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True or false? Fundamental particles

  1. Feb 18, 2012 #1
    True or false?

    1. It is not possible to prove the point of origin of a photon
    2. It is not possible to prove the point of origin of a free electron
    3. It is not possible to prove that protons or neutrons exist inside a nucleus

    Can someone please confirm?
  2. jcsd
  3. Feb 18, 2012 #2


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    There is no way to answer such a question, because you have not given a physics premise to DEFINE what you mean. For example, what is the meaning of "point of origin"? I can point to you a crystal that, when I excite, will emit light. There! I've shown a point of origin of that light! Is that enough? Why not? After all, you left the definition of "point of origin" to me.

    Secondly, what is meant by "prove"? In science, there is no such thing as a rigorous "proof" similar to mathematics. While one can start from "First Principles", the First Principles themselves are arrived at without proofs other than a consistent set of valid observations. So you are asking something out of science that it wasn't meant to do or can't do.

    The thing about science, and physics in particular, is that WHAT WE ASK is often as important as the ANSWERS that we seek. You need to think a little bit more of what exactly it is that you are asking, and how you frame such a question in ways that have clear physics definitions. It is certainly naive to think that these things can be addressed via a simple "True or False" responses.

  4. Feb 18, 2012 #3
    Will you please clarify the first two statements.Prove what about the points of origin?What, exactly,do you mean by "point of origin".As for statement 3. doesn't a nucleus contain protons and neutrons(nucleons) by definition there being cartloads of evidence to back this up?
  5. Feb 18, 2012 #4
    There was (sometimes literally!) cartloads of evidence that all swans were white - then black swans were discovered in Australia.
  6. Feb 18, 2012 #5


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    So where is the "black swan in Australia" in THIS case?

  7. Feb 19, 2012 #6
    Hi there,

    Happy to offer more clarity. One reason for the framing was to keep the post succinct.

    Another way to frame the question is "if we observe a photon, can we prove where it originated from i.e. where it was created?"

    So I guess 1,2,3 could each be rephrased as two questions:
    a. What method do we use to assess where an observed photon/electron/proton/neutron originates from?
    b. Can that method discern the difference between whether the particle was 'freed', 'created', or 'deflected' at that point of origin?

  8. Feb 19, 2012 #7


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    I'm sorry, it seems like you are asking whether we can prove that light in my office comes the light bulb in my ceiling fan. Things in motion can usually have their paths determined fairly easily. For single particles other than light, there is rarely an instance where you would detect them other than in specific experiments where you know the location they are emitted from.

    As for determining whether they were freed, created, or deflected, this is usually determined by where are they coming from. Electrons inside a CRT are quite obviously being freed from the electron gun by high temperature.
  9. Feb 19, 2012 #8
    Yes, I am asking that - trying to establish a subtle point.

    Technically, we can't prove that the light you observe came from your ceiling fan, can we?

    The photon that hits your eye might have come from an air molecule somewhere along the way.

    Similarly, with protons from particle collisions - technically, we can't prove whether they got 'knocked out' of the nucleus, or created instantaneously in the collision. Is that right?
  10. Feb 19, 2012 #9
    Yes technically you can find lots of reading that all agree this is true for all of us.

    Prove it.

    Technically, does it matter? The duration of the nucleus and how it splits is what the experiments are for. Read about them, they write papers so the rest of us can “see” the experiments, you decide what evidence constitutes proof.
  11. Feb 19, 2012 #10


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    You argument really depends on what you mean by "Prove".

    Per dictionary.com :
    to establish the truth or genuineness of, as by evidence or argument: to prove one's claim.
    Law. to establish the authenticity or validity of (a will); probate.
    to give demonstration of by action.
    to subject to a test, experiment, comparison, analysis, or the like, to determine quality, amount, acceptability, characteristics, etc.: to prove ore.
    to show (oneself) to have the character or ability expected of one, especially through one's actions.

    I think it is reasonable to say that I can "prove" the photons originated from my light bulb.
  12. Feb 20, 2012 #11
    I guess - in cases where there is more than one possible explanation, to be sure that the current theory is the most accurate description of what happened

    To use the photon/light bulb example, there are two possibilities
    1. The photon that hits your eye is the same photon that left the lightbulb
    2. The photon that hits your eye is a different photon - the original photon got absorbed by an air molecule along the way, and a new photon got re-emitted

    Or in particle collisions:
    1. The particles that leave the collision got 'knocked out' of the nucleus i.e. they were there all along
    2. The particles that leave the collision got 'created' in that moment i.e. they just came into being

    Is there a way of being 100% sure that the explanation is 1) and not 2)? Please justify.
  13. Feb 20, 2012 #12


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    You can be sure that the standard theories are the most accurate with the least and simplest assumptions to fit our observations.

    I don't believe air is capable of absorbing and then re-emitting an identical photon in such quantities as a light bulb emits.

    There is never a 100% way to be sure of anything. Current explanations is that a particle exists and can be knocked out of the atom. It is conceivable that such particles "meld" together in one big lump and simply create the appearance of a composite particle, however the assumptions we would have to make would be much more complicated and...disagreeable... than current models. It comes down to simply believing that the particles exist as a complete separate part of the system. In the end though, if the way they interact is no different with either theory, then either one is perfectly capable of being used.
  14. Feb 21, 2012 #13
    The very successful modern physics theories requires you to NOT distinguish identical particles (the photon that goes in and the photon that goes out) like this. Meaning, there are experiment whose result cannot be explained if we cling to the classical notion of distinguishing all the photons in the world.

    As far as we know, they just come into being.

    Nucleons is probably not a very good example because they are composite particles (made up of quarks). We have collided electrons with positron, both elementary particles and not made of other. Yet, we observed that other particles (e.g. muon and anti-muon) has came out of the collision with high enough energy. So, you can say that the muon and the anti-muon are created by the collision.

    If we go back to nucleons. Well, protons and neutrons are made up of two kinds of quarks: up and down, but when we collide atoms, we sometimes make particles made up of other kinds of quarks. So you can again say that the new particles/quarks came into being.
  15. Feb 25, 2012 #14
    Okay mathfeel, that helps, thanks.

    But quarks have never been observed, right? So if it's possible that protons and neutrons just 'get created' when something hits a nucleus/nucleon, then it's also possible that quarks don't actually exist.

    Which would change the theory somewhat.
  16. Feb 25, 2012 #15


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    That depends highly on what you mean by "observed". Deep inelastic scattering has revealed that there is a substructure in protons/neutrons that is best explained by having 3 particles forming them. http://en.wikipedia.org/wiki/Deep_inelastic_scattering

    If you are bouncing rocks off of the bottom of a very shallow puddle of water that you can't see past the surface of, and the rocks keep skipping off at different angles instead of the same one every time, you would conclude that the bottom of the puddle is not smooth, but is rough and uneven, giving the rocks their odd bounces. A similar but much more complicated idea explains deep inelastic scattering.

    When particles are smashed together in very high energy collision, the energy of the collision is used to create particles that didn't exist before the collision. However this does not mean that quarks and protons/neutrons don't exist. Smashing atoms and particles together at low energies doesn't create new particles, it simply knocks the existing particles out of the nucleus.
    Last edited: Feb 25, 2012
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