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Nothing can exceed the speed of light

  1. May 22, 2007 #1
    Special relativity tells us information can not be sent faster than the speed of light. I believe Einstein, but was curious what people thought about the following thought experiment:

    The nucleus of an atom has a particular width. With sufficiently advanced technology I could setup a detector that detects motion at the far side of a nucleus. I should be able to “bump” the nucleus from the front side and detect the movement on the back side. One of two things will happen.

    1. The detector will instantaneously detect the movement. This would occur if the nucleus is dense and non-elastic. This would reveal an exception to the “cannot transmit information faster than light” rule (albeit at a very small scale).
    2. The detector will NOT instantaneously detect the movement. This reveals the dense nucleus itself behaves in a wave like fashion.

    I'm an amateur so my question may not even be valid, but I welcome any feedback.
  2. jcsd
  3. May 22, 2007 #2


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    The modern view is that a nucleus is really made up of three quarks, so your example doesn't quite work. But we can generalize a bit.

    It's possible to view quantum mechanical particles as particles, but it's just as possible to view them as waves. When viewed as waves, the waves propagate at 'c'. (For instance, in QED). When viewed as particles, the quantum mechanical particles do NOT have a size - you need to think of them as being points.

    We've actually sidestepped a few tricky issues of interpretation, but these would probably best go in the QM forum. The only coherent interpretation of quantum systems as particles that I'm aware of has the rather odd property that the particles have multiple histories.
    Last edited: May 22, 2007
  4. May 22, 2007 #3


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    The one thing you CAN'T do is treat atoms as little spheres with a diameter.
  5. May 23, 2007 #4


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    Without referring to the specific proerties of a nucleus, there is generally no rigid material in relativity, as a matter of principle.
  6. May 23, 2007 #5

    George Jones

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    Pervect, DaveC426913, and Ich have all made good points.

    At the subatomic level, things are not so clear-cut.

    For simplicity, consider a normal hydrogen atom that has one positively charged proton as its nucleus. This distribution of positive charge doesn't have an edge, it sort of fades away. Getting an extremely accurate handle on the exact form of this fade is a subject of current research.

    Also, this charge distribution is not rigid. In fact, quantum theory shows that there is a small but non-zero probability that hydrogen atom's electron will be found inside the proton!

    If instead of a nucleus, a ruler or steel rod is bumped, the resulting disturbance propgates at the speed of sound in the material out of which the object is made. This is always less than the speed of light.
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