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B 2 events in a planck time not violate conservation of energy

  1. Apr 23, 2016 #1
    This thread is about a topic that unexpectedly came up in my thread titled "Contradiction between Cosmos and what someone here told me?". I will subsequently call that thread "Contradiction" for brevity. I am not sure if this thread belongs on the Quantum Physics forum or the chemistry forum. If the moderators think it should be somewhere else, I suppose the moderators have the ability to move it to wherever forum it should be. The topic of this thread is distinct from the topic of the Contradiction thread, and I feel it deserves its own thread b/c it's so distinct.

    First, let me give a little bit of background on how this topic came up. In my thread "Contradiction" I mentioned how someone here explained to me the three ways that bound state electrons and photons interact with each. Then I mentioned how in the episode Hiding in the Light in the 2014 series COsmos, the host Neil deGrasse Tyson Tyson explained how photons of light and the electrons of an atom interact with each other. Tyson said that when an electron absorbs a photon, the energy of the photon will cause the electron jump up to an orbital with a higher energy level. I've known that for over a decade. But then Tyson said that nobody knows why electrons drop down to lower energy levels. The purpose of the thread Contradiction was to determine what causes electrons to drop to lower energy levels. All my life before I saw the Cosmos episode yesterday, I always thought that the cause of electron's dropping to a lower energy level is the emission of the photon. Then physicsforums member phinds told me that I had it backwards. Phinds told me that the emission of the photon is the effect of the electron's dropping energy levels, not the cause of the electron's dropping energy levels. I asked a physicist about this, and the physicist said that neither the emission of the photon nor the electron's dropping energy levels is the cause of the other. I asked the physicist how he knows this, and the physicist said he knows that neither can be the cause of the other because both events occur simultaneously. The physicist said he knows that both events occurred simultaneously because if they did not occur simultaneously, it would violate the law of conservation of energy. The cause must come before the effect. If two events occur simultaneously, neither could have caused the other.

    Then user phinds says phinds think that if the electron's dropping to a lower energy level happened in an infinitessimaly small amount of time (perhaps a Planck time) before the emission of a photon, the law of conservation of energy might not be violated because the time between the electron's dropping energy levels and the emission of a photon were in such an infinitessimaly small amount of time between each other.

    If the electron's dropping energy levels occurred in an infinitessimaly small amount of time before the emission of a photon, would the law of the conservation of energy be violated? Please elaborate on your answer.
     
    Last edited: Apr 23, 2016
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  3. Apr 23, 2016 #2

    Nugatory

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    The entire question is badly posed. We have a system prepared in one state (excited electron) and we use the rules of QM to calculate the probability that it will transition into another state (ground state electron and a photon). There's nothing in the theory about what happens in between. (This might have been what Tyson was getting at).
     
    Last edited: Apr 23, 2016
  4. Apr 23, 2016 #3
    I don't think that the question is badly posed at all. You say that there is nothing in QM theory about what happens in between. Then why not go beyond QM theory? You did not answer the question.
     
    Last edited by a moderator: Apr 23, 2016
  5. Apr 23, 2016 #4

    Nugatory

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    We have no theory other than quantum mechanics to describe this interaction, so there's no "beyond" to go to.
    Quantum mechanics says there is a before state and an after state but no in-between state. Thus, asking about in-between states (the photon appears while the electron is still excited, or the electron has dropped to the ground state while the photon hasn't appeared quite yet) makes no sense - these states don't exist so there's no way we can say they do or do not conserve energy.
     
  6. Apr 24, 2016 #5
    Assuming for the sake of argument that everything you said is true, asking about in-between states in terms of Quantum Mechanics makes no sense. It appears that you agree with the physicist I talked to on private messages. The physicist says that the electron's dropping energy levels and the emission of the photon takes place simultaneously. If both events occur simultaneously, the implication of this is that neither of the two events causes the other. I have frequently said here that it appears to me that a lot of science at the atomic level relies on a lot of uncashed checks. Even though Quantum Mechanics says nothing about in between states, has it been proven that there are no in-between states in reality? I mean, isn't it possible that there actually is an in-between state in reality but nobody knows anything about it?
     
    Last edited: Apr 24, 2016
  7. Apr 24, 2016 #6
    the Contradiction thread in the Chemistry forum, I mention that Neil deGrasse Tyson said that nobody knows why an electron drops down to a lower energy level. Physicsforums member DrDu wrote the following in response to my comment that Tyson said nobody knows why an electron drop down to a lower energy level: "If he really said so, that's utter nonsense. As has been mentioned before, we can calculate the rate of spontaneous emission with fantastic precision using QED. While QED calculations are in deed involved, the main argument is easy to sketch: The electromagnetic field is a quantum object and as such the electromagnetic field is not completely absent even in the ground state (called vacuum). I.e. although the number of photons is zero in the vacuum, there is still an electromagnetic field present which perturbs the hydrogen atom and causes stimulated emission. Compare the electromagnetic field with the hydrogen atom: Also in the ground state of the hydrogen atom, the momentum and energy of the electron are non-vanishing and the 1s orbital has a finite size."

    In my opinion DrDu is saying the reasons why an electron emits a photon, but DrDu never explained why an electron drops down to a lower energy level. DrDu seems to think he explained why an electron drops to a lower energy level, but I don't see it. Nugatory, what do you think about what DrDu wrote and whether or not it explains why an electron drops to a lower energy level?
     
  8. Apr 24, 2016 #7
    As I have read on numerous other threads on this forum, physics is very good at explaining 'what' happens, but not so good at explaining 'why' it happens. 'Why' is a pretty slippery concept anyway, since it depends on what you consider a good explanation. If a physicist were to say the electron drops to a lower energy level because nature favors a state of lower energy, you could reasonably ask, "Well, why is that?" In the end, nobody really knows, that's just what we always see. (Maybe a real physicist could elaborate here.)
     
  9. Apr 24, 2016 #8
    Sandy, you're saying that science at the atomic level relies on a lot of uncashed checks. When I have stated this at physicsforums, other members here disagree with me.
     
  10. Apr 24, 2016 #9
    I don't think that's what I said. What I mean to say is, physics is a descriptive science, and I don't think any living human can explain 'why' things are the way they are. Of course, this is starting to venture into philosophy, and may attract unwanted attention from the mods.
     
  11. Apr 24, 2016 #10
    If a physicist cannot explain why things are the way they are, the physicist must prove his theories through falsifiable experiment or it's all just baloney. Explaining why things are the way they are is a fundamental part of proving one's theories. I think discovering why things are the way they are is a worthy endeavor.
     
  12. Apr 24, 2016 #11

    Nugatory

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    If you strike out the first part ("If a physicist....the way they are") this would be just fine; falsifiabity through observation and experiment is what it's all about. That's why we take quantum mechanics and quantum field theory so seriously. They are the most solidly confirmed theories in human history, and there is no doubt that they are the best description of how the universe behaves we've ever had.

    That doesn't mean that they explain WHY the universe works the way it does. They don't, and as sandy stone says, that's not a problem because we don't expect any physical theories to do that. Go all the way back to the first modern theory of modern physics - Newton's laws of mechanics and gravity - and there's no WHY there, although there is an accurate description of gravity and mechanics. Why should gravitational force vary as the square of distance? Why is the relationship between force and acceleration ##F=ma## and not something else?

    QM is no different in this regard. What is different is that QM is both wildly at odds with our common sense (also not a problem, because our common sense comes from a lifetime of experience with systems that aren't described quantum mechanically so is irrelevant) and far more mathematically demanding. When the math is beyond our reach, we're tempted to fall back on our misplaced classical intuition... And that leads us to ask meaningless questions like the one that started this thread. Classical intuition tells us that we can't get from "excited electron" to "ground state electron plus a photon" without passing through an intermediate state, and then we need to reason about energy conservation in those states. But as I said above, that entire question is ill-posed - no intermediate states, no question to answer about them, no uncashed checks because the account you're imagining they're written against doesn't exist.
     
  13. Apr 24, 2016 #12
    Nugatory, are you saying that the electron falls to a lower level for no reason, or are you just saying that there is no way for us to know the reason?
     
  14. Apr 24, 2016 #13
    I think Nugatory is saying that a good answer to a "why" question depends on the person asking the question. Perhaps the system dropping to a more stable equilibrium is a good answer. Perhaps because of QED is a good answer. Maybe that's not enough and someday strings or another theory will take it another step. But then somebody else will ask why about that.

    It's not so much that there is no reason or no way to know a reason ; the issue is that there is no unique answer to "why" that everyone can agree unambiguously settles the question (IMO it is still worth asking because it leads you to other questions).
     
  15. Apr 24, 2016 #14
    bluemoonKY, if you accept DrDu's explanation for photon emission then sandy stone completed the picture: "the electron drops to a lower energy level because nature favors a state of lower energy". With the understanding that electron loses energy via emission of photon.

    This sounds a bit fishy to me because it establishes which came first: emission. Next you'll wonder how long between emission and energy level transit, and want to know how the electron moves through intermediate levels. But as said in rest of thread, actually in QM the events are considered simultaneous, and we have no picture for transit.

    As for energy conservation - it's well accepted it can be violated as long as Heisenberg Uncertainty Principle isn't violated. Basically, energy balance sheet can be out of whack if it gets re-balanced in a very short time.

    QM (and physics in general) describes, and specifies calculations to predict, what will happen in a wide range of physical circumstances. Ultimately it doesn't, and can't, explain "why"; it shouldn't try to. Furthermore there are and always will be some circumstances beyond the range of current physics - no matter how far physics develops. Just my opinion.

    Nugatory is correct of course that QM doesn't say (or, I could add, care) what happens, in detail, when the electron transits. But if he's saying QM "proves" that "nothing" happens then - can't agree. Another theory might come along which does address that process. But until it does, why worry about it?

    Finally note there are various speculations about those details, some of which are sensible, which you can google.
     
  16. Apr 24, 2016 #15

    Nugatory

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    I am saying that we don't know whether there is a reason, let alone what that reason might be. We know the exact probability of it doing so in any given time interval, we know that energy and momentum will be conserved across the transition, we know the exact frequency of the emitted photon. But we don't know why this happens, or even whether there's no reason at all.

    That shouldn't surprise you, as every answer to a "Why?" question leads to another one. I already cited the example of Newton's laws which describe the behavior of macroscopic bodies without explaining why macroscopic bodies obey those laws. More than two centuries later, it was discovered that many of the essential properties of Newtonian mechanics are mathematical consequences of the isotropy of space - but now we have to ask why space is isotropic, and it never ends.
     
  17. Apr 24, 2016 #16

    jtbell

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    Remember Newton's famous statement: "Hypotheses non fingo."
     
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