Quantum Experiment Shows Effect Before Cause 465
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.'
causality (Score:5, Interesting)
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.
Reality versus Obeservation (Score:5, Interesting)
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.
Quantum Physics @ Home (Score:5, Interesting)
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.
result of "many worlds" being true? (Score:5, Interesting)
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.
Re:Paradoxical (Score:5, Interesting)
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.
Re:Reality versus Obeservation (Score:5, Interesting)
That phrase is also missing some better note taking and logic. Does it have any meaning? It's not a bad observation at all, altought I'd agree that is a bad description.
To summarize the article, scientists confirm (again) that Quantum Mechanics works as designed. Despite all the naysayers (ok, there aren't many anymore), and the despair of people trying to create any deep understand over what is a purely pragmatic model, the Universe works exactly the way QM says it will. On a related notice, causation is preserved, unless you want any deep understanding of it.
Re:Maybe I'm Understanding This Wrong (Score:4, Interesting)
Ok, let's rephrase the experiment. You have four photons - A, B, C, D. A starts off entangled with B, C starts off entangled with D.
What the experiment appears to show is that if B is then entangled with C, then A is effectively entangled with D. In other words, entanglement is transitive. What it does NOT show is a violation of causality, unless I'm seriously misunderstanding the results.
(There may be other alternative explanations, but I'm satisfied that the results can be explained without resorting to violations of causation.)
However, I am going to throw in another thought -- IF it is established that causation is indeed violated, the Many Worlds theory of quantum mechanics must be false. (The Many Worlds theory says that the universe splits at the event, and that the measurement simply tells you which universe you're in - until then, there's a given probability you're in any of the possible universes. However, the event hasn't taken place at the time of the measurement here, so all probability waves must coexist, so you should observe every possible state. This isn't what's observed. Ergo, one or both of Many Worlds and Violation of Causality must be wrong.)
Re:Time delay - info from the future? (Score:5, Interesting)
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. :)
Re:Is It Effect Before Cause? (Score:3, Interesting)
Apparently you missed a chapter in the Interpretation of QM - one interpretation is that it requires observation by a conscious mind to cause decoherence, meaning that the measuring equipment is itself in a state of superposition up until the moment that Alice and Bob check the readouts. This interpretation has been largely relegated to cocktail conversations by scientists because it appears impossible to test, but has a certain appeal to philosophers, mystics, and New Age types.
One side effect of the interpretation - it makes conscious life inevitable in any universe theoretically capable of supporting it: the entire universe will be in a state of near-infinite superposition, with all possible "timelines" coexisting until one of them gives rise to a conscious mind, at which point the entire system collapses to only those states consistent with that mind's existence.
An interesting theory I heard recently: If you postulate that the fundamental constants of the universe are themselves prone to fluctuation (something we have no data about), especially in the first few moments of the universe, then you have an explanation for why they seem to be fine-tuned to allow the existence of life - all other values belonged to possible states that were lost in the collapse, leaving only the state most conductive to the rapid evolution of consciousness.