Time Travel... the fly in the ointment

[bilby]

Yeah, sure; you can't cross the same river twice. But I have crossed the Brisbane river twice just today. Go figure.

Taken in isolation, but the World itself is moving, once you have crossed the river the World does not have you crossing the river at that moment on that day ever again. Once upon a time there was an England at war with Germany with Churchill as Prime Minister. England still exists but that England has gone forever. The world still exists, but the world of WW2 has long gone.

How can we revisit the events of WW2 with Churchill as PM, a time in the world that (apparently) no longer exists?

Easy - just stand further back. Viewed from Hampsted Heath, WWII is long gone; but viewed from 70-75 light years away, London is still in the grip of WWII.

There are no preferred reference frames.

 

[AdamWho]

Time travel is equivalent to reversing entropy for the entire universe except for the person/machine doing the traveling.

Time travel also implicitly assumes the existence of a preferred observer and time.

Both these violate physics at a very fundamental level.

 

[Jimmy Higgins]

My point is that a time machine is silly. It would need to be a space and time machine because you likely need to change your position in time and space to get where you are going.

Taken in isolation, but the World itself is moving, once you have crossed the river the World does not have you crossing the river at that moment on that day ever again. Once upon a time there was an England at war with Germany with Churchill as Prime Minister. England still exists but that England has gone forever. The world still exists, but the world of WW2 has long gone.

How can we revisit the events of WW2 with Churchill as PM, a time in the world that (apparently) no longer exists?

Easy - just stand further back. Viewed from Hampsted Heath, WWII is long gone; but viewed from 70-75 light years away, London is still in the grip of WWII.

There are no preferred reference frames.

We could put a large mirror 35 light years away instead.

 

[untermensche]

[
Easy - just stand further back. Viewed from Hampsted Heath, WWII is long gone; but viewed from 70-75 light years away, London is still in the grip of WWII.

There are no preferred reference frames.

The events are not the light reflected off them.

The events were the human bodies moving around on a specific planet in specific parts of that planet.

The light is something else entirely.

And of course becoming part of "events" as you describe them, returning to the past as you describe it, means turning into light.

 

[bilby]

[
Easy - just stand further back. Viewed from Hampsted Heath, WWII is long gone; but viewed from 70-75 light years away, London is still in the grip of WWII.

There are no preferred reference frames.

The events are not the light reflected off them.

The events were the human bodies moving around on a specific planet in specific parts of that planet.

The light is something else entirely.

And of course becoming part of "events" as you describe them, returning to the past as you describe it, means turning into light.

All observations are of the past; albeit in many cases the very recent past.

Obviously we can't alter the past - that would lead to paradoxes. But we can observe it. Indeed, we can't stop observing it, while we remain conscious.

We are never a part of the events we observe; our actions are always effective after the event.

 

[untermensche]

The events are not the light reflected off them.

The events were the human bodies moving around on a specific planet in specific parts of that planet.

The light is something else entirely.

And of course becoming part of "events" as you describe them, returning to the past as you describe it, means turning into light.

All observations are of the past; albeit in many cases the very recent past.

Obviously we can't alter the past - that would lead to paradoxes. But we can observe it. Indeed, we can't stop observing it, while we remain conscious.

We are never a part of the events we observe; our actions are always effective after the event.

Given the speed of light the difference in the real world is extremely small.

But there is still the distinction between the events and the light that reflects off the "things" involved in the events.

To go back to the past is to go back to when the events occurred.

 

[T.G.G. Moogly]

The "copy of the past" "exists" in the same "place" as the copy of the "future yet to be".

There exists only now. This is not a problem for the past any more than it is a problem for the future.

It's arguable that "now" even exists. It's basically the infinitely thin edge between the past and the future.

It's all a "now." All of time is an eternal moment. Isn't that what happens at light speed?

 

[DBT]

It's arguable that "now" even exists. It's basically the infinitely thin edge between the past and the future.

It's all a "now." All of time is an eternal moment. Isn't that what happens at light speed?

Time as a measurement of the rate of change....the rate of change being relative to entropy, physical relationships, velocity and mass?

 

[T.G.G. Moogly]

It's all a "now." All of time is an eternal moment. Isn't that what happens at light speed?

Time as a measurement of the rate of change....the rate of change being relative to entropy, physical relationships, velocity and mass?

If I understand correctly, were I able to travel at light speed, from my perspective I would be everywhere all the time. Peez?

 

[Juma]

Time as a measurement of the rate of change....the rate of change being relative to entropy, physical relationships, velocity and mass?

If I understand correctly, were I able to travel at light speed, from my perspective I would be everywhere all the time. Peez?

Only in one direction: the direction you are traveling in. The other two dimensions are unaffected.

 

[EPresence2]

My point is that a time machine is silly. It would need to be a space and time machine because you likely need to change your position in time and space to get where you are going.

Indeed, a space and time machine... that sounds much like a worm hole; Even going to the same place in the future would require traveling in space/time. Of course, the "machine" could only go forward in time, as far as we know. However, there has been speculation that anti-matter is actually matter moving backwards in time.

To the best of my knowledge, most physicists don't believe that antimatter is actually matter moving backwards in time. It's not even entirely clear what would it really mean to move backwards in time, from the popular viewpoint.

If I'm remembering correctly, this idea all comes from a story that probably originated with Richard Feynman. At the time, one of the big puzzles of physics was why all instances of a particular elementary particle (all electrons, for example) are apparently identical. Feynman had a very hand-wavy idea that all electrons could in fact be the same electron, just bouncing back and forth between the beginning of time and the end. As far as I know, that idea never developed into anything mathematically grounded, but it did inspire Feynman and others to calculate what the properties of an electron moving backwards in time would be, in a certain precise sense that emerges from quantum field theory. What they came up with was a particle that matched the known properties of the positron.

Just to give you a rough idea of what it means for a particle to "move backwards in time" in the technical sense: in quantum field theory, particles carry with them amounts of various conserved quantities as they move. These quantities may include energy, momentum, electric charge, "flavor," and others. As the particles move, these conserved quantities produce "currents," which have a direction based on the motion and sign of the conserved quantity. If you apply the time reversal operator (which is a purely mathematical concept, not something that actually reverses time), you reverse the direction of the current flow, which is equivalent to reversing the sign of the conserved quantity, thus (roughly speaking) turning the particle into its antiparticle.

For example, consider electric current: it arises from the movement of electric charge, and the direction of the current is a product of the direction of motion of the charge and the sign of the charge.

Positive charge moving left ( ) is equivalent to negative charge moving right ( ). If you have a current of electrons moving to the right, and you apply the time reversal operator, it converts the rightward velocity to leftward velocity ( ). But you would get the exact same result by instead converting the electrons into positrons and letting them continue to move to the right ( ); either way, you wind up with the net positive charge flow moving to the right.

By the way, optional reading if you're interested: there is a very basic (though hard to prove) theorem in quantum field theory, the TCP theorem, that says that if you apply the three operations of time reversal, charge conjugation (switch particles and antiparticles), and parity inversion (mirroring space), the result should be exactly equivalent to what you started with. We know from experimental data that, under certain exotic circumstances, the combination of charge conjugation and parity inversion does not leave all physical processes unchanged, which means that the same must be true of time reversal: physics is not time-reversal invariant. Of course, since we can't actually reverse time, we can't test in exactly what manner this is true.

Source - http://physics.stackexchange.com/qu...ing-backwards-in-time?answertab=votes#tab-top

 

[Derec]

But I am also space aren't I? Or is the space me? Or are we the same? Time travel seems to necessitate the transportation of space itself into another spacetime. Seems weird to me, and therefore impossible except a la Hollywood.

Does anybody actually believe that every moment of the past is somewhere just sitting there waiting so that it is possible to return to it?

Where is it except the human memory?

Are you familiar with A and B theories of time?
B theory states that past, present and future are equally real as part of the space-time continuum. That model is actually preferred among physicists because it works much better with things like relativity.
Christian apologist William Lane Craig on the other hand is a big fan of the A theory.

 

[untermensche]

Does anybody actually believe that every moment of the past is somewhere just sitting there waiting so that it is possible to return to it?

Where is it except the human memory?

Are you familiar with A and B theories of time?
B theory states that past, present and future are equally real as part of the space-time continuum. That model is actually preferred among physicists because it works much better with things like relativity.
Christian apologist William Lane Craig on the other hand is a big fan of the A theory.

The question remains.

Does anybody actually think that all the configurations of the universe that constitute the passage of time are out there hiding somewhere so that it is possible to return to any point in the past?

If so, where are all those prior configurations hiding?

 

[DBT]

It implies a trail of a near infinite number of earths emanating in a time dimension like an invisible tail behind the current, present earth, which is only an instant in time. Apply that to every matter/energy object in the universe...where does all the mass come from?

 

[Juma]

Are you familiar with A and B theories of time?
B theory states that past, present and future are equally real as part of the space-time continuum. That model is actually preferred among physicists because it works much better with things like relativity.
Christian apologist William Lane Craig on the other hand is a big fan of the A theory.

The question remains.

Does anybody actually think that all the configurations of the universe that constitute the passage of time are out there hiding somewhere so that it is possible to return to any point in the past?

If so, where are all those prior configurations hiding?

Hiding? In space-time of course.

 

[skepticalbip]

The question remains.

Does anybody actually think that all the configurations of the universe that constitute the passage of time are out there hiding somewhere so that it is possible to return to any point in the past?

If so, where are all those prior configurations hiding?

Hiding? In space-time of course.

...

Silly simplistic questions deserve simplistic answers. Well done.

The question seems to have been posed as a "gocha" like when creationists ask how can evolution be true when there is irreducible complexity.

 

[untermensche]

The question remains.

Does anybody actually think that all the configurations of the universe that constitute the passage of time are out there hiding somewhere so that it is possible to return to any point in the past?

If so, where are all those prior configurations hiding?

Hiding? In space-time of course.

So they are not hiding?

Show me where they are then.

Of course they must be within spacetime. That is all we know.

The question is where in spacetime are all those prior configurations of matter and energy hiding?

And if you say they are not hiding point to where they are.

 

[untermensche]

It implies a trail of a near infinite number of earths emanating in a time dimension like an invisible tail behind the current, present earth, which is only an instant in time. Apply that to every matter/energy object in the universe...where does all the mass come from?

Apparently problems like this are nothing.

They can be waved away because some Christian apologist said something or other.

 

[bilby]

Hiding? In space-time of course.

So they are not hiding?

Show me where they are then.

Of course they must be within spacetime. That is all we know.

The question is where in spacetime are all those prior configurations of matter and energy hiding?

And if you say they are not hiding point to where they are.

They are not where you are. That means you can't see them. It doesn't mean they don't exist, any more than New York doesn't exist when you are in LA.

If you stand back far enough, you can see both LA and Manhattan at the same time. If you stand back far enough, you can see what was happening 100 years ago.

 

[bilby]

It implies a trail of a near infinite number of earths emanating in a time dimension like an invisible tail behind the current, present earth, which is only an instant in time. Apply that to every matter/energy object in the universe...where does all the mass come from?

The same place all the mass/energy comes from right now. The first law of thermodynamics implies that the mass/energy that exists in the past and in the future is the same as the mass energy that exists now. You might as well look at a two dimensional slice through a steel beam, measure its width as 100mm, and then ask "If the beam extends in a third dimension, where does all the width come from?".