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back to article Quantum boffins send data ACROSS TIME AND SPACE

Researchers in Israel have pulled a trick that makes quantum physics seem even stranger than an episode of Doctor Who – they've created a pair of photons that was briefly entangled not across space, but across time. The last time El Reg discussed time-like entanglement it was being proposed as a theoretical construct. The idea …

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Anonymous Coward

Agh, my poor brain

Just 'sploded.

OK, so they generated a particle that didn't exist and detected it before it was generated?

Ouch. Even Doctor Who would have trouble beating that.

There is a very controversial theory that antimatter is simply normal matter moving back in time, ie the "Arrow of Time" is reversed. If so then a lot of physics becomes a lot simpler, time paradoxes are avoided and causality simply becomes an observer driven event ie we "see" a glass break because our minds cannot handle the reverse and discard it as garbage data.

Also this means that quasars etc are simply eruptions of antimatter (ie negative time) neatly explaining the anomalous energy production and radiation emission.

AC x520

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Re: Agh, my poor brain

Not surprised thats controversial, it makes not a lot of sense. Arrow of Time is a consequence of entropy that always increasing in the closed system of the universe, not the make up of matter inside it. We don't see glass fragments reform, not because they can't, but because it would take a shed load of energy and time.

As for antimatter, not sure why having opposite charge & spin would effect time in any sense. We already have particles that differ in charge & spin they don't behave differently timewise to the others.

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JDX
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Re: Agh, my poor brain

I don't think it's really a theory, merely a useful mathematical construct - to TREAT anti-particles as regular particles travelling backwards in time is nice for the maths solutions but that's all.

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Holmes

Fear not the paradox is, paradoxically, not going to happen.

I think time paradoxes don't exist - they're just a sci-fi idea... cos, if every decision, even down to the quantum level, creates a new alternate reality then....

Israeli scientist creates time travel.

Mossad think it's fab opportunity to convert the planet to Judaism.

Chap travels back in time and meddles.

BUT

Chap has now create a new 'spur' of reality at the point of devious meddlesomeness which means he's now careened off down another alternate rabbit hole in which he came back from the future to meddle.

Original timeline is secure, he's just now created a new one in which he can move freely back and forth but it's separate and distinct from the timeline he spliced himself from at the point of meddling (likely at the point of his departure in that original 'present' when he pressed the GO button on his time machine).

So the dodgy time-traveller has actually irreversibly left his original timeline/causailty-stream and created a new alternate one by his own actions in which he's now trapped, i.e. he can't 'undo' his own time travel to revert to where he used to be as he can only go back further into the parent of his timeline, creating further splices which may or may not work out more like the original one he left behind.

Hence we don't need to worry about temporal paradoxes, happily.

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Re: Fear not the paradox is, paradoxically, not going to happen.

"So the dodgy time-traveller has actually irreversibly left his original timeline/causailty-stream and created a new alternate one by his own actions in which he's now trapped, i.e. he can't 'undo' his own time travel to revert to where he used to be as he can only go back further into the parent of his timeline, creating further splices which may or may not work out more like the original one he left behind."

The Trousers of Time have a hole in them.

Worse still are the holes in the pockets of the Trousers of Time.

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Re: Fear not the paradox is, paradoxically, not going to happen.

That would mean that to create a time 'spur' you'd need to recreate the mass/energy of the universe less your mass/energy for it to exist with you added to it. Otherwise your 'spur' would just collapse/fail as it would be an incomplete universe.

Proper time travel relies on either all time existing at the same time or the ability to create new universes at will at different time points, the latter might be doable if you don't mind waiting 14 billion years for your new universe to arrive at the required point in history.

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Anonymous Coward

Re: Fear not the paradox is, paradoxically, not going to happen.

Time travel is completely possible, but things just conspire from letting you change anything.

I know, I went back, and nothing I did, stopped either Tony Biar, Gordon Broon, or "Call me Dave" Macaroon from getting into Downing Street.

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Rol
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Re: Fear not the paradox is, paradoxically, not going to happen.

Considering the argument suggesting multiple universes exist having been created from a branch point where a choice to do or not do occurs, then it could be argued, the meddling has already happened.

The "Mossad scientist" has actually travelled back to the branch point and continued on the alternate path, his future alone has deviated, yet he still exists in our branch as the scientist who didn't meddle.

I like to think, reality as we experience it, is the one which takes humanity on the longest journey, with the most favourable outcome, as all the other branches, if followed, result in a premature or even inconclusive end.

In other words, our reality is the product of a quantum state that transcends time, in so much as it chooses the most favourable line to follow based on the outcome of every branching event along it,

So, don't panic, God has the road map and we're not driving, we only think we are.

..and as for time travel, I would like to point your attention to the many events in out history, that are documented as being attended by only a handful of witnesses and not the millions, that would surely have turned up if the ability to travel in time was ever possible.

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Anonymous Coward

Re: Fear not the paradox is, paradoxically, not going to happen.

"That would mean that to create a time 'spur' you'd need to recreate the mass/energy of the universe less your mass/energy for it to exist with you added to it"

Not necessarily. The universe could fork() with copy-on-write semantics.

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Anonymous Coward

Re: Fear not the paradox is, paradoxically, not going to happen.

I tend to believe that there is only one timeline regardless of the existence, or not as the case may be, of time travellers. There is, in other words, only one reality.

If time travel does exist, and the traveller manages to change something "historical", will we even notice it? Our history will change, but we will believe that this is the way it has always been. The only person who will know, with certainty, will be the time traveller who changed the timeline.

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Re: Fear not the paradox is, paradoxically, not going to happen.

Stop! This way lies rump of skunk and madness.

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Re: Fear not the paradox is, paradoxically, not going to happen.

"I think time paradoxes don't exist - they're just a sci-fi idea... cos, if every decision, even down to the quantum level, creates a new alternate reality then...."

Personally I follow the idea of fixed time rather than whole spawned universes every millisecond.

Time is like a movie. You might figure out how to fast forward and rewind but that doesn't change the movie.

Ok imagine you go back in time to kill your father. That would create a paradox. Now with fixed time, any attempt to create a paradox would fail. You get hit by a bus before you can kill him. You go back too far in time and get eaten by a T-Rex etc etc.

Just like the past, the future has already happened and it's only the fact that we don't know what will happen that maintains the illusion of free will...

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Re: Agh, my poor brain

Not to mention that quasars are already known to be active galactic nuclei. The central black hole "eating" matter, causing relativistic jets to form.

The theories expressed by AC were back from the 1960's and 1970's, before spin, color, etc were fully understood and galactic cores were mysterious objects and black holes were unknown.

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Re: Fear not the paradox is, paradoxically, not going to happen.

Itzhak Bars (http://en.wikipedia.org/wiki/Itzhak_Bars) has some interesting ideas on two dimensional Time that might be applicable.

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Holmes

"Another point of view that one can take is that the measurement of the first photon is immediately steering the future physical description of the last photon."

So, they're saying that something happening now can affect something else in the future? And they need to prove this with lasers?

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FAIL

Ah, what mechanism would you propose to explain this ? What links photon 1 and 4 prior to the entaglement ?

If you're going to be a blowhard, can you at least provide a plausible alternative ?

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Anonymous Coward

Re: And they need to prove this with lasers?

Um, we need a reason to prove stuff with lasers now?

Dammit, Igor! Igor! Quick, can you knock together a page on the current and future applications of lasers for this grant proposal I'm writing? Apparently, just writing YAY LASERRRRRRRRS! in 64 point bold italic doesn't cut it any more.

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Anonymous Coward

Nicho

Is the link between 1 to 4 not a possibility though? Just because we have 2 possible options, 1) 4 effects 1 and 2) 1 effects 4 but only recently a theory for 1) with no theory for 2) does not mean 2 is "wrong" until shown so experimentally.

Don't mix theory with experimental data. The data here could show either theory correct, or perhaps both are correct?

The worse argument possible is "oh but what alternative do YOU suggest then". As it fails to show the strength of the original theory, in a false appeal to the "ridiculousness" of thinking about alternatives. If we took that attitude, we'd never have hit QM in the first place... "What alternative to classical mechanics would you suggest, the double slit experiment MUST be classical..." would get us no where. We had to use the data to shape our theory.

Teaching others about such subjects involves showing how the data fits the theory. We can all "imagine" alternatives, that is rather an inexhaustible pursuit, but only 1 will match the experimental evidence. :)

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But isn't there a link between P1 and P4 via the entanglement of P2 to P3 which was entangled with P4?

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Anonymous Coward

@AB - Yes there is. This is the point of the experiment. Let's work it through:

(1) P1 & P2 are entangled.

(2) P1 is measured (destroyed) - This collapses/defines the state of P2

(3) P3 and P4 are entangled

(4) P2 and P3 are entangled

(5) Measurement of P4 shows a correlation with P1 - which no longer exists.

If I'm reading the explanation correctly (and I may not be) then either P1 and P4 are correlated across time, or P2 somehow has a 'hidden variable' that it retains from the P1 entanglement and subsequently passes on to P3 and P4. Now I believe that hidden variables have been fairly comprehensively ruled out by other experiments, leaving us with the interesting prospect that time isn't what it seems to be.

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You know, I have one simple request. And that is to have experimental proofs with frickin' laser beams attached to their heads!

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There should be a hidden variable: light can express itself as both waves and particles which means that there is a "true form" for light, that is neither but permits both. So light must have another, hidden, form/dimension.

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Genius or Brainfart?

Unless these results are repeatable in a meaningful way , this looks more like a case of reading too much into a relatively normal result. At any rate it should inspire discussion and criticism.

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Boffin

Neither... Its a proof...Re: Genius or Brainfart?

Time travel? I think not.

Photon 1, 2, 3 and 4 are linked.

Even though Photon 1 is lost, its state is captured in 2 which is linked to 3 which is linked to 4.

Can you say associative property?

A is to B as B is to C there for A is to C?

No time travel.

A past photon sets the state which is carried forward.

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Anonymous Coward

Re: Neither... Its a proof...Genius or Brainfart?

Yes, at the least they need to (I hope they do in the paper, does not seem to be here in the article) demonstrate the state of particle 4 is not "set" and is QM "undetermined".

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Pfft

Talk about interpreting the figures to mean what you want them to.

All this shows is the proof of reincarnation. Photon 1 is reborn as photon 4.

And there in a nutshell is religion and the sky fairy's explained. Its all quantum not some white guy with a beard.

Can they not ask us some more difficult questions? This is like the Sun crossword vs the Times

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Re: Pfft

I think a more plausible explanation is that P1 was never destroyed, merely shifted into a different state.

But perhaps that level of physics is far too elementary for them.

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Meh

Not as convoluted an explanation as a Dr Who plot.

I don't get either though.

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Smoke and mirrors

I see mirrors, but where is the fire?

I remember years back (could be decades now) some researcher got a photon traveling through an object to register on a detector sooner than its identical twin fired at the same time on a equidistant path. Turned out to be the front part of the photon that traveled through the material had a slightly shifted shape of its wave even though the average of the whole photon took the same time and this shift in the front part was enough to trigger the detector sooner. Was hyped as faster than light due to the photon "tunneling".

I am getting a strong sense of deja vu.

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Re: Smoke and mirrors

This work isn't the same thing as what you just described.

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Anonymous Coward

Re: Smoke and mirrors

not the same thing; what you describe is research on the speed of light, or more subtlety the group velocity of a wave packet (a bunch of photons). In short, light (individual photons) can be measured to travel faster than the speed of light, but "information" (stuff encoded on the photon wave packets, through multiplexing wavelengths or whatever) is garbled if the photons travel at "superluminal" velocities (the group velocity cannot exceed c, speed of light in a vacuum, if you are interested in the information they carry)

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Re: Paul

I know, just thought it would be interesting in light of the questionable interpretations put forth, seems like the more likely explanation is not a surprise and the more suspect one is a ruse to get media attention. Hope that sheds some photons on this matter.

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Yag
Boffin

Help me, Obi Wan Quantobi!

Can someone try to explain in layman terms the whole "entanglement" stuff?

My current comprehension of this phenomenon is probably way off...

Here it is (please don't laugh too much)

Basic entanglement :

Two photon are produced with a given characteristic (let's say polarity) set at opposite value for each photon.

Due to quantic incertainty, you cannot tell which photon is set to which polarity until you measure one of them.

Once one of the pair measured, then the value of the other is also known.

Never could figure out why a lot of physicists seems interested by such an obvious result, so I'm probably missing something...

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Re: Help me, Obi Wan Quantobi!

You need to accept the 'Copenhagen interpretation' of quantum mechanics, or similar, to dig into the quantum weirdness. One way to put it is: 'a quantum mechanical state does not exist before you measure it, but becomes real as soon as you measure it'. So now there is no more causality, because before you measure a QM property of a particle, it didn't exist. Hence there is no mechanistic way to look at its interaction with other correlated particles.

Here, of course, photons 1 and 2 were already forced into a QM state by a measurement on photon 1 before photon 2 interacted with photons 3 and 4. The weirdness of this 'time correlation' is thereby quite a bit reduced and you might argue that this whole experiment is a bit too deterministic.

This whole story of 'entangled in time and not in space' somehow ignores the connection of the two via the propagation velocity (speed of light) -- and from the abstract only I would guess that the authors may have trouble publishing this paper in a real journal due to lack of novelty.

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Anonymous Coward

Re: Help me, Obi Wan Quantobi!

Due to quantic incertainty, you cannot tell which photon is set to which polarity until you measure one of them.

Once one of the pair measured, then the value of the other is also known.

Never could figure out why a lot of physicists seems interested by such an obvious result, so I'm probably missing something...

The bit that you're probably missing is that it's not like knowing that if one side of a coin came up heads, the other side must have been tails. It's more like throwing two seemingly-separate coins at once and finding out that if one comes up heads, the other comes up tails more often than 50% of the time - and the other way round as well, which couldn't happen if one or both of the coins were merely biased. The statistics of the pairs of particles don't match up with the expected probabilities from a coin-like model; google "bell inequality" for more but it gets complicated at that point. A simple

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Re: Help me, Obi Wan Quantobi!

"Two photon are produced with a given characteristic (let's say polarity) set at opposite value for each photon."

That's your mistake. The characteristics are not set. They will be opposite when you actually measure them, but until you measure them they are not set. So if you move them a light year apart and then measure one, the other one becomes set, even though it's a light year away.

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Re: Help me, Obi Wan Quantobi!

How do you know the characteristics are not set? You can't know that for certain because you haven't measured them yet. All this entanglement stuff only makes sense if the characteristics of the pair of photons are actually set at the moment they are created: this way Einstein needn't get upset because there is no spooky action occurring; it's just us reading the state of one, from which we can deduce the state of the other.

IANAPhysicist but I can spot a fallacy when I see one.

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Yag
Pint

I can spot a fallacy when I see one

Same for me, but I also consider that, between :

- Me, with my "basic" education on physics (up to Shroedinger's equation, and I forgot most of the stuff due to lack of use)

and

- An army of overqualified eggheads thinking routinely about this stuff for the last 80 years...

I'm the more likely to be wrong. Occam's overused-till-dullness razor and so on...

<--- maybe this will help to expand my mind and manage to understand it.

And as a final note, for all those who tried to explain : Thank you! :)

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Re: How do you know the characteristics are not set?

Because hidden variable theories make predictions that aren't observed in experiments.

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How do you know the characteristics are not set?

That's how the universe works.

"All this entanglement stuff only makes sense if the characteristics of the pair of photons are actually set at the moment they are created"

No, that's the only way it can match your (and indeed, my) intuitive understanding of how the universe works. How we intuit the universe to work is wrong.

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Re: Help me, Obi Wan Quantobi!

from the abstract only I would guess that the authors may have trouble publishing this paper in a real journal due to lack of novelty

Is Physical Review Letters not a "real journal", then, in your estimation? Last I looked, it was a fairly prestigious, peer-reviewed organ.

I must say I am impressed with the level of boffinry on display on the comments page today, with many confident declarations that there is nothing to see here. Time for a shakeup at the PRL editorial board? Will the Nobel committee be asking the Reg for contact information? Only time will tell (or perhaps it already has).

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Yag

"That's how the universe works."

Well, I'ld say "That's our most accurate explanation of how the universe works" instead...

*Get back to wikipedia article on young's experiment*

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Boffin

Nice Diagram!

I especially like the grave stone and the pram - makes things a little easier to visualise.

However, as for the spooky nature of quantum mechanics, you can gain another insight by sliding all those vertical dash lines and graphics all the way to the left so that they overlap where it says "t=0". You see, as far as a photon is concerned, no elapsed time ever "exists". Its journey begins and ends in *null* time. So what happens to it is like stapling distant pieces of space together.

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Pint

So no surprises?

Everything works exactly as QM says it should.

The fact whether the photon has been destroyed or not is not particulary important.

The problem stems from the fact that people see disconnected "objects" (photons) mysteriously interacting, whereas the whole setup is just one single fat state vector (ket) that is is being heaved around.

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Joke

I can't wait to see the first quantum debugger, where you can fix errors on a code that not been yet written.

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Facepalm

@Tchou

There is already an algorithm for exactly this

Where costs = peanuts + outsourced to Mumbai = bring back in-house

Thus you can fix errors in code that has not yet been written

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Re: @Tchou

Haha! Good one.

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WTF?

Sounds to me like

They copied 1 > 2, then 2 > 3 then 3 > 4

So 4, being copied from earlier data naturally exists in the future, just sounds like a quantum bucket brigade.

IANAQP, however.

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Re: Sounds to me like

That's about what I thought as well - although the copying bit is a kind of inverted copy. Then again, I don't really understand quantum physics either. Could someone with QM knowledge please clarify?

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Re: Sounds to me like

So 4, being copied from earlier data naturally exists in the future, just sounds like a quantum bucket brigade.

Hidden variables. Use those to explain this experiment, and you have to explain all the others that show there aren't any hidden variables.

And yes, this is the first explanation that occurs to most people (see eg Ian Michael Gumby's post above). Since it's obvious, but the editorial board of PRL thought there was something worth publishing here, that ought to be a clue that the obvious explanation is lacking.

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