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Are quantum mechanical effects faster than speed of light?

  1. Jun 3, 2013 #1
    For example - When an electron is excited from an inner shell to an outer shell in an atom, it does the quantum jump between the states. Pretty much it disappears from inner state and appears in outer state. Isn't this happening at faster than light? (The velocity is actually infinity for quantum jumps) That means the electron travelled across the distance between the 2 states at faster than light. I know this may not be observable or usable for anything now. But isn't it still a fact that faster than light travel is possible in universe?
     
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  3. Jun 3, 2013 #2

    Bill_K

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    You must be thinking of the "old" quantum theory, in which the electron travels in an orbit, each state at a different radius, and in order to get from one state to the other it has to "jump".

    In Schrodinger quantum mechanics, an electron state is represented by a cloud of probability. The states overlap, and a transition from one to another takes place at a single point.
     
  4. Jun 3, 2013 #3

    Simon Bridge

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    There are situation where things can go faster than light - but electron transitions are not one of them.

    iirc:
    * the expansion of the Universe means that distant objects can be retreating faster than light
    * if Alice travels to your left at 4c/5 and Bob travels to your right at 4c/5 then you will observe Alice and Bob's separation growing at 8c/5 > c.
    * if you point a laser beam at a screen you get a dot - if you jiggle the laser a little bit, the dot jiggles a bigger bit - how big depends on how far away. Sufficiently far away and the dot moves faster than light.
    * some people describe quantum entanglement as involving information travelling faster than light

    To go FTL, you need to obey relativity and obey causality.

    @BIll_K: I thought the electron "shell"-transitions where basically EM dipole interactions (well, the kind OP is talking about) ... so the time frame is of order 10-23s anyway - so even in the "old QM" it's not faster than light?

    I should check - an atom is of the order of an angstrom across ... time for light to cross an atom is 10^-10/3x10^8 ~ 10^-19 > 10^-23 darn!
     
    Last edited: Jun 3, 2013
  5. Jun 3, 2013 #4

    mfb

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    Dipole interactions with visible light have a corresponding timescale of at least ~500nm/c, which is certainly longer than the time light needs to cross an atom (<1nm/c).
    Some transitions have a higher energy, but then the orbitals are smaller, too.
     
  6. Jun 3, 2013 #5
    The dot does not move FTL. It's not the same dot (or set of photons).
     
  7. Jun 3, 2013 #6

    mfb

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    The position of the dot moves. This position is not an object and no information (or anything else) is transferred along the dot, so it does not violate special relativity.
     
  8. Jun 3, 2013 #7
    Are you saying that if I shine a laser beam towards Pluto and wait 30 minutes so that it makes a dot, I can move the laser source and the dot will move ftl? Clearly this is impossible and a misconception, as we only have photons that travel at c and their source.
     
  9. Jun 3, 2013 #8

    Doc Al

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    Sure. So what?

    It's not impossible. The misconception is thinking that a "dot of light" is a single thing that moves. It's better to think of the moving dot as really new dots that are being formed in different spots along the surface, so it looks like something is moving.
     
  10. Jun 3, 2013 #9

    It's plain wrong.




    False. The dot is made of photons and no photons can move ftl, whatever the mode of their transition from one place to the next.
     
  11. Jun 3, 2013 #10

    Doc Al

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    Let me clarify my response. Shine the laser beam towards Pluto, sweeping it across the surface. When light finally gets there (it will take longer than 30 minutes), the "dot" will sweep across the surface in the time it took you to sweep your laser beam from one side to the other, thus faster than light.

    Don't point at Pluto and wait and then move the laser. You'll of course have to then wait for the light to reach the other side.
     
  12. Jun 3, 2013 #11


    No, the dot will remain in the same place another 30 minutes or so after you move your source. After those 30 minutes, the dot will move across the surface. Remember, we always live somewhat in the past because of c. That's not ftl.
     
  13. Jun 3, 2013 #12

    Doc Al

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    Perhaps you don't know what is meant by a moving dot?

    Nothing is really moving faster than light of course, but the "dot" which is not a stable "thing", looks like it's moving faster than light.

    If you measured the position of "the dot", it would be "traveling" along the surface of Pluto at faster than light speed. (The moon is a more common example.)

    Of course what we call "the dot" is really just a series of reflections of the light as it hits different parts of the surface. No problem in having that series of reflections appear to move faster than light.
     
  14. Jun 3, 2013 #13


    See my previous post, the dot will not move ftl. It's made of photons and no photons have ever been observed to move ftl. You only have a source, photons and SR. Nothing in your set up allows ftl movement of the dot.
     
  15. Jun 3, 2013 #14

    Doc Al

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    Right.

    Right. At what speed? :wink: (That's where the apparent faster than light speed comes in.)

    Next time the moon is out, do this: Point your finger at the moon, sweeping across its surface from left to right, which will take a fraction of a second. If you replace your finger with a laser beam, you will have created a dot on the moon's surface (once the light gets there) that will move across the surface in that same fraction of a second it took to move your finger. That's all we're talking about.
     
  16. Jun 3, 2013 #15

    At c. Photons cannot exceed c.



    This is a different scenario and I agree that it's at least theoretically possible that if you could extend a SOLID rod from the Earth to Pluto that sweeping it across would cause the tip to move ftl. But that's practically impossible and the laser beam dot is demonstrably not going to move ftl. Especially theoretically.

    EDIT: The tip of the rod will not move ftl as the energy required to move the rod as the speed approaches c will be greater than the energy of the Solar system.
     
    Last edited: Jun 3, 2013
  17. Jun 3, 2013 #16

    mfb

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    Maui: I think you don't understand what Simon Bridge, me and Doc Al mean.

    Point a laser pointer towards one edge of the moon, and rotate it so quickly that it points towards the other side of the moon after 1 millisecond. Let's assume that the moon is a flat disk here.
    Roughly 1 second after your experiment started, the laser pointer will make a dot at one edge of the moon. 1 millisecond later, the dot is at the other side, and it has been everywhere in between in this millisecond. If you calculate the speed of the dot, you get a speed above c. No photons moved faster than c, but the dot as "point where photons arrive" did.


    Edit: Oh, new posts.
    No, that is the scenario we were all talking about the whole time.
    NO! Now that is impossible.
    It is possible, both in theory and experiment.
     
  18. Jun 3, 2013 #17

    Cthugha

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    Oh, come on. This is a standard textbook question even given to people in school regularly to demonstrate the difference misconceptions about what physical objects are. Don't tell me you never heard of it.

    And no, this is not what people here are talking about. You shine a laser at Pluto and rotate it. If you rotate it fast enough and track the dots as the position where the light beam hits Pluto versus time, you will see that the dot moves faster than the speed of light. That is no problem as it is not a physical object, carries no information and no single photon ever exceeds c.


    edit: Why do other people always write almost the same answers way faster while I am typing? I must be getting really old now.
     
  19. Jun 3, 2013 #18

    Doc Al

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  20. Jun 3, 2013 #19

    You keep saying the dot will move ftl but I am interested in how and why? How specifically, given that the dot is made of a stream of photons striking the surface. It will take more than assertions, give me the technical details.
     
    Last edited: Jun 3, 2013
  21. Jun 3, 2013 #20
    #



    Same here, only assertions and nothing of technical nature. Are we in the assertions forum?

    We already covered why the dot will not move for at least 30 minutes after you move the laser source but maybe you didn't read the thread.
     
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