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Quantum Experiment Shows Effect Before Cause
steveb3210 writes "Physicists have demonstrated that making a decision about whether or not to entangle two photons can be made after you've already measured the states of the photons."
Here's the article's description of the experiment: 'Two independent sources (labeled I and II) produce pairs of photons such that their polarization states are entangled. One photon from I goes to Alice, while one photon from II is sent to Bob. The second photon from each source goes to Victor. Alice and Bob independently perform polarization measurements; no communication passes between them during the experiment—they set the orientation of their polarization filters without knowing what the other is doing. At some time after Alice and Bob perform their measurements, Victor makes a choice (the "delayed choice" in the name). He either allows his two photons from I and II to travel on without doing anything, or he combines them so that their polarization states are entangled. A final measurement determines the polarization state of those two photons. ... Ma et al. found to a high degree of confidence that when Victor selected entanglement, Alice and Bob found correlated photon polarizations. This didn't happen when Victor left the photons alone.'
AIUI, the notion that information can't be transferred faster than the speed of light is based on the fact that it would violate causality. I have wondered whether causality is an assumption rather than an actual property of the universe.
If it is (I'm not qualified to interpret this experiment), we'll have a lot of new physics coming down the pike over the next few decades.
Sheesh, evil *and* a jerk. -- Jade
I really which quantum people would stop acting like they know what they are talking about.
This is just a really shitty description/way of looking at a series of events and is more or less wrong in the same way that saying your traveling back in time by looking at old stars in the sky from far off distances.
The only thing out of order here is the observers note taking and logic. Due various other quantum flux it may appear to happen in a certain order even though it didn't and its just a matter of appearance due to propagation effects.
Its a bad observation and bad description of that observation, not a causality violation.
The experiment in the article is ... awesome. Though if history is any indication, hoards of raving Slashdoters will try their damnedest to force this into a classical mechanistic world-view.
So here's a fun experiment you can do at home! (Craftsmanship is important for good results.)
1) Start by setting up up a classic double-slit experiment. A laser pointer and some household junk is all you need.
* Observe the interference pattern.
2) Stop denying that you went to see "Avatar" 36 times and grab a couple pairs of 3D movie glasses.
2a) Alternately, you can just buy a polarizing filter sheet. (this is the better way)
3) Being careful to note orientation of the filter, place the filters in front of the slits with one oriented 90 degree to the other. (This is only tricky because the distance between the two slits is so small.)
* What happened to the interference pattern? You "tagged" the individual photons so that you could, in principle, know which slit they passed through, so instead of going through both, they went through just one.
4) Place a third sheet of polaroid between the slits and the detector screen, oriented half-way between the two other filters (if one sheet is vertically oriented and the other horizontally, this sheet will be oriented at 45 degrees)
* The interference pattern is back? WTF? You took the tag away, so that you couldn't know which slit a photon passed through. You "erased" the which-path information so each photon went through both slits, instead of just one of them.
Do the experiment. Accept that the physical world is weird as shit. Shut-up and calculate.
Required reading for internet skeptics
What this article is saying, is that victor's decision to entangle his photons has a direct effect on the results that alice and bob get from their double blind measurements.
So, either there is retrograde communication on time's axis, or....
The decision that victor makes is predetermined, by the act of measurement undergone by alice and bob. (Meaning victor doesn't really have as much free will as he thinks he does.)
Proposed followup experiment:
Alice and bob examine their photons, tell each other, but not victor. Victor decides to entangle or not entangle. Examine new correlation.
This will test "does a correlation between alice and bob indicate that victor will entangle?".
If it does, you have a reasonably strong test case for many worlds.
I'm not sure how serious you are, but I'll point out the problem at the risk of killing the joke. The issue is in step 2. Photons travel at the speed of light (by definition). Because we cannot send information faster than the speed of light, the photons arrive at Victor strictly before any message from Alice and Bob.
Just use a fiber optic cable to make them wait longer. Or bounce between mirrors in a zigzag - this way light trajectory can be long, but the spatial distance can be short.
No problem, I had to have it explained to me once too. They say newborns have an intuitive understanding of some basic physics, but nobody is born understanding quantum mechanics.
Frankly I don't think anyone dies understanding quantum mechanics. :)
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