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.

 

[MoonSong]

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.

Again, because there is no such a thing as absolute coordinates in space. Outside a planet there is not even something as simple as up or down.

The only way to make sense of space is through frames of reference, and while these can be arbitrary, many aren't and are tied to natural phenomena, like say, Earth's gravity.

There's little reason time travel would magically erase that. It would have to be tied to a frame of reference. Be it the entrance to a wormhole and then fly back to the solar system, or something even more fantastical like an energy portal or "altering the subatomic particles that carry time within matter". Just like you can't randomly end up in the magnetosphere just by walking.

 

[GreyLord]

Again, because there is no such a thing as absolute coordinates in space. Outside a planet there is not even something as simple as up or down.

The only way to make sense of space is through frames of reference, and while these can be arbitrary, many aren't and are tied to natural phenomena, like say, Earth's gravity.

There's little reason time travel would magically erase that. It would have to be tied to a frame of reference. Be it the entrance to a wormhole and then fly back to the solar system, or something even more fantastical like an energy portal or "altering the subatomic particles that carry time within matter". Just like you can't randomly end up in the magnetosphere just by walking.

How would you know, we haven't even seen the entirety of the Universe, much less even mapped out the Galaxy (or even mapped out this sector of the Galaxy)?

That said...

I never said absolute coordinates, I said coordinates.

Mapping out the universe with Coordinates is exactly the same as mapping out coordinates on the Earth or the Solar System. The same rules and ideas apply. Just because one is smaller, and one is bigger, does not change the rules of physics or how they are applied. They may be applied on a bigger scale, but they still can be applied.

You are correct, coordinates rely on the relation of spacial objects and our measurements and connections between them. This is how we came up with some of our coordinates.

223 Hillside Road, Manchester, UK is a coordinate of sorts. It enables one to figure out where a location is. You may not know how to find it as it's not a lat/long coordinate, but not all coordinates are specifically as you seem to want to allude to.

Some may be...it's 10 Kilometers from the red farmhouse that you turn left at after you travel 60 Kilometers down the road which is more what you would be saying is impossible as that is talking about a coordinate or location in referencing the location of another object.

However you do it, without a specific way to find the location of things in the past and where they were exactly, it would be impossible to travel to that point in time of their location.

If you traveled back 100 days to the exact location where the Earth is today, you would be in space. You would not find yourself on the Earth. In order to travel to where you want to on the Earth 100 days ago, you would have to first travel to the location where the Earth was at that point, and then move back to the time you wanted to be at.

This is why, if you want to go back to the Earth or a specific point on the Earth in the past, you will need to travel through Space and Time (like a Tardis and a Timelord do) rather than just traveling through time itself.

 

[Umbran]

I think we disagree on what constitutes absolute.

That sounds likely...

It's the only frame in which the cosmological principle holds, and to me that's enough to use it as an absolute reference frame.
EDIT: by "the cosmological principle holds" I meant we observe the universe as homogeneous and isotropic on sufficiently large scales.

So, the problem is that this... isn't a thing. That's not a "principle" of cosmology.

What I really think we have here is a misunderstanding about what is important in Einstein's Special Relativity. That there is an inertial frame in which we might observe the Cosmic Microwave Background as isotropic isn't actually a big deal. It isn't meaningful as far as Special Relativity is concerned.

To a human, "isotropic" would seem to have some special significance. But, indeed, the entire point of Special Relativity is that differences in view of the universe due to constant linear motion don't really matter. That these relative motions are inconsequential is the point! Thus the name!

If you had to be accelerating in order to see the CMB as isotropic would probably tell us that the curvature or expansion of the (observable) universe was asymmetric in some manner, but to do so in an inertial rest frame doesn't actually say much of anything.

I'm not sure I follow, if they are rest with respect to me I can't detect them moving at all.

By the transitive property, if you are instantaneously at rest with respect to the CMB, and they are instantaneously at rest with respect to the CMB, then they are instantaneously at rest with respect to you. The word "instantaneous" here is important, because apparent movement due to cosmological expansion isn't actually movement.

 

[Umbran]

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.

So, here's the issue, right here: "same spot". The universe doesn't actually recognize an absolute "same spot". All spots must be measured relative to some rest frame and chosen center of a coordinate system, but the laws of physics do not recognize any grand universal rest frame. Any time machine, then, cannot be locked into some great universal "same spot".

When we think of the Earth moving under our time traveler, we are implicitly picking a rest frame and origin, but that's a human implicit choice, not one with (to first approximation) special physical meaning. We have established no conceptual reason the time machine has to respect your implicit human choices.

Meaning - you seem to be choosing "the distant stars" (very, very distant, if you're talking about galactic movement) as the source of your reference frame. But, there's nothing special about that choice. The time machine could just as validly be chosen with the ground under it's foundation as the reference frame, with the rest of the Universe whirling around it.

 

[GreyLord]

So, here's the issue, right here: "same spot". The universe doesn't actually recognize an absolute "same spot". All spots must be measured relative to some rest frame and chosen center of a coordinate system, but the laws of physics do not recognize any grand universal rest frame. Any time machine, then, cannot be locked into some great universal "same spot".

When we think of the Earth moving under our time traveler, we are implicitly picking a rest frame and origin, but that's a human implicit choice, not one with (to first approximation) special physical meaning. We have established no conceptual reason the time machine has to respect your implicit human choices.

Meaning - you seem to be choosing "the distant stars" (very, very distant, if you're talking about galactic movement) as the source of your reference frame. But, there's nothing special about that choice. The time machine could just as validly be chosen with the ground under it's foundation as the reference frame, with the rest of the Universe whirling around it.

Obviously it's not the "Same spot" as you put it that way, but even relatively to where it was and everything else...the "same spot" is exactly what I meant.

The only way you end up in Paris 1000 years ago is if the Time Machine and mechanics of time are also held in place is if somehow we only live with Newton's First law and it applies to our movement through time and space in relation to this planet rather than Einsteinian or later applications (which would be where your idea of everything being relative in the first place comes into play).

 

[Ruin Explorer]

I feel like the original article is just wrong because, even just totally ignoring politics and anything a politician from anywhere did or said, the last 5 years are so insanely stupid, that time travel kind of has to exist, and has to being used to mess with us by jerks from the future. Like, I instinctively don't buy that the "no time travel" timeline is also the "absolutely stupidest possible timeline". Particularly if we just look at the decisions and obsessions of billionaires and in general the people with all the money over the last 5-10 years (especially the last 5), it's been absolutely demented, and continues to get worse (and again, this is ignoring politics!). Just incredibly rich people making huge, huge mistakes, that only don't take them out because the other rich idiots are bailing them out/have rigged the system to prevent that, essentially. People with tens and hundreds of billions obsessed about multiple conspiracy theories that a fifteen-year-old from 1993 (or now, even, probably) would correctly say "That's kind of a childish idea, dude" about. People spending tens of billions because they're mad about something meaningless - not to help people, not to raise up humanity or even just their country or city (like honestly even some awful ultra-rich people did in the past), not even really to further to some evil and cunning plan, just because they've got an insane bee in their bonnet about something super-dumb and that's not even true or prima facie plausible. And it's not just one guy either! It's dozens of guys. I feel like to get that many absolute idiots in charge of that much money you kind of need time travel. Someone is doing a Travelers to us to try and screw things up as badly as possible!

To me this isn't plausible as "what happens when there's no time travel", but it's very plausible as "what happens when a lot of people have messed with the timeline to try and make it worse". I'm reminded of The Guns of The South by Harry Turtledove.

I'm not putting this forth as logical or rational position, note, this is vibes, and it ain't nothing but vibes. But that's essence of the 2020s, eh?

 

[Umbran]

Obviously it's not the "Same spot" as you put it that way, but even relatively to where it was and everything else...the "same spot" is exactly what I meant.

So, it isn't the same spot, but you meant the same spot? You meant the thing that you say it obviously cannot be? What?

The only way you end up in Paris 1000 years ago

And you know the "only way"... how, exactly?
I mean, if you are a time traveler, and know, then tell us - we all stand to be able to live quite better using your unique abilities!

is if the Time Machine and mechanics of time are also held in place is if somehow we only live with Newton's First law...

Why? You make this assertion, but give no reasoning behind it.

I will try to restate my previous note, and see if it helps.

Einstein tells us that there's no absolute measure of time, or of space. There is no one fixed inertial reference frame in which the laws of physics work differently than in any other, or that we can refer to for absolute truth. The best we can do is measure things relative to other things.

There is, to first approximation, no reason to choose a "farthest stars" frame, in which the Earth, Sun, and Milky Way are all moving, over a frame that happens to be fixed to the foot of the Eiffel Tower. The "farthest stars" themselves aren't actually fixed, even relative to one another - they only seem that way to use slow-moving, short-lived humans. Neither of those frames is particularly privileged - the only reason to pick one over the other is how hairy the math comes out.

So, if I am travelling in the lifespan of the Eiffel Tower, and start and end someplace mostly at rest relative to the Eiffel Tower, there's no reason the time time machine cannot be working in that frame of reference, and it looks to everyone on the planet like the traveler moved in time, but not in space, relative to the surface of the Earth.

Now, yes, someone sitting around one of the farthest stars would say that the traveler moved in time as well as space. But, WE DO NOT CARE. His view of that isn't special, isn't privileged, isn't more true, than that of a person standing at the foot of the Tower. Folks not in the chosen rest frame of the machine can basically bugger off, we don't need to care about them.