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Some Thought Experiments (simple ones)

  1. Apr 12, 2010 #1
    1. A clock is placed on a light beam.

    Will the clock stop?

    If yes, why doesn't the beam stop altogether?

    2. A ruler is placed on a light beam.

    Will it's length shrink to zero? Why/Why not?

    3. A massless particle is placed on the beam.

    What would happen?

    4. An electron is placed on the beam.

    What would happen?

    The things to be placed on the beam can be even more varied.
  2. jcsd
  3. Apr 12, 2010 #2


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    I am afraid you will have to be more explicit. HOW do you "place a clock on a light beam"? Wouldn't it just fall right through?

    If you mean "move a clock at the speed of light", that's impossible. Nothing with mass can move at the speed of light. And asking "what would happen if they could" is like asking "Suppose relativity were wrong. What would relativity say about this situation?"

    As far as (3) is concerned, where you say "A massless particle is placed on the beam", you are essentially putting a photon into a light beam! Nothing new would happen.
  4. Apr 12, 2010 #3

    I think your contention is wrong because thought experiments are always highly hypothetical and don't tend to be contained in restrictions.

    That's why they are called thought experiments after all!
  5. Apr 12, 2010 #4


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    It's ok for thought experiments to be practically difficult, but they cannot be impossible in principle. To say anything about a thought experiment, you have to be in a conceptual framework in which they are in principle possible.
  6. Apr 12, 2010 #5

    I am also a confused learner, and I did like your questions, because I have the same ones.

    I am not sure I have any answers, so this reply is more of trying to understand the questions.

    First, when you say put a clock on the light beam - should I assume you mean the clock is travelling at the speed of light ?

    If that is your question it seems to me to be very valid. Certainly the wave frequency of light and various particles have been used to measure time, so in a way we can consider a beam of light to be a clock itself.
    So I think your question, and therefore all the following questions, are very worthy of discussion.

    Is what you are asking - what happens to various items when they travel at the speed of light ?

    If so then I'd be pleased to explore the issues through discussion.
  7. Apr 12, 2010 #6

    You got it right my dear!
  8. Apr 12, 2010 #7
    I am nobody to talk of Einstein, but when he did his thought experiments, they were indeed impossible in principle. It's only afterwards that a conceptual framework in which they became possible was devised.

    1. Light is at least 'something' if not matter. How could anything move with the speed of light?

    2. Photons of light have been proved to possess particle nature, How can these 'particles' travel with the speed of light.

    3. The photons of light must be involed in some vibration, oscillation, etc. When time stops at the speed of light, how are such processes possible in the case of photons.

    I have now asked direct questions rather than giving them the shape of 'thought experiments'.
  9. Apr 12, 2010 #8
    Because their rest mass is zero.
  10. Apr 12, 2010 #9
    What you have is an active imagination, not a thought experiment. There is a difference, and when you open with "on a light beam" you're going to get a reaction.

    That said, you have now asked questions, but given #1, it's clear you are leaping to conclusion with 2 & 3.

    As for what light IS, it's just part of the spectrum of Electromagnetism. You haven't asked questions, you've made assumptions, and want things to fit them. You need to learn many MANY more of the basics (such as how to read the diagram that would answer your question) before you form assumptions about what MUST be.
  11. Apr 12, 2010 #10


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    Nope. For example, Einstein's thought experiment where he considered what he would observe if he travelled with a light beam was performed in the framework of Galilean/Newtonian mechanics. In that framework, it is possible to move at any speed. By doing this thought experiment, he discovered that the galilean transformation formulas did not allow Maxwell's electrodynamics to obey the principle of relativity of uniform motions.

    Light particles have no mass. This is why they can move at the speed of light.

    You are imagining photons to be like ordinary objects like balls etc. They are not. They don't vibrate or oscillate. Secondly, a photon is a quantum mechanical idea, and you can't even really think of it as haveing a definite speed or trajectory, so it's better if you stick to classical electrodyanmics when you consider such questions.
    Last edited: Apr 12, 2010
  12. Apr 12, 2010 #11
    Damn it, that's practically like sacrificing a goat to summon the Bohmians! :rofl: (no offense Demyst and Zenith!)

    I think maybe, geometry would be a good place to start too; geodesics do make things far more clear.
  13. Apr 12, 2010 #12
    So, at the speed of light why is their mass of the value that it actually is? Why not more or less? Why not infinite?
  14. Apr 12, 2010 #13
    What do you mean? The photon rest mass is zero, period.
  15. Apr 12, 2010 #14


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    Not sure what you're asking here. Are you asking why is their mass what it is? Or why is the speed of light what it is?

    In the latter case, that is the 64 thousand dollar question.
  16. Apr 12, 2010 #15
    Ok. I ask again.

    You mean to say that light is energy. Does it mean that energy (being at least something) is able to move with the speed of light.

    So, what about mass energy equivalence. Mass can be taken to be as a concentrated (unqualified term, I suppose) form of enegy.
  17. Apr 12, 2010 #16


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    You have not asked a question.
  18. Apr 12, 2010 #17
    I am asking both the questions.
  19. Apr 12, 2010 #18


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    In both cases, they are the 64 thousand dollar question. We don't know.
  20. Apr 12, 2010 #19
    Indeed... I'm wondering if this isn't the crank's version of the Socratic Method. :grumpy: If not, this is still going nowhere with someone who has no interest in anything but confirmation of their bias.
  21. Apr 12, 2010 #20


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    Possibly, but I can give him/her the benefit of the doubt. I think it's more a matter of him/her seeing apparent discrepancies and wanting to explore them but not having enough knowledge to formulate well-defined questions. His/her questions place the burden on us, the answerers, to supply the definitions and terminology.
  22. Apr 12, 2010 #21
    I think you are not inclined to give an answer, or you recognise questions only with the help of the 'interrogation sign'. Otherwise lack of questionmarks doesn't mean that there is no question.
  23. Apr 12, 2010 #22
    Here is an example of "something" that can move at the speed of light or even exceed the speed of light:

    Consider a long straight shoreline and a wave coming into the shore that is not exactly parallel to the shore. If you call the intersection of wave with the shoreline as it hits the shore a thing, then that thing can move along the shoreline at greater than the speed of light. The inportant thing to note here is that thing (the intersection) is not a physical object made of mass and nor can the intersection be used to transmit useful information along the shoreline at superluminal velocities.

    Now consider another analogy. Imagine you have a large calm pond. You throw a pebble into the middle of it and the ripples spread out from where it lands and eventually arrive at the shore. Now none of the molecules of water that were near where the pebble landed are in the ripple that arrives at the shore. What arrives at the shore is energy and information. The individual molecules in the water just bob up and down, but do not travel horizontally. You can think of light as a disturbance in electromagnetic fields that propogates at c and transmits energy and information, but you can not think of the distubance as a particle with rest mass moving at c.
  24. Apr 12, 2010 #23


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    [sarcasm]Yes, it's my fault you're not getting answers.[/sarcasm]

    I cannot psychically guess what parts of your statement you don't understand and want clarification on. The interrogation sign is the indicator of that.

    You spend time responding and criticizing instead of spending that time formulating your questions.
  25. Apr 12, 2010 #24

    I am not doing so. I think according to QM, waves are associated with photons. So, these waves must be involving some activity, some process (even if there is a standing wave). How is this possible when time itself has stopped?

    Plz don't feel pestered.
  26. Apr 12, 2010 #25
    @DaveC: I'd agree, but see above. The combativeness is unusual in someone who is merely curious, but uninformed such as me. I know at least, when to ask and listen, not play word games with the people who are trying to help me.

    That said, I suppose I look for cues in the writing style and approach, and I'm still getting a feeling. The thread title, the reference to "Who am I to talk about Einstein" followed by talk of Einstein.

    I see smoke, but no fire yet. That said, wise man evacuate building filled with smoke, not wait for flames to lick backside! Fortune cookies... the silliest wafers on earth. :biggrin:

    There should be a mechanism in this forum that doesn't ban or kick, but kind of puts some people in a, "how to learn" boot camp. Then again, I do tend to get a bit militant in these situations.

    @Deepak Kapur: He gave you an asnwer that NOBODY enjoys saying, namely: "WHO KNOWS?" Would you prefer that kev or Dave LIE to you? You're asking "Why" instead of "How", in a very complex, contentsted, and incomplete region of physics. Why not take a moment to consider that to ask the right questions is ALWAYS step 1. You need to learn basics so that you have a foundation upon which you can build, and the best way to judge that, is by what questions you ask, and how you respond.

    I'm not criticizing your intellect, just your approach. Is that so terrible?

    EDIT: There is something called Wave-Particle Duality, which is in an of itself a major branch of research, and an outstanding quandry for physics! I know for a fact that there are dozens here who would walk you through the inclined plane, through GR and QM... you're eschewing that for reasons I can only guess at.
    Last edited: Apr 12, 2010
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