Beta
×

Welcome to the Slashdot Beta site -- learn more here. Use the link in the footer or click here to return to the Classic version of Slashdot.

Thank you!

Before you choose to head back to the Classic look of the site, we'd appreciate it if you share your thoughts on the Beta; your feedback is what drives our ongoing development.

Beta is different and we value you taking the time to try it out. Please take a look at the changes we've made in Beta and  learn more about it. Thanks for reading, and for making the site better!

The Possibility of Paradox-Free Time Travel

kdawson posted about 4 years ago | from the fire-up-the-delorean dept.

Science 421

relliker writes in with word of a paper up on the ArXiv by Seth Lloyd and co-workers, exploring the possibility that "postselection" effects in non-linear quantum mechanics might allow paradox-free time travel. "Lloyd's time machine gets around [the grandfather paradox] because of the probabilistic nature of quantum mechanics: anything that this time machine allows can also happen with finite probability anyway... Another interesting feature of this machine is that it does not require any of the distortions of spacetime that traditional time machines rely on. In these, the fabric of spacetime has to be ruthlessly twisted in a way that allows the time travel to occur. ... Postselection can only occur if quantum mechanics is nonlinear, something that seems possible in theory but has never been observed in practice. All the evidence so far is that quantum mechanics is linear. In fact some theorists propose that the seemingly impossible things that postselection allows is a kind of proof that quantum mechanics must be linear."

cancel ×

421 comments

Sorry! There are no comments related to the filter you selected.

First Post? (-1, Offtopic)

Anonymous Coward | about 4 years ago | (#33022408)

Has nobody anything snarky to say?

Re:First Post? (5, Funny)

maxwell demon (590494) | about 4 years ago | (#33022552)

Has nobody anything snarky to say?

They are too busy trying to build postselection time machines. Expect to lose your first post status as soon as one of them succeeds.

Re:First Post? (1)

phantomfive (622387) | about 4 years ago | (#33022790)

First post!!

Also, last post. Try to figure that one out!

Re:First Post? (1)

maxwell demon (590494) | about 4 years ago | (#33022848)

First post!!

Also, last post. Try to figure that one out!

Simple: Only post.

Re:First Post? (1)

icannotthinkofaname (1480543) | about 4 years ago | (#33022908)

Expect to lose your first post status as soon as one of them succeeds.

"The Possibility of Paradox-Free Time Travel"

It would be a paradox if someone other than that AC were to get first post. Obviously, the only reason he got first post to begin with is because he already has a postselection time machine. Or at least, he must acquire one at some point in time, and uses it to make FP on this story.

Also, brb, building a TARDIS.

Re:First Post? (1)

gmuslera (3436) | about 4 years ago | (#33023254)

Won't be a paradox...Another AC will post it,with the very same words, while the 1st one will die in a very improbable, Final Destination style accident. Thats what the universe have prepared for you if you do first posts as AC in Slashdot.

Satruday Morning Breakfast cereal Anticipated this (5, Interesting)

goombah99 (560566) | about 4 years ago | (#33023080)

Yesterdays SMBC had a good point:

You can not have any motivation or objective if you are going to travel otherwise the act of time travel is a paradox. SMBC put it thusly: if you are travelling to change some outcome, and you succeeded, you would not have had the motivation to time travel to make that change.

SMBC's conclusion was that only nitwits have the capacity to time travel and the fact that there seem to be so many confirms that time travel must be going on right now.

But another way to say this is, you can only choose objectives that either already happened in your past or are inevitable no matter what you do.

For example, You could however travel with the objective of sinking the Titanic, but not the objective of preventing the sinking. If you saved the titanic, it would never occur to you to try to save the titanic.

For example, If your objective was to save Abe Lincoln and you succeeded, then it never would have occurred to your pre-travel self that you needed to go back and save abe lincoln.

What all this adds up to I think is that time travel is still forbidden but observational time travel-- gathering information-- is not forbidden.

THere is an interesting proof regarding the computability of any proposition by David Wolpert that shows time travel is forbidden unless the information you gain by doing so is probabilistic or faulty. That is he proves rigorously that it is not possible to answer any arbitrary true/false question about the past with perfect fidelity. Thus time travel that preserves information with fidelity is forbidden. Error prone time travel is however allowed.

Primer, the Movie (1)

goombah99 (560566) | about 4 years ago | (#33023156)

By the way I forgot to mention that there is a known way to Time travel without violating the grandfather paradox or using post selection. It was the basis for the movie primer, and it actually occurs, mathematically at least, for small particles.

The observation is that when a photon splits into a particle+antiparticle and then later those anihilate you can think of this as a particle going forward in time and another particle going backward in time, whose trajectories intersect at the beginning and the end.

Thus it is possible to travel forward in time as long as the person in the future decides to travel backward in time.

THe movie Primer created a machine that ensured that in a way that you could not avoid this. Thus, as far as I know, it's the only time travel movie that actually is technically possible and devoid of grandfather problems.

By the way if you rent this, plan on watching it twice. Then again a few days later after you google the shit out of all the things that you did not understand. Let me just tell you that EVERYTHING in this movie, no matter how little sense it makes to you, actually makes perfect sense. But the movie does not explain itself.

Re:Satruday Morning Breakfast cereal Anticipated t (2, Interesting)

ceraphis (1611217) | about 4 years ago | (#33023224)

Yes, but what if with the act of time travel with motivation you also create an alternate reality where the titanic didn't sink or abe lincoln survived? Is there some rule where you must be attached to your original timeline?

Time Cube? (5, Funny)

Lythrdskynrd (1823332) | about 4 years ago | (#33022414)

http://timecube.com/ [timecube.com] ... obviously.

Re:Time Cube? (1)

JohnRoss1968 (574825) | about 4 years ago | (#33022642)

This is truly truly sad.
This guy needs serious medications

Re:Time Cube? (0, Offtopic)

JohnRoss1968 (574825) | about 4 years ago | (#33022680)

After reading just a little of that page I feel the need to wash my computer.
and my self...
and my soul
come to think of it my cat is in need of a bath.
Hell while i'm at it maybe I should wash the car.
i take it back...
after reading 1 page of that site I NEED SERIOUS MEDICATION.

Re:Time Cube? (2, Insightful)

J.J. Dane (1562629) | about 4 years ago | (#33022874)

"after reading 1 page of that site I NEED SERIOUS MEDICATION"

Call the webmaster, you can probably get a pretty good deal since he's obviously not using his..

Re:Time Cube? (0)

Anonymous Coward | about 4 years ago | (#33022706)

While posted for fun, there could possibly be some truth to it.

For a moment, let us simplify reality so that there is only two particles in existence, void as space, and reality is contained within a Tesseract
Then we assign both particles the name of a and b to track them through 4D space.
The 4th spacial dimension of the tesseract could be the rate of change in the universe (time), and the movements through this "tesseract" are interpreted as time.

Of course, this could just be the wooziness caused by these pain killers i'm on.
Still interesting to think about. Ruling an idea out because it sounds crazy is crazy. Quantum mechanics is the craziest thing humans have come up with, well, outside of Steve Ballmer.

Re:Time Cube? (0)

Anonymous Coward | about 4 years ago | (#33022894)

Yes, but Slashdot is EDUCATED EVIL

Re:Time Cube? (4, Funny)

hvm2hvm (1208954) | about 4 years ago | (#33022886)

fuck, it looks like a neural network with a few weeks of training on conspiracy theory material has been put on repeat

Re:Time Cube? (1)

bdenton42 (1313735) | about 4 years ago | (#33022912)

I like the wallpaper.

Ohh, I see (0)

Anonymous Coward | about 4 years ago | (#33022430)

No, wait, what?

No need to read the article (1)

JimboFBX (1097277) | about 4 years ago | (#33022440)

I learned everything about this already from Futurama

Re:No need to read the article (1)

WarpedCore (1255156) | about 4 years ago | (#33022468)

Yeah, don't put metal in the microwave while observing a super nova.

Re:No need to read the article (1)

JWSmythe (446288) | about 4 years ago | (#33023268)

    Which is almost as good advice as "don't observe metal while putting a super nova in the microwave"

And Back to the Future. (5, Insightful)

khasim (1285) | about 4 years ago | (#33022518)

The problem with all of those approaches is that they assume a "meta-time" (even if not stated as such) that will alter the PRESENT based upon changes in the FUTURE.

That's how a photograph that you have right (taken in the future) now will change based upon events that have not happened yet.

Once you get past that, you understand that there is no "grandfather paradox". If it exists in the current time then it exists in the current time. The future will not reach back and "clean up" the present to make it more acceptable to the future.

Obligatory cartoon linkage:
http://www.smbc-comics.com/ [smbc-comics.com]

Re:And Back to the Future. (4, Insightful)

Schadrach (1042952) | about 4 years ago | (#33022752)

So, what you are suggesting in the case of the grandfather paradox is that you kill your grandfather, are never born, and yet continue to exist unscathed? Essentially being a causeless effect, or rather an effect that causes it's cause never to have occurred?

Essentially the opposite of a closed temporal loop where something is it's own cause.

Of course the SMBC leaves out the third possibility: You go back in time and only change things that were unintended, causing you to not notice any changes because they "were always like that" as of the moment you made said changes. But that of course assumes that changing something that would effect you in some way actually does effect you, and not cause you to live without ever being born (for example).

Re:And Back to the Future. (5, Insightful)

Cylix (55374) | about 4 years ago | (#33022864)

The post selection method would be...

You killed the man you thought to be your grand father, but it turns out you are from an illicit relationship. Your grand mother quickly remarries and the man assumed all the roles the other fellow would have done.

That version of the grand father post selection paradox can go soap opera silly really fast. It would get really strange if everyone you kept killing in your family tree resulted in discovering that each generation was conceived in a series of illicit relationships. Take that days of our lives!

Pretty much, yep. (3, Interesting)

khasim (1285) | about 4 years ago | (#33022904)

Essentially being a causeless effect, or rather an effect that causes it's cause never to have occurred?

Pretty much, yep. Because you are now existing at a prior point in the chain of causality. So you've already accepted either:
1. circular causality (and a supreme janitor who cleans up the past to keep the future tidy) (and how would you tell the difference) or

2. effect without cause (because you exist prior to your parents giving birth to you).

Re:No need to read the article (0)

Anonymous Coward | about 4 years ago | (#33022544)

I know XKCD is traditional, but today's SMBC [smbc-comics.com] is obligatory.

Re:No need to read the article (1)

MichaeLuke (50412) | about 4 years ago | (#33022942)

Anyone who read Superman comics in the sixties understands that you can go back in time and change things, but the results are always the same. Or, as Stewart Brand and David Byrne would say, the future wants to be the same as it ever was.

The Paradox of Possibility-Free Time Travel? (1)

Doc Ruby (173196) | about 4 years ago | (#33022450)

One of these quantum universes has to have every quantum event probability = 0%, and one p=100%. Those two together are the paradox of possibility-free time travel. In one, there's no chance of free will. In the other, a time machine is certain.

Re:The Paradox of Possibility-Free Time Travel? (1)

maxwell demon (590494) | about 4 years ago | (#33022536)

One of these quantum universes has to have every quantum event probability = 0%, and one p=100%.

No. That would violate the laws of quantum mechanics. Show me a measurement event which has 0% probability and another one having 100% probability where the two are not separated by a superselection rule, and I readily show you a measurement event on the same system which has 50% probability.

Re:The Paradox of Possibility-Free Time Travel? (1)

Doc Ruby (173196) | about 4 years ago | (#33022602)

OK. I just flipped a coin. It came up heads. Its coming up tails is now 0% probability. Its coming up heads is now 100% probability. As a coin I used the spin of a photon.

Re:The Paradox of Possibility-Free Time Travel? (1)

maxwell demon (590494) | about 4 years ago | (#33022818)

The spin of a photon is better known as its polarization. To get a heads/tails result, you had to choose a direction to measure the spin on. According to your claim, the photon is still there, so you managed a demolition-free polarization measurement, so the photon now actually has that polarization (i.e. if you repeat the same measurement again, you get a 100% probability, while measuring the opposite polarization you get a 0% probability). Ok, here's one way how you get a 50% probability:

  • If you measured a circular polarization, then now measure a linear polarization (any linear polarization will do).
  • If you measured linear polarization, measure linear polarization at an angle of 45 degrees to your original linear polarization measurement.
  • If you measured elliptical polarization, choose linear polarization at 45 degrees to the main axes of the corresponding ellipse.

Note that these cases cover all possibilties.

Re:The Paradox of Possibility-Free Time Travel? (1)

Artraze (600366) | about 4 years ago | (#33022696)

The idea of free will v.s paradox free time travel, and really all time travel paradoxes in general, is built upon the notion that the universe is causal. That seems like a pretty basic and safe assumption, but we are making it based how things seem, not direct observations and experiments. If our technology gets to the point where we're manipulating space and time, than we would be in a much better position to know. For now though, all we 'know' is time travel doesn't exist and the universe is causal. If you don't assume the former, there's no real reason to assume the latter either.

Sign me up! (1)

EWAdams (953502) | about 4 years ago | (#33022452)

But if I'm going BACK in time, I'm taking some aspirin, toothpaste, deodorant, and toilet paper with me. I hope the machine is big enough.

Re:Sign me up! (5, Funny)

ThePangolino (1756190) | about 4 years ago | (#33022884)

But if I'm going BACK in time, I'm taking some aspirin, toothpaste, deodorant, and toilet paper with me. I hope the machine is big enough.

Don't forget your towel you insensitive clod!

Quantum mechanics is WEIRD. (0)

Anonymous Coward | about 4 years ago | (#33022462)

> However, if nonlinear behaviour is allowed, time travel will be possible wherever it takes place.

This seems obvious, but if you think about it, it starts to make a different, twisted kind of sense...

Just like magic (-1, Offtopic)

Cathoderoytube (1088737) | about 4 years ago | (#33022466)

Wow Kdawson did you choose that red colour all by yourself? This surely is big important news. Seeing as time travel is possibly the most pressing scientific issue of the day. Plus the inclusion of non specific aspects of quantum mechanics really lends a sense credibility to the theory.

I used quantum mechanics to type this message out.

Re:Just like magic (1, Informative)

Anonymous Coward | about 4 years ago | (#33022516)

If you see it without comments it'll be red.

Re:Just like magic (1)

maxwell demon (590494) | about 4 years ago | (#33022582)

There's no red. Probably you've seen a summary from the mysterious future. Post-selected time travel has been used successfully on Slashdot for quite some time now. :-)

Does this mean... (0)

Anonymous Coward | about 4 years ago | (#33022474)

I can go back and see where the hell I left my keys?

Re:Does this mean... (2, Funny)

maxwell demon (590494) | about 4 years ago | (#33022586)

Yes, but the keys will be postselected to be in the worst case possible.

Caution about ArXiv (4, Informative)

vsage3 (718267) | about 4 years ago | (#33022482)

I have neither the capacity nor the will to vet the paper, but it should be noted that ArXiv is not peer reviewed. While experimentalists use it as a place to publish pre-prints of their papers and will typically only put them up after the papers have been accepted, but theorists use the medium as a substitute for publishing and so many wacky and untrue claims get put up there.

Re:Caution about ArXiv (2, Informative)

Anonymous Coward | about 4 years ago | (#33023116)

"...many wacky and untrue claims get put up there."

This is demonstrably untrue. Since the mid-90s ArXiv has been the standard way in which theorists communicate their papers to a wider audience. Although you're technically correct that it's not peer reviewed in the traditional sense, it does have quite a strict authentication and author endorsement procedure that filters out 95% of the garbage that would appear there if it were open to all. In addition, reading a paper's abstract and looking at the names and institutions of the authors is enough to determine whether a paper is obvious garbage. Reading it and going through the arguments is then enough to determine its value conclusively. The paper in question passes all of those initial tests easily.

Really, ArXiv works astonishingly well and is an excellent resource.

Time travel leads to Parallel universes that make (4, Insightful)

Joe The Dragon (967727) | about 4 years ago | (#33022504)

Time travel leads to Parallel universes that make paradox not happen in the one you left.

It's the Dark Side (5, Funny)

greyworld (802114) | about 4 years ago | (#33022512)

Am I the only person who has noticed the Authors name? Seth Lloyd = Sith Lord I think we should be very cautious of these findings.

Re:It's the Dark Side (1)

rubycodez (864176) | about 4 years ago | (#33022996)

and at least one of the others must be watched with caution, as sith lords always appear in pairs.

Re:It's the Dark Side (1)

OrangeCatholic (1495411) | about 4 years ago | (#33023072)

You should be cautious because he's a Mech E. If his CS and physics ideas had any merit, he'd switch departments.

Dress it up! (5, Insightful)

Beelzebud (1361137) | about 4 years ago | (#33022520)

You can dress up pseudoscience with a bunch of equations, but tell me how this is based on any type of actual science. If this is science, then Deepak Chopra must be an actual genius...

I'm not an expert but it does seem like a lot of physicists are just lost in their own little worlds. I realize science is a process, but spending valuable time "researching" time travel, before we can even explain what time or even gravity is, seems like skipping over the hard work to spend time on "fun stuff".

Re:Dress it up! (1)

noidentity (188756) | about 4 years ago | (#33022672)

The concept of time travel itself shows misunderstanding of what time is. It basically posits an extra fifth dimension that's the "real" time, and treats our time as a dimension just like one of the three spatial dimensions. At that point, you could then talk of meta-time-travel, and so on.

Re:Dress it up! (3, Interesting)

maxwell demon (590494) | about 4 years ago | (#33022910)

The concept of time travel itself shows misunderstanding of what time is. It basically posits an extra fifth dimension that's the "real" time, and treats our time as a dimension just like one of the three spatial dimensions.

No. That may be a common misconception in SF time travel, but it's not the essence of the idea of time travel. The misconception which leads to this "hidden meta-time" is the idea of an universal, absolute time. But for our real universe, we already know since Einstein that there's no absolute time, and two different observers may disagree about how much time was between two events, and may even disagree about the temporal order of causally unrelated events. Time travel is nothing than a logical extension of this concept, where observers also disagree about the temporal order of causally related events. There's absolutely no meta-time needed.

Re:Dress it up! (1)

OrangeCatholic (1495411) | about 4 years ago | (#33023132)

>and two different observers may disagree about how much time was between two events, and may even disagree about the temporal order of causally unrelated events.

How did you get from time being stretchy to it being able to invert itself?

Time is a yardstick used to measure the distance between events. If the events are practically simultaneous, time is zero. Photons, for example, always have time set to zero, because photons do everything instantly. Distance, likewise, can also be near zero, if two objects are right on top of each other.

But you can't have negative time any more than you can have negative distance. You can walk backwards and call it negative, but from God's eye it still looks like a positive translation from one place to another.

Re:Dress it up! (2, Interesting)

maxwell demon (590494) | about 4 years ago | (#33022718)

I don't think the authors expect to see post-selection time machines any time soon. However such work may enable us to understand better the structure of quantum mechanics. By exploring what is and isn't possible in theory, we get guidelines in which directions it makes sense to look for adaptions of quantum theory (e.g. for quantum gravity), and which directions are better avoided.

Re:Dress it up! (1)

Artraze (600366) | about 4 years ago | (#33022740)

Exactly.

From TFS:
>In fact some theorists propose that the seemingly impossible things that postselection allows
>is a kind of proof that quantum mechanics must be linear.

Work like this is not meant to be practical; it's a thought exercise. It provides a sort of preview of different things that may or may not be possible, and can inspire other scientists. For example (given the above context), it may generate a testable idea which can then be used to disprove non-linear quantum mechanics.

Pseudoscience or not, it's still interesting (3, Insightful)

impaledsunset (1337701) | about 4 years ago | (#33023142)

I'm completely with the parent.

While I don't think that you can call pseudoscience the exploration of the implications of a theory that are unlikely to be possible in practice, let's assume it's pseudoscience. So what? It was pretty interesting read, at worst it will serve as an inspiration to some science fiction author. It's interesting. You know why? Because it is "fun stuff"! Also, as the parent stated, exploring the theoretical possibilities provides better understanding of the model, allows you to improve the model and might allow you to find the boundaries where the model stops being correct. Also, making advances in the part that don't apply in practise might improve the understanding of the practical part of the model. Infinitesimals don't seem to exist in our universe, but the models that explore their properties closely have been the basis for most of the physics.

Also, I might not understand the article completely, but it, along with another report that was posted to Slashdot less than an year ago, seems to show a method of time "travel" that doesn't allow to send information back in time at all. Seems reasonable, and not against anything that I know about the world I'm living in. It would completely blow out my idea of time -- I'm a firm believer that only the current moment exists, and you can't affect or travel to previous ones, and that other interpretations of time are merely implementation details of the physical models we use -- but these results wouldn't be against any physics I know. Also, even if there is an experiment that confirms that this paper isn't bullshit, and it is empirically proven that this kind of time "travel" is possible, the results won't have a single interpretation. I wouldn't be surprised even if someone builds another model that doesn't involve any time travel that explains the same empirical results.

Yes, someone needs to verify the premises and the conclusions, I don't have good enough knowledge to do that myself on the first read, but I didn't see any mistakes pointed out by the GP, only baseless claims, so I'd rather go with the article and/or the paper. I have a direct question for the GP: We have no idea what time is, OK. Suppose that our current theoretical model allows for time travel (which would seem to be the case unless the article is full of mistakes). Are you denying that testing them would allow us to be closer to understanding what time is?

Re:Dress it up! (5, Insightful)

Lifyre (960576) | about 4 years ago | (#33022726)

Having been part of the physics community for a few years (got a BS in it for some reason) I can say those little worlds often result in some useful science. Usually when someone not in the fantasy land looks in sees that one or two things that guy is working on might have merit and looks into it. Who knows if one of these guys working on time travel might actually figure out what time is? If we don't know what time or gravity even is who is to say that this work might not be instrumental in figuring it out? As long as it is a minority working on the fantastical, science will still make progress with a few boosts here and there by some crazy idea that actually works.

Re:Dress it up! (1, Interesting)

Anonymous Coward | about 4 years ago | (#33022732)

but spending valuable time "researching" time travel, before we can even explain what time or even gravity is, seems like skipping over the hard work

Except that most of the progress in science involve poking at theories around the edges, and seeing where it unravels.

I imagine that, if you were in 1900, you would be criticizing Einstein "Albert, it's no use day-dreaming about what would happen if you could ride a beam of light! We don't know what light is made of, let alone how you would attach a seat to it!"

The point is, these sorts of crazy thought experiments allow you to see where the theories are thin or fall apart. Hell - it's in TFS. If this scientist is correct, this sort of time-travel holds implications about how the world behaves, but only if quantum mechanics is non-linear. Unless I'm mistaken, what will happen is a bunch of researchers will poke and prod at the theory, and come up with a simple, doable experiment (almost certainly *not* involving time travel in any sort of "Back to the Future" sense, and probably not involving anything bigger than a proton). "If results are A, we prove that QM is non-linear. If results are B, we can conclude that it *is* linear." Anytime theory departs from intuition, it's a great time to test the theory. e.g. the double split experiment: "Quantum mechanics seems to require that the particle go through both slits simultaneously! That's nonsense! I'll just do an experiment to show those quantum mechanics people how nature *really* works ..."

The ultimate result of Lloyd's work isn't going to be an H.G. Wells-style time machine, it'll be a new, better understanding of the nature of physics.

Re:Dress it up! (1)

maxwell demon (590494) | about 4 years ago | (#33022934)

"If results are A, we prove that QM is non-linear. If results are B, we can conclude that it *is* linear."

More like: "If results are B, we know that any possible nonlinearity in QM will be less than (small value)".

Re:Dress it up! (1)

whiplashx (837931) | about 4 years ago | (#33022822)

Yes, its valuable to research the hard, practical stuff. But come on, do you really want to live in a world where no one explores the interesting possibilities?

This post strikes me as narcissistic and pessimistic. Reminds me of those wise words, "If man was meant to fly, God would have given him wings."

Re:Dress it up! (1)

garompeta (1068578) | about 4 years ago | (#33023206)

You must be new in science. About a century ago our science used to believe in flogiston. About two or three centuries ago, we used to believe that we were in the center of the universe. About three or four centuries ago we believed in homunculus growing out from semen. Science is just a slow process that filters out the options.

Dressing up Particle Man! (0)

Anonymous Coward | about 4 years ago | (#33023240)

You can dress up pseudoscience with a bunch of equations, but tell me how this is based on any type of actual science. If this is science, then Deepak Chopra must be an actual genius...

The part about particles should help. In other words if particles could do what the paper says, then the question should be, what would we observe if true?

So, as Dawkins would say... (0)

Anonymous Coward | about 4 years ago | (#33022554)

Where are all the time travelers?

Re:So, as Dawkins would say... (1)

maxwell demon (590494) | about 4 years ago | (#33022736)

They got eaten by dinosaurs.

Primer (5, Interesting)

kylben (1008989) | about 4 years ago | (#33022614)

The movie "Primer" had an interesting take on avoiding paradoxes. http://video.google.com/videoplay?docid=3909854615539675694# [google.com] (entire movie online)

Re:Primer (1)

whiplashx (837931) | about 4 years ago | (#33022854)

I do think that movie had a good solid scientific background, except for the part where the paradoxes started to affect the character's health. I don't see any evidence to the contrary, but its kind of an odd leap to make.

Re:Primer (1)

Angry Rooster (972166) | about 4 years ago | (#33022976)

I don't think it was the paradoxes that were causing the health problems. My impression was that it was a side effect of their method of time travel. The farther back they go (like when trying to fix problems, which also happens to be when they would be most likely to introduce paradoxes) the higher the level of degradation. - Rooster

Re:Primer (1)

swb (14022) | about 4 years ago | (#33023028)

That was a great movie, but difficult to keep track of the time travel.

I don't think they ended up with specific paradoxes, but I seem to recall a lot of attempts to fix things by going further and further back; perhaps not explicit paradoxes, but a lot of manipulation of the past to try to impact the future.

Re:Primer (1)

impaledsunset (1337701) | about 4 years ago | (#33023176)

The main thing I learned about time travel from Primer is that it leads to headaches when it gets involved. I'm really hoping I'll never be one of the people who are supposed to comprehend what's happening should we invent it.

Or... (2, Insightful)

haystor (102186) | about 4 years ago | (#33022624)

Or, we go with the simple, elegant solution to the problem...it's not possible.

the other angle (1)

Lord Ender (156273) | about 4 years ago | (#33022644)

Did he prove that paradox-free time travel is possible thanks to possibility that quantum selection is non-linear, or did he prove by contradiction that quantum selection is linear?

Re:the other angle (2, Informative)

maxwell demon (590494) | about 4 years ago | (#33023002)

Neither. Assuming he's right (and the Technology Review article correctly reproduced his claims; I haven't actually read the arXive article yet), he proved that you cannot have non-linear quantum mechanics without time travel. Given that some people try to resolve the measurement problem by adding nonlinearities, that's certainly an interesting result.

Re:the other angle (3, Interesting)

icannotthinkofaname (1480543) | about 4 years ago | (#33023052)

I think it's the first one, but only because I'm pretty sure it can be rephrased thus:

"He proved that paradox-free time travel is possible through postselection of quantum teleportation by postselecting the condition that quantum mechanics is non-linear."

However, I believe this phrasing assumes that the probability of quantum mechanics being non-linear is nonzero, so if I just divided by zero, I apologize.

Finite Probability (3, Funny)

fahrbot-bot (874524) | about 4 years ago | (#33022656)

...anything that this time machine allows can also happen with finite probability anyway.

Now, if we can just hook in the logic circuits of a Bambleweeny 57 Sub-Meson Brain to an atomic vector plotter suspended in a strong Brownian Motion producer (say a nice hot cup of tea)...

Huh ? (1)

lbalbalba (526209) | about 4 years ago | (#33022658)

Well you lost me right after 'the grandfather paradox'. I even read the article and I *still* don't understand. Is there a summary for dummies ?

Re:Huh ? (2, Informative)

CODiNE (27417) | about 4 years ago | (#33022978)

Grandfather paradox: Go back in time, shoot your grandfather... now your father wasn't born, you weren't born. Which means your grandfather doesn't get shot, so you get to be born, etc... the universe flip-flops your time travel forever, the record is skipping for eternity.

I personally like Primer and it's time machine concept. You could go back to yesterday and kill yourself, that version of you would die but you would not disappear. Also if you kept the yesterday you from going in the time machine you now have 2 of yourself that you're stuck with. No paradoxes, it runs more a computer program or a flowchart.

Grandfather paradox requires a sort of feedback loop in time where the universe seemingly gets stuck in a cycle until things resolve themselves.

Re:Huh ? (1)

lbalbalba (526209) | about 4 years ago | (#33023104)

Yeah, thanks... I got the 'grandfather paradox', and even the concept of the fact that you doing something in the past will not retro-actively change the future as well (paradox free time travel). I just failed on all the 'quantum mechanics' and 'postselection' etc. stuff... That is, I understand the concepts that make this work, just not the individual mechanisms that make up the solution.

Re:Huh ? (1)

maxwell demon (590494) | about 4 years ago | (#33023066)

If you use quantum mechanics to combine a nondeterministic Turing machine with a teleportation device, you get a time machine. However you still can't correct the past.

Other issues exist (3, Interesting)

AnAdventurer (1548515) | about 4 years ago | (#33022704)

And if you travel outside of your light-cone? (other then math breaking down)

We know we know.... (1)

deepershade (994429) | about 4 years ago | (#33022748)

It's all down to the doom field.

Quantum Mech. is a Sexy but Deceptive Siren (3, Funny)

Tablizer (95088) | about 4 years ago | (#33022768)

Quantum mechanics is a big tease. It seems whenever it's about to give you Jetsons or Stargate technology, there's always a big fucking loophole or caveat. You can go into the past, but you can't come back and/or die; you can travel faster than light, but the universe will end before you reach your destination; you can predict the future, but will change it in the process without knowing what the change is; you can date 3-breasted aliens, but they all have penises, or whatever. (Okay, I made up the last one.)

There must be a God, because nature wouldn't find a way to tease us with so many Almost's and fuck with our minds in so many different ways that QM does.

Or maybe it's the anthropic principle keeping us from destroying the universe with time weapons?

Something odd is going on. Time for a congressional investigation.

Re:Quantum Mech. is a Sexy but Deceptive Siren (1)

Gorobei (127755) | about 4 years ago | (#33023030)

Sheesh, you get semiconductors, lasers, quantum bomb detectors and god knows what else, and you want a stargate right now?

Re:Quantum Mech. is a Sexy but Deceptive Siren (1)

Tablizer (95088) | about 4 years ago | (#33023138)

Similar devices may have worked in a Newtonian model as well. But my complaint is the repeated almost-ness of QM. If it was a clear-cut "no", it may be easier to live with. With questions such as, "Can you go faster than light?", "Can you travel back in time?", "Can you predict/see the future?", the answer is usually "Probably, but.....".

Re:Quantum Mech. is a Sexy but Deceptive Siren (0)

Anonymous Coward | about 4 years ago | (#33023270)

You are mistaking Hollywood's view of reality with, well, reality. In Hollywood, there's almost always a consequence to every super action. You can travel in time, but one slip and you'll erase the future. Superman can fly and see through walls, but there's this green rock that completely neutralizes all his powers. Green Lantern has issues with yellow, etc etc etc.

This attempt at balance pervades pop culture not because it's real but because these things make for better, more fun stories. They provide a convenient antidote for "god mode" power, whatever it may be. Because being a god is really rather dull. Every god needs an Achilles heel to be interesting.

But nature and the universe don't know about Hollywood twists or comic book powers, or anything about heels. And the universe doesn't care either.

If time travel exists, it will be (or has been) possible without artificial consequences. Exposure to exotic radiation is likely the biggest problem. The human body is not designed for much outside of our little comfort zone here. We need lots of air and food and water and meds, and flush toilets, and booze, and TV, but still die early and easily. We are not made to be physical explorers of the hostile universe.

Side note: writers don't think much about this, but if you gain the power of time travel, you also gain the power of space travel because time and space are the same, really. So once you gain the power to go back and see Lincoln get shot, you also could be anywhere else in the universe, any when. With that power and ability, why would you care about Lincoln? The most important thing to ever happen to the human race, aside from existing at all, would be the moment the first person uses that power and ability to go somewhere else. There would be little reason to ever come back.

Dr. Who can do this kind of thing, but he hangs out in England a lot often in the odd rock quarry. But that's only because it's cheaper to film the show, not because a real Dr. Who would actually bother with some rock quarry somewhere in England. Ah yes, Dr. Who has his convenient weakness too, his own kryptonite. His machine is barely controlled, he dies a lot, and a lot of things want to kill him (but nearly always fail). Entertaining, yes. Amy Pond, yes. Realistic time travel? No.

Hitchhiker's Guide (0)

Anonymous Coward | about 4 years ago | (#33022770)

Kinda seems like the Infinite Improbability Drive

Maybe they mean this... (2, Interesting)

bar-agent (698856) | about 4 years ago | (#33022772)

As far as I can tell, the article is saying that if you impose a condition in the present, you cause the past to change so that it matches. This process of imposing a condition must affect the quantum mechanical properties of whatever you are checking, similar to a quantum computer.

So basically, if your granddad rigs up a machine that kills him depending on the quantum state of a particle, and then he leaves that particle in an indeterminate quantum state until he has your dad and your dad has you, and then you collapse that particle's waveform into the state that would have killed him, he will have died back then. And somehow paradox is avoided.

Wha?

idea of time travel (3, Interesting)

phantomfive (622387) | about 4 years ago | (#33022860)

I can accept FTL travel as maybe possible, but time travel seems farfetched to me. It means that every single state that the universe has ever been in is preserved (somewhere) in it's exact state. We're not talking about the awareness of the state being preserved on the speed-of-light boundary away from the location of the state, it's the actual state, in a way that can be modified and changed. Does this even seem reasonable? How could all that be stored?

Not only that, it means that a change in one of those states will instantly change every subsequent state. So when you travel back, everything will be different. This is really hard to believe.

Re:idea of time travel (2, Informative)

maxwell demon (590494) | about 4 years ago | (#33023216)

I can accept FTL travel as maybe possible, but time travel seems farfetched to me.

You can't have one without the other.

Re:idea of time travel (1)

phantomfive (622387) | about 4 years ago | (#33023230)

Why? By current physics, you can't have either. FTL travel could possibly come up in the future as we come to a better understanding of physics. My argument was that we will never come to time travel, no matter how our understanding of physics changes.

Re:idea of time travel (2, Interesting)

OrangeCatholic (1495411) | about 4 years ago | (#33023244)

>Does this even seem reasonable? How could all that be stored?

Yep. People who believe in parallel universes don't seem to comprehend the vast amount of data that would need to be stored to make such a thing possible. Every electron twitch - boom, new universe, with all of its state intact, loaded into a new memory location far away from the previous one.

I guess with time travel they are saying all the previous states are still stored. Pretty close to the same thing. You would need a whole other universe whose job is to calculate every combination of ours.

The universe is big, but not arbitrarily powerful where you can just ascribe all sorts of amazing abilities, like the ability to remember every state it's ever been in. Too much.

we're already moving through time at light speed (0)

Anonymous Coward | about 4 years ago | (#33022866)

because most of us refuse to 'see' what's going on around us, we continue to 'wait' for the right 'time', or some way to get there. the change is not exactly seamless.

meanwhile (there's always one (while) somewhere); the main distraction to unimpeded 'travel', the corepirate nazi illuminati, is always hunting that patch of red on almost everyones' neck. if they cannot find yours (greed, fear ego etc...) then you can go starve. that's their (slippery/slimy) 'platform' now. see also: http://en.wikipedia.org/wiki/Antisocial_personality_disorder

never a better time to consult with/trust in our creators. the lights are coming up rapidly all over now. see you there?

greed, fear & ego (in any order) are unprecedented evile's primary weapons. those, along with deception & coercion, helps most of us remain (unwittingly?) dependent on its' life0cidal hired goons' agenda. most of our dwindling resources are being squandered on the 'wars', & continuation of the billionerrors stock markup FraUD/pyramid schemes. nobody ever mentions the real long term costs of those debacles in both life & any notion of prosperity for us, or our children. not to mention the abuse of the consciences of those of us who still have one, & the terminal damage to our atmosphere (see also: manufactured 'weather', hot etc...). see you on the other side of it? the lights are coming up all over now. the fairytail is winding down now. let your conscience be your guide. you can be more helpful than you might have imagined. we now have some choices. meanwhile; don't forget to get a little more oxygen on your brain, & look up in the sky from time to time, starting early in the day. there's lots going on up there.

"The current rate of extinction is around 10 to 100 times the usual background level, and has been elevated above the background level since the Pleistocene. The current extinction rate is more rapid than in any other extinction event in earth history, and 50% of species could be extinct by the end of this century. While the role of humans is unclear in the longer-term extinction pattern, it is clear that factors such as deforestation, habitat destruction, hunting, the introduction of non-native species, pollution and climate change have reduced biodiversity profoundly.' (wiki)

"I think the bottom line is, what kind of a world do you want to leave for your children," Andrew Smith, a professor in the Arizona State University School of Life Sciences, said in a telephone interview. "How impoverished we would be if we lost 25 percent of the world's mammals," said Smith, one of more than 100 co-authors of the report. "Within our lifetime hundreds of species could be lost as a result of our own actions, a frightening sign of what is happening to the ecosystems where they live," added Julia Marton-Lefevre, IUCN director general. "We must now set clear targets for the future to reverse this trend to ensure that our enduring legacy is not to wipe out many of our closest relatives."--

"The wealth of the universe is for me. Every thing is explicable and practical for me .... I am defeated all the time; yet to victory I am born." --emerson

no need to confuse 'religion' with being a spiritual being. our soul purpose here is to care for one another. failing that, we're simply passing through (excess baggage) being distracted/consumed by the guaranteed to fail illusionary trappings of man'kind'. & recently (about 10,000 years ago) it was determined that hoarding & excess by a few, resulted in negative consequences for all.

consult with/trust in your creators. providing more than enough of everything for everyone (without any distracting/spiritdead personal gain motives), whilst badtolling unprecedented evile, using an unlimited supply of newclear power, since/until forever. see you there?

"If my people, which are called by my name, shall humble themselves, and pray, and seek my face, and turn from their wicked ways; then will I hear from heaven, and will forgive their sin, and will heal their land." )one does not need to agree whois in charge to grasp the notion that there may be some assistance available to us(

boeing, boeing, gone.

Fact (5, Funny)

Charliemopps (1157495) | about 4 years ago | (#33022920)

“The Encyclopedia Galactica has much to say on the theory and practice of time travel, most of which is incomprehensible to anyone who hasn’t spent at least four lifetimes studying advanced hypermathematics, and since it was impossible to do this before time travel was invented, there is a certain amount of confusion as to how the idea was arrived at in the first place. One rationalization of this problem states that time travel was, by its very nature, discovered simultaneously at all periods of history, but this is clearly bunk. The trouble is that a lot of history is now quite clearly bunk as well.”

has never been observed in practice... (1)

countertrolling (1585477) | about 4 years ago | (#33022930)

Well DUH!

Time travel never involves paradoxes (3, Insightful)

tomhudson (43916) | about 4 years ago | (#33022950)

The concept of a paradox is entirely a human concept - in other words, it's in the eye of the observer. The universe wouldn't "classify" you going back in time and killing your great-grandparents before you were born as a paradox, simply because the universe is not an observer. It would happen - so what - "It is what it is". That would just be part and parcel of the way the universe works in that particular case.

Attempting to say that this would result in a paradox as far as the universe is concerned is anthropomorphizing the universe to an absolutely unforgivable degree. Sure, it makes for a good time travel story, but the universe won't lose any sleep over it, any more than it does for me writing "The next phrase is false." "The previous phrase is true." "Both the previous phrases are true" "The previous phrase is true" There's no paradox. The universe doesn't suddenly go wonky, and cats mate with dogs, etc.

Re:Time travel never involves paradoxes (0, Flamebait)

OrangeCatholic (1495411) | about 4 years ago | (#33023274)

>simply because the universe is not an observer.

Wow. You're amazingly stupid. So if a tree falls in a forest, it doesn't make a sound?

I know it makes for a great philosophical question, and even leads into discussions about cats in boxes, but...on a practical level, yes, we all know that it does make a sound.

Quantum Leap (1, Informative)

thetoadwarrior (1268702) | about 4 years ago | (#33022966)

Clearly no one has watched Quantum Leap. You can only time travel within your own life. Time travel is so far off that we won't see anyone traveling back in time yet.

wait... what ? (3, Funny)

koolfy (1213316) | about 4 years ago | (#33023112)

it does not require any of the distortions of spacetime that traditional time machines rely on.

Wait, did I missed the part where time machines were something traditional or common or anything like that ?

Seriously, time travel became mainstream and nobody told me ?

If (2, Insightful)

Dunbal (464142) | about 4 years ago | (#33023172)

If time travel existed at some point in the future, we would have had evidence of its existence in the past...

Great Scott! (1)

dkleinsc (563838) | about 4 years ago | (#33023178)

Was Seth Lloyd perhaps inspired to design paradox-free time machines by the great Christopher Lloyd?

Future unknown (1)

gmuslera (3436) | about 4 years ago | (#33023192)

Knowing (not predicting, not hinting, but knowing) our future is, from the future perspective, altering the past. That means that physic proved that we won't be able ever to know for sure our future?

Well, is not so bad, at least we won't go extinct because of blue butterflies.
Load More Comments
Slashdot Login

Need an Account?

Forgot your password?

Submission Text Formatting Tips

We support a small subset of HTML, namely these tags:

  • b
  • i
  • p
  • br
  • a
  • ol
  • ul
  • li
  • dl
  • dt
  • dd
  • em
  • strong
  • tt
  • blockquote
  • div
  • quote
  • ecode

"ecode" can be used for code snippets, for example:

<ecode>    while(1) { do_something(); } </ecode>