Time Travel

[keith99]

Make a side trip to the 24th century and borrow Captain Pickard's Universal Translator....

Yup, that one seems to be magic and somehow magic fails when it comes to metaphors!

 

[dms1972]

Transwarp itself was never really satisfactorily explained.

You are right on that, but maybe a problem about it was expounded pretty well by McCoy and Spock:

McCoy "You present the appearance of a man with a problem."

Spock: "You're perception is correct Dr. In order to return us to the exact moment we left the 23rd century, I have used our journey back through time as a referent, calculating the co-efficient of elapsed time in relation to the acceleration curve."

McCoy: "So what's your problem?"

Spock: "Acceleration is no longer a constant."

McCoy (smiling smugly): "You'll have to just use your best guess."

In Star Trek time-travel is not explained much and I think now that the best part about the times they touch on it. Not because I think they might give mad scientists ideas - but because no one would really know for sure how it would work - they'd just have to attempt it, and then they might not even know how they did it if it worked (which in my view is impossible anyway)

So in Star Trek four, they do a sling shot round the sun. Then it looks like Kirk was tripping on some high dose of something (maybe McCoy had given him a tranquilliser) - which was weird - and didn't make a lot of sense, but might have had something to do with Kirk having given the order to go at warp speed and that he was now in charge of the mission. I thought it turned in a pretty good film all the same.

In fact in the dialogue from the above scene it seems to me now that McCoy was smug because he had the best clothes for life in the late 20th Century, compared to the other crew still in their star fleet uniforms. That time period was clearly preferable to him with his common sense approach to problems. And he could likely have done pretty well as a doctor.

But one theory is that if it was possible and you went back in time even the rain would go through you, because nothing could be changed.

 

[dgiharris]

One aspect that Star Trek failed to coherently address is time travel, which is a side effect of FTL. If one has warp drive, one can use it at will in a temporally inconsistent manner, since warp drive requires we reject causality. ....

Believe it or not, you are wrong. You are correct in the sense that if we go faster than the speed of light then yes, it is possible to go back in time, that is, if you could figure out a way to punch through the infinite mass/infinite energy required to go faster than the speed of light...

but the rules are different in regards to space and space time. A mathematician figured out that theoretically it is possible to wrap yourself in a bubble of space-time and that bubble of space-time can propagate through the universe faster than the speed of light without breaking Einstein's theory or relativity.

https://en.wikipedia.org/wiki/Alcubierre_drive

kinda cool. So Star Trek sorta got it right, it is possible to zip around the galaxy faster than the speed of light

 

[Wgw]

Believe it or not, you are wrong. You are correct in the sense that if we go faster than the speed of light then yes, it is possible to go back in time, that is, if you could figure out a way to punch through the infinite mass/infinite energy required to go faster than the speed of light...

but the rules are different in regards to space and space time. A mathematician figured out that theoretically it is possible to wrap yourself in a bubble of space-time and that bubble of space-time can propagate through the universe faster than the speed of light without breaking Einstein's theory or relativity.

https://en.wikipedia.org/wiki/Alcubierre_drive

kinda cool. So Star Trek sorta got it right, it is possible to zip around the galaxy faster than the speed of light

The problem is that the Alcubierre drive violates causality. It is still "FTL." The local bubble of space time in an Alcubierre type warp bubble is entirely local. Thus, a spacecraft using this type of drive could easily violate causality and cross back into its own past. Without causality, there is nothing to ensure that the local time in the Alcubierre bubble corresponds with that of two or more external locations.

All proposed systems for violating the limit imposed by the speed of light violate causality. You cannot have both. What is more, even at a quantum level, there is mich reason to believe that one cannot use "spooky action at a distance" to transmit information on an FTL basis, because that information is lost with decoherence.

 

[dgiharris]

The problem is that the Alcubierre drive violates causality. It is still "FTL." The local bubble of space time in an Alcubierre type warp bubble is entirely local. Thus, a spacecraft using this type of drive could easily violate causality and cross back into its own past. Without causality, there is nothing to ensure that the local time in the Alcubierre bubble corresponds with that of two or more external locations.

All proposed systems for violating the limit imposed by the speed of light violate causality. You cannot have both. What is more, even at a quantum level, there is mich reason to believe that one cannot use "spooky action at a distance" to transmit information on an FTL basis, because that information is lost with decoherence.

Could you please give an example of how going FTL in a warp bubble violates causality. It does not.

There are examples of things going faster than the speed of light in nature. In quantum mechanics, phase transitions of electrons from one level to another are faster than the speed of light.

If you are in a warp bubble and travel from here to the next Star System at 10X the speed of light. Then decide to go back home and travel at 20X the speed of light. You don't arrive "before" you left. The physics of the Alcubierre bubble states that inside the bubble you can't go faster than light, so you maintain your same velocity or acceleration that you had prior to entering that bubble. It is the actual bubble itself that warps space-time and moves. So no matter how fast the bubble is going, it doesn't violate causality. It can't return prior to when it left.

Basically, as I understand it, the alcubierre bubble basically sorta cheats. Since you can't go faster than light within space-time, the bubble gets around it by directly manipulating the actual space time. We know that gravity does manipulate space-time and we know that gravity actually "warps" space time. So we have empirical evidence that you "can" warp space time. So it isn't that big a stretch to say that it is "plausible" to have FTL travel "if" you could figure out a way to manipulate space time.

Or put another way, why doesn't gravity lead to violations of causality when we know gravity does in fact bend space-time?

I will admit, my math is limited, I'm decades out of practice in upper level math. However, from the perspective of concepts, the alcubierre bubble seems elegant and plausible. At the quantum level FTL occurs, at the macroscopic level, warping of space-time occurs. A warp bubble still manages to adhere to the laws of the universe because it is directly manipulating the fabric of the universe-- that is, directly manipulating space-time. So if you could manipulate space-time, I think it is plausible you can achieve FTL.

 

[Tina W]

Late 1800s and early 1900s. Vintage things fascinate me, and I think I like that time period best. I love looking at old vintage pictures, it makes me feel something I can't explain. I don't know why it just fascinates me. I love the idea of time travel because of that even though it's not possible, unless God does it, with Him all things are possible. But yeah I think I would go to the 1800s or 1900s.

 

[Wgw]

Could you please give an example of how going FTL in a warp bubble violates causality. It does not.

There are examples of things going faster than the speed of light in nature. In quantum mechanics, phase transitions of electrons from one level to another are faster than the speed of light.

If you are in a warp bubble and travel from here to the next Star System at 10X the speed of light. Then decide to go back home and travel at 20X the speed of light. You don't arrive "before" you left. The physics of the Alcubierre bubble states that inside the bubble you can't go faster than light, so you maintain your same velocity or acceleration that you had prior to entering that bubble. It is the actual bubble itself that warps space-time and moves. So no matter how fast the bubble is going, it doesn't violate causality. It can't return prior to when it left.

Basically, as I understand it, the alcubierre bubble basically sorta cheats. Since you can't go faster than light within space-time, the bubble gets around it by directly manipulating the actual space time. We know that gravity does manipulate space-time and we know that gravity actually "warps" space time. So we have empirical evidence that you "can" warp space time. So it isn't that big a stretch to say that it is "plausible" to have FTL travel "if" you could figure out a way to manipulate space time.

Or put another way, why doesn't gravity lead to violations of causality when we know gravity does in fact bend space-time?

I will admit, my math is limited, I'm decades out of practice in upper level math. However, from the perspective of concepts, the alcubierre bubble seems elegant and plausible. At the quantum level FTL occurs, at the macroscopic level, warping of space-time occurs. A warp bubble still manages to adhere to the laws of the universe because it is directly manipulating the fabric of the universe-- that is, directly manipulating space-time. So if you could manipulate space-time, I think it is plausible you can achieve FTL.

The problem with what you are saying is that you ignore the raison d'etre for having FTL - bypassing time dilation. Sidestepping time dilation is inherently a causal violation.

Polaris is roughly 1,000 years from Earth. Now, let us say you travel there at one thousand times the speed of light. You reach Polaris "this year" in Polaris time, instead of the thirty first century, which is when you would arrive otherwise. You then immediately return at two thousand times the speed of light. If you left today, you would arrive at Earth in the eleventh century.

An Alcubierre drive may be possible, but if it is, say goodbye to causality.

 

[dgiharris]

The problem with what you are saying is that you ignore the raison d'etre for having FTL - bypassing time dilation. Sidestepping time dilation is inherently a causal violation.

Polaris is roughly 1,000 years from Earth. Now, let us say you travel there at one thousand times the speed of light. You reach Polaris "this year" in Polaris time, instead of the thirty first century, which is when you would arrive otherwise. You then immediately return at two thousand times the speed of light. If you left today, you would arrive at Earth in the eleventh century.

An Alcubierre drive may be possible, but if it is, say goodbye to causality.

This is the beauty of the Alcubierre drive, it allows you to bypass time dilation to an extent.

Your problem is that you are thinking 3 dimensionally. You are absolutely right, if we stay in 3 dimensions then yes, exceeding the speed of light will result in a causal violation.

however, Alcubierre bubbles and the theoretical alcubierre drive works in the 4th dimension because it directly influences space-time.

The way you are thinking about FTL, you are thinking about it in terms of traveling through space. And again, if we were "only" traveling through space faster than FTL then you are correct, we violate causality.

However, the warp bubble doesn't travel through space, it travels through space-time. There is a difference. If you do not understand the difference between space and space time, then you cannot understand how causality is NOT violated.

not to sound like an authoritarian on the subject. I will admit I am just regurgitating arguments and explanations of people way smarter than me.

 

[Wgw]

This is the beauty of the Alcubierre drive, it allows you to bypass time dilation to an extent.

Your problem is that you are thinking 3 dimensionally. You are absolutely right, if we stay in 3 dimensions then yes, exceeding the speed of light will result in a causal violation.

however, Alcubierre bubbles and the theoretical alcubierre drive works in the 4th dimension because it directly influences space-time.

The way you are thinking about FTL, you are thinking about it in terms of traveling through space. And again, if we were "only" traveling through space faster than FTL then you are correct, we violate causality.

However, the warp bubble doesn't travel through space, it travels through space-time. There is a difference. If you do not understand the difference between space and space time, then you cannot understand how causality is NOT violated.

not to sound like an authoritarian on the subject. I will admit I am just regurgitating arguments and explanations of people way smarter than me.

If one can manipulate spacetime to produce an Alcubierre drive, one creates the possibility of closed timelike curves. It is a fundamental characteristic of the drive. Because basically what you are doing is exploiting spacetime curvature to cut out the time that would have elapsed at C.

The speed of light provides for causality in the face of different frames of reference. So when we sidestep it, we lose non-local causality. Causality is preserved only locally, in the inertial frame of reference inside the Alcubierre "bubble."

 

[dgiharris]

If you were able to create an Alcubierre bubble, then in effect, you have created your own pocket universe that is INDEPENDENT from the surrounding universe. So, within this pocket universe (i.e. bubble) time progresses at whatever rate. Outside of the bubble in the surrounding universe, time proceeds at a different rate. When you exit the bubble and reenter normal space, then you have progressed X amount into the future. However, there is no way for you to go "back" in time in either reference frame. So causality is still not violated.

The only way causality could be violated is if you could go back in time.

Maybe we need to step through an example.

Lets say that there is a star that is 10 light years away. This means it takes light from that star 10 years to make it to us. Lets say we are able to create an A-bubble and travel at 1 light year per hour. So, we would be able to reach this star in 10 hours. So, we travel to this star in ten hours and we set up our equipment at an orbit of 0.1-AU from this star, then we spend one week in front of this star making enormous planet sized shadow puppets. Then we travel back to our original location at maximum speed and arrive 10 hours after our 1-week puppet show.

now, when we arrive our engineers have figured out a way for us to now travel at 1 light year per second. The wavefront of our shadow puppets has now been traveling at the speed of light for one week + 10 hours and has yet to reach us. It will take 10 years to reach us.

just because we can now travel at 1 light year per second which is a rate 3600 times faster than what we could have traveled before, we still can't arrive at that star "before" we left. If we travel at 1 light year per second inside our A-bubble we arrive 10 seconds later, but that is still one week + 10 hours + 10 seconds later. So how can causality be violated?

notice that with the above example I didn't have to get into any time dilation and that is the beauty of the A-bubble.

If one can manipulate spacetime to produce an Alcubierre drive, one creates the possibility of closed timelike curves. It is a fundamental characteristic of the drive. Because basically what you are doing is exploiting spacetime curvature to cut out the time that would have elapsed at C.

The speed of light provides for causality in the face of different frames of reference. So when we sidestep it, we lose non-local causality. Causality is preserved only locally, in the inertial frame of reference inside the Alcubierre "bubble."

I'm having a little bit of trouble understanding what you are saying here.

Can you provide an example of HOW causality is violated. The only way that causality can be violated is if you can come up with an example of the A-bubble allowing you to do something akin to the grandfather paradox (i.e. you go back in time and kill your grandfather).

You said that "Because basically what you are doing is exploiting spacetime curvature to cut out the time that would have elapsed at C." I think that if this is true, you have taken it too far? You aren't cutting out "all" the time, just most of the time. Time still progresses in the surrounding universe and when you reenter that universe or come out of your bubble (either or) then time has progressed some unit of measure into the future. And as long as time has progressed, then you can't go backwards in time and violate causality.

Just because you can do something that violates the speed of light doesn't necessarily mean you have violated causality. As I understand it, "if" you could break the speed of light in normal space then yes, you could violate causality. However, the A-bubble moves through space-time... hmmm... I'm just parroting what I read....

Can you give me an example of violating causality with the A-bubble?

 

[Wgw]

If you were able to create an Alcubierre bubble, then in effect, you have created your own pocket universe that is INDEPENDENT from the surrounding universe. So, within this pocket universe (i.e. bubble) time progresses at whatever rate. Outside of the bubble in the surrounding universe, time proceeds at a different rate. When you exit the bubble and reenter normal space, then you have progressed X amount into the future. However, there is no way for you to go "back" in time in either reference frame. So causality is still not violated.

The only way causality could be violated is if you could go back in time.

Maybe we need to step through an example.

Lets say that there is a star that is 10 light years away. This means it takes light from that star 10 years to make it to us. Lets say we are able to create an A-bubble and travel at 1 light year per hour. So, we would be able to reach this star in 10 hours. So, we travel to this star in ten hours and we set up our equipment at an orbit of 0.1-AU from this star, then we spend one week in front of this star making enormous planet sized shadow puppets. Then we travel back to our original location at maximum speed and arrive 10 hours after our 1-week puppet show.

now, when we arrive our engineers have figured out a way for us to now travel at 1 light year per second. The wavefront of our shadow puppets has now been traveling at the speed of light for one week + 10 hours and has yet to reach us. It will take 10 years to reach us.

just because we can now travel at 1 light year per second which is a rate 3600 times faster than what we could have traveled before, we still can't arrive at that star "before" we left. If we travel at 1 light year per second inside our A-bubble we arrive 10 seconds later, but that is still one week + 10 hours + 10 seconds later. So how can causality be violated?

notice that with the above example I didn't have to get into any time dilation and that is the beauty of the A-bubble.



I'm having a little bit of trouble understanding what you are saying here.

Can you provide an example of HOW causality is violated. The only way that causality can be violated is if you can come up with an example of the A-bubble allowing you to do something akin to the grandfather paradox (i.e. you go back in time and kill your grandfather).

You said that "Because basically what you are doing is exploiting spacetime curvature to cut out the time that would have elapsed at C." I think that if this is true, you have taken it too far? You aren't cutting out "all" the time, just most of the time. Time still progresses in the surrounding universe and when you reenter that universe or come out of your bubble (either or) then time has progressed some unit of measure into the future. And as long as time has progressed, then you can't go backwards in time and violate causality.

Just because you can do something that violates the speed of light doesn't necessarily mean you have violated causality. As I understand it, "if" you could break the speed of light in normal space then yes, you could violate causality. However, the A-bubble moves through space-time... hmmm... I'm just parroting what I read....

Can you give me an example of violating causality with the A-bubble?

Edit: misread the question.

From Wikipedia:

Calculations by physicist Allen Everett show that warp bubbles could be used to create closed timelike curves in general relativity, meaning that the theory predicts that they could be used for backwards time travel.[28] While it is possible the fundamental laws of physics might allow closed timelike curves, the chronology protection conjecture hypothesizes that in all cases where the classical theory of general relativity allows them, quantum effects would intervene to eliminate the possibility, making these spacetimes impossible to realize (a possible type of effect that would accomplish this is a buildup of vacuum fluctuations on the border of the region of spacetime where time travel would first become possible, causing the energy density to become high enough to destroy the system that would otherwise become a time machine). Some results in semiclassical gravity appear to support the conjecture, including a calculation dealing specifically with quantum effects in warp-drive spacetimes that suggested that warp bubbles would be semiclassically unstable,[10][29] but ultimately the conjecture can only be decided by a full theory of quantum gravity.[30]

Miguel Alcubierre briefly discusses some of these issues in a series of lecture slides posted online,[31] where he writes "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)." In the next slide he brings up the chronology protection conjecture, and writes "The conjecture has not been proven (it wouldn’t be a conjecture if it had), but there are good arguments in its favor based on quantum field theory. The conjecture does not prohibit faster-than-light travel. It just states that if a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole."

 

[Wgw]

So basically, wishful thinking on the part of Alcubierre that a magical undefined effect will somehow destroy anyone who seeks to misuse the Alcubierre Drive.

It's wishful thinking piled on more wishful thinking, given that the Alcubierre drive requires exotic matter and is probably impossible.

 

[dgiharris]

So basically, wishful thinking on the part of Alcubierre that a magical undefined effect will somehow destroy anyone who seeks to misuse the Alcubierre Drive.

It's wishful thinking piled on more wishful thinking, given that the Alcubierre drive requires exotic matter and is probably impossible.

Imagine we were sitting at a cafe in 17th century France and someone put forth the notion that one day we would be able to travel to the moon...

We've only been in the space age for 60 or so years, barely the blink of an eye as far as human civilization is concerned. As human beings, we have a knack for realizing our dreams. We define ourselves by doing the impossible time and time again. There just are very few problems we are unable to solve and I see the FTL problem as a challenge we as a species will someday beat.

Granted, it won't be in my lifetime but I do believe it is possible. Someone will figure out a way to do it.

 

[Wgw]

Imagine we were sitting at a cafe in 17th century France and someone put forth the notion that one day we would be able to travel to the moon...

We've only been in the space age for 60 or so years, barely the blink of an eye as far as human civilization is concerned. As human beings, we have a knack for realizing our dreams. We define ourselves by doing the impossible time and time again. There just are very few problems we are unable to solve and I see the FTL problem as a challenge we as a species will someday beat.

Granted, it won't be in my lifetime but I do believe it is possible. Someone will figure out a way to do it.

It would be much easier to just extend the human lifespan to compensate for the extended duration of interstellar voyages. Or use suspended animation. Generation starships though, not a huge fan of those...

 

[dms1972]

Ok, where did this notion that travelling faster than light results in going back in time come from? Im not at all sure about it - so please explain.

In any case I don't believe it will become practically possible, theres a limitation placed on us by God because we are fallen, and by nature. And there's plenty of other things to be understanding better, so its no restriction on proper earth science.

Also we are looking back in time in a sense when we look at the sun, which of course you shouldn't do directly, because we are seeing the sun as it was about 500 seconds ago. Yes or No?

 

[Wgw]

Ok, where did this notion that travelling faster than light results in going back in time come from? Im not at all sure about it - so please explain.

In any case I don't believe it will become practically possible, theres a limitation placed on us by God because we are fallen, and by nature. And there's plenty of other things to be understanding better, so its no restriction on proper earth science.

Also we are looking back in time in a sense when we look at the sun, which of course you shouldn't do directly, because we are seeing the sun as it was about 500 seconds ago. Yes or No?

In a sense, yes, we are looking back in time when we view the sun or distant suns, in particular, which is why astrophysicosts are able to rely on astronomy to provide information about the beginning of the universe.

Now, FTL results in causality violations where two points are moving away from each other (which is normal, due ro the expanding universe), owing to the fact that the speed of light becomes relative to either endpoint. It becomes possible to arrive before you left, resulting in a closed timelike curve.

From an SF perspective closed timelike curves are actually interesting; Doctor Who relies on them frequently as plot devices, and Star Trek has more than once employed one.

 

[dms1972]

But we are not talking about real back in time travel surely even if some of these hypothetical notions are correct? I mean I don't mind discussing it as science-fiction, or science-fantasy, but not as real science.

With regard to films I haven't come across a major film that seriously attempts to explain time travel - even the Back to the Future trilogy only make a fairly comic explanation. The reason being that it is the potential paradoxes that are more interesting in this sort of story, and also the culture clash elements provide a big part of the interest - a theory of time-travel less so.

 

[dms1972]

I haven't really read enough Scifi besides CS Lewis who obviously isn't hard science fiction, perhaps because some more recent works look pretty daunting in length - I've read a bit of Ray Bradbury however which I thought was good - I love his prose.

I prefer the cautionary tales sort of sci-fi - there has to be a mad genius, or bordering on or crossing over into madness, someone else who faces the temptation to power but doesn't succumb, and a few other elements. Standard staples for me.

But Bradbury is in a different class, I wouldn't call it hard science-fiction - for instance its not how a rocket works but things like the heat a rocket gives off during launch on a winter's day he attempts to convey - how it would almost for a few moments change the climate.

 

[keith99]

I haven't really read enough Scifi besides CS Lewis who obviously isn't hard science fiction, perhaps because some more recent works look pretty daunting in length - I've read a bit of Ray Bradbury however which I thought was good - I love his prose.

I prefer the cautionary tales sort of sci-fi - there has to be a mad genius, or bordering on or crossing over into madness, someone else who faces the temptation to power but doesn't succumb, and a few other elements. Standard staples for me.

But Bradbury is in a different class, I wouldn't call it hard science-fiction - for instance its not how a rocket works but things like the heat a rocket gives off during launch on a winter's day he attempts to convey - how it would almost for a few moments change the climate.

You might want to give The Past Through Tomorrow' by Heinlein. Not recent and parts rather dated, but offhand I think if Robert were still alive he could very quickly rewrite most of it to still work today. It is huge, but it is not one story and they all pretty much stand alone.

I hadn't realized the reason why there isn't that much short stuff. Or perhaps I should say good short stuff. The problem is with science fiction for just about any story you need to create the entire world and that is as much work for a short story as a trilogy, and in practice authors get paid by the word or at least by the page count.

 

[dms1972]

Its Bradbury's gift for writing short Sci-fiction vignettes I think has rarely been surpassed. CS Lewis was an admirer of Bradbury's The Silver Locusts.

Take this excerpt from it for example - its really well written, but not technical:

"The ship came down from space. It came from the stars and the black velocites, and the shining movements, and the silent gulfs of space. It was a new ship; it had fire in its body and men in its metal cells, and it moved with a clean silence, fiery and warm...Now it was decelerating with metal efficiency in the upper Martian atmospheres. It was still a thing of beauty and strength. It had moved in the midnight waters of space like a pale sea leviathan..."