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What is Identity in Physics ?

  1. Jun 10, 2004 #1
    What is Identity in Physics ???

    The universe is made of varieties of elementary particles. Physicists classify these into fermions and bosons. Modern cosmology can even give the ratios of number of particles such as the ratio of number of protons and photons in the universe.

    Quantum mechanics tells us that an electron in the universe is identical to every other electron. They have exactly the same electric chage, same rest-mass, exactly the same responses to the interaction of the weak and strong force, and the same total spin. All other particles are also identical to these distinctive properties.

    But what can distinguish two electrons is their quamtum states. The quantum state is a probability. And there are many kinds of probability. The probability of being in a particular location of spacetime. The probability of the spin pointing in a particular direction. And the probability of having a particular velocity and energy.
  2. jcsd
  3. Jun 10, 2004 #2


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    Was that a statement or a question?
  4. Jun 10, 2004 #3
    Sorry. I leftout the key question for this thread. Thanks for noticing the problem. The question is without knowing the quantum states (probability) how can same species of particles be identified?

    And given some states, how can one be very sure that it's the true identity and not some false identities of other particles?
  5. Jun 10, 2004 #4


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    the same particles have the same probability function.
  6. Jun 10, 2004 #5
    Should have known there is simple answer to unfounded complicated question. Thanks. But still my question about true identity and false identity would not go away. This has to do with the search for the elusive graviton.

    Theory says that graviton has zero mass, zero charge, spin 2 , it only interact with itself and other gravitons, it always travel at the speed of light in vacuum but yet their quantum state is also zero. How can a particle with almost all its properties zero be found?
  7. Jun 12, 2004 #6


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    I think string theory has some answers from the topical study i've done on the subject. It's supposed to predict a particle of zero mass, zero charge, and spin 2, better known to its friends as the graviton. So the fact that many of the properties are zero doesn't really matter when it comes to finding the particle. The fact that these properties are zero is information in itself. The only problem where all these zeros will manifest themselves is in actual experiment, to experimentally determine its existence.
  8. Jun 13, 2004 #7
    The approach of string theory is from the small (qunatum mechanics) to the large (general relativity). The approach of loop quantum gravity is from the large to the small. The arrival of both approaches is at a state of perfect symmetry which is a state of zero information content and a state of infinite entropy.
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