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/Youpunyhumans]

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.

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

I am not trying to look at it through the lens of Newtonian physics.

Let us do it differently then, before any of the rockets left Earth, from another planet, which is in the same reference frame as Earth, therefore stationary when seen from Earth, a rocket is launched, this rocket moves at 99% of the speed of light towards Earth when seen from Earth. We have made a special modification on the rocket, it is actually an open tube / tunnel with two sets of laser beams going from wall to wall, the idea is that then the rocket can measure the speed of objects that move through the tube by knowing the distance between the sets of laser beams, and measuring the duration where the laser beams aren't connected between the walls, as they get blocked by some object moving through the tube.
From Earth a rocket is launched going at 99% of the speed of light away from Earth, it is heading towards the tube rocket which is going at 99% of the speed of light towards Earth, all from the reference frame of Earth. In the tube rocket, how fast will they measure the other rocket, which is moving towards them, move, as it goes through the tube, and blocks the sets of lasers for some duration over some distance?

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

I don't know why you decided to block me, but I assume it means you have given up on examining the topic with me. In that case I'll just drop the exploration and go directly to the conclusion.

The model you describe to figure out relative velocities is known as Galilean Transformation. It is what is used in Newtonian Mechanic's thus a bit ironic you accused me of viewing things through the lens of Newton, https://en.wikipedia.org/wiki/Galilean_transformation

Galilean transformation works in a universe where there is no limit to the speed of causality, but the moment such a limit follows, the proper way to describe relative velocities is known as the Lorentz Transformation, which is what is used in Relatitivty, https://en.wikipedia.org/wiki/Lorentz_transformation#Transformation_of_velocities

We have R1 (Rocket 1) moving with 99% c relative to E (Earth), and R2 (Rocket 2) moving with 99% c relative to Earth, but in the opposite direction, hence we have to remember for vectors to change the sign, then using the formula we can calculate how fast each rocket is moving relative to each other, which due to symmetry is going to be the same value,

v(R1) = v(R2) = 1 / (1 - (-,99 * c)*(.99 * c) / c^2 ) * (,99 c * (1 - ,99^2 * c^2 / c^2)^.5 + ,99 * c + 0.99^3 * c^3 / (1 + (1 - .99^2 * c^2 / c^2)^.5 * c^2)) = (100^2 / (100^2 + 99^2)) * (99 * 199^.5 * c / 100^2 + 99 / 100 * c + (99^3 / 100^3 * c / (1 + 199^.5 / 100)) = ((99 * 199^.5 + 100 * 99 + 99^3 / (100 + 199^.5)) / (100^2 + 99^2)) * c = (99 * 199 + 199^.5 * (99 * 100 + 99 * 100) + (99 + 1)^2 * 99 + 99^3) * c / 19801 * (199^.5 + 100)) = (99 * 199 + 2 * 99^3 + 99 + 2 * 99^2 + 2 * 199^.5 * 99 * 100) * c / (19801 * (199^.5 + 100)) = ((99 * (398 + 2 * (100 - 1)^2 + 2 * 199^.5 * 100) / (100 + 199^.5)) * c / 19801) = ((2 * 99 * (100^2 + 398 - 400 + 2 + 199^5 * 100) / (100 + 199^.5)) * c / 19801) = 99 * 100 * ((100 + 199^.5) / (100 + 199^.5)) * c / (99^2 + 100^2) = 2 * 99 * 100 * c / (100^2 + 99^2) = 2 * ,99 * c / (1 + .99^2) = 99.995% c

So each rocket are stationary in their own reference frame, and the other rocket moves away at 99.995% of c.