Time travel doesn't exist because time travel wiped out the timelines where it did

[Umbran]

In order to move through time you need to be put outside of the current time.

Since, per Einstein, time is not fundamentally different from space, that's like saying in order to move in space, you need to be put outside space.

However, it appears Umbran is arguing against this idea, and as such I pointed out what I understood he was claiming (so if it is a misunderstanding on my part, he can clarify. I think it is possible we are saying the same thing but talking past each other as well...but it is unclear to me).

I will try again.

Frames of reference are arbitrary. You may choose the frame for your convenience.

So, put a battlemat on a table. Put a miniature on the battlemat. Now, that miniature has a frame of reference, for what it left, right, forward, and back, along gridlines of the mat. The miniature can move anywhere, and you can describe that as so many squares forward/back, and so many left/right.

Put an object (say, a can of soda) on any point on the surface of the table you like.

You will always be able to rotate the battlemat under the figure and can, such that the path toward the can is directly along one set of gridlines.

Similarly, local to me, the traveler in the time machine, there is a frame in which travelling in time is just travelling in the time dimension.

You are correct that other people, in other parts of the universe, may see it as travelling in both the space and time dimensions. The person sitting on Mars will think you have to adjust for the orbit and spin of the Earth. The observer at Alpha Centauri will say you also have to adjust for the motion of the Sun through the Galaxy. A person outside the Local Group would say you also have to adjust for the movement of the Milky Way itself.

What you seem to repeatedly fail to get is that those people don't matter. The fact that someone sees it as both time and space motion doesn't change anything! That is the very essense of Einsteinian Relativity - that different observers see different time and space coordinates does not change the physics of the situation for the traveler!

 

[Umbran]

... specifically because movement due to cosmological expansion is actual movement.

Not really. This is most readily seen in how the observable universe has a measurable radius (some 46.5 billion light years)

Anything beyond the edge of the observable universe is receding from us faster than light, such that light from it will never reach Earth. But, Einstein tells us that real motion faster than light is impossible. Therefore, the measured motion due to cosmological expansion isn't real.

With cosmological expansion, the motion is an illusion. What is actually happening is that space is added between objects. This is extremely important when we speak about whether expansion is accelerating or decelerating or steady-state. For accelerating expansion, if it were actual motion, you'd need to be exerting forces on objects to accelerate them. But, there is no such force.

 

[MoonSong]

Since, per Einstein, time is not fundamentally different from space, that's like saying in order to move in space, you need to be put outside space.



I will try again.

Frames of reference are arbitrary. You may choose the frame for your convenience.

So, put a battlemat on a table. Put a miniature on the battlemat. Now, that miniature has a frame of reference, for what it left, right, forward, and back, along gridlines of the mat. The miniature can move anywhere, and you can describe that as so many squares forward/back, and so many left/right.

Put an object (say, a can of soda) on any point on the surface of the table you like.

You will always be able to rotate the battlemat under the figure and can, such that the path toward the can is directly along one set of gridlines.

Similarly, local to me, the traveler in the time machine, there is a frame in which travelling in time is just travelling in the time dimension.

You are correct that other people, in other parts of the universe, may see it as travelling in both the space and time dimensions. The person sitting on Mars will think you have to adjust for the orbit and spin of the Earth. The observer at Alpha Centauri will say you also have to adjust for the motion of the Sun through the Galaxy. A person outside the Local Group would say you also have to adjust for the movement of the Milky Way itself.

What you seem to repeatedly fail to get is that those people don't matter. The fact that someone sees it as both time and space motion doesn't change anything! That is the very essense of Einsteinian Relativity - that different observers see different time and space coordinates does not change the physics of the situation for the traveler!

Exactly. I could declare my house as the center of the universe and then describe how the sun, moon and the planets move and the physics equations to describe these movements. It all would be a messy affair, but it is doable and nothing would break even Newtonian physics.

 

[Autumnal]

I remember Dad sort of resenting having to dig out his immediately-post-WW2 (Caltech class of 1946) linear algebra when working on ranging systems for Voyager’s encounter with Jupiter. He felt that a proper frame of reference wouldn’t require that. But his bosses at JPL hadn’t put him in charge of reference frame choice, so he didn’t get to.

He also sometimes said he felt Copernicus was just being difficult.