did n’t make something unlike occur .
The option to this notion is that every time you travel back in time and change clobber , you ’re creating a new universe , harmonize to Hugh Everett ’s “ many world ” hypothesis . This young realness would still have a consistent past , which everybody would remember the same way — but it would be the yesteryear that resulted from the sentence traveller ’s changes .
articulate Brun :
There could be multiple universes , and castrate the past will cause the universe to branch . Weird and skill fictional as that sounds , we have to take it seriously in quantum shop mechanic , where one of the leading interpretations of the possibility is the so - call “ Many Worlds ” picture , in which every quantum event causes the universe to branch . But even in this case , one can not go back and change the history of one ’s own universe .
You might only be able to travel back as far as the distributor point where the first prison term simple machine was invented .
Unless someone ’s invent a time auto and we just do n’t roll in the hay about it yet , this would mean that we ’re dependable . Explains Vanderbilt University Physics ProfessorThomas J. Weiler :
prison term travelling to the past , if at all possible , may only go back as far as the first time machine . Since the past of our civilization lacks a time machine , it is changeless , whereas the past of more in advance civilisation may be mutable .
Nobody ’s show off
The only clear means to know if sentence travelers were going back and messing with the past times would be if they bragged about it . Which , being masses , they belike would . Says Huggett :
Suppose tomorrow you go back in time 2 days ( i.e. , to yesterday ) , and sensationally appear on home TV making correct prognostication about today . It would be well known today that your future self had affected the past .
Although , even then , we would n’t know for certain if these boastful clock time travelers had actually changed anything , since we ’d only remember one past times .
There would be physical traces
Depending on the method of time travel people were using , you ’d expect there to be forcible traces , says Huggett . In Carl Sagan ’s refreshing Contact , metre change of location is possible using a path in spacetime that leads to the past . According to Huggett , this sort of wormhole “ requires exotic forms of matter to hold it candid , which may be detectable . ” Also , there ’s the interrogation of preservation of energy — when you appear in the past , you wo n’t be take shape out of matter and energy that were already there , but rather you ’ll fundamentally be importing free energy from the future tense . This could make hint that might be noticeable — if anybody really was coming backwards in prison term to our earned run average . So in a nutshell , the results of clip travel would be unnoticeable to those of us who are stick in linear time , but the method of time travel would probably leave some trace .
Screwing with cause and gist would convert the laws of probability
If someone really could go back and change the past , this could intend that effects would precede causes , Brun tells io9 . And that , in twist , could mean that everything would go topsy - turvy , leading to logical repugnance that we might well notice . Brun explain :
The mix up of reason and effect can make otherwise unconvincing outcome become much more probable . So if we suddenly encounter the laws of luck plunder our common sense estimate of chance , that could mean that time travel is going on nearby . In rule this means that one could detect the existence of clock time machine — perhaps even before they are ever built ! But it ’s hard to know exactly what to take care for .
chronicle proves they ’re not doing it .
Let ’s give the last word to Harvard University Physics ProfessorGary Feldman : “ If time traveler are incessantly changing the past , they are not very good at it . Why did they not obviate two disastrous and senseless world war in the preceding century ? ”
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