What would happen to you if you went back in time and killed your grandfather? A model using photons reveals that quantum mechanics can solve the quandary—and even foil quantum cryptography.
Recent experiments offer tentative support for time travel's feasibility—at least from a mathematical perspective. The study cuts to the core of our understanding of the universe, and the resolution of the possibility of time travel, far from being a topic worthy only of science fiction, would have profound implications for fundamental physics as well as for practical applications such as quantum cryptography and computing.
The source of time travel speculation lies in the fact that our best physical theories seem to contain no prohibitions on traveling backward through time. The feat should be possible based on Einstein's theory of general relativity, which describes gravity as the warping of spacetime by energy and matter. An extremely powerful gravitational field, such as that produced by a spinning black hole, could in principle profoundly warp the fabric of existence so that spacetime bends back on itself. This would create a "closed timelike curve," or CTC, a loop that could be traversed to travel back in time.
http://www.scientificamerican.com/article/time-travel-simulation-resolves-grandfather-paradox/
[Related]: http://www.scientificamerican.com/article/according-to-current-phys/
[Abstract]; http://www.nature.com/ncomms/2014/140619/ncomms5145/full/ncomms5145.html
(Score: 2, Funny) by Horse With Stripes on Thursday September 04 2014, @09:53AM
I say "pictures or it didn't happen".
I want one of these scientists to actually go back in time and kill their own grandfather before the scientist's father is conceived, and before the scientist's grandfather donates sperm. And I want pictures of it. Then we'll talk. Until then it didn't happen ;-)
(Score: 5, Funny) by GreatAuntAnesthesia on Thursday September 04 2014, @10:03AM
> Until then it won't will haven't happened ;-)
FTFY.
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @11:40AM
at least until someone goes back and then it will have going to have happened.
(Score: 1) by Horse With Stripes on Thursday September 04 2014, @01:21PM
Nice! :-)
(Score: 2) by choose another one on Thursday September 04 2014, @03:38PM
At this point in any time travel conversation one needs to defer to Douglas Adams who predicted just this confusion, plus in fact the solution to the grandfather paradox in TFA, several decades ago... http://pages.cs.wisc.edu/~param/quotes/guide.html [wisc.edu]
(Score: 2) by nitehawk214 on Thursday September 04 2014, @09:41PM
And this is why the Campaign for Real Time was formed... because temporal grammar is hard.
"Don't you ever miss the days when you used to be nostalgic?" -Loiosh
(Score: 2) by q.kontinuum on Thursday September 04 2014, @11:38AM
Interestingly, this is what I posted to the previous article as well [soylentnews.org]
You went back in time and stole my quote? (How does copyright work in this context?)
Registered IRC nick on chat.soylentnews.org: qkontinuum
(Score: 1) by Horse With Stripes on Thursday September 04 2014, @01:22PM
I'll wait for your DMCA take down notice. ;-)
(Score: 3, Funny) by q.kontinuum on Thursday September 04 2014, @03:05PM
In that case I will sent it one month ago and file a complaint for not reacting upon. Check and mate, mate!
Registered IRC nick on chat.soylentnews.org: qkontinuum
(Score: 1) by Horse With Stripes on Thursday September 04 2014, @08:32PM
D'oh!
See everyone? This stuff is so easy to understand that I, um, did and forgot ... or I forgot that I didn't understand it. I'll just wait for q.kontinuum to let me know which one it was ;-)
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @09:56AM
...scientist, engineer and mathematician jokes
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @10:09AM
queue.pop()
(Score: 1, Funny) by Anonymous Coward on Thursday September 04 2014, @10:20AM
I'm sorry, but there are no jokes in the queue. The jokes I've put in tomorrow will have been going to have had been removed already five years ago. Please try again yesterday.
(Score: 1) by panachocala on Thursday September 04 2014, @11:55AM
cue... it's cue.
(Score: 1) by GeminiDomino on Thursday September 04 2014, @02:48PM
Works either way once jokes.count >= 2.
"We've been attacked by the intelligent, educated segment of our culture"
(Score: 2) by tangomargarine on Thursday September 04 2014, @03:01PM
Or enqueue, i.e., to put into a queue.
"Is that really true?" "I just spent the last hour telling you to think for yourself! Didn't you hear anything I said?"
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @10:17AM
I like Earth in the 20th century.
(Score: 4, Informative) by geb on Thursday September 04 2014, @10:29AM
The frame dragging effect of large rotating bodies can twist spacetime in unintuitive ways, and radically change the "angle" at which space and time sit when compared to the universe further out. It's called a Tipler cylinder type time machine. However, you can only use the effect to go back in time if you have a cylinder of infinite length or can produce repulsive gravitation via negative energy.
Searches for negative energy have produced only shaky evidence, and there's no good reason to expect that it exists.
Until somebody manages to find a solid example of negative energy, you might as well be speculating about a time machine powered by rubbing magic lamps.
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @10:38AM
Oooooo negative energy. The same stuff that powers real-world warp drive!
(Score: 1, Funny) by Anonymous Coward on Thursday September 04 2014, @10:43AM
Yes, the (US) plug has been filed, so it will go in to the socket either way and connected the wrong way, which gives negative energy.
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @01:33PM
And stabilizes wormholes.
Has anyone figured out how to build a Heisenberg compensator using negative energy?
(Score: 2) by EvilJim on Friday September 12 2014, @05:04AM
You'll need one big set of balls to compensate for Heisenberg.... oh... you mean the original one.
(Score: 2) by Blackmoore on Thursday September 04 2014, @01:50PM
this isnt 4chan.
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @05:36PM
0 - (-energy) = energy.
Using no energy at all, our equation nets us free energy AND time travel.
(Score: 2) by darkfeline on Thursday September 04 2014, @07:28PM
Maybe you could power the black hole with another black hole?
Join the SDF Public Access UNIX System today!
(Score: 4, Interesting) by HiThere on Thursday September 04 2014, @07:59PM
Your statement is too strong. The math has been solved for cylinders of infinite length, yes, but that's not proof that those are the only methods. I suspect that rotating rings (take the cylinders, and close them into a circle...use a large enough length that you can set them rotating just as the cylinders are rotating) would also work. This is still far beyond our capabilities, but gets rid to the requirement for infinite length...which, IIUC, is only there in the first place to simplify the math. Just saying "long enough" would probably suffice, though I'm not sure. If the ends of the cylinders are well outside of our current light-cone, though, wouldn't that count as "effectively infinite in length"?
N.B.: I've never worked through the math, but I believe that this is just one special case solution that was easy.
Also, doesn't the Casimir effect count as negative energy? I can't see how you could use it to stabilize a worm-hole, but it seems to demonstrate the existence of negative energy.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by geb on Friday September 05 2014, @11:19AM
Theorists didn't stop working on the problem after doing the easy maths with infinite cylinders.
http://journals.aps.org/prd/abstract/10.1103/PhysRevD.46.603 [aps.org] - "Chronology protection conjecture", by none other than Stephen Hawking is the best known paper studying it. It concludes that it's impossible to produce a causality loop, i.e. local backwards time travel, without negative energy.
(Score: 2) by HiThere on Friday September 05 2014, @05:49PM
I'm afraid the terms used in the abstract leave me not quite able to understand what he's saying, though I accept that they increase the precision of what he was saying. But the final sentence of the abstract is stronger than I've seen generally asserted, in fact stronger than your assertion.
OTOH, Hawking was clearly biased against time travel before he even started working on the math (for that paper). So I'm not sure I can accept that the math actually says what he claims, until it's verified by several people with less investment. That said, it sounds like it *does* place very strong limits on the possibilities.
WRT negative energy, doesn't the Casimir effect count as an existence proof? Even though that particular method of generating negative energy is useless for most purposes, it still exists. And, AFAIK, there is no proof that implies that it's the only way to create negative energy.
I think it's best to not decide based on existing information. If you wanted to assert that "Time travel into the absolute past is probably impossible." I'd have no argument. I agree that that's what the existing evidence seems to show. But that's a very different statement.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by evilviper on Thursday September 04 2014, @11:20AM
Occam's razor says time-travel isn't possible. It's generally best to accept that if your equaltions result in a paradox, it's because your equations are less than perfect.
We know that Relativity isn't perfect, hence the need for a grand unified theory, so it seems completely mind-numbing that people would believe that time travel is possible, just because the equation says 1+1=3.
Hydrogen cyanide is a delicious and necessary part of the human diet.
(Score: 2) by opinionated_science on Thursday September 04 2014, @11:54AM
I am inclined to agree, but as stronger principle is thermodynamics, that causality cannot be broken locally - and perhaps not at all.
I am thinking that feedback from he exchange in energies would close it down very fast. More importantly, nothing in the universe is "static". To go back in time 1 year on Earth would be a very long way away! Our solar systems is moving at 200km/s so that's 6.3*10^9 km away already!!!! (That's approximately the distance to pluto...)
I would say building ships that can go 200km/s is a prerequisite...
(Score: 2) by No.Limit on Thursday September 04 2014, @12:16PM
200km/s relative to what?, our galaxy?
It's an interesting question where we'd pop up if we moved in time (back and forth) and with what speed relative to what.
However, I think it'd depend a lot on how one travels in time, e.g. if you had to jump into a black hole, your position and speed would probably be determined by the black hole.
(Score: 4, Informative) by opinionated_science on Thursday September 04 2014, @12:27PM
"Using speed measurements of the gas at different distances from the Galactic center, the Sun appears to be cruising along at 200 kilometers per second and it takes 240 million years to complete the grand circuit around the Galaxy."
http://solar-center.stanford.edu/FAQ/Qsolsysspeed.html [stanford.edu]
The universe is expanding too...our galaxy is moving 1.3*10^6 miles/hr (361 miles/sec)
(Score: 1, Informative) by Anonymous Coward on Thursday September 04 2014, @01:39PM
That fully depends on in which direction you go back in time. There's no absolute "at the same place at another time" (and for that insight, it didn't need Einstein, already Galileo knew this; Einstein added that there's also no absolute "at the same time at another place" either.
(Score: 2) by BasilBrush on Thursday September 04 2014, @12:16PM
Occam's razor says time-travel isn't possible.
You don't say why. It may be because we haven't seen any evidence of time travellers from the future. But their are other possible explanations for that.
Perhaps time travel needs a device at the target time. No such device exists yet, so time travellers from the future can't get back this far.
Or perhaps the planet is destroyed by a giant asteroid next year, before the means to travel in time is developed.
The good thing is that based on the argument in TFA, the time travel paradox isn't one any more.
Hurrah! Quoting works now!
(Score: 2) by opinionated_science on Thursday September 04 2014, @12:22PM
My guess is thermodynamics and feedback. A "time portal" would simply explode due to the differences in energy (entrop). between the past and the future.
In fact in the very best science fiction movie effect, this would be like a bubble moving outwards at the speed of light!
It may turnout that blackholes since the "crimp" our reality (by creating an event horizon), may indeed make it possible by stopping this catastrophic feedback.
Stephen Hawking has already thrown his part for time travellers, and noone came!!!
(Score: 2) by tangomargarine on Thursday September 04 2014, @02:53PM
Wouldn't a more accurate math analogy be dividing by zero? Depending on context it might be defined but we can't come up with one yet.*
And besides, in math you use proof by contradiction all the time. Isn't that the same as a paradox?
*Maybe we have. IANAMathematician.
"Is that really true?" "I just spent the last hour telling you to think for yourself! Didn't you hear anything I said?"
(Score: 3, Insightful) by HiThere on Thursday September 04 2014, @08:05PM
Two points:
1) What Occam's razor says depends on your Bayesian priors.
2) Occam's razor isn't always right.
To me Occam's razor says that one should believe the equations until there's a good reason not to. This doesn't count as a good reason.
N.B.: This doesn't mean that time travel is an engineering possibility. It may well always remain as theoretic as the magnetic monopole is currently.
OTOH, using your interpretation of Occam's razor, one might be inspired to search for a more complete theory that excludes the possibility of time travel. That would justify your adopting the stance you propose.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by evilviper on Friday September 05 2014, @01:37AM
Trusting equations that cause paradoxes is never the simpler solution
Hydrogen cyanide is a delicious and necessary part of the human diet.
(Score: 2) by HiThere on Friday September 05 2014, @05:34PM
What is a "paradox"? Simply a conclusion that you don't want to accept. Quantum theory was originally rejected for that reason, but that didn't end the argument, and now it's accepted. And it's FULL of "paradox"es.
OTOH, perhaps there will be a successor to quantum theory. That's why I said that if it inspires you to work on a replacement theory, then it's reasonable. But unless you do, you it's simpler to accept the math. Often what you originally thought was a paradox turns out to actually be the way things work.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by evilviper on Saturday September 06 2014, @06:16AM
No. "A paradox is a statement that contradicts itself".
Nonsense. Quantum theory is full of things that seem illogical when compared with classical physics, but absolutely no paradoxes. Nothing in quantum theory is self-annihilating, which is required for a paradox.
No, it's simpler to accept plain old logic, and just recognize that the math isn't perfect, and will sometimes give nonsensical answers.
The grandfather paradox is the simplest, most straight-forward paradox there can be. There's no way out of it, unless there are severe constraints on time-travel that have not yet been imagined.
Hydrogen cyanide is a delicious and necessary part of the human diet.
(Score: 2) by HiThere on Saturday September 06 2014, @07:06PM
There are actually multiple solutions to the "grandfather paradox" that don't involve violation of any known physical law. One of them invoke the EWG multi-world hypothesis, and states that the past you arrive in is immediately forked from the past you remember, so you don't end up in the same branch. (Actually, you never arrived in your actual absolute past, you just arrive in something that is indistinguishable from it except that you are present). That's my preferred interpretation.
Another was elaborated in novel form by Dr. Robert Forward in his book "Timemaster". This uses the approach from Physical Review D44 (15 August 1991): pg 1077-1099: Echeverria, Fernando, Funnar Klinkhammer, and Kip S. Thorne. "Billiard Balls in Wormhole Spacetimes with Closed Timelike CurvesL Classical Theory"
(There are other references. I don't read the math, but I trust the author.)
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by evilviper on Monday September 08 2014, @03:24PM
That would be one of those "severe constraints on time-travel" that I mentioned.
It's also VASTLY more complicated that just saying that time-travel isn't possible.
Hydrogen cyanide is a delicious and necessary part of the human diet.
(Score: 2) by HiThere on Monday September 08 2014, @07:03PM
Not necessarily. I you accept the EWG multiworld interpretation of quantum physics then it seems a trivial modification. And the injection of energy would almost seem to require it. (I haven't even looked at the math in decades, and never worked through it, so this is just my guess, which isn't worth much when compared with someone who understands the math. But that's what it would seem it would need to require.)
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by khakipuce on Thursday September 04 2014, @11:45AM
I'd like someone to explain why time isn't just our perception of the propagation of "change" though the universe. I've asked this question of a number of people and on a number of forums over the years and never really got an answer.
The speed of light is constant, meaning a photon emitted by the sun does not instantly appear on the earth. Whilst the change in the universe due to the sun emitting a photon propagates towards us other change occurs more locally, such as me typing this. Hence we perceive "time" as having elapsed since we did things between the photon being emitted and it hitting my eye.
In this model time is not reversible since any attempt to undo a change is actually creating more change, not less.
(Score: 3, Informative) by geb on Thursday September 04 2014, @12:22PM
Just by moving around, you can experience a different rate of time compared to other observers around you. The effect is tiny in any normal human context, but it's not difficult to measure with an accurate enough clock. The rate at which you move through spacelike dimensions affects the rate at which you move through timelike dimensions. At the extremes, moving close to the speed of light means that you experience time very very slowly compared to everything else. This isn't just some vague guess - it can also be observed directly with any high energy particle. Unstable particles moving close to the speed of light decay at a much, much lower rate than the equivalent static particle, in accordance with the predictions of relativity.
Time isn't just a concept, it is demonstrably a dimension. It is to some degree interchangeable with the dimensions of space. The summary even mentions this.
The universe does not run on words, it runs on equations, so trying to describe things in terms of "change" isn't useful unless you can define it mathematically.
The mathematical framework describing time as one dimension of spacetime is known as Minkowski space: http://en.wikipedia.org/wiki/Minkowski_space [wikipedia.org]
(Score: 3, Interesting) by khakipuce on Friday September 05 2014, @07:12AM
As I understand it, your experience of time does not change however relative to a stationary observer (or one moving more slowly) time will be dilated - i.e. you will not age as much. Einstein asked the question "if I was traveling at the speed of light, would I be able to see my face in a mirror?" He concluded that he would if length and time dilated. This is the same as saying that the propagation of change takes longer.
(Score: 2, Interesting) by cubancigar11 on Thursday September 04 2014, @12:27PM
Change is time. Specifically, we have found out that a lot of unit changes can accumulate into a bigger change. For example, a lot of changes into the position of electrons and protons can be summed-up as The Sun, or The Earth, or The Universe.
There is no perception of change. Time is, literally, change. Imagine a universe where nothing is changing. No electron is moving etc. Everything is fixed. Entropy is 0. Then there is no time. And there is no way for you to know because in such a universe no light will travel into your eyes and there cannot be an observer.
Now, the reason time goes forward and not backwards has got something to do with entropy and 2nd law of thermodynamics. Any physicists here to explain further?
(Score: 2) by khakipuce on Friday September 05 2014, @07:17AM
I agree. The second law of thermodynamics says entropy always increases, you can't undo work. Which is what I meant when I said that trying to undo change just causes more change. Putting the photon energy back into the atom that released it requires more energy.
(Score: 2) by monster on Friday September 05 2014, @02:30PM
Actually, well...
Imagine the event horizon of a black hole. A particle-antiparticle pair gets generated. The antiparticle goes into the black hole, never to be seen again. Nearby, another pair gets generated with the same result. Now, the two newly created particles collide, releasing a photon, and drop into the black hole. Purely by chance, the photon travels into the atom that released your photon. Meanwhile, your photon goes into the blackhole. Would it be possible for your atom to end with the same energy as before, thereby undoing the work and reducing entropy?
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @03:13PM
Actually, from the point of view of the photon, it does instantly appear on Earth.
(Score: 2, Informative) by dak664 on Thursday September 04 2014, @06:38PM
"The speed of light is constant, meaning a photon emitted by the sun does not instantly appear on the earth. "
Actually that is exactly what it does, the apparent paradox resolved by the fact that all observers measure the same speed of light, even when they are all moving with different relative velocities. The math then says that reality can only involve the distance between events as measured by the length of 4 vectors (there being no absolute 4-origin as far as we know).
The 4-distance between the event "emitted by the Sun" and the event "appear on the Earth" is by definition zero for all observers. They are the same event and there are no events in between.
Poincare's insights came from this insight of differential geometry.
http://en.wikipedia.org/wiki/Henri_Poincar%C3%A9 [wikipedia.org]
(Score: 2) by HiThere on Thursday September 04 2014, @10:41PM
There are several ways of explaining this with different metaphysical significances, but with, AFAICT, no way of testing between them.
One answer is that the "present" is the only time that currently exists, and what we have as memories of the past are not causally linked to anything existing. In this interpretation "time" is the state of the universe resulting from the uncertainty in position of every quantum particle in it. This make it a "reasonable fiction" to talk about the past and the future. They don't exist, but the current state of the universe being uncertain they "might".
I don't like this one, but I think it's logically possible. This one seems to imply the Copenhagen interpretation of Quantum physiics.
Another answer is that the past and the future do, indeed, exist. Our memories and expectations for the results of probabilistic state transitions between them. This one seems to imply the EWG multi-world interpretation of quantum physics.
Etc. Note that there are around five basic flavor of answer, and each one corresponds to one of the interpretations of quantum physics. And there is no known way to choose between the interpretations, as the all make the exact same predictions.
I didn't get into psychological interpretations of time, as that isn't something I feel competent to deal with. (Even for physics it's a bit beyond my competence...but you did want an answer.)
P.S.: The model you mentioned is, indeed, reversible. From the photon's perspective it is emitted
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2, Funny) by segmentation on Thursday September 04 2014, @12:00PM
Why always the grandfather? Why not the grandmother? Or father? Or great-great grandfather? What has my grandfather ever done to you?
(Score: 1) by hendrikboom on Friday September 05 2014, @04:08AM
It should be your mother's mother. You're more likely to know for sure who she is.
(Score: 2) by mechanicjay on Thursday September 04 2014, @12:01PM
...your own grandpa, maybe you can become your own grandpa!
Though there is a song on the topic, which doesn't require time-travel, just a complicated situation where a dude marries a widow with a daughter. His widower Dad marries the daughter. Add a couple kids into the mix and he becomes his own grandpa. No incest or time travel needed.
This is the problem with scientists, always trying to do something theoretical with science, when a simple real world solution already exists! ;)
My VMS box beat up your Windows box.
(Score: 3, Funny) by WizardFusion on Thursday September 04 2014, @01:09PM
They already do something like this - it happens all the time with rednecks
(I have the mod points to burn)
(Score: 2) by Thexalon on Thursday September 04 2014, @01:55PM
Which of course presents a possible solution to the grandfather paradox: If your grandfather is killed, you have to take his place.
The only thing that stops a bad guy with a compiler is a good guy with a compiler.
(Score: 1) by jbWolf on Thursday September 04 2014, @06:47PM
I have to admit that I couldn't quite visualize everything the article was trying to say, but this visualization of my grandfather maybe being dead and maybe not being dead (because I'd kill him before I was born) does not make sense in the macro world. It doesn't fix the grandfather paradox. I think the difference is that the articles focuses on particles and not groups of particles.
For instance, when astronauts are accelerated in a rocket, space-time is dilated / contracted between them and us. We know that and have proved that. However, if one part of that astronaut (like a hand) is dilated / contracted differently than another part of the same person (like a leg), then that person would be ripped apart. Groups of particles vs a single particle. It makes a difference.
Of course, I'm no physicist and I could be completely wrong. The research they are doing with meta materials and invisibility is absolutely stunning and fascinating. By my reasoning, that kind of stuff should not exists and yet it does.
www.jb-wolf.com [jb-wolf.com]
(Score: 2) by digitalaudiorock on Thursday September 04 2014, @07:22PM
Homer Simpson: Oh, no! If Marge marries Artie, I'll never be born.
(Score: 2) by kaszz on Thursday September 04 2014, @10:41PM
So what happens if one travels far enough back in time and prevent your own existence?
(it may be enough to be curious and in the interaction make one of your parents in love with you instead of each other)
(Score: 0) by Anonymous Coward on Thursday September 04 2014, @11:46PM
just a blurb but one way to "fix" the paradox is to postulate a required "beacon".
so one can say that time travel is possible but only if a beacon has been placed.
thus one can return ONLY to the time that the beacon was activated (in the past).
it's not a "clean" solution because one could travel to the moment were the beacon was
activated in the past (from any time in the future) and shot the gun upon arrival.
at least it sounds like suicide (kill oneself) instead of murder (kill the grandfather) though.
maybe this postulate might give a clue on how the "activator"(human) and "beacon"(inanimate object)
need to be entangled?
(Score: 1) by SunTzuWarmaster on Friday September 05 2014, @07:44PM
Time Travel!
When do we want it?
It doesn't matter!
What do we want?
Time Travel!
When do we want it?
It doesn't matter!
(Score: 2) by EvilJim on Friday September 12 2014, @05:06AM
I think if I went back my grandfather would kick my arse. we're computer geeks these days, back then they did manual labour.