# Will time travel ever be possible?



## Itullian

Do you think its possible?


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## echo

probably not -- otherwise wouldn't someone have already come back and mentioned it


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## joen_cph

Based on the undeniable evidence provided by countless sci-fi movies: of course ;-). 

Probes rather than humans at first, probably. But perhaps we´re talking about 200 - 500 years in the future, before it will be possible.


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## amfortas

I travel through time every day. At a fairly constant rate.


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## aleazk

There are certain solutions of Einstein's equations that allow closed causal curves, but the mechanisms are not realistic.

In this paper, Kip Thorne investigates exhaustively the possibilities of your question in the context of classical general relativity.


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## joen_cph

As far as I vaguely remember to know, it´s at least sometimes thought of as mainly a question of being able to create and concentrate enough energy ?


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## BRHiler

Actually, we are the Ancient Aliens from the H2 Ancient Aliens series. We just don't know it yet 

You see all the TV shows (i.e. Quantum Leap, Dr. Who, various episodes of Star Trek) and all the movies and you think that it would be impossible because of all the changes made to history, but in reality, if someone did go back in time and change history, we wouldn't know because to our minds it is history.

Take the JFK assassination. Maybe he wasn't killed, but someone went back in time and killed him. We wouldn't know any different, because to us, he died.

But in all likelihood, as cool as it would be (I want to see how they built Stonehenge), I doubt it's possible. Too many space-time/continuum thingies.

3.21 Jigawatts!???!?!?!?!?!


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## TxllxT

πάντα χωρεῖ καὶ οὐδὲν μένει Everything changes and nothing stands still.

δὶς ἐς τὸν αὐτὸν ποταμὸν οὐκ ἂν ἐμβαίης. You could not step twice into the same river.


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## aleazk

joen_cph said:


> As far as I vaguely remember to know, it´s at least sometimes thought of as mainly a question of being able to create and concentrate enough energy ?


The problem with these things (closed causal curves, wormholes, etc.) is that they require the violation of certain 'energy conditions' and also tend to be quite unstable. Normal matter obeys these 'energy conditions', and matter that does not obey them is called 'exotic matter'. What Thorne says is that according to quantum field theory you can actually violate these conditions and also that the instabilities can be avoided. But, even if that's true, there's no known mechanism able to provide the enormous quantities of exotic matter required for a realistic thing.

So, the answer is basically: general relativity says that there are matter configurations that could allow the effect, but it's not clear if these matter configurations are possible!


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## SeptimalTritone

aleazk said:


> The problem with these things (closed causal curves, wormholes, etc.) is that they require the violation of certain 'energy conditions' and also tend to be quite unstable. Normal matter obeys these 'energy conditions', and matter that does not obey them is called 'exotic matter'. What Thorne says is that according to quantum field theory you can actually violate these conditions and also that the instabilities can be avoided. But, even if that's true, there's no known mechanism able to provide the enormous quantities of exotic matter required for a realistic thing.
> 
> So, the answer is basically: general relativity says that there are matter configurations that could allow the effect, but it's not clear if these matter configurations are possible!


I haven't taken a GR course yet, but are energy conditions essentially stuff like you can't have negative mass or whatever weird stuff is needed to generate a wormhole? Or the wikipedia article says rho + 3p > 0 what does pressure have to do with it?

And qft can allow, in principle, this kind of stuff WTF? But there's no mechanism for such a solution to be reached? Hmm... looks like killing our mother before we were born is a long shot.


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## Whistler Fred

Actually, as much fun as it can be in sci-fi, I hope not. Although not strictly about time travel, I'm thinking of Issac Asimov's excellent short story "The Dead Past" about a device that allowed you to see into the past (the "chronoscope") and it's unforseen consequences for humanity. It's worth checking out.


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## Guest

Itullian said:


> Do you think its possible?


Itullian, I already told you tomorrow.


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## aleazk

SeptimalTritone said:


> I haven't taken a GR course yet, but are energy conditions essentially stuff like you can't have negative mass or whatever weird stuff is needed to generate a wormhole? Or the wikipedia article says rho + 3p > 0 what does pressure have to do with it?
> 
> And qft can allow, in principle, this kind of stuff WTF? But there's no mechanism for such a solution to be reached? Hmm... looks like killing our mother before we were born is a long shot.


You make too many questions!, but that's good. 

In relativistic theories, the mechanical properties of matter are described by a tensor called the Stress-energy tensor. Basically, the only way to describe energy, momentum, pressure, etc., in a relativistically coherent and meaningful way is to mix all of these things in one single tensor (the same happens with the electric and magnetic fields, which are mixed into a single tensor, the Electromagnetic tensor). So, that's why things like pressure appear when you bring up this tensor.

Once you have the tensor, physicists start to impose certain conditions that they would like to see in this tensor. One of them is called WEC (weak energy condition), and it basically says that the total energy density is always positive 'for any observer travelling at a speed less than c' (this is a loose and imprecise way of interpreting a technical term, 'observer with timelike four-velocity', but for the sake of simplicity I mention it). In this case, only the energy density appears since WEC only involves the 00 component of the tensor. For a particle at rest, this simply implies asking its mass to be positive (since the only energy in this case is the rest energy of the particle and so E=m, in natural units, i.e., c=1). All classical matter is believed to meet WEC... and for a good reason, nobody ever saw a particle with negative mass! (put a -m, with m>0, in Newton's laws of motion and you will get pretty non-sensical properties).

All wormhole solutions (and the 'time-travel solutions' too) ask for negative mass, or, more generally, violation of WEC, NEC. So, it's not a capricious thing: the wormholes simply need the violation of things that seem 100% reasonable (classically, at least...)

Relativists are so convinced that these energy conditions hold (at least macroscopically and on average), that in all of the important theorems in advanced GR (the singularity theorems, the laws of black hole mechanics, etc.) you will find some of these energy conditions in the hypothesis.

Yes, in QFT you have a thing called the Casimir effect that could allow some of the required violations of the energy conditions. But to construct a time-machine from that is a very long shot, as you say.


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## joen_cph

At least I understood that so far most seem to think that travelling into the future would be "easier" than going to the past, and that there are clear differences between these "genres".


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## Guest

I hope not. It seems it would only be a recipe for disaster. We already screw up so many things thinking we know how to plan things perfectly. Could you imagine a bunch of do-gooders thinking they are going to travel back in time and right some of history's greatest wrongs? As they say, the road to hell is paved with good intentions. 

And anyways, I travel through time all the time. Why, just now, I traveled from 4:44pm to 4:45pm local time while writing this.


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## aleazk

joen_cph said:


> At least I understood that so far most seem to think that travelling into the future would be "easier" than going to the past, and that there are clear differences between these "genres".


Yes. In non-relativistic theories, all observers measure the same elapsed time for the same two events. Suppose you have three different events, one in the future of the other: e3>e2>e1. t12, t13, and t23 are the elapsed times between these events (the notation should be obvious). Now, suppose that I start at e1. Some time later, my clock says 't12'. So, in a non-relativistic theory, I should be in the event e2 now. If, for some reason, I end in e3 instead, then I would say that I travelled into the future, since I only had enough elapsed time for reaching only e2. In a non-relativistic theory we would call that a 'travelling to the future scenario'. In these theories, it seems quite a strange thing since there's no obvious mechanism that could allow that. What happened to t23? Why I suddenly popped in e3 instead of reaching only e2?

The thing is that in relativistic theories the time elapsed between two different events depends on the observer. In relativity, time and space merge into a single four-dimensional thing (spacetime, the set of all possible events) and with its own metric structure. An observer has a path in spacetime, and the time measured by this observer is the _spacetime length_ of this path according to this metric. So, different observers measure different times between the same two events in the same sense that two different paths in space connecting the same points can have different length.

So, suppose I stay in Earth and that my twin brother goes to explore the universe in a very fast ship (close to c). If he returns, he will be quite more younger than me. That's because the Earth's spacetime trajectory is a spacetime geodesic, and these geodesics _maximize_ the spacetime length, i.e, the time between events. So, one could say that 'my brother travelled to the future' (because, according to him, in a blink he's suddently in a more sophisticated future with flying cars, robots, etc.). So, you could say that relativity is giving you an actual mechanism for the effect. _And it's happening all the time!_ But I don't like to call that 'travelling to the future' because in relativity the technical definition of 'future' is different.

All this is special relativity, i.e., zero-spacetime curvature.

Now, what's travelling to the past? The idea is that I depart from e1, go to e2, then to e3, and, for some reason, after this I end in e1 again!

From the spacetime viewpoint, we say that your spacetime path is a closed curve. You cannot have these things in special relativity. That's why we go to general relativity, where the spacetime geometry is curved and you can have all kind of crazy things!, like these closed spacetime paths, singularities (these are the 'edges' of spacetime, where time and space both 'end'), etc.


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## Itullian

joen_cph said:


> At least I understood that so far most seem to think that travelling into the future would be "easier" than going to the past, and that there are clear differences between these "genres".


But the future hasn't happened yet, no?


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## Lukecash12

While there are such things as time distortion and time dilation, because time is relative to space and energy, implying an actual jump in time would be implying a different state of affairs altogether. Or to put it more simply: all of reality would have to follow suit, either reverting back to a previous state or somehow jumping ahead. When we start to talk about alternate universes we step into the realm of "well maybe, scientifically speaking it's possible" but there isn't a shred of evidence for it, so assuming a priori that this is the case would be an egregious violation of Occam's Razor.

However, as aleazk pointed out the rate of the process of time is relative in both special and general relativity, so we can come to some pretty interesting conclusions from there.


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## amfortas

SeptimalTritone said:


> Hmm... looks like killing our mother before we were born is a long shot.


Fortunately, the option is always there afterwards.


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## Couchie

I say yes. 

Personally I do not see much gain in betting against technological development. You can only be proven wrong. The alternative is being proven correct, or falling back on "it just hasn't happened yet".


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## Blake

I'll say no, for now. But if it works then I'll come back to change my answer.


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## Posie

If it were possible, the concept of time would cease to exist.


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## Morimur

If the human mind can conceive it, it is possible. I just don't think we (humans) will be around long enough to achieve time travel.


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## KenOC

Of course we travel in time every day. But in a usual and uninteresting way...


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## techniquest

> I'll say no, for now. But if it works then I'll come back to change my answer.


I think I'll do the same...

EDIT: Just come back from 2147 to let you know that it is possible. Woot!  By the way, they found Sibelius' 8th symphony - it wasn't burned after all. It's not bad, but you'll have to wait until 2088 to hear it.


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## joen_cph

Itullian said:


> But the future hasn't happened yet, no?


I can give you Sorabji´s complete name in writing, but I´m unable to really answer this, except from my own experience: correct. Some advanced scientists might have other theories.


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## Badinerie

There is no such thing as time. Time is just a philosophical abstraction we use to describe the path of entropy.
Just as when many of us approach death, delirium or become victim to dire circumstances, suddenly develop a belief in God and an Afterlife, when the scientific community reach the same point they suddenly declare Time Travel may be possible. 
Time is a bag of useful adverbs nothing more.


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## Andreas

Time travel is an indispensible psychological topos. Since we have memory as well as a conception of the future, we time-travel a lot mentally. Events of the past or of the future can immediately affect us in the present when we think of them. Physical time travel is really secondary in this regard. But the notion of being able to change the past and see the future is of course incredibly powerful. Its the notion of a life without regret and fear, maybe even without death. Easy to see why time travel is so fascinating a concept.


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## BRHiler

I think I short-circuited my brain reading all these responses.

Time is something we experience in only one direction, but cannot perceive. Kinda sucks :/


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## Guest

Couchie said:


> I say yes.
> 
> Personally I do not see much gain in betting against technological development. You can only be proven wrong. The alternative is being proven correct, or falling back on "it just hasn't happened yet".


What about back in the day when the great technological aspiration was to turn lead into gold?

We also can't fathom what the situation will be like were we to ever reach the point where we were technologically developed enough to attempt something like time travel. By then such an idea might be obsolete.


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## Taggart

SeptimalTritone said:


> Hmm... looks like killing our mother before we were born is a long shot.


There's a delightful Heinlein story called All You Zombies which has nothing to do with zombies and is a fascinating time travel paradox on this subject.


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## Guest

Taggart said:


> There's a delightful Heinlein story called All You Zombies which has nothing to do with zombies and is a fascinating time travel paradox on this subject.


Oh, that's not the weirdest from Heinlein on time travel. I believe he also has a full novel where the protagonist goes back in time and fathers himself - he is his own father. How about that for a paradox? That, and just really disturbing, with the other concept of incest.


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## Varick

Itullian said:


> Do you think its possible?


Yes I think it's possible. Do we, now, at this time in human existence have any idea how we can go about doing it? Absolutely not.

Everything we know right now about physics, quantum physics, and the universe and all the natural laws associated with it is a drop in the bucket compared to what we will know 50, 100, 200 years from now. Those 200 years from now are going to look at our knowledge and technology today, the way we look back at the knowledge and technology of humanity 500 or 1000 years ago.

I think it is enormous hubris in human nature to think that we can predict in our current time what we can, can not, will and will not do some years from now. We have absolutely NO IDEA.

Just because we have not been able to perform alchemy from thousands of years of trying, does not mean we never will.

V


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## Varick

Here's some other food for thought (which is why I laugh when I hear people talking about our future energy crises). Many scientists say that there is enough energy in a single standard pencil that could fuel your home's energy needs (heat, electricity, etc) for weeks.

At present time, we just have absolutely no idea how to access that energy. Our level of ignorance today is always enormous to the knowledge we will gain tomorrow.

V


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## BRHiler

Varick said:


> Here's some other food for thought (which is why I laugh when I hear people talking about our future energy crises). Many scientists say that there is enough energy in a single standard pencil that could fuel your home's energy needs (heat, electricity, etc) for weeks.
> 
> At present time, we just have absolutely no idea how to access that energy. Our level of ignorance today is always enormous to the knowledge we will gain tomorrow.
> 
> V


And you can bet that the oil companies and their in-pocket politicians will a) pay off said scientist(s) billions to not reveal how it's done, b) call the science blasphemy, and/or c) make said scientist(s) disappear right quick, if and when this happens


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## Varick

BRHiler said:


> And you can bet that the oil companies and their in-pocket politicians will a) pay off said scientist(s) billions to not reveal how it's done, b) call the science blasphemy, and/or c) make said scientist(s) disappear right quick, if and when this happens


That's a bit too cynical and "conspiracy theory" for me (I still have yet to see *one* conspiracy theory turn out to be true). Once the science of this comes out, it will be more of a scientific "community" discovery and everyone will know about it. So much so, as to suppress it will be impossible.

The smarter "energy" companies and other smart entrepreneurs will just find a way to make money from it, and old energy technology will eventually become obsolete.

V


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## Couchie

DrMike said:


> What about back in the day when the great technological aspiration was to turn lead into gold?
> 
> We also can't fathom what the situation will be like were we to ever reach the point where we were technologically developed enough to attempt something like time travel. By then such an idea might be obsolete.


Recall that while 19th century chemists declared transmutation impossible, 20th century physicists discovered it was indeed possible, at least in minute quantities through nuclear bombardment and radioactive decay. My understanding is they have even achieved lead to gold, although more naturally gold decays to lead.

That is a development on the timescale of decades. What about 100,000 years from now? I think it is total folly to bet against scientific development.


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## Guest

BRHiler said:


> And you can bet that the oil companies and their in-pocket politicians will a) pay off said scientist(s) billions to not reveal how it's done, b) call the science blasphemy, and/or c) make said scientist(s) disappear right quick, if and when this happens


Sorry, that would never happen. More likely the oil companies would finance the discovery on their own so that they could profit off of the INSANE revenues they would gain from it. Think about it - an energy source where the input material is so cheap and plentiful? What company would jump at that opportunity?


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## aleazk

Vesuvius said:


> I'll say no, for now. But if it works then I'll come back to change my answer.


I'm afraid that will not be possible:

http://en.wikipedia.org/wiki/Novikov_self-consistency_principle

So, better to take responsibility for what you write!


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## Guest

Couchie said:


> Recall that while 19th century chemists declared transmutation impossible, 20th century physicists discovered it was indeed possible, at least in minute quantities through nuclear bombardment and radioactive decay. My understanding is they have even achieved lead to gold, although more naturally gold decays to lead.
> 
> That is a development on the timescale of decades. What about 100,000 years from now? I think it is total folly to bet against scientific development.


I think it is totally folly to bet for or against scientific and technological advancement. 100,000 years from now? Who would even be around to collect on that bet? The thing is, these are the things we think are cool now. Will there still be such interest 10 years from now? 100? 1000? We think space ships are cool now. What if, before we get better technology for space ships, we develop some other technology for transportation that makes a space ship completely obsolete? Scientific and technological knowledge will advance - that is true, and it would be folly to bet against that. But it is foolish to try and place bets on which direction they will go. We are applying our own knowledge to future technology, of which we know nothing.


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## Radames

Whistler Fred said:


> Actually, as much fun as it can be in sci-fi, I hope not. Although not strictly about time travel, I'm thinking of Issac Asimov's excellent short story "The Dead Past" about a device that allowed you to see into the past (the "chronoscope") and it's unforseen consequences for humanity. It's worth checking out.


Have not read that story in years. One of my favorite time travel stories is called Millennium by John Varley. The time travelers don't want to change the past so they take people off of airplanes that are about to crash and kill everyone. Then they replace the people with fake corpses. But then something goes wrong.....

So the time travelers ARE here - they just keep their presence secret.


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## SeptimalTritone

aleazk said:


> I'm afraid that will not be possible:
> 
> http://en.wikipedia.org/wiki/Novikov_self-consistency_principle
> 
> So, better to take responsibility for what you write!


Indeed, I'm pretty sure that closed timelike curves are hugely different form "time paradoxes", because time paradox implies an unphysical contradiction, whereas a closed timelike curve is a consistent thing. At each point in the closed timelike curve, there is one thing that happens!


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## PetrB

Don't despair: within your lifetime, there is always _Astral Projection_, ready at your fingertips at a moment's notice if you so choose :tiphat:


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## aleazk

SeptimalTritone said:


> Indeed, I'm pretty sure that closed timelike curves are hugely different form "time paradoxes", because time paradox implies an unphysical contradiction, whereas a closed timelike curve is a consistent thing. At each point in the closed timelike curve, there is one thing that happens!


Yes, that's the idea. The real thing behind all that is that, regarding the ontological nature of time, relativity favors Eternalism rather than Presentism. So, spacetime exists and it's immutable. A CTC is simply a closed curve in this spacetime; therefore, because of the latter immutability, the 'history' in the curve has to be consistent (even if it is a closed curve, i.e., even when you can go back and try to alter things). And that's because if this is not the case, that would be in contradiction with the immutability of spacetime. The example given in wiki from the Terminator 1 film is very good and easy to grasp for the 'layman'. The film depicts Novikov's principle in a very precise way.


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## brianvds

Time travel will be discovered in the past. It's going to have been great.


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## Blake

aleazk said:


> I'm afraid that will not be possible:
> 
> http://en.wikipedia.org/wiki/Novikov_self-consistency_principle
> 
> So, better to take responsibility for what you write!


Wouldn't even us traveling to the future be a paradox? Hypothetically, we travel to the future and are influenced by the future, which would consequently change said future. It doesn't matter who initiates the conference... the past or the future. It's the meeting that counts. Unless we can't come back... but even then, who knows what butterfly effect has been thwarted by simply the travel. I think it's all a paradox.


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## Blake

I don't get this closed time-curve stuff. What's the point? Is it really just a perpetual loop?


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## SeptimalTritone

Vesuvius said:


> Wouldn't even us traveling to the future be a paradox? Hypothetically, we travel to the future and are influenced by the future, which would consequently change said future. It doesn't matter who initiates the conference... the past or the future. It's the meeting that counts. Unless we can't come back... but even then, who knows what butterfly effect has been thwarted by simply the travel. I think it's all a paradox.


Yes, one can travel to the future, but not back. Traveling to the future is just the twin paradox which is just special relativity. By going supah fast in a rocket (like a good fraction of the speed of light supah fast) away from earth and back, you'll age less than the people staying at earth. Indeed, without any forces you'll go in a straight line, the path of most aging, but in any other path connecting two points (you'll have to be influenced by forces) you'll age less!

To be clear: when I say "age less" that really means travel to the future in layman's terms.

Indeed, I would agree with aleazk and say that eternalism is the way modern physics thinks of things rather than presentism, especially since "clocks may go at different rates if they take different paths through spacetime".


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## SeptimalTritone

Vesuvius said:


> I don't get this closed time-curve stuff. What's the point? Is it really just a perpetual loop?


Now this is a neat question that I would love to hear from the master math/physics dude from! It seems that if the total proper time around the loop is finite, then yes it's just a perpetual loop of living the same thing over and over again with no memory! This is really hard to imagine for a clock or human being (if the total proper time is something, how can the clock go around the loop and register what it initially did?). Actually... okay I'm confused too.

Unless... somehow the clock gets "destroyed" at some point in spacetime like things get destroyed to a singularity when going to a black hole.


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## Lukecash12

Vesuvius said:


> Wouldn't even us traveling to the future be a paradox? Hypothetically, we travel to the future and are influenced by the future, which would consequently change said future. It doesn't matter who initiates the conference... the past or the future. It's the meeting that counts. Unless we can't come back... but even then, who knows what butterfly effect has been thwarted by simply the travel. I think it's all a paradox.


That's because you're thinking of this in terms of the popular conception of time travel. Time is space and matter related and vise versa in relativity so you can't "meet yourself" because you are the object in question, time itself can't be in question here because time is relative to space and matter. Also, unless we subscribe only to general relativity time moves in one direction.


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## aleazk

Vesuvius said:


> Wouldn't even us traveling to the future be a paradox? Hypothetically, we travel to the future and are influenced by the future, which would consequently change said future. It doesn't matter who initiates the conference... the past or the future. It's the meeting that counts. Unless we can't come back... but even then, who knows what butterfly effect has been thwarted by simply the travel. I think it's all a paradox.


No, because you are not 'altering' the future, it's simply your future. There's not a well defined notion (in the same reality) of the future in the case you didn't make the travel (this would be the initial future that is 'changed'), and in this sense producing a paradox; there's only one possibility occurring in this reality: you either stay, or you travel to the future; both are different options and cannot occur simultaneously in the same reality. Your future in this reality is simply the one you choose (stay or travel). If you travel, the events that you experience are simply the events that make your future, that's all, _since that's your future_.

The problem are the backward travels, since your own past is in your own future now, and then if your future is different from your past, you get a paradox.



Vesuvius said:


> I don't get this closed time-curve stuff. What's the point? Is it really just a perpetual loop?


Actually, you don't need a closed curve in order to travel to the past. A curve at the verge of being closed would also work (i.e., rather than coming back to the exact same event, you end in one whose spatial location is close to the one of this first event). But a small perturbation would produce closed curves. So, it's believed that in any time-machine scenario, closed curves will always arise, and so they are used as the natural markers for this phenomenon. That doesn't mean that these curves are the most useful for you (since you will get stuck in a time loop); indeed, close to these curves, there will be curves at the verge of being closed, and these are the useful ones.


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## Varick

SeptimalTritone said:


> Yes, one can travel to the future, but not back. Traveling to the future is just the twin paradox which is just special relativity. By going supah fast in a rocket (like a good fraction of the speed of light supah fast) away from earth and back, you'll age less than the people staying at earth. Indeed, without any forces you'll go in a straight line, the path of most aging, but in any other path connecting two points (you'll have to be influenced by forces) you'll age less!
> 
> To be clear: when I say "age less" that really means travel to the future in layman's terms.
> 
> Indeed, I would agree with aleazk and say that eternalism is the way modern physics thinks of things rather than presentism, especially since "clocks may go at different rates if they take different paths through spacetime".





SeptimalTritone said:


> Now this is a neat question that I would love to hear from the master math/physics dude from! It seems that if the total proper time around the loop is finite, then yes it's just a perpetual loop of living the same thing over and over again with no memory! This is really hard to imagine for a clock or human being (if the total proper time is something, how can the clock go around the loop and register what it initially did?). Actually... okay I'm confused too.
> 
> Unless... somehow the clock gets "destroyed" at some point in spacetime like things get destroyed to a singularity when going to a black hole.


To put it in an example (layman's terms): If you traveled in a vehicle at the speed of light away from the Earth for one Earth year (as you feel it), then turned around and headed back to Earth, when you land, you will have only aged two years. However, everyone you know would probably have been dead for years (not sure how many years). Your grandchildren would probably be dead for years, yet you would only be two years older.

Time is relative to speed. They have actually done this experiment with atomic clocks (the most accurate clocks in the world). They put a clock in the fastest jet fighter plane, synced that clock with a clock on the east coast (of the US) and one on the west coast. The pilot flew the plane at it's fastest speed across the country, and when he landed the clock in the plane was a few 100ths (or thousandths I can't remember) of a second behind than the ones on land. Ie: time "slowed down" in the plane.

Pretty cool stuff. If you want to read a good book on this sort of thing, read Stephen Hawking's "A Brief History Of Time". It's written in laymen's language so anyone can read it. Great book!

V


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## aleazk

SeptimalTritone said:


> Now this is a neat question that I would love to hear from the master math/physics dude from! It seems that if the total proper time around the loop is finite, then yes it's just a perpetual loop of living the same thing over and over again with no memory! This is really hard to imagine for a clock or human being (if the total proper time is something, how can the clock go around the loop and register what it initially did?). Actually... okay I'm confused too.
> 
> Unless... somehow the clock gets "destroyed" at some point in spacetime like things get destroyed to a singularity when going to a black hole.


lol, is that me?

Actually, this is a subtle question. Suppose a digital clock starts at event e (the reading is 0:00). It goes through a closed timelike curve and ends in e again (the reading is 1:00). But this clock (the one reading 1:00) is not the same as the one reading 0:00. You will see two clocks, one starting the journey and the other one also starting it but after a complete ride (remember that the consistency principle doesn't say that the two clocks have to 'smoothly' transform into the other, it simply says that the second clock can't do anything that could prevent the original one from departing, for example, by exploding at 1:00 and thus destroying itself and the other one). It's like in the movies when you go backwards and you see your younger self, we have two people. So, the clock keeps reading. But now you have a big problem. By the same reasoning, people (older versions of you or the clock) start to pile up as they make more complete rides. So, the net result is that at any event in the closed curve, you actually have an infinite quantity of clocks! Or imagine a light ray, you get a ray of infinite intensity as the initial ray piles up with itself. And this is catastrophic, since it produces a divergence in the stress-energy tensor and therefore a divergence in the curvature of spacetime (because Einstein's equations relate this curvature to the stress-energy tensor). Of course, what this means is that as soon as you produce the closed curve, matter is going to start circulating through it and therefore to produce an immense spacetime curvature that ultimately will destroy the closed curve (or time-machine). Even in vacuum, quantum vacuum fluctuations will do the pile up job!

This is Hawking's main argument against time-machines. In this book by Thorne you will find many insights on these more subtle points.


----------



## Ian Moore

Time is an irreversible variable. I think you can speed up and slow down our view of time but the images we see would only be apparent. It's already happened. When we look at the stars, what we are seeing is history. The distances are so vast that the light has taken a huge amount of time to get to us. That is the effect of distance and the speed of light. We would have to travel at speeds completely unimaginable to get to the star before our minds are able to perceive the stars light. That would be real time travel. To be able to travel faster than thought and perception...or we could just get in an aeroplane and fly across the date line..


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## Lukecash12

According to H.G. Wells it would be pretty messed up anyways, with weird subterranean monsters.


----------



## Antiquarian

Didn't Benjamin Franklin invent time travel? I hear it occurs twice every year in America, and it's scientific term is Daylight Savings Time.


----------



## Guest

I am actually from the future, and have come back with a wonderful investment opportunity that is going to make all of you millionaires in about 10 years time. Just send $1000 each to:
DrMike


----------



## Guest

Oh, and not to spoil too much, but . . . zoot suits are going to make a HUGE comeback.


----------



## Kieran

Sometimes when I come here and see the thread "Is Mozart Overrated?" back on the top of the page, I dunno if I've traveled back in time but it definitely appears that time has successfully stood still...


----------



## Guest

Kieran said:


> Sometimes when I come here and see the thread "Is Mozart Overrated?" back on the top of the page, I dunno if I've traveled back in time but it definitely appears that time has successfully stood still...


I think that somehow Salieri discovered time travel and has come to our time to besmirch Mozart and play up his own stuff. Just a theory.


----------



## Blake

aleazk said:


> Actually, you don't need a closed curve in order to travel to the past. A curve at the verge of being closed would also work (i.e., rather than coming back to the exact same event, you end in one whose spatial location is close to the one of this first event). But a small perturbation would produce closed curves. So, it's believed that in any time-machine scenario, closed curves will always arise, and so they are used as the natural markers for this phenomenon. That doesn't mean that these curves are the most useful for you (since you will get stuck in a time loop); indeed, close to these curves, there will be curves at the verge of being closed, and these are the useful ones.


Now, that's interesting. So, the closed-loop is more beneficial as a 'road map' to give an idea that there are more open curves around to make use of. Is it part of the theory that one could use these to go back and forth to the future, or can you only go forward? Because if you can come back, then you can take someone from the future with you and they will be traveling to the past... causing a paradox. I'd imagine that even simply the information you'd bring back could potentially cause a paradox.


----------



## aleazk

Vesuvius said:


> Now, that's interesting. So, the closed-loop is more beneficial as a 'road map' to give an idea that there are more open curves around to make use of. Is it part of the theory that one could use these to go back and forth to the future, or can you only go forward? Because if you can come back, then you can take someone from the future with you and they will be traveling to the past... causing a paradox. I'd imagine that even simply the information you'd bring back could potentially cause a paradox.


You always go forward in time, the thing is that spacetime itself "rolls up". Spacetime is the set of all events, where an event is a point of space at an instant of time. Now, we could draw flat spacetime (let's use only one spatial dimension) as a two-dimensional plane with two axis: a space axis x and a time axis t; an event then is a point in that plane, since it corresponds to a particular value of x and t.

Now, imagine a particle: how we describe that particle in spacetime? as a point? No, because as time passes, the particle occupies different events. So, in spacetime, the 'history' of a particle is a line which extends in the direction of the time axis. This line is called its 'world-line'. Something like this:

http://astarmathsandphysics.com/a-l...-cones-the-order-of-events-html-m17a23344.gif

Note that these 'light cones' are simply the world-lines of light rays diverging from the origin of coordinates. Massive particles can't travel at the speed of light or fast. So, their world-lines _always stay inside_ the light-cones.

In special relativity, that's all you got. Spacetime is flat and then the particle just keeps advancing in time, into its always new future. Like you advance in a spatial dimension in infinite space.

But in general relativity, spacetime can be curved! and so crazy things can happen to this spacetime. For example, take the spatial line with t=0 (this is simply the x axis), and the spatial line with t=10 (also a x axis, but at t=10). Cut all of the rest of the initial spacetime (so, we get a piece that starts a t=0 and ends in t=10; you can keep the spatial part infinite). Now, roll up this piece and indentify t=0 with t=10. You get a cylinder:

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/relativistic_cosmology/time_travel_3.gif

This is a completely valid spacetime in general relativity (it's even flat, i.e., no curvature!; the difference with the flat spacetime of special relativity is in the global topology of the spacetime: in special relativity, the global topology can only be that of ordinary euclidean space, 4-dimensional though, while here is a cylinder, S1xR^3, S1 is the circle, the time part, and R^3 is ordinary 3-dimensional euclidean space).

But look close at this thing. Even when you keep advancing forward in time, you turn around the cylinder and after a while you end in the same event from which you departed (not only in space, but also in time!). That's a classic example of a 'closed timelike curve' (we call 'timelike' to the curves inside the light cone, and 'null' to the ones that make the cone; actually, you can define the notions of timelike and null indenpendently from all that; an axiom of relativity is that massive particles have timelike world-lines; and here it comes the good part: you can show as a theorem that null lines always form that cone structure and with the timelike lines inside; therefore, that puts a limit on the speed of the timelike lines, the limit is the speed of the null lines; using the equations of electromagnetism, you can see that light travels in null lines, and there you have the value of your speed limit, the speed of light)

You can also create these closed timelike curves in less 'artificial' ways. For example, by keeping the ordinary R^4 topology, but now 'twisting' the light cones structure (this spacetime is not flat):

http://vivakademia.com/blog/wp-content/uploads/Figure-2.-Light-Cone.jpg

And, in fact, this is the type of spacetime that is often considered for time machines.

As for the paradoxes, yeah, sure, you could go back and kill your mother. That's a paradox since now you are not going to be conceived and therefore you are not going to travel backward in time in order to kill your mother. That's why we were talking about Novikov's principle, earlier with SeptimalTritone.


----------



## hpowders

Just take me back to 1965, so I could make it right with my first love.


----------



## Blake

aleazk said:


> You always go forward in time, the thing is that spacetime itself "rolls up". Spacetime is the set of all events, where an event is a point of space at an instant of time. Now, we could draw flat spacetime (let's use only one spatial dimension) as a two-dimensional place with two axis: a space axis x and a time axis t; an event then is a point in that plane, since it corresponds to a particular value of x and t.
> 
> Now, imagine a particle: how we describe that particle in spacetime? as a point? No, because as time passes, the particle occupies different events. So, in spacetime, the 'history' of a particle is a line which extends in the direction of the time axis. This line is called its 'world-line'. Something like this:
> 
> http://astarmathsandphysics.com/a-l...-cones-the-order-of-events-html-m17a23344.gif
> 
> Note that these 'light cones' are simply the world-lines of light rays diverging from the origin of coordinates. Massive particles can't travel at the speed of light or fast. So, their world-lines _always stay inside_ the light-cones.
> 
> In special relativity, that's all you got. Spacetime is flat and then the particle just keeps advancing in time, into its always new future. Like you advance in a spatial dimension in infinite space.
> 
> But in general relativity, spacetime can be curved! and so crazy things can happen to this spacetime. For example, take the spatial line with t=0 (this is simply the x axis), and the spatial line with t=10 (also a x axis, but at t=10). Cut all of the rest of the initial spacetime (so, we get a piece that starts a t=0 and ends in t=10; you can keep the spatial part infinite). Now, roll up this piece and indentify t=0 with t=10. You get a cylinder:
> 
> http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/relativistic_cosmology/time_travel_3.gif
> 
> This is a completely valid spacetime in general relativity (it's even flat, i.e., no curvature!; the difference with the flat spacetime of special relativity is in the global topology of the spacetime: in special relativity, the global topology can only be that of ordinary euclidean space, 4-dimensional though, while here is a cylinder, S1xR^3, S1 is the circle, the time part, and R^3 is ordinary 3-dimensional euclidean space).
> 
> But look close at this thing. Even when you keep advancing forward in time, you turn around the cylinder and after a while you end in the same event from which you departed (not only in space, but also in time!). That's a classic example of a 'closed timelike curve' (we call 'timelike' to the curves inside the light cone, and 'null' to the ones that make the cone; actually, you can define the notions of timelike and null indenpendently from all that; an axiom of relativity is that massive particles have timelike world-lines; and here it comes the good part: you can show as a theorem that null lines always form that cone structure and with the timelike lines inside; therefore, that puts a limit on the speed of the timelike lines, the limit is the speed of the null lines; using the equations of electromagnetism, you can see that light travels in null lines, and there you have the value of your speed limit, the speed of light)
> 
> You can also create these closed timelike curves in less 'artificial' ways. For example, by keeping the ordinary R^4 topology, but now 'twisting' the light cones structure (this spacetime is not flat):
> 
> http://vivakademia.com/blog/wp-content/uploads/Figure-2.-Light-Cone.jpg
> 
> And, in fact, this is the type of spacetime that is often considered for time machines.
> 
> As for the paradoxes, yeah, sure, you could go back and kill your mother. That's a paradox since now you are not going to be conceived and therefore you are not going to travel backward in time in order to kill your mother. That's why we were talking about Novikov's principle, earlier with SeptimalTritone.


Impressive. Thanks for that. I see what you've been pointing to, now. I just need some time for marination.

What's important here is the actual bending of space-time, correct? You continue to go forward, but the 'fabric' bends around you.


----------



## aleazk

Vesuvius said:


> Impressive. Thanks for that. I see what you've been pointing to, now. I just need some time for marination.
> 
> What's important here is the actual bending of space-time, correct? You continue to go forward, but the 'fabric' bends around you.


Yes, of course, it's a lot of information, I don't expect you to get it all 'instantly'. I myself learned all these things through years of study. But I have the philosophy that it's better to give all the information rather than imprecise things, and most importantly: in the way we physicists think about these things, i.e., no bull**t. And that's because, in my experience, I only understood the concepts in all of their profundity when I saw them in their natural context. You could say that my approach is 'what you see is what it is, then it's up to you to make your own intuition about it'. Of course, I can make comments about my personal intuition, but I found that sometimes we have different forms of understanding things at the intuitive level. And the relativity theories really, really conspire against your common sense intuition. It can be a real nightmare for students to grasp it intuitively at the beginning. In fact, the intuition comes from your constant contact with the material through the years (i.e., in the hard way)

But yes, you got it right. The cylinder example is very good, it's a textbook example (in fact, I took it from a relativity textbook  ). And remember: it's not the curvature of 'space', it's the curvature of _spacetime_ (so, don't try to imagine some curved three-dimensional space, etc.; in fact, notice that in the cylinder example all the action is happening in the time part, the spatial part is trivial; ok, yes, the cylinder is a two-dimensional thing, but that's because I eliminated two spatial dimensions, it's just a device for drawing these things in actual paper, if you go to the actual reality, you will not 'see' any cylinder, you will perceive this odd topology by the fact you can travel backward in time; so, yes, it can be a little abstract, that's other of the difficulties for intuition)

Maybe it was my fault since I started using the term without explaining it (blame SeptimalTritone and his technical questions , that distracted me)

Check the Wald book I mentioned to you before.


----------



## Blake

aleazk said:


> Yes, of course, it's a lot of information, I don't expect you to get it all 'instantly'. I myself learned all these things through years of study. But I have the philosophy that it's better to give all the information rather than imprecise things, and most importantly: in the way we physicists think about these things, i.e., no bull**t. And that's because, in my experience, I only understood the concepts in all of their profundity when I saw them in their natural context. You could say that my approach is 'what you see is what it is, then it's up to you to make your own intuition about it'. Of course, I can make comments about my personal intuition, but I found that sometimes we have different forms of understanding things at the intuitive level. And the relativity theories really, really conspire against your common sense intuition. It can be a real nightmare for students to grasp it intuitively at the beginning. In fact, the intuition comes from your constant contact with the material through the years (i.e., in the hard way)
> 
> But yes, you got it right. The cylinder example is very good, it's a textbook example (in fact, I took it from a relativity textbook  ). And remember: it's not the curvature of 'space', it's the curvature of _spacetime_ (so, don't try to imagine some curved three-dimensional space, etc.; in fact, notice that in the cylinder example all the action is happening in the time part, the spatial part is trivial; ok, yes, the cylinder is a two-dimensional thing, but that's because I eliminated two spatial dimensions, it's just a device for drawing these things in actual paper, if you go to the actual reality, you will not 'see' any cylinder, you will perceive this odd topology by the fact you can travel backward in time; so, yes, it can be a little abstract, that's other of the difficulties for intuition)
> 
> Maybe it was my fault since I started using the term without explaining it (blame SeptimalTritone and his technical questions , that distracted me)
> 
> Check the Wald book I mentioned to you before.


Excellent. I think I'll scoop that book up. Pretty exciting stuff.

Not sure if it has been mention yet... or maybe I over-read. Are physicist making a particular headway in finding or creating these space-time curves? Speed, explosions, implosions, wormholes, glitches in the matrix....


----------



## Lukecash12

Vesuvius said:


> Excellent. I think I'll scoop that book up. Pretty exciting stuff.
> 
> Not sure if it has been mention yet... or maybe I over-read. Are physicist making a particular headway in finding or creating these space-time curves? Speed, explosions, implosions, wormholes, glitches in the matrix....


No, they aren't. Every once in a while someone comes up with an exciting new idea in that area but we're no closer to opening up a wormhole or something.


----------



## Blake

Lukecash12 said:


> No, they aren't. Every once in a while someone comes up with an exciting new idea in that area but we're no closer to opening up a wormhole or something.


That's one hell of a mission to be on. Whoever figures this out will be in the ranks of Einstein, if not greater.


----------



## Lukecash12

Vesuvius said:


> That's one hell of a mission to be on. Whoever figures this out will be in the ranks of Einstein, if not greater.


If it's even something to be figured out. Depending on what camp you're in this stuff can make about as much sense as a round square.


----------



## Blake

Lukecash12 said:


> If it's even something to be figured out. *Depending on what camp you're in this stuff can make about as much sense as a round square.*


That's normally how things are until someone makes sense of them. I know too little to be a part of any camp. I'll remain a free agent.


----------



## aleazk

Vesuvius said:


> Excellent. I think I'll scoop that book up. Pretty exciting stuff.
> 
> Not sure if it has been mention yet... or maybe I over-read. Are physicist making a particular headway in finding or creating these space-time curves? Speed, explosions, implosions, wormholes, glitches in the matrix....


There was some interest in the 80s and early 90s, you can find a good summary of all the investigations in the review paper by Kip Thorne I mentioned before. It's a technical paper, but Thorne's style is didactic. Also, you can check Thorne's 'pop' book on the topic. The paper and the book are more than 20 years old, but the situation today is more or less the same.

Within the small community of hardcore relativists, Thorne is the most enthusiastic about wormholes, time travel, etc. He was one of the first who proposed a solution of Einstein's equations describing a stable and traversable spherical wormhole. Though the concept goes back to Einstein himself: in the 20s, I think, he realized that the Schwarzschild solution (the one we use to describe the curvature around spherical objects like stars and black holes, in fact, in complete vacuum the solution is simply a spherical black hole) was actually more complex; the part we were using was only a small section of the solution; in its full extention, the solution describes two different 'universes', each one containing a white hole, then these white holes collapse and a wormhole connecting the two universes is formed, but, again, this wormhole collapses and you end with two black holes, one in each universe (animation). And this happens so quickly that you would need to travel faster than light in order to use the wormhole for going to the other universe; if you enter in the wormhole, the thing collapses into a black hole before you can leave! At that time, this wormhole was called 'Einstein-Rosen bridge'. But it is unstable.

Thorne advised Carl Sagan in the stellar travel part through wormholes when the latter was writing his novel 'Contact'. He's also one of the advisers in a new science fiction movie about similar topics. I mention all this so you can have an idea of Thorne's role in all this.

The famous mathematician Kurt Gödel was one of the first who found a solution with closed timelike curves (CTC) and to recognize them. He dedicated his paper to Einstein in his 70 birthday (they were very close friends). Einstein found the existence of closed timelike curves in his theory as a very disturbing thing. Later, Thorne discovered that you can also use wormholes for producing CTCs.

As for your question here's a summary: Einstein's equations relate the curvature of spacetime with the matter distribution in it; so, you can input some matter configuration into these equations and see what kind of curvature it produces; or, you can input some curvature and see what kind of matter configuration you would need in order to produce that curvature; and this second case is the one we need, since we already know the kind of spacetime geometry we want (one with CTCs or wormholes); you give that input to the equations and they say you would need matter with negative energy; but that kind of things do not exist classically. Thorne argues that you could use a quantum effect called the 'Casimir effect', which effectively produces the desired 'negative energy'. There are some problems, but in principle it could work. But, in the case of the wormhole, that only is useful for stabilizing the wormhole, and this is because the solution we put into the equations was one describing an eternal and stable wormhole, so the equations only say what you would need for achieving that configuration (an eternal wormhole). We don't know any mechanism to build the wormhole in the first place! (white holes are nonsensical entities, since they violate the second law of thermodynamics, so the Einstein-Rosen bridge is not realistic). Thorne speculates that at the level of 'quantum spacetime', wormholes are constantly being created and destroyed as quantum vacuum fluctuations of this quantum spacetime. So, Thorne says, we don't need to create them, we only need to 'catch' one and stabilize it with the negative energy. mmmmm, okay... 

The next problem are the matricide/killing my grandfather paradoxes. But these are solved by the consistency principle we were talking earlier. And Thorne showed that the laws of physics predict consistent histories, putting solid ground to the principle in this way (though there are some subtleties and complications).

The next problem are the particles 'looping in time' and piling up with older versions of themselves, thus producing a divergent energy and therefore a divergent curvature that ultimately would destroy the time machine moments after its creation. Thone says that the negative energy needed for the wormhole will 'deflect' the trajectory of the particles and thus avoid the pile up. Nevertheless, both Hawking and him agree that there will be a moment with a peak in the energy, and say that we need a theory of quantum gravity in order to know the correct way in which energy produces curvature and in this way knowing if this peak would destroy the wormhole or not. Hawking is convinced that it will be destroyed. Thorne says we don't know yet. LOL, and that's basically the state of the thing.


----------



## Blake

So, it's still in the highly speculative stage, I see. Is this something that you have faith in it's resolve, or do you think it's a flop? Is there a particular area physics that garners most of your attention?


----------



## aleazk

Vesuvius said:


> So, it's still in the highly speculative stage, I see. Is this something that you have faith in it's resolve, or do you think it's a flop? Is there a particular area physics that garners most of your attention?


Well, to be honest, I wouldn't say I read and study these things because I think they will be realized at some moment. Of course, it would be fun and interesting if that's the case, and in fact there's some interest from my part in that sense (i.e., I sympathize with Thorne's enthusiasm). But it's all just pure speculation, even based at some moments on speculations about theories we don't even have.

I like these topics because the situations are so extreme and bizarre that it's a real challenge to your understanding of the theories involved. So, I like to study these things in order to sharpen my understanding of the fundamental concepts of the main theories we use. Do I believe time travel is possible? I think we really have not enough information for a definite answer. But I think an important thing to share with the public is that, unlike 100 years ago, we now have complex theories about space and time and we can actually see that time travel is a possibility that could be allowed by the theories, we know how to formulate the question in our theoretical frameworks and how to explore it in them. That's quite something considering that 100 years ago the question was nonsensical or at least unapproachable. It's quite an advance.

In the end of the 18th century and using Newtonian gravity, Laplace speculated about the existence of 'black stars', i.e., stars in which gravity is so strong that light cannot escape from their surface. That's very close to the modern idea behind black holes, though in a very embrionary state, the true general relativistic explanation is far more complicated and intricate. 200 years later, we found with our telescopes that these things really seem to exist. So, very likely, if time travel is possible in the future, these contemporary speculations will be seen in the same way we see today Laplace's speculations: very embrionary and rough attempts. Possibly, with new theories, the true theoretical explanation will be far more sophisticated, with explanations that use new concepts and new physical laws, all unknown to us today.


----------



## joen_cph

Aleazk, one cannot but be impressed, concerning your knowledge and approach in these matters. Interesting posts.


----------



## Lukecash12

joen_cph said:


> Aleazk, one cannot but be impressed, concerning your knowledge and approach in these matters. Interesting posts.


I need to give this thread a rest. It's like smoking crack, reading all of these references. I wonder what Newton and Leibniz would have thought if they knew the sorts of things people would be writing a relatively short time later?


----------



## Blake

aleazk said:


> Well, to be honest, I wouldn't say I read and study these things because I think they will be realized at some moment. Of course, it would be fun and interesting if that's the case, and in fact there's some interest from my part in that sense (i.e., I sympathize with Thorne's enthusiasm). But it's all just pure speculation, even based at some moments on speculations about theories we don't even have.
> 
> I like these topics because the situations are so extreme and bizarre that it's a real challenge to your understanding of the theories involved. So, I like to study these things in order to sharpen my understanding of the fundamental concepts of the main theories we use. Do I believe time travel is possible? I think we really have not enough information for a definite answer. But I think an important thing to share with the public is that, unlike 100 years ago, we now have complex theories about space and time and we can actually see that time travel is a possibility that could be allowed by the theories, we know how to formulate the question in our theoretical frameworks and how to explore it in them. That's quite something considering that 100 years ago the question was nonsensical or at least unapproachable. It's quite an advance.
> 
> In the end of the 18th century and using Newtonian gravity, Laplace speculated about the existence of 'black stars', i.e., stars in which gravity is so strong that light cannot escape from their surface. That's very close to the modern idea behind black holes, though in a very embrionary state, the true general relativistic explanation is far more complicated and intricate. 200 years later, we found with our telescopes that these things really seem to exist. So, very likely, if time travel is possible in the future, these contemporary speculations will be seen in the same way we see today Laplace's speculations: very embrionary and rough attempts. Possibly, with new theories, the true theoretical explanation will be far more sophisticated, with explanations that use new concepts and new physical laws, all unknown to us today.


You have skill at explaining this stuff, my friend. Any particular theory or question you're researching at this point?


----------



## clara s

The fourth dimension...

has the philadelphia experiment really happened?

There are two events happening now in different parts of the world, 
which make me wish that travel in time was possible

I want to travel in specific years


----------



## aleazk

joen_cph said:


> Aleazk, one cannot but be impressed, concerning your knowledge and approach in these matters. Interesting posts.





Vesuvius said:


> You have skill at explaining this stuff, my friend. Any particular theory or question you're researching at this point?


Oh, thanks guys!

I actually greatly enjoy thinking about how to explain these things, how to 'pack' them in a logically coherent and self-contained 'narrative'. I have a good number of notebooks and monographs which I fill with ideas, points of view, alternative proofs of theorems (or directly entire topics written under my points of view), which I pretend to use as the basis for my courses once I become an established teacher and researcher (I'm still a graduate student, though fortunately at the end of the journey by now!). And if all goes well, maybe even textbooks.

Re areas of interest: I'm in a field called 'mathematical physics', we like to investigate (in a very rigorous way from the mathematical point of view) the mathematical foundations, and related developments, of the theories we have. Most physicists would simply say we are mathematicians. And, to some extent, that's correct; we are mathematicians working on some aspects of the physical theories and using the mentality of mathematicians (which is a little different than that of physicists). But these aspects are actually very important from the 'conceptual foundation of the theories' point of view. And it often involves very subtle problems in mathematics and even epistemology.

At the more concrete level, my interest now is an alternative formulation of quantum mechanics called the 'algebraic approach', whose main application is in the formulaton of quantum field theory in curved spacetime ('particle physics in the presence of, classical, gravity', although you have a lot of new phenomena which are absent in ordinary particle physics, the very notion of 'particles' is not absolute for example; since we don't have yet a 'working' theory of quantum gravity, this working semiclassical approximation is the best thing we have right now, and it is widely used in cosmology for example)


----------



## SeptimalTritone

After looking a bit at that Kip Thorne review article...

He does mention that infinite energy build up from a particle going round and round in a coil in spacetime that builds up infinite energy density and kaboom, or even diverging vaccuum polariztion when the spacetime survature gets heavier near the time machine can destroy the macroscopic wormhole. That seems in line with what your earlier post was, so indeed, that's interesting!

I wonder what happens when the particle goes around the loop and interacts with itself... does that prevent the infinite energy density buildup due to the particle going round and round? I mean... maybe at least vaccum polarization divergence is enough to prevent macroscopic time machines anyway, but... it's sort of amazing to think of an electron interacting with itself (not in the usual qed self energy way but literally scattering with "another" electron). That's... weird. And if the ultimate theory of quantum gravity allows something like Planck-volume wormholes with high energy particles zipping around them and scattering with themselves... wow.


----------



## aleazk

SeptimalTritone said:


> After looking a bit at that Kip Thorne review article...
> 
> He does mention that infinite energy build up from a particle going round and round in a coil in spacetime that builds up infinite energy density and kaboom, or even diverging vaccuum polariztion when the spacetime survature gets heavier near the time machine can destroy the macroscopic wormhole. That seems in line with what your earlier post was, so indeed, that's interesting!
> 
> I wonder what happens when the particle goes around the loop and interacts with itself... does that prevent the infinite energy density buildup due to the particle going round and round? I mean... maybe at least vaccum polarization divergence is enough to prevent macroscopic time machines anyway, but... it's sort of amazing to think of an electron interacting with itself (not in the usual qed self energy way but literally scattering with "another" electron). That's... weird. And if the ultimate theory of quantum gravity allows something like Planck-volume wormholes with high energy particles zipping around them and scattering with themselves... wow.


Actually, for ordinary particles, Thorne says that the negative energy needed to stabilize the wormhole will 'defocus' the particles and thus avoid the 'piling up' (check this, if you input negative energy into that equation, you get the opposite effect, i.e., defocus rather than focusing). So, at least for wormhole-based time machines, this doesn't destroy it. He thought the same would happen for vacuum fluctuations circulating through the machine, but after he did the calculation, he saw that the situation was different and that they indeed might destroy the machine in the moment of its creation. And that's really tough, since the calculation was done using abstract CTCs (i.e., it applies to all types of time machines). So, it may actually be the 'protecting mechanism' desired by Hawking.

Thorne made calculations about billiard balls looping in time, coming back and hitting their past selves before entering the time machine. A 'billiard ball' version of the grandfather paradox. He found that the laws of physics _do_ predict solutions in which the ball hits its past self but not with enough intensity as to prevent its entrance into the time machine. So, it's a self-consistent history that agrees with the self-consistency principle. So, the particle can indeed interact with its past self and without producing paradoxes.


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## SeptimalTritone

aleazk said:


> Thorne made calculations about billiard balls looping in time, coming back and hitting their past selves before entering the time machine. A 'billiard ball' version of the grandfather paradox. He found that the laws of physics _do_ predict solutions in which the ball hits its past self but not with enough intensity as to prevent its entrance into the time machine. So, it's a self-consistent history that agrees with the self-consistency principle. So, the particle can indeed interact with its past self and without producing paradoxes.


The fact that this is possible is fascinating from a quantum gravity standpoint, especially renormalization of divergences. Indeed, in the usual qed the infinite self energy of the electron can be removed by imagining that there is a limited small length scale (or equivalently high energy) where the bare electron has small mass, and the dressed electron mass is the bare electron plus the photons and electron/positron pairs around it with ultraviolet cutoff. Taking the limit to zero, this is a good solution to the infinities of qed.

But if we have the electron freaking _scattering_ with another electron (i.e. itself) rather than just exchanging a photon with itself... that seems to be so much more divergent.

Just the simple Einstein Lagrangian is nonrenormalizable in qft because you're expanding in 1/G and 1/G blows up at short length scales but that's not so intuitive. There is probably "deep down there" so much crazy stuff happening on the quantum gravity scale man that makes it so divergent. Exciting.

For Christ's sake I need to learn my stuff if I take a quantum field theory class this Fall!


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## EddieRUKiddingVarese

^ can you repeat that backwards


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## Capeditiea

:O no mention of John Titor? :O


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## EddieRUKiddingVarese

But we can discredti him, he never mention the Donald


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## Guest

Yes.

_Last edited by dogen; May-13-2112 at 09:25._


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## Capeditiea

EddieRUKiddingVarese said:


> But we can discredti him, he never mention the Donald


in a way Titor did... Stating that the president who started WWIII would be the last US president.  
plus we are well on our way to a Civil War II. (which i would want to leave the country before this happens.) :O

:3 Most of my knowledge comes from anime.  Steins;Gate in this case. (which for about 3 or so months i thought John Titor was just some amazing subject for the show... turns out... that there was a John Titor in meat space... not sure how i found this out... but turns out i ended up reading the posts and getting all obsessed. :3 i can accurately state that even though the dates are completely incorrect. (probably from what ever he changed in 1975... which is funny about Omaha. because the last tornado we had was in 1975. and he probably had to get the computer from the "before the tornado destroyed Nebraska Furniture Mart (which always sold computers as well.) just to get the IBM5100. But then also... little did he know... I have always planned on dominating the world.  creating Omaha as the capital of the United States. :3 so there is that.

Ironically... these are all probably just delusions of my mind.


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## EddieRUKiddingVarese

^ Funny how these things always happen in the US


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## David OByrne

Time travel is possible, I can prove it because I'm from 1 second in the future


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## EddieRUKiddingVarese

I'm one day ahead, the dateline gives me automatic power to time travel :lol:


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## KenOC

EddieRUKiddingVarese said:


> I'm one day ahead, the dateline gives me automatic power to time travel :lol:


One day ahead is an advantage, but it's offset by always being upside down.

BTW people in your climes are often spoken of as "antipodean." So what exactly do you have against podeans? Why this inveterate and stubborn opposition?


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## EddieRUKiddingVarese

Less wear on my shoes that way, I'll report back in 9hrs and 15mins how the 6th June is shaping up...............


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## Larkenfield

"Will time travel ever be possible?"

In time, it will.  But I imagine that some passengers will still be late for their flights... The good news is that one will probably be able to hear new music before it's ever composed.


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## EddieRUKiddingVarese

^ Even 4'33"


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## Capeditiea

did i make it back in time? *looks at posts before. 

phew, good.


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## Pat Fairlea

Time travel?
Yes, of course it will having been possible one day.


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## Dan Ante

No not never not nohow..........


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## Capeditiea

Dan Ante said:


> No not never not nohow..........


but you're traveling through time now...


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## Dan Ante

Capeditiea said:


> but you're traveling through time now...


Relative to what?


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## SixFootScowl

EddieRUKiddingVarese said:


> ^ Even 4'33"


4'33" was the first music ever. Cage didn't compose it. He only named it.


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## SixFootScowl

Capeditiea said:


> did i make it back in time? *looks at posts before.
> 
> phew, good.


Beware if you go back in time to before the internet and CDs you will have no way to communicate with us and will have to spin vinyl oldies until you can find your way back. What if you got stuck there? Then you would have to go for cryogenics to preserve your body until TC is created.


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## EddieRUKiddingVarese

Fritz Kobus said:


> 4'33" was the first music ever. Cage didn't compose it. He only named it.


Yeah, it even predates the Big Bang


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## Cosmic Cowboy

I'm desperately hoping that time travel is possible. Based on the length of my hair, the mustache, the denim jacket, the snakeskin boots, the painted-on faded jeans, and what I'm listening to (The Eagles - _Desperado_) it seems as if I've been trapped in the year 1973 for over 45 years now.


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