Time travel is not logically possible

[Boltzmannbrain]

Time travel is not logically possible if we assume Einstein's theory of relativity is more or less the true explanation of the universe.

Einstein's theory of relativity implies a block universe, which is to say that the future already exists, and the past still exists.  Ultimately, the universe is a 4d structure that doesn't change. 

And as far as we know, this entire structure doesn't age in another temporal dimension, and it doesn't exist inside of another spacial dimension either - it simply just exists.

Now for an example of the illogical idea of time travel.  Imagine that it is the 3rd of January at 11:00pm.  You are alone in your house.   A week later you have the opportunity to travel back in time (somehow).  You decide to visit yourself on the 3rd of January at 11:00pm.

Structurally speaking, what happened here, how did this static 4d structure (the universe) change?  How can a nonmechanical structure just change itself from being A to being B? 

Moreover how is it logical that the universe has only you in your house and also has two of you in your house? 

From what I understand, the universe is made up of events that have their set x,y,z,t locations in spacetime.  None of these events move, and none of them appear or disappear.  Everything just is the way it is.  In general, nothing physical actually changes or moves (I put "physical" because you could argue that a nonphysical consciousness moves through the human parts of this structure, but that's a whole other discussion).

How is time travel logically possible?

[MigL]

Not that I have high regard for the 'logic' used in your argument, but I don't recall anyone saying that it was possible.

[Boltzmannbrain]

53 minutes ago, MigL said:

Not that I have high regard for the 'logic' used in your argument, ...

So your response is that my logic isn't good, but you don't say why.  Okay, you have my ear, so please tell me what part of my logic is not up to your standards.

 

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but I don't recall anyone saying that it was possible.

And this just tells me that you don't know much about this topic.  

Edited January 4 by Boltzmannbrain

[exchemist]

59 minutes ago, Boltzmannbrain said:

Time travel is not logically possible if we assume Einstein's theory of relativity is more or less the true explanation of the universe.

Einstein's theory of relativity implies a block universe, which is to say that the future already exists, and the past still exists.  Ultimately, the universe is a 4d structure that doesn't change. 

And as far as we know, this entire structure doesn't age in another temporal dimension, and it doesn't exist inside of another spacial dimension either - it simply just exists.

Now for an example of the illogical idea of time travel.  Imagine that it is the 3rd of January at 11:00pm.  You are alone in your house.   A week later you have the opportunity to travel back in time (somehow).  You decide to visit yourself on the 3rd of January at 11:00pm.

Structurally speaking, what happened here, how did this static 4d structure (the universe) change?  How can a nonmechanical structure just change itself from being A to being B? 

Moreover how is it logical that the universe has only you in your house and also has two of you in your house? 

From what I understand, the universe is made up of events that have their set x,y,z,t locations in spacetime.  None of these events move, and none of them appear or disappear.  Everything just is the way it is.  In general, nothing physical actually changes or moves (I put "physical" because you could argue that a nonphysical consciousness moves through the human parts of this structure, but that's a whole other discussion).

How is time travel logically possible?

Nobody claims it is, apart from Dr. Who.

10 minutes ago, Boltzmannbrain said:

So your response is that my logic isn't good, but you don't say why.  Okay, you have my ear, so please tell me what part of my logic is not up to your standards.

 

And this just tells me that you don't know much about this topic.  

Can you provide a reference to show who thinks it possible?

[Boltzmannbrain]

12 minutes ago, exchemist said:

Nobody claims it is, apart from Dr. Who.

Can you provide a reference to show who thinks it possible?

I took the first reference that came up because this is not what I had expected to have to defend.  

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Dr. Ana Alonso-Serrano, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained the possibility of time travel and how researchers test theories. 

Space and time are not absolute values, Alonso-Serrano said. And what makes this all more complex is that you are able to carve space-time.

“In the moment that you carve the space-time, you can play with that curvature to make the time come in a circle and make a time machine,” Alonso-Serrano told USA TODAY. 

She explained how, theoretically, time travel is possible. The mathematics behind creating curvature of space-time are solid, but trying to re-create the strict physical conditions needed to prove these theories can be challenging. 

 

 

from https://www.usatoday.com/story/tech/science/2022/09/10/time-travel-possible-science/7847346001/

 

 

[exchemist]

1 hour ago, Boltzmannbrain said:

I took the first reference that came up because this is not what I had expected to have to defend.  

 

from https://www.usatoday.com/story/tech/science/2022/09/10/time-travel-possible-science/7847346001/

 

 

That article is an elaborate way of saying time travel is not possible. What does it mean to say you can "carve" spacetime? It's bullshit. 

[joigus]

5 hours ago, Boltzmannbrain said:

Time travel is not logically possible if we assume Einstein's theory of relativity is more or less the true explanation of the universe.

OTOH, I wouldn't expect Einstein's theory as a standalone to be more or less the true explanation of the universe. In particular, and as concerns time, I would expect left-right asymmetry and charge conjugation asymmetry (time-inversion asymmetry) in the standard model (and how they play out in combination with gravity) to play a very deep role in it.

[Genady]

19 minutes ago, joigus said:

OTOH, I wouldn't expect Einstein's theory as a standalone to be more or less the true explanation of the universe. In particular, and as concerns time, I would expect left-right asymmetry and charge conjugation asymmetry (time-inversion asymmetry) in the standard model (and how they play out in combination with gravity) to play a very deep role in it.

As I see it, time travel and time inversion are two different things. The former involves "time mixing", i.e., the traveler's body maintains its time while being transferred into a different time of its environment. Of course, the time travel into the future is very much possible - e.g., the SR twins.

[joigus]

41 minutes ago, Genady said:

As I see it, time travel and time inversion are two different things. The former involves "time mixing", i.e., the traveler's body maintains its time while being transferred into a different time of its environment. Of course, the time travel into the future is very much possible - e.g., the SR twins.

Agreed. There are other differences that are relevant. Time inversion is a discrete transformation, like all inversions.  It bears the question: Could it be that certain solutions of GR continuously transform both time and space so that a continuous evolution brings local observers to a state in which the universe is everywhere the same (including the particular observer) except for a parity transformation? I don't know if that's been considered, but I'm sure it has.

I made my comment essentially because I don't think Einstein's theory can be claimed to be "more or less the true explanation of the universe". That's too strong a statement and I don't think there's any hope of that.

[MigL]

10 hours ago, Boltzmannbrain said:

Okay, you have my ear, so please tell me what part of my logic is not up to your standards.

i) GR is not a 'complete' theory; while extremely accurate where applicable, it fails to make valid predictions in many cases, as it fails miserably when dealing with hi mass/energy and extremely short separations.

ii) Gr does allow for closed timelike loops, which would enable time travel through the use of 'wormholes'.
However, keeping those wormholes open requires the use of 'exotic' matter ( negative mass/energy ), which, as far as I know, can be harvested from immediately inside the event horizon of a Black Hole ( the 'counterpart' to Hawking radiation ), and may not exist or be realizable. 
See K Thorne's work.

What does that tell you about my knowledge of the subject ?

[Boltzmannbrain]

9 hours ago, exchemist said:

That article is an elaborate way of saying time travel is not possible. What does it mean to say you can "carve" spacetime? It's bullshit. 

[swansont]

17 hours ago, Boltzmannbrain said:

Einstein's theory of relativity implies a block universe, which is to say that the future already exists

I don’t think this is a true statement. How does it imply a block universe, where “the future” already exists?

What future? 

17 hours ago, Boltzmannbrain said:

Now for an example of the illogical idea of time travel.  Imagine that it is the 3rd of January at 11:00pm.  You are alone in your house.   A week later you have the opportunity to travel back in time (somehow).  You decide to visit yourself on the 3rd of January at 11:00pm

IIRC the possibility of time travel is limited, it’s not arbitrary. You can only go back to a time after you built a time travel device.

edit: #8

https://www.preposterousuniverse.com/blog/2009/05/14/rules-for-time-travelers/

[Boltzmannbrain]

11 hours ago, MigL said:

i) GR is not a 'complete' theory; while extremely accurate where applicable, it fails to make valid predictions in many cases, as it fails miserably when dealing with hi mass/energy and extremely short separations.

I didn't say it was a complete theory.  This thread is about my claim that time travel is not possible with GR.  

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ii) Gr does allow for closed timelike loops, which would enable time travel through the use of 'wormholes'.
However, keeping those wormholes open requires the use of 'exotic' matter ( negative mass/energy ), which, as far as I know, can be harvested from immediately inside the event horizon of a Black Hole ( the 'counterpart' to Hawking radiation ), and may not exist or be realizable. 
See K Thorne's work.

What does that tell you about my knowledge of the subject ?

 

It tells me that you agree with me that time machines are possible, at least theoretically.  But I am not only saying that it is unknown whether or not they are possible, I am flat out saying that they are impossible.  I gave my reasons in the OP.  It makes no sense to even talk about them in GR (except to say that they make no sense).

4 hours ago, swansont said:

I don’t think this is a true statement. How does it imply a block universe, where “the future” already exists?

What future? 

Take a 1 dimensional particle that lasts for 5 seconds for example.  Assume there is no force applied to it so that its worldline looks like a long straight 2 dimensional line on a spacetime diagram.  In GR, this line doesn't have a past, present or future; it just simply exists.  Past, present and future are human constructs.  They are concepts in the mind.  Since everything is made up of these eternal particles, so is the universe.

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IIRC the possibility of time travel is limited, it’s not arbitrary. You can only go back to a time after you built a time travel device.

edit: #8

https://www.preposterousuniverse.com/blog/2009/05/14/rules-for-time-travelers/

 

What if advanced aliens built a time everyday for the past 10,000 years, and they have them, oh I don't know, under some mountains?  It allows a lot of opportunity to do illogical things like change the past.  

Edited January 5 by Boltzmannbrain

[MigL]

The 'block' universe is an interpretation.
One of many, and the one you are choosing to use is called 'eternalism' and has many counter-arguments.
See here for other interpretations 

 

[Boltzmannbrain]

12 hours ago, MigL said:

The 'block' universe is an interpretation.
One of many, and the one you are choosing to use is called 'eternalism' and has many counter-arguments.
See here for other interpretations 

 

If you watched the video or truly understand GR, you would know that there is really only one option.

[swansont]

14 hours ago, Boltzmannbrain said:

Take a 1 dimensional particle that lasts for 5 seconds for example.  Assume there is no force applied to it so that its worldline looks like a long straight 2 dimensional line on a spacetime diagram.  In GR, this line doesn't have a past, present or future; it just simply exists.  Past, present and future are human constructs.  They are concepts in the mind.  Since everything is made up of these eternal particles, so is the universe.

If there’s no past, present or future, because they are human constructs, how is it that “relativity implies a block universe, which is to say that the future already exists”? If future is a human construct, not part of relativity. These statements are inconsistent.

[Boltzmannbrain]

1 minute ago, swansont said:

If there’s no past, present or future, because they are human constructs, how is it that “relativity implies a block universe, which is to say that the future already exists”? If future is a human construct, not part of relativity. These statements are inconsistent.

Imagine you are looking at a typical coffee table.  It's measurements are not important, but let's say it's 2 meters long, 1 meter wide and 1/2 meter high.  Does this object have a past, future or present?  Of course not, it is just an object. 

In one second, this coffee table extends through the 4th dimension. 

Now let's isolate this coffee table again.  It is now an object measuring 2m x 1m x 0.5m x 300,000,000m (remember the coffee table went through 1 second of time). *this is of course uses the Minkowski metric which is different metric than Euclidean metric, but the idea is the same. 

At the beginning of the second, the coffee table was in our present.  But for someone or something else travelling at a certain speed, the coffee table at the end of the second is in their present.  We are all correct GR.   

The reason why the coffee table had a past, present and future is because we say it does; that's it.

Imagine that someone spills a coffee at the end of that second.  If we are at the beginning of that second, there will be other frames of reference where the coffee was already spilled.  Of course they can't communicate to us that this will happen, but it happens in their present nonetheless.  

So if the coffee spills at what may be an infinite number of "presents" in the universe, what or who is to say that it should have a present at all.  Or a past or a future.  It (along with the rest of the universe) just exists eternally in GR. 

 

[swansont]

2 hours ago, Boltzmannbrain said:

Imagine you are looking at a typical coffee table.  It's measurements are not important, but let's say it's 2 meters long, 1 meter wide and 1/2 meter high.  Does this object have a past, future or present?  Of course not, it is just an object. 

The object exists at t=0 and at t= t1, the current time. It is likely to exist at some time t2>t1

One can speak of its past and future, though those are colloquial expressions.

2 hours ago, Boltzmannbrain said:

In one second, this coffee table extends through the 4th dimension. 

Now let's isolate this coffee table again.  It is now an object measuring 2m x 1m x 0.5m x 300,000,000m (remember the coffee table went through 1 second of time). *this is of course uses the Minkowski metric which is different metric than Euclidean metric, but the idea is the same. 

That's not how this works.

2 hours ago, Boltzmannbrain said:

At the beginning of the second, the coffee table was in our present.  But for someone or something else travelling at a certain speed, the coffee table at the end of the second is in their present.  We are all correct GR. 

Which is why we don't speak of present except in relation to our own frame. And my objection has nothing to do with the present. It's the assertion that relativity says that the future already exists. No additional conditions were specified.

 

2 hours ago, Boltzmannbrain said:

The reason why the coffee table had a past, present and future is because we say it does; that's it.

Imagine that someone spills a coffee at the end of that second.  If we are at the beginning of that second, there will be other frames of reference where the coffee was already spilled.  Of course they can't communicate to us that this will happen, but it happens in their present nonetheless.  

So if the coffee spills at what may be an infinite number of "presents" in the universe, what or who is to say that it should have a present at all.  Or a past or a future.  It (along with the rest of the universe) just exists eternally in GR. 

You've introduced other frames into the discussion, but that's just a distraction. We have an observer, who has a frame of reference. There are events that happened in the past, and some will (presumably) happen in the future. Relativity tells us that clocks in different frames will disagree as to when an event occurred, or the order in which events occurred. But regardless of your frame, there will be a time where the coffee had not yet been spilled.

[Boltzmannbrain]

4 hours ago, swansont said:

The object exists at t=0 and at t= t1, the current time. It is likely to exist at some time t2>t1

One can speak of its past and future, though those are colloquial expressions.

That's not how this works.

I wasn't sure what you knew.  That is why I simplified it to spatial distance.  However, I did add that it is a different kind of metric than Euclidean. 

The idea is practically the same.  The spacetime interval for the coffee cup is s^2 = - (ct)^2, x^2, y^2, z^2 = - (3x10^8m/s*1s)^2, 0, 0 , 0.  

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Which is why we don't speak of present except in relation to our own frame. And my objection has nothing to do with the present. It's the assertion that relativity says that the future already exists. No additional conditions were specified.

I am just trying to explain it to you the best I can.

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You've introduced other frames into the discussion, but that's just a distraction. We have an observer, who has a frame of reference. There are events that happened in the past, and some will (presumably) happen in the future. Relativity tells us that clocks in different frames will disagree as to when an event occurred, or the order in which events occurred. But regardless of your frame, there will be a time where the coffee had not yet been spilled.

The only reason why the coffee hadn't spilled in your present is because of the angle that your present occupies in spacetime.  It is in the present of other frames because their presents in spacetime are at a different angle.  

 

Edited January 6 by Boltzmannbrain

[MigL]

You cannot 'see' an extension along the time axis, as that implies an 'outside' PoV.
If you understand the concept of the block universe, you must realize that you are embedded in it. 
There is no outside.

[Boltzmannbrain]

22 minutes ago, MigL said:

You cannot 'see' an extension along the time axis, as that implies an 'outside' PoV.
If you understand the concept of the block universe, you must realize that you are embedded in it. 
There is no outside.

Where did I say you can?