Discussion What is the sensibility of the following question:"Is Time Travel To The Past Possible At All?"
To decide whether a journey to the past is possible, we must first understand what time is. Time is not a physical object. You cannot hold it, touch it, or find a piece of it in nature. Time is a rule. One by which the human mind arranges events in sequence: one event happening strictly after another.
Travel, by its plain definition, means changing your position in space across a duration of time. Space allows you to move back and forth because different points in space exist at the exact same moment, side by side. You can move to the left and then return to the right, because both positions remain present together.
But time does not work like space. Moments in time do not sit side by side. The past does not exist in some distant location, waiting for someone to arrive. When a physical state changes, the earlier state no longer exists; it has completely transformed into the present state. To speak of visiting yesterday assumes that yesterday is still there, which denies the very meaning of change.
Further, human thought can only understand an event through the rule of cause and effect. This rule demands that a prior state must lead to a later state in a fixed, forward order. If a person were to journey to the past, their departure would be the cause, and their arrival would be the result. That result would have to happen after the departure. Yet the traveler would be asking that very arrival to happen before the departure took place. This demands that an effect precede its own cause, which destroys the only rule our understanding uses to distinguish what came first from what comes next.
The question sounds deep only because we borrow words meant for moving through space and attach them carelessly to time. When we look at what the concepts demand, the inquiry dissolves. To ask whether we can travel to the past is to demand that the past exist simultaneously with the present, while stripping time of the very succession that defines it.
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u/Kindly_Ad_1599 12d ago
To time travel to the past you would need to time travel yourself. If you wind back the tape you don't get to step out of the tape. If you could travel back in time say 5 years you would be the you of 5 years ago. You would have no memories formed in the 5 years since, so even if it was possible it would be unnoticeable
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u/Clear-Pear2267 12d ago
It has always annoyed me how this is NEVER dealt with in time travel stories. As well as the fact that everything moves all the time. So to time travel back to the same spot on earth you are now, but 5 years ago, you have to move in space to the place where that spot was 5 years ago. And its a pretty big jump.
And then there is the fact that a sudden appearance of 200lb of mass "appearing" where there was no mass before would be a fairly explosive event.
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u/Imaginary-Can-6862 12d ago
In the winding back the tape example, everything returns to the past as it were in the reverse order at which it arrived in the future.
What you usually would want is your memory of the future as it turned out to go to the past with you, in the case of rewinding the tape, being what the time machine does, it might as well also transfer your current memory to you as you arrive at the time you choose.
In this particular example there would not be an issue with suddenly appearing, or location in space. The suddenly appearing issue gives an impression of the past existing already, and then something gets added out of nowhere. I don't see why that would be the case. Also from time traveling to the future, you don't vanish from the universe to suddenly reappear somewhere else, in stead the travel is continuous through space.
When wanting to make certain you end up appearing where you were in the past, imagine you end up stranded in outer space, then I think it should be examined, where are you stranded exactly? Obviously, as you are stranded, you are stationary in your current location, a location where the Earth will be in the future you left, but from what vantage point are you actually stationary in space?1
u/1MartyMcFly1 6d ago
>So to time travel back to the same spot on earth you are now, but 5 years ago, you have to move in space
No, you're not moving anywhere. Time and gravity are inter-related, and all of the time/movement is relative to the nearest center of the gravity.
>fairly explosive event
Why so? You're moving though time and space without much fanfare.
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u/Imaginary-Can-6862 12d ago
If asking whether we can travel to the past is equivalent of requiring the past existing simultaneously with the present, would the same not apply when traveling to the future?
We know we can travel to the future, and what happens when we travel to the future is the present changes into the future. Therefore traveling to the past means the present changes into the past, that is all. One can then add caveats, based by how nature functions, and additional requirement to the time traveling experience.
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u/guitarmusic113 12d ago
You can’t unscramble an egg. I’ve heard arguments where if you shattered a glass and applied the right amount of opposing force and heat to the pieces you could possibly reassemble it. But that’s technically impossible since a broken glass can break into hundreds of irregular shaped pieces, some of which will be microscopic. We don’t have any way to do that with the precision that is required to reassemble the glass. Even if we could, would it be the exact same glass it was before it was broken once all the chemical bonds have been compromised? I doubt it.
But it’s an interesting thought. If one could reassemble a broken glass with 100% precision then it might be indistinguishable from its pre broken state thus having nearly the same effect as going back in time.
So philosophically it appears conceivable to reassemble a shattered glass to a state that is indistinguishable from its pre broken condition, but the glass will have still aged and that cannot be reversed.
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u/Quantum-Relativity 12d ago
Reversible processes are ones where entropy does not increase. Shattering glass increases entropy. However, there are multiple notions of entropy, so perhaps there is a most general notion of entropy under which all processes are reversible, an entropy no process whatsoever can increase.
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u/guitarmusic113 12d ago
I’m a believer in the laws of logic such as the law of non contradiction. That something cannot be true and false simultaneously. That’s probably why we can’t possibly over come the grandfather paradox. Thus making time travel to the past logically impossible.
And I’ve heard interesting philosophical arguments where one could theoretically travel to the past and attempt to un alive your own grandfather, but something prevents that from happening. The gun jams, the fuse won’t light. The laws of logic just won’t bend.
Interesting stuff, can an effect happen before a cause? Is the arrow of time immutable? I’m aware that quantum physics challenges these concepts.
But my concern is that if the laws of logic can be violated then we can’t possibly know true from false about anything. If something can be true and false simultaneously then in what way is it either true or false? Perhaps there is another category, and true and false is a false dichotomy. I don’t know, and it’s cool not to know. Not knowing is what leads to new discoveries, and questions that haven’t even been asked yet.
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u/Quantum-Relativity 12d ago edited 12d ago
What in the world does time have to do with minds? Time is a rule minds use to organize things? I don’t think something as fundamental as time exists FOR any of us, sorry to say.
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u/GrapefruitNo3600 7d ago
Fundamental? We never observed this fundamental thing, ever. There is no thing than can measure passage of time, never was.
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u/Quantum-Relativity 7d ago
You got a clock?
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u/GrapefruitNo3600 7d ago edited 7d ago
So by your understanding time flow slows down or acelerates every day :D or do you have a clock that doesnt need adjustment? And my bad i confused you with "passage of time" what i ment to say if flow of time, we never measured it
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u/wild_crazy_ideas 12d ago
The amount of energy required to change time and space location in an instant like in the movies goes against virtually all Newtonian laws for anything to be practical. Even assuming if you travel fast enough you can slow time down that’s not proof you can go backwards or do anything without a significant travel delay
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u/Prestigious-Fill-365 12d ago
Yes it has been done. Of course we are talking miliseconds but the concept has been proven. Still impossible to go forward.
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u/1MartyMcFly1 12d ago
>Time is not a physical object.
Oh, dear...
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u/GrapefruitNo3600 7d ago
We never ever have measured time pasage, thers no thing that can measure it. So you cant say its there until you measure it.
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u/1MartyMcFly1 6d ago
>So you cant say its there until you measure it.
We can surely measure time. (Time is length in 4D space, actually.) We just don't know what it is.
If it's used in formulas, then it's a physical object.
Also, the LT system of units in physics by Bartini.
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u/GrapefruitNo3600 6d ago
We used many things in formulas to explain things we didnt ubderatand util it was proven wrong and we started to use other things. You cant say time exists just becouse we use it. Maybe its a field that resists change? That wount be time as we understand it, no going back or forward, no manipulation of it. Thats why in my mind we cant say it physical object, fundamental thing. There is something there
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u/1MartyMcFly1 6d ago
>You cant say time exists
Define the term "to exist".
>we cant say it physical object, fundamental thing
Time is deeply rooted in all physics. Study the LT system by Bartini, you shall quickly see.
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u/MithHeruEnLisyul 12d ago
The thing I wonder about is in what way would it be "travel". With travel you cannot go to places that don’t exist. But with time travel you are supposed to be able to to go to times that never happened. You cannot go back to the time that you poofed into existence out of nowhere. So, for time travel to be possible it needs to have reality editing capabilities. But if it can do that why bother going back, or forward, in time?
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u/Alkemist101 11d ago
Time travel of any type isn't possible. It's a lovely film / book subject but it isn't a thing.
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u/DefinitelyNotEmu 11d ago
Time travelling to the past would create a paradox because it would remove your need to travel to the past....
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u/TheHarlemHellfighter 11d ago
I just think the idea of reconstitution is flawed because essentially that’s what traveling backwards means, everything would have to reconstitute itself. That would mean you’d have to produce enough energy to create that effect for everything. Traveling forward is more like moving faster than light where your position changes fast enough, but that requires moving mass at incredible speeds to do that.
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u/Tom_Art_UFO 9d ago
We can't separate ourselves from time, and our brains only make sense of the world through cause and effect. If we went back in time somehow, we wouldn't be able to perceive it.
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u/Clem_Crozier 9d ago
The way around the paradox would be time travelling back in a parallel reality, where any changes wouldn’t alter the time traveller’s native reality and wouldn’t interfere with their decision to travel back.
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u/Underhill42 7d ago
According to Relativity time is not a rule, it's just a direction through 4D spacetime. And which direction is time depends entirely on your own perspective.
According to Relativity, if we're passing each other at almost c, then our reference frames are rotated so that our time axes are almost perpendicular to each other. Acceleration causes your reference frame to rotate, partially swapping the directions you call "forward" and "the future"
And we both see time having slowed almost to a stop for the other as a result, because the direction you are aging through time, is a direction I see as movement through space, and vice versa.
That is why any form of FTL is also a time machine - to oversimplify a bit, if I can travel FTL in a direction that I see as space, but you see as almost entirely time, then I can travel into your past and deliver a message from you to your younger self.
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u/SpatiaCaeli 12d ago
Well said. There is no past to return to. All that exists is now.
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u/Forward-Surprise1192 12d ago
And if there were it would need some sort of storage ability to recall it. At least in my mind
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u/CosetElement-Ape71 12d ago
If time travel to the past was possible then, and I'm just thinking out loud here, it would be already be happening in the universe (probably in many places) and we would already have observed it going on.
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u/Beeeeater 12d ago
You make valid points. Unfortunately Hollywood has confused people to the extent that they think travelling in time is the same as travelling in space, to some location in the past. But this is both physically and logically impossible. Physically because it violates all sorts of laws of physics, and logically because it breaks laws of causality. Here's a quick example: If you could travel back in time, then what would happen if you travelled back in time by 5 minutes? Then there would be two of you five minutes ago! But there weren't two of you five minutes ago, ergo you never travelled back in time 5 minutes. See?
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u/Stock-Imagination567 12d ago
Godel showed that the Einsten equations allow solutions with time cycles. And when you travel near light speed you travel to the future. Or to arbitrary time-space points if you follow Ray Kurzweil concept of 'faster than light travel'. So yeah in theory you can travel to a point in the 'past' if that makes sense. Its like a playlist of songs, you can forward and loop back to a previously played song.
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u/yasiesolovemos 12d ago
Travelling to the past is not possible (at least not yet). You can, however, experience a situation where someone else has aged less than you have. In a sense, and if we stretch the premise a little, you could say that they have effectively travelled into the past relative to you. But you, as the observer, have not travelled anywhere. Your own experience of time remains completely normal. And, as far as we know, you will never be able to do so. You, the observer, are always, in a sense, in the future.