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

[Retros_x]

But also, proving a negative like, "there is no way to travel backwards in time" is incredibly difficult.

Proving in a scientific way, yes it is difficult. But I was just responding through the premise of the thread - I think its much more reasonable that back wards time travel is impossible as an explanation for missing time travelers than they explanation they destroyed every timeline where its possible. Contrary to time travel to the future we have no physical and theoretical model of how time travel to the past could work.

 

[GreyLord]

Proving in a scientific way, yes it is difficult. But I was just responding through the premise of the thread - I think its much more reasonable that back wards time travel is impossible as an explanation for missing time travelers than they explanation they destroyed every timeline where its possible. Contrary to time travel to the future we have no physical and theoretical model of how time travel to the past could work.

Here's a humorous reason why time travel may be possible, but we don't have any time traveler's.

They go back in time, but go back to the wrong location!

The Earth's movement is not static in space. It moves at a very fast speed. It moves thousands (if not faster, it's hard to calculate for me, but it's not just it's rotation, it's a movement around the Sun, then the Solar System's movement around the Galactic Center, then the Galaxy's movement through the Galactic Cluster and the Galactic Cluster's movement through space itself which may be up to millions of miles (Kilometers) per day/week/month/year).

Without knowing the size and mapping the universe, it may be impossible to know the exact location the Earth has been in the past (you would need a coordinate map for the Universe...for starters). So...the time traveler's go back in time, but as this would be breaking the time/space relationship to a degree, they go back to where the Earth is currently...except...there is no Earth in that location at that time period.

 

[MoonSong]

Except,

Here's a humorous reason why time travel may be possible, but we don't have any time traveler's.

They go back in time, but go back to the wrong location!

The Earth's movement is not static in space. It moves at a very fast speed. It moves thousands (if not faster, it's hard to calculate for me, but it's not just it's rotation, it's a movement around the Sun, then the Solar System's movement around the Galactic Center, then the Galaxy's movement through the Galactic Cluster and the Galactic Cluster's movement through space itself which may be up to millions of miles (Kilometers) per day/week/month/year).

Without knowing the size and mapping the universe, it may be impossible to know the exact location the Earth has been in the past (you would need a coordinate map for the Universe...for starters). So...the time traveler's go back in time, but as this would be breaking the time/space relationship to a degree, they go back to where the Earth is currently...except...there is no Earth in that location at that time period.

Except there's no such a thing as absolute coordinates. Even space itself changes over time. It's impossible to know if something is truly moving or not moving. Is that car approaching or are you getting close to the car? Or maybe both of you are moving. That's where frames of reference factor in.

 

[briggart]

Except,
Except there's no such a thing as absolute coordinates. Even space itself changes over time. It's impossible to know if something is truly moving or not moving. Is that car approaching or are you getting close to the car? Or maybe both of you are moving. That's where frames of reference factor in.

Well, the cosmic microwave background sort of provides an absolute definition of “truly moving”. If you are locally at rest with the CMB, the only motion you’ll detect with respect to other observers who are also locally at rest with the CMB, is the one given by the expansion of the universe.

 

[Umbran]

Well, the cosmic microwave background sort of provides an absolute definition of “truly moving”.

No, it is still a relative one - it would only be absolute if physics changed in that rest frame, which it does not. As far as has been measured so far, the CMB rest frame is not special. Choosing the CMB as a rest frame is the same as choosing a frame in which the center of the Milky Way, or the Sun, or the Earth is at rest.

If you are locally at rest with the CMB, the only motion you’ll detect with respect to other observers who are also locally at rest with the CMB, is the one given by the expansion of the universe.

So, all you are doing here is adding the CMB in as an extra step - if you are locally at rest in your own frame, the only motion you'd detect with respect to other observers who are locally at rest with respect to you is the movement given by the expansion of the universe.

 

[tomBitonti]

Whether the past still exists or if only a thin membrane exists is not answered. One (apparently) popular view, called the block universe, has all times existing, with time having meaning on space like trajectories within the four dimensional universe. If there is just one block universe, time travel requires .. interesting .. connections within the universe. If there is one, unchanging block, travel to a causally connected prior time would not enable one to change the past. Perhaps if there were multiple connected blocks, there could be the appearance of change.

As a side note .. consider the topology of multiply connected block universes. Note that events with different outcomes occur all over, and that blocks which have not received information from events in distant regions possibly should be the same until a split is forced. I struggle to conceptualize this topology, let alone reason about it.

TomB

 

[briggart]

No, it is still a relative one - it would only be absolute if physics changed in that rest frame, which it does not. As far as has been measured so far, the CMB rest frame is not special. Choosing the CMB as a rest frame is the same as choosing a frame in which the center of the Milky Way, or the Sun, or the Earth is at rest.

I think we disagree on what constitutes absolute. 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.

So, all you are doing here is adding the CMB in as an extra step - if you are locally at rest in your own frame, the only motion you'd detect with respect to other observers who are locally at rest with respect to you is the movement given by the expansion of the universe.

I'm not sure I follow, if they are rest with respect to me I can't detect them moving at all. But we can be in any sort of motion with respect to the cosmological expansion. But if I am locally at rest with respect to the expansion, and you are locally at rest with respect to the expansion, there is only one way in which we can move with respect to each other.

EDIT: by "the cosmological principle holds" I meant we observe the universe as homogeneous and isotropic on sufficiently large scales.