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Astro_Bot
09-04-2013, 03:45 PM
"Spooky action at a a distance" via quantum entanglement has been known about for some time, and is used in quantum cryptography, but just how fast is it?

According to a new paper, reported in this article (http://www.theregister.co.uk/2013/04/08/chinese_entanglement_transfer_exper iment/), the lower bound is 1.38 x 10^4 times c. That's around Warp Factor 9.999*. That's the lower bound, i.e. it's no slower than that.

At that speed, a message could be sent to Alpha Centauri in under 3 hours! Not exactly the equivalent of a Star Trek sub-space channel, but still pretty good.

I'm not sure how a quantum communication device would be used in practice, since this experiment involved entangled photons and lasers ... and quantum crypto uses fibre optics ... not exactly practical for space ships outside the solar system. Still, it seems like a step in the right direction.

Astronomy angle? Sure! The optical transmitting/receiving devices included telescopes; ED80s to be precise. ;)

The orginal paper is here (http://arxiv.org/pdf/1303.0614v1.pdf).


* The various Star Trek series weren't consistent with their definitions of warp speed, but there's a summary of it here (http://www.ex-astris-scientia.org/treknology/warp6.htm) - I'm using the STNG scale.


Edit: While this post was meant as infotainment, it wasn't meant to be wrong. Sadly, like much infotainment, it's a load of old codswallop. I've changed the thread title so that it won't become a troll magnet.

Dave2042
10-04-2013, 02:52 PM
This is certainly interesting, however it's not going to allow faster-than-light communication. I'll try to explain.

At each (physical) end of the experiment, what each experimenter (call them A and B) sees is a particle, with the two possible states (call them Up and Down), which he measures and unpredictably comes up with one of those two states. Entanglement means that if experimenter A sees 'Up', then experimenter B must see 'Down'. So it looks like, when experimenter A performs his measurement, the information about which state he has measured is 'instantly' (faster than light) communicated to the other particle.

The problem, however, is that what experimenter A measures is random, so until he has measured the state, he doesn't know whether it will be Up or Down. Similarly, experimenter B also doesn't know whether his particle will be Up or Down until he measures it.

The only way experimenter B would know his particle's state in advance of measuring it himself, is if experimenter A sent him a message about the state of the other particle. But experimenter A can only send that message at the speed of light (since this is no longer being communicated via entanglement).

So, in fact entanglement doesn't actually communicate anything in a way that violates relativity. Prior to a 'classical' message arriving, each experimenter simply sees an unknown state particle, measures it and gets a random result.

pluto
10-04-2013, 03:21 PM
Good explanation Dave :)

A related article from today: http://www.theregister.co.uk/2013/04/10/spooky_action_in_space/

Dave2042
10-04-2013, 04:00 PM
In the same vein, here is something really loopy I remember from uni days.

Note - very simplified explanation glossing over plenty of details.

When you're running a particle accelerator, you slam two particles into each other, and then see two 'jets' of various particles shoot out in opposite directions. Now you can explain the angle of the jets based on the initial particles types, energies etc and QM.

Now the way the jets get started is that after the collision, one initial particle goes one way and the other another. Of course you don't know which particle went which way, and is in which jet (because each jet is a complete mess with lots and lots of particles in it), and this affects the answer QM gives you. And that's the result you get in a real experiment.

But...

You could decide to use an initial pair of particles which were different in some specific way - let's say one is positively charged and the other negatively. Now if you could get a good enough snapshot of the jets, you could simply cancel off particles in either jet, until you found one jet was net positive and the other net negative.

At this point you would know which way the initial particles went. And QM would now give you a different answer for the jet angle. And then what? Which answer do you get? The answer for where you don't know, or the one where you do?

And in either case you have some serious problems.

If you get the 'don't know' answer, then our current understanding of QM would seem to be wrong, which is unfortunate because it works so well.

If you get the 'do know' answer, then the implication is that QM knows in advance that you are going to do this trick, which is flat out nuts - even nutser than QM. And what if you were going to do it and then didn't get around to it?

No one has done this, and (as per my comment above) no one would suggest it would ever be possible in practice, but I find the possibility quite unsettling.

Of course one answer is that the QM (and the laws of physics generally) turn out to forbid you from doing this, so the problem simply doesn't arise.

Dave2042
10-04-2013, 04:02 PM
Thanks - I'm relieved to think I do still understand this a little after a couple of decades away from the field.

astronobob
10-04-2013, 09:30 PM
coool, Internet with 60million million gig a second :D but then, 12months later, thatll be too slow :rolleyes:

madbadgalaxyman
11-04-2013, 09:58 AM
The seductive and attractive idea of faster-than-light communication is apparently an impossibility in practise.

How about another seemingly impossible idea:

Find the exact distance from the Earth that the wavefront of a specific signal has reached in space (at the present moment in time), the wavefront of electromagnetic radiation (e.g. visible light) which is coming from a specific historical event occurring at a specific time in human history.
Then (very!) quickly station a telescope a little further from the Earth than the current radius of the sphere (centred on the Earth) that the light from the event has currently reached!
Then point this "ultra-ultra-high-resolution" telescope back at the Earth, just before the light from the "historical event of interest" reaches the location of the telescope which is x light years from the Earth.
Then use the telescope to view the historical event of interest, and find out if the historians are right about what happened!

[[
Me, personally, I think it would be cool to see Jesus walking on the water! (at least, if you do believe in miracles, you will find out whether or not your belief is justified. Or did the water somehow develop enough surface tension to support a human being?)

As an aficionado of medieval and "dark ages" and pagan history, I would also like to view some specific battles that occurred between mighty and war-hardened knights, and to view the little-known pre-Christian worship practices of Anglo-Saxon England.

It would also be interesting to see Neanderthal culture and technology, as actually practised by the Neanderthals!

I would also like to see the first metazoans that evolved on our planet, slithering in the primeval slime! ]]

avandonk
13-04-2013, 10:43 AM
Shirly these two entangled objects are just the two ends of the same string connected via an/other dimension/s.

Bert

Astro_Bot
13-04-2013, 11:31 AM
Thanks for explaining that, Dave.

It certainly is as a seductive idea, but I was wrong thinking that information (in any quantity at all) could be sent faster than light. The worst part is that I have a relativity book on my shelf (Kenyon, General Relativity) and have actually read it, albeit 20-odd years ago. I should have known I was posting rubbish.

Tip for young players: Flu medication and six hours a week of Star Trek will addle your brains. If you're similarly afflicted, stay of the web or you might post something silly like I did.

avandonk
15-04-2013, 12:35 PM
Just watch this and be more surprised!

http://www.youtube.com/watch?v=2DIl3Hfh9tY

And this

http://www.youtube.com/watch?v=NZ-ElsvYKyo

Bert

Dave2042
16-04-2013, 08:35 AM
You're way too hard on yourself. This is very strange stuff. And while the no-faster-than-light thing is core to relativity, I'd hesitate to say that we understand the universe so well that it couldn't turn out to have exceptions.

g__day
21-04-2013, 09:28 AM
Current issue of New Scientist has an interesting article that asks what happens to entanglement in space - reviewing as one passes quantum encrypted messages via satellites we should see what happens to entanglement as your carrier leaves Earth's main gravitational field...