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Physicists Create Quantum Link Between Photons That Don't Exist At the Same Time 364

sciencehabit writes "Physicists have long known that quantum mechanics allows for a subtle connection between quantum particles called entanglement, in which measuring one particle can instantly set the otherwise uncertain condition, or 'state,' of another particle—even if it's light years away. Now, experimenters in Israel have shown that they can entangle two photons that don't even exist at the same time. Anton Zeilinger, a physicist at the University of Vienna, says that the experiment demonstrates just how slippery the concepts of quantum mechanics are. 'It's really neat because it shows more or less that quantum events are outside our everyday notions of space and time.'"
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Physicists Create Quantum Link Between Photons That Don't Exist At the Same Time

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  • by quax ( 19371 ) on Thursday May 23, 2013 @12:49AM (#43800341)

    Special Relativity makes quite clear that if two particles are spacelike [wikipedia.org] separated when measured, that the concept of "instantaneous" is devoid of meaning.

    If you have this kind of distance than you will have just one special reference frame where this is true, and infinite more where the events are arbitrarily separated in time. This is already at the core of the EPR paradox [wikipedia.org].

    I.e. that you can have entanglement across time follows trivially from SR and the EPR paradox.

    It's just astounding how many times the very same insight can get repackaged and sold as new.

  • Re:Science (Score:5, Interesting)

    by Black Parrot ( 19622 ) on Thursday May 23, 2013 @01:01AM (#43800383)

    The latest Scientific American has an article about a newish bayesianized quantum theory. To the limited extent that I understand it, the wave function is just the bayesian priors - what you think before you collect the evidence. The only thing that collapses when you measure something is your ignorance about the state of the universe.

  • Observation vs model (Score:3, Interesting)

    by Anonymous Coward on Thursday May 23, 2013 @01:09AM (#43800417)

    If I had a machine, and it could only see the large circles, then all I would see is the large circles.
    If I then made a model of how the large circles appear and disappear, that model would be correct, it would fit the data, it would show the probability of the circles appearing as they jump around. Those circles will jump, they'll go backwards in time, they'll do kinds of weird things.
    So my equations all work, and my model of jumping circles works, ergo my model is correct?

    Except it isn't, its a function of the limitations of the machine used to observe the underlying effect.

    "then the Rayleigh-Jeans formula is completely correct. Never mind that it predicts that all blackbodies should be emitting radiation with infinite power"
    So how fast is light really traveling in this crazy new world?

  • by tonywestonuk ( 261622 ) on Thursday May 23, 2013 @02:31AM (#43800681)

    I believed as you did. Then I read this http://quantumtantra.com/bell2.html [quantumtantra.com] - Its like Quantum physics , without the maths, and for the it literate.

    Changed my ideas on what QM was all about.

    Go read it. Seriously.

  • by master_p ( 608214 ) on Thursday May 23, 2013 @03:40AM (#43800895)

    According to the article, particles 1 and 4 do not coexist. Therefore, one must be destroyed before the other is created.

    But if 1 is destroyed before 4 is created, then the entanglement of 1 and 2 is broken before 3 and 4 are created (because 3 and 4 are created together, and then 2 and 3 are entangled).

    So, by the time 2 and 3 are entangled, 1 does not exist, because 3 already exists and is entangled with 4.

    The question that arises is then how do they know that 1 and 4 are entangled?

    It could simply be that 1 and 4 show the same state when measured, because 1 and 2 were entangled, then 3 and 4, then 2 and 3. Which means that whatever entanglement existed between 1 and 2 will exist between 1 and 3 and 1 and 4, even if 1 does not exist.

    That does not mean particles are entangled across time. It may mean that entaglement is simply peristent and transmiitable.

    Most probably there is a misunderstanding somewhere between the announcement and the article, so please anyone that knows more, elaborate.

  • by michelcolman ( 1208008 ) on Thursday May 23, 2013 @07:04AM (#43801437)

    Space itself can expand at much greater velocities than c.

    To explain this in a little more depth, what we call "space" is really just tied to an arbitrary choice of space-time coordinates. If we choose a different reference frame, distances and times will be different. Just to give a silly example, if I define a meter to be the width of an atom, or if I define a second to be the time required for the earth to go around the sun a thousand times, I can easily travel faster than c. So how does this apply to cosmology and general relativity?

    Depending on the coordinate system you choose, the universe can really look radically different, even to the point of no longer being infinite. I will give two possible views, both equally valid even thought the first may appear strange. (So read the rest as well before labeling it as rubbish).

    You can apply a classic "special(ly?) relativistic" coordinate system to the universe, with us at the center. The speed of light is the same everywhere, relative to us, just like Einstein said in the beginning. Things that are far away from us are moving away at high speed (but less than the speed of light) and are therefore aging more slowly. This means that some far away galaxy isn't just younger (defined as the amount of local evolution after the big bang) because we had to wait for its light to get to our telescope, it actually is younger "right now" even if we take the traveling time of light into account. Local clocks are really advancing more slowly. The effect increases with increasing distance, and at a distance of c times the age of the universe, the big bang is happening as we speak. Right now. This also means that the universe is finite (assuming nothing existed before the big bang, which is a big assumption). Not that it matters much, because we could never reach this "edge" anyway. It is retreating at the speed of light.

    This model is quite interesting but a bit cumbersome for cosmology, so most people prefer to use the "cosmological model". They simply adjust the coordinates of time and space so that the whole universe is the same age and looks roughly the same everywhere, "right now". See, we just changed the definition of "now" and chose a coherently matching set of space coordinates so everything looks rougly the same size, that's all we did. In General Relativity, we are completely free to do so, you can pick pretty much any coordinate system you like. Things can move from the future into the past and back again as we change our variables, without impacting causality (which is all that matters).

    Using the cosmological model, the universe is now truly infinite, the big bang is in the distant past everywhere and all the clocks are running at the same speed (as long as they are stationary relative to "space", i.e. moving away from us at the same speed as the average local galaxy). Now, however, the assumptions of special relativity no longer hold. In particular, the speed of light is no longer the same everywhere. Light speed is still the same everywhere locally, relative to "space" (the speed of the average galaxy in that area), but you have to take the properties of our peculiar coordinate system ("expanding space") into account. If at some distance, "space" and the objects in it are expanding away from us faster than the speed of light, the light from those galaxies will never reach us since it will actually be retreating as if it were running towards us on a conveyor belt moving the other way at a higher speed. The conveyor belt isn't "real", it's just an artifact of our choice of coordinates which does not comply with special relativity.

    In the first model, those distant galaxies simply never come into existence since the local "space" is asymptotically stuck at a time shortly after the big bang. Things over there are moving away from us at increasing velocity approaching c, and time (rate of aging of that part of the universe) is grinding to a halt.

    But do those places exist or not?

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