>"Because time travel is limited to these special particles, it is not possible for a man to travel back in time and murder one of his parents before he himself is born, for example. However, if scientists could control the production of Higgs singlets, they might be able to send messages to the past or future,"
This is absurd. If you can send messages back in time, then someone can build a robot that can accept messages from the future, and kill your parents that way^. Sending information back in time is a paradox. Period.^^
^An option available to everyone after the robot is built. Maybe this is actually enough to resolve the issues they're concerned with. I'm somewhat suspicious.
^^To be clear, I am not saying that the work is wrong or impossible, merely that their example is less than explanatory about why their theory does not have the problems of other theories.
>This is absurd. If you can send messages back in time, then someone can build a robot that can accept messages from the future, and kill your parents that way^. Sending information back in time is a paradox.
That's only true if "free will" exists, i.e. if its really "changing" the past instead of fulfilling it.
Well, to be fair, the ability to send messages to the past really only becomes useful once such messages are receivable. For the messages to be receivable, we must know the properties of the medium that the messages are to be sent on.
Basically, this limits the very earliest that such backwards going messages can be received to around now-ish.[1]
As a consequence it may not be a paradox anymore, since there will be no point at which we can operate without a suspicion of future interference, thereby reducing the paradox effect, and in fact encouraging it by divining the outcomes of our actions. Of course this divining rapidly loses power anyway, as multiple messages are sent conflicting with one another - it gets messy quick. It gets messier quicker if you choose to go with a "many worlds" interpretation of the universe. Further, there are questions regarding observation: does observing a message from the future cement that future into being, possibly incurring an absurd chain of events but not causing any paradox (the 12 monkeys thing)?
What I am really driving at here is this: Your paradox claims are amusingly simplistic, the real deal here is that we just don't know enough to say.
[1] I am ignoring two big possibilities: 1. that this has been known for some time by, say, evil wall street bankers with secret labs (and their ilk -- governments, supervillians and such). 2. That there aren't aliens at some point in the very distant past whom we get messages to, and convince to do our bidding, who show up at various key stages of history an change everything. This may or may not have happened, depending on what you believe from the history channel (ancient aliens) and Stargate.
If you can send messages back in time, then someone can build a robot that can accept messages from the future, and kill your parents that way^. Sending information back in time is a paradox. Period
Not quite. You're neglecting quantum mechanics.
Very roughly speaking, in quantum field theory, the probability of an event happening is a weighted sum over all the possible ways that it could happen (a path integral), and (again, speaking roughly) the options that seriously violate the various conservation laws tend to have negligible contributions. Presumably the path integral formulation would still hold up even with closed timelike curves (time travel curves).
An inconsistent state, like one where you kill off your own parents, would probably exhibit all sorts of self interference that would make its contribution to the path integral very small, whereas the consistent states would be much better represented, and much more likely to happen. Weird "violations" of physics could quite easily present themselves as the most probable outcomes, especially if the alternative was an inconsistent outcome.
A good way to think about this is in terms of waves on a string, or the "wave" of an electron in orbit around a nucleus - the only vibration frequencies that "survive" are the ones that reinforce themselves, and the rest get cancelled out by interference (in the electron case, because they don't "match up" when they meet themselves after an orbit). If you think of the state of the universe as the electron, and the orbit as a loop through the closed timelike curve, then it's easy enough to imagine quantum mechanics doing all the hard work for us, and making sure that only consistent outcomes are reinforced, while all the other ones are destroyed by interference.
I should point out, though, the fact that it's theoretically conceivable that the universe could support time travel doesn't necessarily mean that it does, and I suspect most physicists think it's highly unlikely.
Every time I think of possible FTL travel now I remember that it takes light about 8 minutes to get from our sun to the earth. If we ever do get to FTL travel, we really need to figure out the FT part or we still aren't going to be going anywhere too quickly.
Edit: And as soon as I typed that out I remembered relativity effects, and my brain hurts.
In the general case, time travel is only good for making terrible, terrible science fiction plots.
However, FTL is well justified. Even with various limitations (e.g. you can only use it for instant communications, or only robots can travel), it'd be an utterly fantastic leap for mankind.
To anyone who might not be familiar with the applications of timelike paths in computers: it's the quickest way to the singularity. Basically, you get a computer and wire its output (in the future) to its input (in the present or past). Depending on how time travel works, this either gives you an infinite amount of computational power (actually an infinite amount of time to perform operations in, but it looks rather similar to outside viewers) or a way to quickly find the conditions under which a program returns the input as output (which gives you instantaneous protein folding, factoring of large primes, and so on, if you design your programs properly. If you don't, it gives you a good way to find buffer overflows and other failure conditions). Of course, both of those would look pretty much exactly the same to outside viewers.
I hate to break this to you, but there's no such thing that you can describe as simply as "the present". Since we're talking about time travel here, it's important to remember that relativity is real, and that every single atom in your body exists at a different "present".
Our GPS satellites are time travellers. Their distance away from Earth is so great that we must compensate for their different perception of time, or our reported locations would be way off.
You should not be so absolutist in your views, especially when you don't have an accurate understanding of time.
I also take issue with you suggesting that nothing dangerous should be "bothered" with in research. Look where the Manhattan project got us: We have safe nuclear reactors all over the planet, we have sent a few spaceships outside the solar system (Voyager 1 and 2, for example) that run on nuclear reactors. We know that nuclear research presents great danger, but the rewards are so huge it's worth it.
All knowledge is worth some danger, especially the truly incredible knowledge like finding out that our understanding of physics is incorrect, or building spaceships that leave the solar system.
You laid it out pretty clearly, but one detail ought to be corrected -- Voyagers etc. use Radioisotope Thermoelectric Generators rather than reactors.
The difference is, there is no chain reaction in RTGs; the natural spontaneous fission is relied on. The construction (lack of neutron moderator) prevents chain reaction for safety reasons -- no chance of runaway reaction. On the other hand, it means the thermal output cannot be regulated at all, and drops steadily in parallel to the half-decay time.
The RTGs are so safe they even used miniature ones in some heart pacemakers implanted years ago; they are still going [1].
I find your objection quite strange. If it _is_ possible, and it _is_ dangerous, than it would be critical to understand how it works, lest it be abused.
That being said, understanding time travel (even if it's impossible) is very important for getting a better understanding by what we mean by 'time' in the first place.
Maybe in this context it would be useful to reflect about time.
Our life is a continuous concatenating of present moments. And every present moment is new. But where do they come from? Nobody knows.
I see time as a continuous gift from Someone outside of our physical Universe. No other explanation is logically possible, because any explanation would come from inside our Universe -- but fresh new present moments can't be located inside, otherwise they would not be new.
If it's the former, well, I guess you're probably right. Nobody knows. At least, nobody thinking about it in a purely philosophical/theological context.
Very interesting but its bad speculation to say we can send any type of message this way. Messages sent to the past have already been received. So unless scientists have been getting messages from their future experiments...
What about limited forms of time travel such as the one in the film Primer - where they have a device that does indeed allow travel through time but only back to the point where it was turned on and only at the normal rate (i.e. it takes an hour subjectively to go back an hour in time).
In the article under discussion, this is not what the authors are talking about: they're literally proposing a model under which every time you traverse the extra spatial dimension, you end up at an earlier point in time. So the reach of that communication would be, in theory, unlimited.
The article postulates that time-traveling communications can exist, via devices yet to be invented. Since we currently have no way of interpreting the backwards-flowing information, we would not see evidence of its invention today.
I hate judging based on stupid journalist writeups of stuff like this, so I'll actually look through the article, for once.
The first part of the abstract: "We construct a simple class of compactified five-dimensional metrics which admits closed timelike curves (CTCs), and derive the resulting CTCs as analytic solutions to the geodesic equations of motion. The associated Einstein tensor satisfies the null, weak, strong and dominant energy conditions; in particular, no negative-energy "tachyonic" matter is required."
Translated for the layman: "We completely arbitrarily and quite deliberately construct a geometry that allows normal particles to time travel. It turns out that under this geometry, if a particle time travels, it doesn't have to be abnormal."
Let's just say I'm not impressed so far...we'll see how the rest of the article goes...
Ok, so I gave this a really quick read-through, and as usual the article way overstates what's been done.
I'll try to put this briefly, without too much technical detail.
General relativity permits (and we've always known this) solutions that involve closed timelike curves (CTCs). Usually, when we look into these solutions in detail, we find that they require strange sorts of matter to sustain themselves, or that they're unstable. Hawking proposed that the universe in some way bans any geometry that would allow these solutions.
The authors here claim that by adding a fifth dimension that's curled up like a circle, and adding a specific sort of curvature to it (the metric contains a periodic cross term between the usual time dimension and the curled dimension), you can get CTCs that are geodesics (i.e. paths that particles will take with no forces on them), and you don't get weird energy conditions or anything like that.
Then they go on about the potential consequences if the universe is actually curved up like this, and so on, but don't offer any particularly compelling reason to believe that it is.
So basically, they claim to have shown that it's possible for the universe to be like this, and that no condition we currently know of prohibits it.
Meh. Could be, if we assume that they're right - it's always, of course, possible that there's some other problem with their solution that wouldn't be covered by the usual energy conditions, I'm not sure, I've been out of the GR circuit for a while so I don't know what people currently consider to be the most reasonable set of constraints that the Real World follows. But even if they're right, there are plenty of solutions to Einstein's equations that could have happened and don't, so there's nothing too revolutionary here, and mentioning a whole lotta shit about branes and the Higgs boson doesn't change that.
This is still quite useful theoretical physics to do, mind you, but it's nowhere near as important as the article makes it out to be.
Theoretically, traveling forward in time has already been figured out. Its just a matter of traveling REALLY fast. Traveling backwards seems to be the biggest issue...
You build a mechanism now that sends a particle to the future upon receipt of a particle from the future, then later on you send a particle back in time to that mechanism.
The faster you travel, the faster time goes relative to things stationary to you it could be possible that some particles are spinning off at some multiple of the speed of light, and is aging much faster than we are, and having spent millions of years existing in the space next to the collision, evaporates or flips into another place by chance.
An "instance in time" is simply the state of all energy at any one moment. I say "all energy" because our reality is comprised of 1) energy in many different forms and 2) space - the container of the energy.
To travel back in time would be to reverse the course of all energy - from the perspective of a segment of the energy. Given the total amount of energy in our reality - that's not very likely.
38 comments
[ 3.1 ms ] story [ 93.1 ms ] threadThis is absurd. If you can send messages back in time, then someone can build a robot that can accept messages from the future, and kill your parents that way^. Sending information back in time is a paradox. Period.^^
^An option available to everyone after the robot is built. Maybe this is actually enough to resolve the issues they're concerned with. I'm somewhat suspicious.
^^To be clear, I am not saying that the work is wrong or impossible, merely that their example is less than explanatory about why their theory does not have the problems of other theories.
That's only true if "free will" exists, i.e. if its really "changing" the past instead of fulfilling it.
Basically, this limits the very earliest that such backwards going messages can be received to around now-ish.[1]
As a consequence it may not be a paradox anymore, since there will be no point at which we can operate without a suspicion of future interference, thereby reducing the paradox effect, and in fact encouraging it by divining the outcomes of our actions. Of course this divining rapidly loses power anyway, as multiple messages are sent conflicting with one another - it gets messy quick. It gets messier quicker if you choose to go with a "many worlds" interpretation of the universe. Further, there are questions regarding observation: does observing a message from the future cement that future into being, possibly incurring an absurd chain of events but not causing any paradox (the 12 monkeys thing)?
What I am really driving at here is this: Your paradox claims are amusingly simplistic, the real deal here is that we just don't know enough to say.
[1] I am ignoring two big possibilities: 1. that this has been known for some time by, say, evil wall street bankers with secret labs (and their ilk -- governments, supervillians and such). 2. That there aren't aliens at some point in the very distant past whom we get messages to, and convince to do our bidding, who show up at various key stages of history an change everything. This may or may not have happened, depending on what you believe from the history channel (ancient aliens) and Stargate.
Not quite. You're neglecting quantum mechanics.
Very roughly speaking, in quantum field theory, the probability of an event happening is a weighted sum over all the possible ways that it could happen (a path integral), and (again, speaking roughly) the options that seriously violate the various conservation laws tend to have negligible contributions. Presumably the path integral formulation would still hold up even with closed timelike curves (time travel curves).
An inconsistent state, like one where you kill off your own parents, would probably exhibit all sorts of self interference that would make its contribution to the path integral very small, whereas the consistent states would be much better represented, and much more likely to happen. Weird "violations" of physics could quite easily present themselves as the most probable outcomes, especially if the alternative was an inconsistent outcome.
A good way to think about this is in terms of waves on a string, or the "wave" of an electron in orbit around a nucleus - the only vibration frequencies that "survive" are the ones that reinforce themselves, and the rest get cancelled out by interference (in the electron case, because they don't "match up" when they meet themselves after an orbit). If you think of the state of the universe as the electron, and the orbit as a loop through the closed timelike curve, then it's easy enough to imagine quantum mechanics doing all the hard work for us, and making sure that only consistent outcomes are reinforced, while all the other ones are destroyed by interference.
Nobody should have the power to alter events outside of the present in any way, shape or form. Period.
Edit: And as soon as I typed that out I remembered relativity effects, and my brain hurts.
However, FTL is well justified. Even with various limitations (e.g. you can only use it for instant communications, or only robots can travel), it'd be an utterly fantastic leap for mankind.
I hate to break this to you, but there's no such thing that you can describe as simply as "the present". Since we're talking about time travel here, it's important to remember that relativity is real, and that every single atom in your body exists at a different "present".
Our GPS satellites are time travellers. Their distance away from Earth is so great that we must compensate for their different perception of time, or our reported locations would be way off.
You should not be so absolutist in your views, especially when you don't have an accurate understanding of time.
I also take issue with you suggesting that nothing dangerous should be "bothered" with in research. Look where the Manhattan project got us: We have safe nuclear reactors all over the planet, we have sent a few spaceships outside the solar system (Voyager 1 and 2, for example) that run on nuclear reactors. We know that nuclear research presents great danger, but the rewards are so huge it's worth it.
All knowledge is worth some danger, especially the truly incredible knowledge like finding out that our understanding of physics is incorrect, or building spaceships that leave the solar system.
Oh, the irony!!!
The difference is, there is no chain reaction in RTGs; the natural spontaneous fission is relied on. The construction (lack of neutron moderator) prevents chain reaction for safety reasons -- no chance of runaway reaction. On the other hand, it means the thermal output cannot be regulated at all, and drops steadily in parallel to the half-decay time.
The RTGs are so safe they even used miniature ones in some heart pacemakers implanted years ago; they are still going [1].
--
[1] http://duckduckgo.com/?q=nuclear+pacemaker&v=
That being said, understanding time travel (even if it's impossible) is very important for getting a better understanding by what we mean by 'time' in the first place.
Yet in practice it's better to publish that openly, lest the blackhats abuse the vulnerabilities for their purposes unhindered.
Our life is a continuous concatenating of present moments. And every present moment is new. But where do they come from? Nobody knows.
I see time as a continuous gift from Someone outside of our physical Universe. No other explanation is logically possible, because any explanation would come from inside our Universe -- but fresh new present moments can't be located inside, otherwise they would not be new.
If it's the latter, you might find sections of RobotRollCall's first comment in this reddit discussion enlightening: http://www.reddit.com/r/askscience/comments/fjwkh/why_exactl...
If it's the former, well, I guess you're probably right. Nobody knows. At least, nobody thinking about it in a purely philosophical/theological context.
I hate judging based on stupid journalist writeups of stuff like this, so I'll actually look through the article, for once.
The first part of the abstract: "We construct a simple class of compactified five-dimensional metrics which admits closed timelike curves (CTCs), and derive the resulting CTCs as analytic solutions to the geodesic equations of motion. The associated Einstein tensor satisfies the null, weak, strong and dominant energy conditions; in particular, no negative-energy "tachyonic" matter is required."
Translated for the layman: "We completely arbitrarily and quite deliberately construct a geometry that allows normal particles to time travel. It turns out that under this geometry, if a particle time travels, it doesn't have to be abnormal."
Let's just say I'm not impressed so far...we'll see how the rest of the article goes...
I'll try to put this briefly, without too much technical detail.
General relativity permits (and we've always known this) solutions that involve closed timelike curves (CTCs). Usually, when we look into these solutions in detail, we find that they require strange sorts of matter to sustain themselves, or that they're unstable. Hawking proposed that the universe in some way bans any geometry that would allow these solutions.
The authors here claim that by adding a fifth dimension that's curled up like a circle, and adding a specific sort of curvature to it (the metric contains a periodic cross term between the usual time dimension and the curled dimension), you can get CTCs that are geodesics (i.e. paths that particles will take with no forces on them), and you don't get weird energy conditions or anything like that.
Then they go on about the potential consequences if the universe is actually curved up like this, and so on, but don't offer any particularly compelling reason to believe that it is.
So basically, they claim to have shown that it's possible for the universe to be like this, and that no condition we currently know of prohibits it.
Meh. Could be, if we assume that they're right - it's always, of course, possible that there's some other problem with their solution that wouldn't be covered by the usual energy conditions, I'm not sure, I've been out of the GR circuit for a while so I don't know what people currently consider to be the most reasonable set of constraints that the Real World follows. But even if they're right, there are plenty of solutions to Einstein's equations that could have happened and don't, so there's nothing too revolutionary here, and mentioning a whole lotta shit about branes and the Higgs boson doesn't change that.
This is still quite useful theoretical physics to do, mind you, but it's nowhere near as important as the article makes it out to be.
To travel back in time would be to reverse the course of all energy - from the perspective of a segment of the energy. Given the total amount of energy in our reality - that's not very likely.