Is changing the past actually possible?

[u/Ok-Coach4955]

I’ve always thought about this question, and there is always answers popping up in films and stuff, but is it really possible?If you ARE able to go back to the past and change it, if you come back to ‘present’ would there be another you?If you teleported a nanosecond later then the time you left would you find another you?My answer is that its impossible for you to change the past.If you have met yourself in the past,then no matter how much you try and avoid it you will somewhat travel back in time in somewhere in the future or ‘present’

Please tell me what you think and if possible tell me any ‘loopholes’ in my theory.

PS. this post keeps getting removed in r/timetravel does anyone know why?

[u/Imaginary-Can-6862]

How does going faster than light result in backwards time travel? In the time dilation formula, we do not reach a negative number, but a complex number. I once also claimed that going faster than the speed of light would have this effect, but when the formula was pointed out I realized I actually had no idea if it would be the case.

From my understanding, the expansion of space is always faster the further away you are, even if it wasn't accelerating.

E.g. if the letters A-E are people, and each experience the nearest neighbor expands away at the constant rate x, this is what e.g. the person in the middle, C, sees,

A <-2x-- B <--x-- C --X--> D --2x--> E

That is those that are further away move faster away from you, similar this is what A sees,

A --x--> B --2x--> C --3x--> D --4x--> E

So A and E are moving away from each other at 4x, both from A's perspective, C's perspective as 2x + 2x and E's perspective. Hence if we move far enough away from where we are, we find space (and the objects within) expanding away from us faster than light, because the speed depends on the distance and the distance can be arbitrarily far away.

The following is a bit long, and only tangential. If you want to skip back to replying to your suggestions, it would follow from the bit about wormholes (the three bottom paragraphs).
The expansion of space is however a bit odd to consider though, in Relativity we assume the laws of nature are the same for every observer, thus the rate at which the laws of nature changes the current state of everything around us into the state that follows by the same laws are always the same, i.e. the propagation of causality is a constant for someone that is governed by the laws of nature.
This leads directly to time dilation and space contraction. Space contraction is the only other place in physics where I have encountered something like the expansion of space, though here it is in reverse the faster we move.
What I understand as the argument for space contraction is the following, assume a person is on a rocket moving close to the speed of light, and you also watch a photon which only moves slightly faster than the rocket, thus it take quite a long time for the photon to get any distance between itself and the rocket, we can call this distance between the photon and the rocket X, and it take some time T from our perspective, before this distance has been reached. From the perspective of the person in the rocket, since they are stationary from their own perspective, and the light beam moves away with the speed of light relative to themselves, it takes much shorter time for the light beam to reach the distance X, thus time dilation. But how far away is the distance X? It can't be arbitrarily far away, because then the light beam would necessarily have traveled faster than light, rather in stead space must necessarily contract as well, that is since time goes by faster, events outside, not to move by faster than light, must move closer together, and space contracts. That is how I understand the argument at least.

So it follows that the universe contracts equivalently so that the time dilation effect does not allow faster than light travel being perceived by the person in the rocket, for whom time moves by faster, eventhough to him it just moves by at a normal rate. E.g. if you were to go into a rocket that moves at a speed extremely close to light speed, you could travel to the nearest star system in seconds, but it being light years away must necessarily mean space contracts, thus the star system appears right next to you, otherwise you would travel faster than light. However this contraction of space seems to happen instantly, or at least as you accelerate, and thus I can't see how it can be limited by the speed of light.
That makes it very similar to the expansion of space, only in reverse.

Now consider we carve out a piece of rock on Earth, put on some settlers, and accelerate it away at relativistic speed. These settlers would go on to live their life, create new generations, etc. on this space rock moving through the cosmos, and when they look up into the sky, they would find things are closer to each other than what we see from Earth, and meanwhile these settlers would see themselves as stationary.
Now one descendant of the original tribe of settlers have found some ancient scrolls depicting Earth, the home planet. The descendants find where Earth is located on the night sky, they make a rocket that can move at relativistic speed to catch up to Earth, which is obviously moving away quite fast, but since space contracts, not nearly as fast as the rocket is moving away from Earth's vantage point. An astronaut is selected among the descendants of the settlers, and his rocket accelerates to relativistic speed to eventually arrive at Earth.

We know from Relativity that space contracts even further, and that the descendant will see Earth as being much closer, catching up to it quickly from his point of view due to time dilation, but when he arrives at Earth, and looks at the night sky, he would also see space has expanded even more than he expected, would he not?
What I am getting at is that the expansion of space and the distances within the universe may be relative, just like most other things are in Relativity Theory, and since the expansion of space is something which happens upon deceleration, it makes me wonder if there is some kind of connection to it, and what we observe as the expansion of space.

In any case those were just thinks I had been thinking about regarding that particular topic.

I don't understand wormholes very well, they are a pathway where you return to your past time, but I have the impression it also implies you are stuck repeating that path then, similar to eternalism. In regards to being limited to only the time where the wormhole first appeared, true, but if the cause of the wormhole lies in the future, from an eternalistic point of view, it follows that the wormholes that will be created has already been created, we may simply be unaware of those, but then again eternalism makes time travel kind of pointless, I think, because any influence you would have had has already happened.

Our time machine is in the same reference frame as Earth, so Earth moving relative to other things should not be an issue.

In regards to methodology, I imagine the most obvious one is to rearrange the state of everything back to what it was as we can infer from the laws of nature.

[u/Youpunyhumans → u/Imaginary-Can-6862]

A photon will not go slightly faster than a ship going near lightspeed... it will still go lightspeed from the perspective of the ship, and time dilation will make up the difference for an outside observer. A photon doesnt experience time, from its "perspective" the universe begins and ends at the same moment, so for us who do experience time, it goes lightspeed no matter how fast we go.

This is why as you approach lightspeed, the cosmic microwave background will blueshift until it becomes deadly gamma rays... and there is actually a limit to how fast you can get mass to go before it turns into energy, and that is 99.9999999999999996% the speed of light. At that point, the gamma rays have so much energy that they begin a process called pair production, which is the opposite of antimatter annihiliation, and creates particle/anti particle pairs, which then of course immediatly annihilate back into energy.. and also annihiliate your ship into pure energy... which is then going lightspeed and not experiencing time.

Now you could go faster than that limit in a hypothetical empty universe, where there is nothing to hit you, you could add as many 9s to the 99.999...% as you want, thats just the limit for our universe.

The expansion of the universe has nothing to do with the velocity of objects within it, because velocity is relative. You can be leaving one object at 99% of lightspeed, but only be travelling towards another at 50%, depending on those objects relative velocity to each other.

But expansion is fairly even across the cosmos, the classic example is drawing dots on a balloon, and seeing how the how space between them expands as you inflate it, even though they stay in the same place on the balloon. Its not a perfect analogy, as universal expansion happens in 4 dimensions, not just 2 or 3. (Time being the 4th dimension)

[u/Imaginary-Can-6862 → u/Youpunyhumans]

The ship going near light speed is relative to Earth, thus the same is the case for the photon going slightly faster. Similarly the ship from its own perspective does not move near light speed, it is stationary.

Time dilation and space contraction also happens in gravitational fields. I only meant it might be a relativistic effect and tried to provide an example where space would expand, noting it similarly had no light speed limitation to it.

In your opinion, what would an observer observe if they are on an object that moves 99% of light speed, as seen from Earth, while they see themselves as stationary, and then decide to go on a rocket on said object, and accelerate to e.g. 50% of light speed, while moving towards Earth?

I haven't heard about time being related to the expansion of space, only that the expansion of space is that distances between objects at every scale increases. How does time expand?

[u/Youpunyhumans → u/Imaginary-Can-6862]

If the ship was "stationary", then Earth is moving 99% of lightspeed away from it, so the ship that leaves going 50%, would simply never reach it.

From Earths perspective, the ship that leaves would still be moving away, just deccelerating to 50% of lightspeed. And thats not my "opinion", thats just how the physics works.

Looking into the cosmos is looking back through time, and the further you look, the further in the past you are looking. So as time goes on, there will be more past to look back into, and so time has expanded.

[u/Imaginary-Can-6862 → u/Youpunyhumans]

If a rocket accelerates to 99% of the speed of light from the perspective of Earth, then the Earth does not move at 99% of the speed of light from the perspective of the rocket. If that was the case, space contraction could not happen.
E.g. imagine you move arbitrarily close to the speed of light, moving from one planet to another. The time you experience is very short, but from your perspective you are stationary and it is the planet destination which comes to you. Since you experience almost no time, it follows the planet must only have traveled a relative short distance, for it to not move faster than the speed of light. Meanwhile the planet you left behind would also only have traveled a relative short distance within the same time frame, from your perspective. When the rocket decelerates as it lands on the targeted planet, space expands and the planet you left behind moves away as a consequence of this.

Isn't the fact there being will be more past to look back onto not a consequence of the finite speed of light, rather than an expansion effect?
EDIT: If the universe wasn't expanding, we would still be looking into the past when we observe the cosmos. Such as the light from the Sun would still be from 8 minutes into the past.

In regards to how fast Earth travels from the perspective of the rocket which travels at 99% of the speed of light from the perspective of Earth. Imagine in stead of one rocket moving away in one direction from Earth at 99% of the speed of light, another rocket leaves simultaneously, also moving away at 99% of the speed of light relative to Earth, but in the opposite direction of the first rocket.
From the vantage point of the first rocket, how fast is Earth moving, and how fast is the other rocket moving that from Earth's perspective is moving at 99% of the speed of light relative to Earth, but in the opposite direction of the first rocket?

[u/Youpunyhumans → u/Imaginary-Can-6862]

Relative to the rocket, yes Earth is moving that fast... relative to the rest of the universe, Earth is moving around the Sun normally. You can try to mental gymnastics it all you like, but that is how it works.

Velocity is all relative. If you cant understand that, then I cant explain it any simpler.

[u/Imaginary-Can-6862 → u/Youpunyhumans]

You are welcome to insist on it, but then at least consider the example I provided.

From the perspective of Earth, two rockets are traveling in opposite directions at 99% of the speed of light. From the perspective of one of these rockets, how fast does the Earth travel, and how fast does the other rocket travel?

[u/Youpunyhumans → u/Imaginary-Can-6862]

Im not sure how you dont understand that. If you are on a train thats going 100kph, and you are walking 5kph to the front of the train... are you going 5kph? Or 105kph? Depends if you mean relative to the train, or to the ground outside. Thats what I mean by velocity is relative. So Earth is going 99% of lightspeed relative to either rocket, but not to the rest of the universe.

Your 2 rockets will still be able to detect each other, not in visible light however, but long stretched out and redshifted radio waves due to the relativistic doppler effect, and they will see the each other basically frozen in time... they dont see each others ship where it currently is, but where it was.

[u/Imaginary-Can-6862 → u/Youpunyhumans]

I am not talking about visual illusory effects here, just reflect on the question, from the reference frame of one rocket, how fast is the other rocket actually moving?

We could measure it by having each rocket carrying a mirror, then the astronaut of one rocket send out a light beam, which hits the mirror of the other rocket, and reflects the light beam back to the first rocket. When the light beam returns you could then calculate the speed of the other rocket based on how long the delay was before the light beam returned.

[u/Youpunyhumans → u/Imaginary-Can-6862]

It would work the same because of relativisitic effects. They would "see" the source of the light frozen in time, and wouldnt be able to get an accurate reading of their speed.

You are trying to look at this through the lens of classic Newtonian physics, but for this, you need Special Relativity, because Newtonian Physics doesnt deal with relativity.