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

[gban007]

Sabine Hossenfelder says they're not actually paradoxes. And that the faster moving/accelerating person is the one who takes the longer path through Proper Time

That isnt how I understood the video. The accelerating person has a longer spacetime path, as is moving through time and distance, compared to Alice just moving through time. But video specifies that Alice has the maximal / longest proper time, and that Bob will always have a shorter proper time due to the acceleration.
Now as there isnt a true rest, both are moving distance wise as well as time, and both are accelerated, just Alice is moving less distance and is less accelerated, which means she takes a longer proper time to get to same end point than Bob.

 

[GreyLord]

I pointed out the problem before, and I don't really think it's been addressed, so I'll try to explain it more plainly.

You live in Paris, France. It is the year 1898.

You move to Barcelona, Spain in 1910.

You move to New York City, USA in 1920.

You move to San Francisco, USA in 1935.

You move to Honolulu, Hawaii in 1948.

You move to Tokyo, Japan in 1958.

Now, a time traveller wants to go back to see what you were like in 1908. If they go to where you live now and travel back in time at your spot, they will still be in Tokyo, Japan. They will be nowhere close to where you actually lived in 1898.

The Earth orbits around the Sun. Hopefully this is not a hard concept for anyone to understand.

If this was static, it means that it takes 365 days (+/- a little bit) to get to the same spot you were before. If you went back in time 100 days ago, you would literally be in open space if you went to the same spot. You have to know where in the orbit the earth was and plan to travel through space as well as time in order to get to that location...otherwise you are 1/3 (+/- a bit) away from where the earth actually is.

However, the Solar System is NOT static in space. It rotates as well. It rotates around the Milky Way. This takes around 200-250 MILLION years from what I looked up. That means, if you went back in time 2000 years to see the Roman Empire, you would need to know exactly WHERE the solar system was at that moment in relation to the Milky Way, and then where Earth was in relation to the Solar System or you would still end up in the blackness of space. You cannot go to the same spot you start from.

And....if that's not enough, the Milky Way Galaxy is not static in Space. It is moving through space at a guestimated speed of 2.1 Kilometers/hr (1.3 million miles per hour for those in the US).

This means, you cannot simply travel back in time, you need some sort of coordinate system in order to go back to where the Earth was at the time period you intend to go to, otherwise you will be millions of kilometers/miles away from the earth in the blackness of space.

Most likely you would need to have some sort of coordinate system that maps out the entire universe in order to figure it out.

We haven't even seen the entire universe yet, much less mapped it out with coordinates.

This is one of the real reasons why Time Travel would be incredibly difficult. It's not just figuring out how to go back in time, but to figure out the exact spacial coordinates that you will want to go back to.

Otherwise you just end up in the blackness of space in whatever time you go back to.

 

[GMMichael]

That isnt how I understood the video. The accelerating person has a longer spacetime path, as is moving through time and distance, compared to Alice just moving through time. But video specifies that Alice has the maximal / longest proper time, and that Bob will always have a shorter proper time due to the acceleration.
Now as there isnt a true rest, both are moving distance wise as well as time, and both are accelerated, just Alice is moving less distance and is less accelerated, which means she takes a longer proper time to get to same end point than Bob.

My big takeaway was that each person doesn't get younger/older, relative to each other. Alice ages more. But yes, I guess Alice had the longer proper time path, which is why she aged more. A "true rest" isn't needed to objectively determine who did more aging/whose clock ticked longer. In fact, I suspect that true rest is either impossible or an invitation to being obliterated by quantum particles.

Most likely you would need to have some sort of coordinate system that maps out the entire universe in order to figure it out.

Skynet figured this out back in . . . 2112? It's why terminators never arrive next to their termination targets.