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Direction of a photon is fixed upon creation or observation?

  1. Feb 10, 2015 #1
    My question is a little more broad than I could make clear in the title but here goes.

    First of all, to be clear, am I correct in understanding that light behaves in precisely the same way as an electron in the sense that there exists a wave function which evolves according the Schrodinger equation and which is "collapsed" upon some kind of observation to produce measurable properties?

    Secondly, suppose we have an electron oscillating in a vacuum. This will create an electromagnetic wave correct? Now my first question is this, is there any directional bias in that wave as a result of the direction in which the electron oscillates? So if the electron is oscillating back and forth along some arbitrary x-axis, does the conservation of momentum play any part in fixing the direction of travel of the electromagnetic wave from the start? Of course, there has to be something else involved to make the electron oscillate... So suppose we had two electrons attached by a spring and we offset the system such that it would oscillate back and forth as a closed system. The totally energy and momentum are conserved, so does this mean that there is no need for any emitted photons to assume a particular direction right from the start? And presumably this hypothetical system would gradually lose energy and come to rest as a result of this emission? (This is starting to sound like the old classical issue with the electrons spiralling into the nucleus)

    More generally, does an EM wave actually have a direction of travel? Or is it more correct to think of it like a ripple on a pond spreading out in all directions from the source, and the apparent direction of travel is merely something which is inferred from the place in which it is later detected?

    After all, we never truly observe a photon. We can observe that electron A at position, x and time, t has lost a certain amount of energy, and we can observe that electron B at position, (x + dx) and time, (t + dt) has gained an equal amount of energy and so we say that "a photon has travelled in a straight line from electron A to electron B". But in reality, all we have are two electrons exchanging energy at a distance over an amount of time which is related to that distance by the so called "speed of light". Does a photon really need to exist to explain this or have we merely inferred the existence of a photon to explain the way that the energy moves from one place to another?

    Apologies for squeezing 4 or 5 questions into one post but I believe this is really one general discussion/question about the nature of light.
     
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  3. Feb 10, 2015 #2

    mfb

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    Light is more complicated, but there are some similarities.
    In general, yes.
    Yes, but I would not use conservation of momentum to analyze this because then you need the momentum change of the electrons. Anyway, you'll always get some angular dependence for the emitted radiation, but never a fixed ideal direction.
    Every part of it has, like water waves.

    Have you ever seen an electron? Physics just provides models to describe experimental results. Asking if something actually exists is pointless.
     
  4. Feb 10, 2015 #3
    First of all thanks for the response. Secondly, I must disagree that asking if something actually exists is pointless. Physicists used to believe in the existence of the Aether but we have since done away with that. I agree that asking aimless existential questions may not be helpful but there is a point to it. Suppose we are barking up the wrong tree? What if the apparent existence of this thing we call "light" is really a red-herring. Suppose that in truth, the two electrons are really right on top of one another (in some other abstract sense, I'm aware of the exclusion principle) and that the energy transfer does not actually take place over a distance/time but that in fact the distance/time difference that we observe is purely an illusion. This could explain very well why the ratio of distance to time is always the same, c.

    Fro the perspective of a photon, there was no journey, the time dilation would be such that strictly speaking, the birth and death of a photon occur at the same instant. This would fundamentally change our interpretation of light. Asking questions about what does and does not exist will never be pointless.

    And perhaps we haven't been able to poke an electron with a stick and say "Yep, found it" but at least we can directly interact with it. It has mass and charge. A photon is almost pointless in it's existence, it has no mass, no charge, nothing that can actually be measured. It is purely an inferred particle.
     
  5. Feb 10, 2015 #4

    bhobba

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    You need to ask sensible questions.

    Why would you think something we encounter all the time doesn't exist?

    Remember when thinking about this physics is not concerned with philosophical semantics.

    Why do you think a photon has a perspective - meaning it can be looked at in a classical sort of way such as having properties when not observed.

    It has energy, momentum and spin all of which can be measured. And this takes us back to your existence thing - since it has these measurable properties why would you doubt its existence.

    Just a suggestion - instead of wild speculation based on misconceptions it would be more useful to learn the facts. The reason Einstein was able to successfully challenge the orthodoxy of his time was he knew that orthodoxy very well.

    Thanks
    Bill
     
    Last edited: Feb 10, 2015
  6. Feb 10, 2015 #5

    Because we don't encounter it! Try to think outside the box here... Isn't it apparent that with all difficulties in interpretations we're facing in QM that it's time to think a little differently. The only way we experience light is as a transfer agent for energy, we never directly deal with photons. We watch as some energy disappears over here and reappears over there. Having clearly seen that locality is not a fundamental part of the universe, is it really necessary to postulate the existence of a photon anymore? Can we not explain all of the effects of light in the manner I describe above?

    Personally I think I am asking sensible questions, physics has been going around in circles for the last 20 years because no one is willing to question the accepted understanding. I'm not saying it's wrong, but why can't we have a discussion about it? Have you ever considered that light is not "fast", instead, we are slow? What if we are looking at the universe from a 'stretched out' perspective, wherein things which are actually occurring at the same place and time, appear to us to be separated by some distance in space time. Then suppose the apparent time take is always directly proportional to the apparent spacial distance and that the proportionality constant is universally the same. We call it the "speed of light" but maybe it's time to stop think in terms of "speed" and start think in terms of space to time ratios. Does this make any sense at all? I'm trying not to sound like a mad man here but I think this perspective has some merit to it.
     
  7. Feb 10, 2015 #6

    bhobba

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    Why would you believe we don't encounter photons?

    We can measure their energy for example.

    What difficulties are you referring to?

    IMHO QM has one difficulty, and one difficulty only. It doesn't matter what issue worries you in QM we have an interpretation that fixes it - but we cant decide experimentally between them.

    Thanks
    Bill
     
  8. Feb 10, 2015 #7
    Ahh but not directly! This is the point I'm making my good man. The only way for us to measure that energy is for the photon to interact with something else such as an electron. We measure the energy of the electrons that the light interacts with, not the light itself. Combining this fact with the knowledge that photons have no mass, no charge... Surely it's not too great a leap to question if photons are anything more than a useful mathematical construct.
     
  9. Feb 10, 2015 #8

    bhobba

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  10. Feb 10, 2015 #9
    I'm not sure you understand my point. That experiment perfectly demonstrates what I'm talking about. They do not directly detect the photon but instead measure the energy of something that the photon interacts with. It specifically states in the first paragraph "When an incoming X-ray hits the xenon gas, it will transfer its energy to the xenon atom, causing an electron to be knocked off."

    This is what I'm talking about. We do not measure the energy of the photon directly, we measure the effect it has on electrons.
     
  11. Feb 10, 2015 #10

    bhobba

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    You missed the bit I added - we learn about anything by interacting with it. The electrons whose energies we measure to determine the photons energy how do we measure that?

    Thanks
    Bill
     
  12. Feb 10, 2015 #11

    anorlunda

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    Would you care to explain to us how you measure anything, big or small, without it interacting with something? How would you define measurement as distinct from interaction?
     
  13. Feb 10, 2015 #12
    Well exactly, it's an endless chain of things observing other things but the difference is that electrons for example has mass and charge, they have observable properties. An electron can exist on it's own, a photon is only a packet of energy.

    If there's one vital thing QM has taught us, it's the principle of retrospective collapse. That is, that certain events do not really happen as the experiment is ongoing but are instead filled in retrospectively upon collapse. Take the set up in the delayed choice experiment, it's clear that the decision about which route the photon takes is not determined until after detection. The route is determined retrospectively.

    In exactly the same way, when a photon travels from one electron to another, can it really be said to exist in the time between emission and absorption? I do not believe that a photon can exist independently. It only exists as an inferred particle to describe the mechanism by which electrons exchange energy. That is not a good enough reason to suggest that it has a physical presence in the universe. The simple fact that a photon can never be directly observed is testament to this. Only the end result can be observed. And electron can be interacted with directly, you can move it around with an electric field, it can be excited, it can exist independently. There is absolutely no evidence that a photon can exist on it's own.

    It's like saying "I saw John at the start line" and "I saw John at the finish line"... 100 years ago that would have been enough to say that John ran the race but not with QM. We know that the state of the system between observations is not precisely defined.
     
  14. Feb 10, 2015 #13

    bhobba

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    Photons are like anything else - we know about them by their interactions some of which we call observations. A measurement is simply an observation a number has been assigned to.

    Now we are getting to a genuine issue in QM - defining an observation rigorously as distinct from interaction. In any actual application its easy to see where an observation occurred - eg in Schroedinger's Cat its at the particle detector - but defining it exactly is another matter. That however has now been solved - its just after decoherence. It doesn't solve the measurement problem but has shifted it to why do we get any outcomes at all. Of course a theory always has some unexplained things and that is simply one QM has. Of course some interpretations try to answer that question - with varying degrees of success.

    Added Later:
    I apologise for the first line I have now removed. It was addressed to James White who was not the poster of this valid point.

    Thanks
    Bill
     
    Last edited: Feb 10, 2015
  15. Feb 10, 2015 #14

    bhobba

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    Why do you consider mass and charge existing on their own and not momentum or energy?

    Thanks
    Bill
     
  16. Feb 10, 2015 #15

    bhobba

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    Why do you think a photon has the property of travelling when you are not observing it?

    Thanks
    Bill
     
  17. Feb 10, 2015 #16
    I would love to.

    This is the measurement problem with a different suit on is it not? The question really is, what is a measuring device? Where does the Von Neumann chain end?

    But I never meant to imply that you can observe elementary particles without them interacting with something else. But there is an important difference between a say, an electron and a photon. Electrons can exist on their own, you can leave an electron in the middle of empty space and it will hang out there until something comes along and moves it. A photon is not like that at all. A photon has absolutely no properties that make it sound at all physical, it has no mass for a start. It supposed whizzes around at this incredible speed. My suggestion is that it doesn't whiz around at all and that we are wrongfully asserting it's existence where it is not needed.

    Imagine if you will that two electrons which appear to be separated by some distance, are actually right next to eachother (in some abstract sense beyond normal space and time). And by being next to eachother they exchange an amount of energy. Simple, no need for a 3rd particle to carry the energy. Now what if our view of reality is not fundamental and we are in fact seeing a distorted version of events? What if our perspective is distorting this event in such away as to separate the two electrons in spacetime. And because space and time are always related by this fixed proportionality constant, c - we always observe an apparent constant "speed" of energy transfer. Purely because the ratio of apparent distance to time taken is always the same.
     
  18. Feb 10, 2015 #17

    bhobba

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    It isn't - and that's not the question from the modern viewpoint with out knowledge of decoherence. We know exactly where to put the Von-Neumann cut - just after decoherence. A measuring device is simply anything capable of producing decocohence and it happens all the time - the environment is constantly observing everything. For example a dust particle is decohered by a few stray photons from the CBMR and given a definite position. The answer to Einsteins question to Bohr is the moon there when its not being observed is its never not being observed. BTW the joke wasn't on Einstein for being wrong - the joke was they were both wrong:
    http://www.fisica.ufmg.br/~dsoares/cosmos/10/weinberg-einsteinsmistakes.pdf

    The measurement problem is actually quite subtle and has a number of parts. Its explained in section v2.5 of the following:
    https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

    Decoherence explains many parts such as the preferred basis problem and what is an observational 'apparatus', but the problem it cant explain without further assumptions is why do we get any outcomes at all - technically it's how an improper mixture becomes a proper one:
    http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

    QM's answer to the questions that have been asked here is we only ever know anything by interactions, specifically the interaction called decoherence, and they are called observations. QM is a theory about those observations - ie its a theory about interactions. What its doing, being, travelling etc etc as has been mentioned in this thread when not observed the theory says nothing - its silent. Photons, electrons, protons etc etc are al exactly the same in that respect.

    Thanks
    Bill
     
  19. Feb 10, 2015 #18

    bhobba

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    Can I ask where you got that from?

    It most definitely is NOT what QM says.

    Electrons and photons are exactly the same thing - excitations in an underlying quantum field - for photons its the EM Field - for electrons it the electron field.

    Thanks
    Bill
     
  20. Feb 10, 2015 #19
    Remember that decoherence produces what looks like a definite position - in actuality decoherence does nothing more than entangle the particle with the rest of the environment (and not produce collapse).
     
  21. Feb 10, 2015 #20

    anorlunda

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    A photon may not have rest mass but it most certainly has momentum. For a down to earth application of that, consider the detonation of a two stage thermonuclear warhead. The momentum of those photons is enormous!

    http://en.m.wikipedia.org/wiki/Thermonuclear_weapon#Radiation_pressure
     
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