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.

 

[Umbran]

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.

Exactly.
However, for purposes of talking about movement within your house, no other choice makes a whole lot of sense, either.

 

[GMMichael]

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.

I'm seeing this differently. Time stops for those traveling at C. So there's a point when all observers can agree: that fast guy's clock just isn't ticking. The opposite end of the spectrum is speed zero. All observers can agree: that guy's clock is broken (doesn't exist?). Between the two, you're at some combination of vector or time speed. Neither has stopped / reached a limit.

If I'm not too far off, and your time machine doesn't change your location, then you're at speed zero, and different observers can agree that you're doing only one thing: traveling in time. Or violating the Heisenberg uncertainty principle.

I've seen how some travelers can age faster than others, but is there a reasonable suggestion on how some travelers can age backwards? (And even more extreme, make the entire universe age backwards with you? AKA time travel?)

 

[Maxperson]

I don't think you understand what I mean by the same spot.

I've tried to explain it.

Going to the same spot on earth today is like going to the same spot in the solar system today.
If it is relative to the solar system, it is relative to the Earth.

For example, the earth rotates. You put yourself stationary above the Earth for one hour, where you are not turning in rotation with the earth (aka, the same spot you are at right now but 100 feet up, but not turning in relation to the earth), you will find yourself a ways away from where you started.

Yes, it is relative, but we still have mapped it out on the Earth, and the same laws apply to the Earth as the rest of the universe (the Earth is not excluded from the laws of the universe as you seem to want to imply).

However, as you seem to want to twist the semantics and do no matter how simple I try to explain it, you do not wish to understand it, I have no idea how to explain what I am intending.

We don't know if there's some way to decouple movement through time from movement through space with regard to time travel. Every year we learn new things and revise old understandings. If they can be decoupled, you could move through time, but not space and end up in the past, but in the same spot on Earth as you left.