Time Travel is Impossible and if not Impractical

[HPositive]

Time Travel would very well break the laws of the Universe yes there are some thing for why it is possible but here is my reasoning for why it is Impossible

 

Okay, lets say you have built a time machine in 1999 that time machine cannot come back in time past the year 1999 because the time machine did not exist yet, and is destroyed in the year 2012 so it cannot further than 2012 but still the time machine cannot even go into the future because the future has not happened yet. Then also when you travel in time saying it is even possible you' go back to 2003 and let's say you were in Rhode Island in 2003 but the time machine was in Ohio you would suddenly be in Rhode Island and the Time machine would be in Ohio. Thinking in the Grandfather Paradox "If you go back in time before your Grandmother and Grandfather met and you killed your Grandfather you would not exist but you were not born yet so how could you kill your Grandfather if you did not exist" maybe not the best example but still go to show how Impossible time travel would be and if not Impractical.

[Robittybob1]

The molecules existed before and after the "time machine". So does the machine exist before the machine is made?

What is the machine going to do to go backward or forward in time? Was it some mechanical manipulation?

What do you imagine a time machine will do?

 

Is it just like going to a museum, being surrounded by a bit of the past?

[Delta1212]

Assuming, for a moment, that you could travel back in time. Why would you be sent back to where you were when that time originally happened? That seems more like a time rewinder than a typically envisioned time machine, which would be fine if there was a mechanism for making such a machine that seemed more feasible than a normal time machine, but if anything I think the opposite is true.

[HPositive]

The molecules existed before and after the "time machine". So does the machine exist before the machine is made?

What is the machine going to do to go backward or forward in time? Was it some mechanical manipulation?

What do you imagine a time machine will do?

 

Is it just like going to a museum, being surrounded by a bit of the past?

Well, I see what your saying I don't even believe time travel would be possible so far but yes I do see the molecules would exist before the time machine even existed I may have not seen that because there is room for mistakes because my ideas I have had I have discussed with many and may have not picked it up

Edited August 6, 2015 by HPositive

[studiot]

When you say 'time travel' you need to start by explaining exactly what you mean.

 

Here is an example to think about.

 

Say you have a ruler 10 units long.

When the ruler is lying still on the bench at time. t =0, all the particles (represented by the numbers 0 to 10) are in the same time zone together.

 

Now say you push it along from the zero end so it travels in space.

The push on the piece of ruler from 0 to 1 pushes on the piece of ruler from 1 to 2 and so on.

Each push takes a finite time to transmit so the push from 9 to 10 comes a finite time after the push from 0 to 1 .

 

So you are not moving the entire ruler at the same time or each piece of ruler follows its own trajectory in time.

 

If you now turn this argument round to discuss time travel, not space travel, how do you describe 'time travel' ?

[singhsippi]

I have always heard that physicists do not know about whether backward time travel is done or not. I think that is absolutely wrong. Physicists say they have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light — demonstrating that outside science fiction, time travel is impossible.

[ajb]

Physicists say they have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light — demonstrating that outside science fiction, time travel is impossible.

Okay, the speed of light locally is a speed barrier that we cannot cross, this is true. So the time travel based on simply going faster than light seems completely impossible on physical grounds.

 

However, you can have time travel in general relativity that does not break this principal by changing the local geometry. Wormholes provide an example where you can beat light signals by 'taking a short cut'. In no way have you broken the local maximal speed, but globally it seems that you have. One can arrange a wormhole such that we have a time machine. Warp drives also allow one to break the global speed limit while not breaking the local one.

 

The problem with such strange scenarios is that they require exotic matter to support them and there are questions about the quantum stability of these configurations. However, in general there seems to be no deep reason why time travel is not allowed in general relativity, but case by case proposed time machines run into problems.

[neutrinosalad]

The problem with such strange scenarios is that they require exotic matter to support them and there are questions about the quantum stability of these configurations. However, in general there seems to be no deep reason why time travel is not allowed in general relativity, but case by case proposed time machines run into problems.

 

This is something I am curious about. If the exotic situations aligned to make time travel possible, what would happen if you traveled back in time and altered historical events?

 

What I am getting at is, is there the possibility of parallel universes existing where pathways in time splintered off into two by an event happening in two different ways?

[ajb]

This is something I am curious about. If the exotic situations aligned to make time travel possible, what would happen if you traveled back in time and altered historical events?

Good question and this gets at the idea of causality.

 

What I am getting at is, is there the possibility of parallel universes existing where pathways in time splintered off into two by an event happening in two different ways?

There seems to be three basic solutions here;

 

i) The 'Back to the future' solution in which you simply can change history and all the complications with causality that follow.

ii) The 'Novikov consistent history' solution in which you were always part of history and cannot change anything. Then we lose free will, but at least in terms of physics this seems okay.

iii) The 'parallel universe' solution which is what you say above. By travelling you end up in another universe and so do not mess up your history.

 

Or maybe something else...

[Strange]

i) The 'Back to the future' solution in which you simply can change history and all the complications with causality that follow.

ii) The 'Novikov consistent history' solution in which you were always part of history and cannot change anything. Then we lose free will, but at least in terms of physics this seems okay.

iii) The 'parallel universe' solution which is what you say above. By travelling you end up in another universe and so do not mess up your history.

 

I wonder if (i) and (iii) are actually any different - in terms of what is perceived. Taking Back to the Future as the canonical example, is it possible to distinguish between being in the same timeline that has changed versus a different timeline? Maybe Marty et al. actually ended up in a different universe but then managed to get back to the original one (or one very similar to the original).

 

All of these can create great fiction. Case (ii) is used in several stores, for example Twelve Monkeys and the gag in Bill and Ted where they hide a key for their earlier selves to find so they can escape from the police station.

 

Bill and Ted also introduces the paradox of where information comes from: the only way that Bill and Ted know Rufus's name is because the future Bill and Ted tell them. How do the future Bill and Ted know, because they were told by Bill and Ted. But maybe his name isn't Rufus at all ...

[ajb]

I wonder if (i) and (iii) are actually any different - in terms of what is perceived. Taking Back to the Future as the canonical example, is it possible to distinguish between being in the same timeline that has changed versus a different timeline? Maybe Marty et al. actually ended up in a different universe but then managed to get back to the original one (or one very similar to the original).

Marty McFly's actions were leading to events that were coursing him to have never existed. I therefore thing he was changing his history rather than entering another universe. The first film seems to be a twist on the 'grandfather paradox'. Why he would fade away in pictures and eventually in person, I have no idea!

 

These are the sort of problems one would have to resolve somehow if i) is realised in nature.

[Strange]

Why he would fade away in pictures and eventually in person, I have no idea!

 

Well, really, just for dramatic effect! But you could interpret that as his "present" changing around him as his past actions catch up with him. Or that he is slowly being pushed from one timeline to another.

 

But any of these explanations only make sense in terms of the world that is created in a particular story or movie. All one can expect is that the storytellers remain consistent with their own assumptions and conventions.

[jajrussel]

In order to get from one place to another place you pretty much have to know where the other place is going to be in the future. How does a worm hole solve this problem?

[neutrinosalad]

There seems to be three basic solutions here;

 

i) The 'Back to the future' solution in which you simply can change history and all the complications with causality that follow.

ii) The 'Novikov consistent history' solution in which you were always part of history and cannot change anything. Then we lose free will, but at least in terms of physics this seems okay.

iii) The 'parallel universe' solution which is what you say above. By travelling you end up in another universe and so do not mess up your history.

 

Or maybe something else...

 

Thank you for this response. Time and perception of reality have been topics of interest for me lately.

[Cuba]

The OP would make sense if it just said that whis theory depends on space-time being predetermined. Then yes if you went back the atoms making up your body and mind would probably return to where they were supposed to be at that particular moment in space time, beyond that you wouldent even remember going back and you would live your life in a perpetual loop. Though I personally do not think the universe is dictated by some kind of space-time fate.

[Fred Champion]

Just what is it that makes anyone believe that time is a real thing?

 

I suggest that time is an artifact of memory. We have some record of events, or states of objects relative to one another, and by comparing the events or states we deduce time.

 

I suggest that no inspection of the current state (the only state which may be inspected) will produce any result that could be interpreted as time. In other words, time will not be seen to be a component of any state.

[Strange]

Just what is it that makes anyone believe that time is a real thing?

 

Physics.

[swansont]

Just what is it that makes anyone believe that time is a real thing?

 

I suggest that time is an artifact of memory. We have some record of events, or states of objects relative to one another, and by comparing the events or states we deduce time.

 

I suggest that no inspection of the current state (the only state which may be inspected) will produce any result that could be interpreted as time. In other words, time will not be seen to be a component of any state.

 

!

Moderator Note

Please stick to the topic. This is about the physics of time travel, not the ontology of time.

[TakenItSeriously]

The worm hole time travel went like this: you start with two ends of a worm hole in the present time.

 

Then Bob takes one end of the worm hole on a trip traveling near the speed of light while John stays with his wormhole that moves forward in local time

 

Bob comes back 1 month later his time, only 13 months has gone by in local time due to his relativistic travel

 

Now either bob can go back 1 year to end up 1 month from when he left getting back in sync with John, or John may jump forward 1 year in time.

 

I think this may be the limitations that were discussed, when traveling back in time had its limitations though going forward in time can be open ended by taking that worm hole on another trip to extend the jump ahead.

 

Also Back to the future and the parallel universe theory are the same concept. The parallel universe theory was based on Quantum Mechanics Many Worlds Theory which allows for changes because nothing is certain. However you end up diverging the timeline you grew up in so you can't go back to the original state from which you started. You just move forward to to the new future which you will determine with each action you take..

 

Edit to add: the Polaroid gimmick was due to the writers paradox of not having any way to increase the drama in the movie unless they had a way to show how they screwed up the future in the first place.

 

Edit again to add: I always figured Heisenberg Theory was just Gods version of creating save points in case he screwed something up because starting again from the beginning is such a drag.

 

Like when he realized dinasours would never get any more interesting so could go back to where mammals came along and send an asteroid to try again.

Edited December 16, 2015 by TakenItSeriously

[TheGeckomancer]

There are a bunch of reasons why time travel is not realistic. And a few wonky ways it MAY be possible, such as gravitational shenanigans involving orbiting blackholes. But we have a lot of evidence suggesting it's not possible. Or if it is, that the universe STRONGLY frowns on it, or creates separate time lines for different instances of time travel. See Stephen Hawkings Time Traveler Party.

[Fred Champion]

 

Physics.

The moderator tells me that saying time doesn't exist isn't fair in this thread, that I should stick to the physics of it.

 

I find it impossible, actually absurd, to attempt a physics of something that has no physical manifestation.

 

I encourage everyone who would contemplate "doing" physics on, about, of for any topic to first determine whether or not that topic has any physical manifestation. Time certainly does not. Time "travel" is fiction, not physics.

[ajb]

I encourage everyone who would contemplate "doing" physics on, about, of for any topic to first determine whether or not that topic has any physical manifestation. Time certainly does not.

Though we do have a parameter 't' that appears in many of the equations of physics! And we can even use this physics to get a meaningful measurement of a duration.

 

Time "travel" is fiction, not physics.

I think this is the question really posed in the opening post.

 

If time travel is not allowed in nature, then why?

[Strange]

I encourage everyone who would contemplate "doing" physics on, about, of for any topic to first determine whether or not that topic has any physical manifestation. Time certainly does not.

 

And yet the timestamps and ordering of posts in this thread would appear to prove you wrong.

 

 

Time "travel" is fiction, not physics.

 

Currently, yes.

[geordief]

Fair to say "simultaneous time" does not exist?

[ajb]

Fair to say "simultaneous time" does not exist?

Right, different observers will in general not agree on duration of events nor will they generally agree on simultaneity of events.