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What exactly is a quantum fluctuation

  1. May 16, 2012 #1
    This comes from Paul Davies' the Cosmic Jackpot:

    I just assumed a quantum fluctuation was more or less what a fluctuation in normal English is like: something that goes up and down. So I thought a quantum fluctuation was the act of some magical pure energy going up and down. But from the above text it looks like it refers the tendency of particles to pop in and out of existence.

    Any further details would be appreciated.
  2. jcsd
  3. May 16, 2012 #2


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    that means, it isn't, but pretending it is helps you to understand it :wink:
  4. May 16, 2012 #3
    OK. The way that the classical world works is that I have a box, and I ask how many shoes are in that box, and I get a single integer. There is one shoe in the box. Two shoes in the box. Fifty shoes in the box.

    Particles don't work that way. I have a box. I ask how many electrons are in the box, and it's not an exact number. Instead of having two electrons in the box, I have two-ish or three-ish electrons. There are a lot of complicated rules for how to answer that question, but the short answer is that electrons don't work like shoes, and when you ask how many electrons are in the box, you don't end up with an integer answer.

    So I ask how many electrons are in the box. I come up with the answer "two-ish". I then do an experiment that forces the system to an exact integer answer, and when I do the experiment, I come with the answer "three." "Three" is close enough to "two-ish" that it's the right answer.

    So some of the ideas about the universe is that if you ask "how many universes" are in the box, you end up with the answer zero-ish. You then do an experiment to force the system to give you an integer answer, and "one" is close enough to "zero-ish" for the universe to exist.
  5. May 17, 2012 #4


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    We obviously have one universe, so that is the default value. Any more than that is speculation, any less is illogical.
  6. May 18, 2012 #5
    You need to be clear about what you mean by universe. If you mean all there is, then by definition there is only one universe, but if you mean all there is began at the BB 13.7 billion years ago, then that would exclude the possibility of the multiverse.
  7. May 18, 2012 #6
    Define "all" and "is"

    Seriously, mathematicians and philosophers spend a ton of time on these sorts of things. Defining "all" and "is" turns out to be much harder than it looks.

    Also, it's speculation, but sometimes it turns out to be useful speculation. For example, we can ask what would happen if there was "another bang" which is not directly observable. It turns out that this would change the CMBR.

    One more thing "quantum fluctuations" are not speculation. Seeing particles "sort of exist" is something that physicists see everyday. The way we describe electromagnetism involves exchanges of "ghost particles".

    The speculative part is to extend what we see every day to the beginning of the universe. You see "ghost electrons" all the time. But there is a difference between an "electron" popping out and an entire universe.

    It's one of those "we don't know if it works until someone tries to make it work" things, so it's a good thing to *try* to take the processes we can observe and tinker with the math to see if it can create the entire universe. However, no one I know has gotten this to work.

    Even getting things to the point where you can describe things in the language that people use to describe what happens at CERN is something that no one I know has been able to do.

    I can go pretty deep into the how QM works, but it might be easier if someone asks a specific question.
  8. May 19, 2012 #7
    agree mostly
  9. May 19, 2012 #8
    Keep in mind that it's your observations that are fluctuating. The field doesn't have some classical value that fluctuates with time. Instead, it has an eigenvalue - a set range of possible classical values. Each possible classical value has a certain probability amplitude assigned to it. When you take an observation, you get a classical value. But since it depends on probability which one you observe, different observations reveal slightly different values. This makes it appear as though the field is 'fluctuating'.

    Of course, the term gets thrown around now like nothing.
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