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Why perrin's experiment demonstrates the existence of atoms?

  1. Nov 14, 2011 #1
    Perrin got his nobel prize because of his experiment on brownian motion, which is thought to have proven the existence of atoms

    i cannot understand that

    if you see an array of dots from STM, i believe you demonstrate the existence of atoms

    but i cannot see why perrin's experiment or brownian motion has anything to do with atoms

    it cannot be explained without atoms?
  2. jcsd
  3. Nov 14, 2011 #2


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    The tiny specks in the fluid were doing a movement considered a random walk, so something had to be impinging on the specks to cause this motion. Since this something could not be seen with the naked eye or with using lenses, this something had to be very very small. Rather than the fluid being a continious medium down to the infinite, it was determined that matter was composed of tiny particles called atoms or molecules than were still large enough to have momentum enough to have an effect upon the specks.
  4. Nov 14, 2011 #3
    this reasoning is not compelling, i would say

    the fluid can have fluctuations and give momentum to the specks.
  5. Nov 14, 2011 #4


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    You don't think there were dozens or more scientists that investigated these kinds of possibilities both before the experiment and between then and the award of the Nobel Prize?

    Edit: Also, remember that nobel prizes are usually awarded years after the experiments to verify that they are accurate/important.
    Last edited: Nov 14, 2011
  6. Nov 14, 2011 #5


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    On its own, yes, not very compelling. But Perrin was using his experiment to verify Einstein's equation on Brownian motion. You should realize that this was mainly a conformation of Avogardo's number, and not that there was such a thing as atoms or molecules Even though Perrin's experiment confirmed Einstein's equation ( so they said back then ) his value was out by 20% or so from the accpeted value.
  7. Nov 14, 2011 #6

    Ken G

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    I'd say the problem is that the story gets simplified to fit in a "sound byte" like "atoms must exist to produce random motions" or some such oversimplified claim. When it gets boiled down that far, you are right to question the conclusion, but the real story is much more complete.

    What's really interesting about the story is that for eons, philosophers debated whether a fluid was continuous or atomic in nature, but here Perrin and Einstein are joining forces to basically say, "rather than debate it, let's look at it". A test particle should react differently in the two cases, and that's what they were able to determine, in favor of the atomic model.
  8. Apr 19, 2012 #7
    Just because they function like we would perceive a incredibly small concrete particle to function that does not mean that atoms are really incredibly small concrete particles but they could be something else such as a condensed system located within a field that interacts with other condensed systems within the field to give us properties that we typically conceptualize as being physically concrete.

    Am I close to what you are attempting to describe?

    Anyway I think from the perspective of physics it is not important if they are really physically concrete. It is only important that they function as something that we would perceive as being physically concrete would function.

    Ontology of a particle as per the stanford encyclopedia of philosophy below:
    Particles are defined as being discrete. Particles are countable individuals in contrast to a liquid or a mass. Obviously this characteristic alone cannot constitute a sufficient condition for being a particle since there are other things which are countable as well without being particles, e.g., money or maxima and minima of the standing wave of a vibrating string. It seems that the so-called primitive thisness or haecceity is missing to make up a sufficient condition for a particle, i.e., it must be possible to say that it is this or that particle which has been counted in order to account for the fundamental difference between ups and downs in a wave pattern and particles (see also the entry on quantum theory: identity and individuality). In Teller 1995 primitive thisness as well as other possible features of the particle concept are discussed in comparison to classical concepts of fields and waves as well as in comparison to the concept of field quanta. A critical discussion of Teller's reasoning can be found in Seibt 2002. There is still another feature which is commonly taken to be pivotal for the particle concept, namely that particles are localizable in space. While it is clear from classical physics already that the requirement of localizability need not refer to point-like localization, it will turn out later in this section that even localizability in an arbitrarily large but still finite region can be a strong condition for quantum particles.
  9. Apr 19, 2012 #8
    sorry for the bulk but i also wanted to include this post which details problems associated with a physical interpretation of a particle in a vacuum state: Further Problems for a Particle Interpretation of QFT

    The standard definition for the vacuum state |0> is that it is the energy ground state, i.e., the eigenstate of the energy operator with the lowest eigenvalue. It is a notable result in ordinary non-relativistic QM that the ground state energy of e.g., the harmonic oscillator is not zero in contrast to its analogue in classical mechanics. Not only is the same true for the vacuum state in QFT, the relativistic vacuum of QFT displays even more striking features. The expectation values for various quantities do not vanish for the vacuum state. The label ‘|0>’ does not indicate that the energy is zero in the vacuum state. It rather stems from the assumption that there are no particles present in the vacuum state: an N-particle state can be built up from the vacuum state by the N-fold application of a creation operator. Non-vanishing vacuum expectation values prompt the question what it is that has these values or gives rise to them if the vacuum is taken to be the state with no particles present. Since the vacuum state |0> is closely linked to N-particle states where N is not zero, properties of the vacuum state have a great impact on the particle interpretation as such. If particles were the basic objects about which QFT speaks how can it be that there are physical phenomena even if nothing is there according to this very ontology?
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