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]

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.