| From: Matt | 3/09/99
22:08:32 |
| Subject: Instantaneous or back in time | post id:
35107 |
| Say you had two magic coloured
balls - separated by an arbitrary distance and these balls have the
property that they are always opposite in colour (this is the simplest
version). Then if you look at one you know instantly that the other must
be the opposite colour. However the colour that your ball will be is
totally random! Therefore how could you tell the difference between there being instant "communication" between the two, so that they always have the opp colour, or that this communication travels back in time, finds out the correct answer to what your colour is and then travels back to the present to tell the other ball what its colour should be? | |
| From: Chris W (Avatar) | 3/09/99
22:52:34 |
| Subject: re: Instantaneous or back in time | post id:
35116 |
| You're describing the behaviour
of entangled photons. Two photons, separated by an extended
distance, are mysteriously linked: measuring one affects the other
instantaneously. This behaviour is predicted by quantum theory.
Entanglement does not fit well with classical theories. Einstein felt that this 'action at a distance' was a flaw in quantum theory, but current experiments seem to confirm the quantum theory's prediction. New Scientist: http://www.newscientist.com/ns/970628/nlight_nf.html Light's spooky connections set distance records. I cannot shed light on how quantum theory explains this apparently instantaneous transfer of effect in a way that is consistent with absolute limit of the speed of light. I'm not even sure that it needs to! As a thought experiment: In order for a signal to leave your ball (A), travel backward and then forward through time to advise the other ball (B) what colour to be you are faced with a problem. Let's say that A changes colour a t=0. The signal advising B of the colour change can leave A at some t<0 in order to reach B before t=0: how does it know what information to transfer (ball A's colour hasn't been determined yet)? If you take the approach that the signal leaves at t=0, travels backward in space-time for some distance, turns around and travels forward in space-time to reach B: what causes the signal to change it's direction of propagation? | |
| From: Matt | 3/09/99
23:05:25 |
| Subject: re: Instantaneous or back in time | post id:
35118 |
| I guess this could happen - the
instant A's colour is measured then the info goes back in time to B so
that when B is measured it is the right colour.
| |
| From: Dr. Ed G (Avatar) | 4/09/99
5:19:17 |
| Subject: re: Instantaneous or back in time | post id:
35148 |
| From a Quantum Mechanical
perspective the conundrum comes from talking about the situation in the
wrong way. Specifically, to talk about the two balls as separate enitities
is incorrect if they are truly entangled - and the behaviour
you're talkingabout would not occur if they weren't entangled. So they're
referred to as two entangled particles, but they're not "two" systems,
they're one system which contains two particles that are actually
indistinguishable from each other... until you make a measurement, at
which time you destroy their entanglement. In other words, you can't
distinguish one particle from the other until you measure one of them, and
until you can distinguish them you can't talk about them as two different
things. This may seem rather pedantic, and splitting hairs, but the quantum world tends to be a really pedantic place. And it may seem irrelevant that you can't distinguish two particles, but such inability to distinguish two particles, such as coupled electrons in the electronic states of a molecule, leads to profound effects... one of which is the Pauli Exclusion Principle which effect the chemical and physical properties of all the elements and their compounds. Now, if we accept the fact that the two photons are indistinguishable, and accept the fact that we're talking here about one system, not two related systems, the upshot of this is that no information can pass between the "two" photons - a single entity can't "pass" information to itself - and so causality, and Einstein's assertion that no influence can travel faster than the speed of light is preserved because no influence is travelling faster than the speed of light, in this case. The unfortunate consequence of this, for technology, is that we can't use this phenomenon as a means of faster-than-light communication. I'm not denying it's not really wacky - it is really wacky... but then the Universe is a very wacky place! Soupie twist, Ed G.  | |