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Another double-slit question

  1. Jan 15, 2015 #1
    When performing the double slit experiment using one photon at a time does the single photon wave front hit both slits at the same time?
  2. jcsd
  3. Jan 15, 2015 #2


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    The wavefront does diffract off of both slits at the same time, and it is that wavefront that will describe the interference pattern on the screen.

    As far as whether the photon itself went through one slit or another, that is an unresolved issue in quantum metaphysics.
  4. Jan 16, 2015 #3
    Sounds to me like the real question is whether the photon is a particle or simply a disturbance of space/time?
  5. Jan 16, 2015 #4


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    It is not a particle like you're imagining, not in any way, shape, or form. It is also not a 'disturbance of spacetime'. It is the quantized interaction of an EM wave. We have almost a complete understanding of what a photon is and how it behaves. It's only in the regime of extraordinarily high energy that our understanding breaks down.
  6. Jan 16, 2015 #5


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    Where you got the idea its a disturbance in space-time has me beat.

    If you want to see a real explanation of the double slit check out:

  7. Jan 16, 2015 #6


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    Quantum mechanics is only really prepared to answer what you are likely to find when you measure things.

    Over time, you can get an idea of what a photon is like due to working with quantum theory, but quantum "objects" are in classes all their own.
  8. Jan 17, 2015 #7
    As far as I know there is only one way of creating a photon and that is by accelerating a charged particle. Now where does the photon come from if not from disturbed space/time.
  9. Jan 17, 2015 #8


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    Accelerating a charged particle doesn't create a photon, it creates electromagnetic radiation, and it does so by disturbing the electromagnetic field without (significantly) disturbing space-time. Purcell's first-year E&M textbook has a very good heuristic explanation of how the electromagnetic field is "disturbed" to produce radiation.

    To bring photons into the picture, you have to look at what happened when the electromagnetic radiation interacts with some other matter (or you have to be talking quantum electrodynamics, and that's not what we're talking about here).
  10. Jan 17, 2015 #9


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    Electromagnetic fields are fundamental objects. There's nothing simpler you can derive it from, at least not within our understanding of nature today. The source of the electromagnetic fields are electrically charged particles. The electric charge itself is also a fundamental property of matter that cannot be derived from some more simple principles (today). Physics is, despite an astonishing success to summarize the description of nature into a few natural laws formulated in terms of mathematics (which is the only adaquate language to describe nature), an empirical science, i.e., fundamental principles have to be deduced from many careful and accurate observations and quantitative measurements. Some notions turn out to be very fundamental.

    In this way around 1900 it turned out that we cannot understand the behavior of atoms and subatomic particles (which were discovered by observation around this time) in terms of classical physics, and finally a lot of thinking lead to the discovery of quantum theory.

    One of the most complicated issues are photons, because they are very far from our daily experience. E.g., it is quite difficult to produce precisely one photon, which is a certain quantum state of the quantized electromagnetic field. The concept of quantum field theory has to be envoked necessarily when it comes to relativistic phenomena, because the inner structure of relativistic space-time leads to the conclusion that quanta can be destroyed and created in reactions of other quanta.

    An accelerating charged particle doesn't simply create a single photon but another kind of states of the quantized electromagnetic fields, socalled coherent states. In such a state the electromagnetic field does not consist of a certain determined number of photons but it's a socalled superposition of many-photon states with any photon number.

    To create a single photon, one way is to excite an atom to a higher energy state and then wait a while (the mean time you have to wait, the lifetime of the excited state, can, by the way, also calculated with quantum theory) until the atom relaxes again to a lower-energy state. The energy difference is usually radiated as a single photon.

    In more recent times the discovery of a process in certain birefringent crystals, called parametric downconversion, has made it much more easy to create single-photon states, and that boosted a whole industry in high-precision quantum-optics experiments using such a photon source. There you shoot a strong laser beam, which also is a coherent state of the electromagnetic quantum field, into the crystal and sometimes a photon from this coherent state is picked up by the material which in turn emits two photons of lower energy (usually two photons with have the energy of the original photon from the laser beam). These two photons have very fascinating quantum features in being in a socalled polarization-entangled state, and you can do a lot of fascinating experiments to test very fundamental predictions by quantum theory which contradict our current everyday experience dramatically. Another application, however, also is to have a "heralded single-photon source". The above described parametric downconversion processes ensures you that precisely two photons get emitted, and you can detect one of those photons at one place. Usually in doing so it's absorbed by the detector, but then you know for sure that there's one (and only one!) other photon.

    Although the very structure of relativistic quantum field theory follows to a large extent from a careful mathematical analysis of the structure of (special) relativistic space time in terms of its symmetries, it's wrong to say that a photon comes from "disturbed space-time". That simply doesn't make sense. It's coming from reactions involving charged particles (like an electron and a positron annihilating into two photons) or light with matter (which after all also consists of charged particles) as in the example with the parametric downconversion.
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