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How do we know atoms exist?

  1. Jan 4, 2009 #1
    I've studied enough of physics and chemistry to find out this question has not yet been answered. All theories derive mathematical formulas that deal with point particles. But how are scienstists convinced that nature is made of point particles.
     
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  3. Jan 4, 2009 #2

    Vanadium 50

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    I think you may have ended your studies a wee bit soon. Atoms can be handled one-at-a-time, so "how we know they exist" has the same answer as "how do we know gumballs exist" - hold out your hand and I'll give you one.

    Atoms, however, have definite size and are not points.
     
  4. Jan 4, 2009 #3

    malawi_glenn

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  5. Jan 4, 2009 #4

    DaveC426913

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    I believe though, that the OP's question might better apply to subatomic particles, which are point particles.

    The answer to "how we know they exist" is that we only define them by their effects on the world around them. We may not be able to say exactly "what" an electron is, but we can definitely describe its properties such as mass and charge, etc.

    Q: What is an electron?
    A: That whateveritis which is observed to have a given charge and a given mass.



    ... Well, that's not quite true is it? We do say things like "electrons do not have substructures" and "electrons are not distinguishable from each other". So we are claiming to know something about them beyond what we can directly observe. Though I do believe this is where empirical observation ends and theory begins.
     
  6. Jan 4, 2009 #5
    If we know atoms to be what they are, then we certainly know that they are NOT point particles. Atoms have a 3D shape or volume, they can translate, rotate, and vibrate etc. We also know that these atoms are not the ultimate "smallest" particle of matter as they are built up out of even smaller particles.

    The "point particle" in classical physics is used as a simplification so that the translational motion of an object can be considered by itself without also considering the rotation of an object (anything larger than a point particle will exhibit rotation unless it is pushed precisely along its center of mass). Just because the concepts of physics are originally introduced with point particles does not mean that the truths of the laws are limited to point particles! The point particles are used as an approximation to simplify the analysis of the first few laws for beginning students, you can do all the same analysis with "real" objects that are not point particles...it would just be more difficult to do it this way for a beginner (as you would now have to consider the ideas of center of mass and rotation along with translational motion all at the same time if taught this way).
     
  7. Jan 4, 2009 #6

    DaveC426913

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    I am not so sure their point-particality is just a simplification for clarity, as you are suggesting.

    I do believe that in SR, the subatomic particles have to be point particles or the math doesn't work. In fact, the pointness or non-pointness is an important factor in why SR and QM are currently incompatible. String theory was invented partly in an attempt to explain how point particles could interact without generating numbers that approach inifinity.
     
  8. Jan 4, 2009 #7
    I feel that the problem with the question is... what is a point-particle? Do you mean an eigenstate of the position operator x in QM, or the one particle states |p> in QFT?

    If you are considering the first one, then nature certainly isn't made of point particles, in fact, most objects have a finite width in their position representation.

    The latter (the QFT case) can hardly be considered as a "point" particle at all, since it's wave function is spread out and none of them has a definite position.

    The concept of particles in physics can best be described as an intuitive notion rather than something that's like a solid particle existing at a point with infinite density. The classical case is merely an approximation to the quantum ones.
     
  9. Jan 4, 2009 #8
    The concept of a dimensionless "point" particle comes from classical physics. That's all I really know for sure. So far there is no experimental evidence of a dimensionless subatomic particle within the nucleus. So the question more or less becomes is SR an approximation? or is it exact, and is the approximation being made with QM? Most of us want to believe that the approximation is made with QM because it just "feels" incomplete (and most of the new theories are trying to "complete" it), but if the last century indicates anything it is that the answer (if we even get one) is not going to be what we want.
     
  10. Jan 4, 2009 #9

    Vanadium 50

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    Perhaps. There's a limit to how much second guessing one can do. "How do we know atoms exist?" might really mean "Are there such things as point particles?" Or it might mean something else.

    I don't think anything in particle physics requires particles to be point-like. It's simply that quarks and leptons have shown no evidence at all of substructure at any scale we can probe.
     
  11. Jan 4, 2009 #10
    Until the LHC or something experimentally demonstrates that quarks exist I am not necessarily sure that they exist much less that they have or don't have substructure. It could BE that the fundamental particles are simply the things that don't show evidence of decay such as a proton or an electron and that we are staring at them right now and don't even know it. Until (if) we actually smash a proton apart we won't know for sure.

    Anyways, you can't even talk about sub"atomic" particles or what have you unless you first believe in the atom. The topic of how we "know" atoms exist is a very long one and would take quite a bit of time to develop properly. I suggest reading Issac Asimov's book "Atom" if you want an enjoyable easy ride from the initial discoveries to the present state of affairs.
     
  12. Jan 5, 2009 #11

    malawi_glenn

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    tim_lou, a (charged) point particle is something who's charge distribution is dirac delta function, and that one can measure by scattering.

    DaveC426913, If the OP claims to have studied physics on the level that he/she knows that proofs for existence of atoms don't exists, then I think it is adequate to answer that question since it was raised and formulated in that way.
     
  13. Jan 5, 2009 #12

    Vanadium 50

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    • Deeply inelastic scattering measurements, beginning with the Nobel prize winning work of Friedman, Kendall and Taylor.
    • Quarkonium energy levels, beginning with the Nobel prize winning work of Ting and Richter.
    • Dozens of confirmed predictions of hadrons, beginning with the Panofsky prize winning work of Samios.
     
  14. Jan 5, 2009 #13

    malawi_glenn

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    You should get updated, the existence of quark is old. The task of LHC is not to find quarks since that question is solved long long time ago.

    And as far as we know, leptons and quarks are point particles, no furter structure has been found.

    You should recapitulate scattering theory and form factors in quantum mechanics.
     
  15. Jan 5, 2009 #14

    DaveC426913

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    It was covered in posts 2 and 3. Unless the OP chooses to disbelieve, I would say that part of the discussion is resolved.
     
  16. Jan 5, 2009 #15
    I disagree here. One of the primary tasks of the LHC is to show clear *experimental* evidence that protons do in fact have a definite substructure. The math might tell you that its there, but there is no guarantee that the problem is not with the math (especially since our math does have holes in it that we do not understand such as the aforementioned infinities that are brushed aside). Most physicists agree that electrons for instance do not have a substructure. And yet aside from the mass, the electron is very similar to the proton in many observable ways (no evidence of decay for example, or the simple fact that the two things interact directly with each other in low energy matter).

    I realize that being a modern day physicist is a frustrating thing in that you have to take "half answers" from untested theories (and the quark is still an old untested theory...despite the theoretical argument for it, we haven't directly detected a quark from any experiment thus far). However, I would urge caution...to bring up recent history, the michelson-morely experiment was supposed to confirm the existence of the ether at the turn of the century. If I was standing by waiting for that experiment to be performed at the time and chatting about it with a bunch of physicists, I am sure that the best and brightest of the bunch would be there trying to convince me that this would prove the ethers existence and that this whole thing was a formality. Except when they actually did the experiment they found out the interpretation of the experiment was something else that nobody had thought of. I would not be surprised if they found a quark in the LHC and that could put things to rest, but I would not be all that surprised if they found something entirely different either.
     
  17. Jan 5, 2009 #16

    malawi_glenn

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    Renge_Ishyo, there has been Nobel Prices given to quark findings.

    Before proceeding, please show your sources for your claim regarding what LHC are looking for.

    http://public.web.cern.ch/Public/en/Research/COMPASS-en.html

    Compass is just "finetuning", its task is not to find the existence of quarks.

    Under which stone have you lived?
     
  18. Feb 25, 2009 #17
    I also want to know if atoms or molecules truly exist. What proof do we have to substantiate this claim?
     
  19. Feb 25, 2009 #18

    malawi_glenn

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    we have just discussed such evidence.
     
  20. Feb 25, 2009 #19
    Renge Ishyo,
    The top quark has been detected, and it doesn't hadronize before decaying. So in that sense, we have seen free quarks in collider experiments already.
     
  21. Feb 25, 2009 #20
    I understand that we've seen atoms using Electron Tunneling Microscopy.

    We also have good models concerning Brownian motion, Atomic Theory, and other Mathematical Derivations that according to the scientists prove it true without a doubt.

    But, I still don't understand how this actually proves that that's an atom and not something else; which could create a bias in what we're seeing in these pictures.

    However, I do admit that at least viruses have a certainty of existence, being the smallest things seen. But I'm not sure about molecules or anything smaller than that due to lack of proof.

    Do the molecules appear to be the perfect geometric structures we see with the computers and instruments? And if this is so, back up why our instruments aren't falsifying the results with a bad way to measure the size of the molecule? Are we sure our measuring methods have the accuracy and true image we think they do?

    Are you sure that other forces and fields aren't just making it "look" like an atom, when it could in truth be a much smaller or larger particle?

    I'm not even sure about the existence of the electron. What if it were something else? What if light were something else too, what other imaging techniques and equations do we have that actually prove these assumptions true?

    Please use whatever mathematical methods, figures, constants, and scientific proof or evidence to disprove me. I'm not a philosopher, I'm not sure about it if we haven't seen it with human eyes or photographic cameras instead of electron cameras.

    I will listen to whatever you say, but I may question the truth in each of your statements.
     
    Last edited: Feb 25, 2009
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