Electrons and Their Properties

I’ve been quite busy with some physics research this week, but I have nevertheless managed to finish a new article on electrons, part of my Structure of Matter series, which aims (among other things) to introduce a non-expert to particle physics, step-by-step.  The completion of this article feels like a significant step for this website.  After all, the electron was the first subatomic particle and the first of the apparently-elementary particles to be discovered, about 115 years ago, and its discovery really gave birth to the field of particle physics we know today.  Moreover, it was the failure to describe the behavior of electrons within and outside of atoms that forced physicists to go beyond Newtonian views of physics processes, and introduce the theory of quantum mechanics.  Electrons, tiny as they are, are enormous in human life; they play a key role in all chemical reactions, including those that sustain our bodies.  Beyond that, they lie at the heart of much modern technology — electronics!  And there’s more.  So no particle physics website can be complete without an electron webpage.

Looking ahead, a question I sometimes get asked is whether I’m sure electrons (or any other elementary particles that physicists talk about) really exist.  After all, it is true I’ve never seen a picture of one taken with any sort of microscope!  Well, in answer to this question, I want to write an article on why we particle physicists are so confident that electrons (and atomic nuclei) exist… explaining the types of experiments and the types of logical reasoning that lead to this conclusion.  I suspect a lot of readers will find such an article interesting; after all, why should one take expert knowledge for granted just because it appears in a textbook or on a website?  Readers should demand to know where the knowledge came from — and a writer should be prepared to answer.

35 responses to “Electrons and Their Properties

  1. It’s almost certainly not something you want to get into in this article, but there are interesting ontological questions about whether electrons “really exist” per se, or rather whether it’s more correct to say that the electron field really exists, and stable quantized waves are one of the things the electron field can do.

    I understand this isn’t the same sense as your article means the question, though.

  2. Matt, thanks for the time and effort you’re putting into these articles.

    Do you think that perhaps in this day and age, people should try to focus on an electron having a probablity wave associated with its position, rather than it being a “wave”?

    If you get a chance, take a look at this paper by Hans Ohanian on spin:

    American Journal of Physics — June 1986 — Volume 54, Issue 6, pp. 500
    What is spin?
    Hans C. Ohanian
    According to the prevailing belief, the spin of the electron or of some other particle is a mysterious internal angular momentum for which no concrete physical picture is available, and for which there is no classical analog. However, on the basis of an old calculation by Belinfante [Physica 6, 887 (1939)], it can be shown that the spin may be regarded as an angular momentum generated by a circulating flow of energy in the wave field of the electron. Likewise, the magnetic moment may be regarded as generated by a circulating flow of charge in the wave field. This provides an intuitively appealing picture and establishes that neither the spin nor the magnetic moment are ‘‘internal’’—they are not associated with the internal structure of the electron, but rather with the structure of its wave field. Furthermore, a comparison between calculations of angular momentum in the Dirac and electromagnetic fields shows that the spin of the electron is entirely analogous to the angular momentum carried by a classical circularly polarized wave.

  3. The proof of the existence of anything like here in this field is given in terms of effects. First we say it is there, it will have such and such characteristics and predict that it will behave in abc way in xyz conditions AND if we are able to prove such effects (abc way) in those xyz conditions, then we say its proof and thus the things exist.

    • True — but the point is to provide the details. What strategies are used? How convincing are they? For many non-scientists, electrons seem no more real than fairies… they’ve never seen them. For those of us who work in the field, they are as real as rocks or wind or the moon. It’s worth explaining why we’re so confident about something you can’t observe with your senses, or with any simple extension of your senses (such as a microscope).

  4. You’re a legend.

  5. Oh dear. I do so hate ‘How do you KNOW’? questions. They ca so easily degrade into unscientific nonsense and philosophical jargon. I remember having to write a paper on just what a ‘thing’ was, and I still have terrible memories of an entire group of people who seemed to be under the impression that imagination and science were equally valid ways of looking at the world.

    • Natural science historically developed out of philosophy or, more specifically, natural philosophy. It is considered to be the precursor of natural sciences such as physics. At older universities, long-established Chairs of Natural Philosophy are nowadays occupied mainly by physics professors. Modern meanings of the terms science and scientists date only to the 19th century.

      • Indeed, but it seems to me that there has developed a sort of ‘crank philosophy’ of late, individuals with no formal philosophical qualifications who misuse some profound philosophical arguments and systems to reinforce their own ignorance. A particularly aggravating example is the use of ‘Science cannot absolutely prove anything, ergo whatever I say *could* be true, ergo I am right.’

        • If anybody is pioneer to say something “right”, he must break established notions. If we say philosophy( rational argument) is a rule and authority, then innovations “may” be the casualty. There is also Theosophy.
          But Mr.Kudzu, you are 100% correct that, tangential arguements leads to headache, wasting the invaluable time of good experts.

        • I’m not sure this is “of late”. I suspect it has been around for centuries; perhaps someone has done a study.

          • It probably has, but was more excusable I think when science and philosophy were joined at the hip and ‘God did it’ was a valid excuse. Possibly it’s just postmodernism and the modern suspicion of authority but it seems to me the past few decades have seen ‘ignore all contrary evidence’ become more valid as a debate strategy.

  6. Just a technical point of doubt here: is it the electrons which carry current in the neurons, or the ions such as Na+ in solution?

    • Oh… right. Drat! Thanks… that was a dumb blunder, presumably a product of writing that part too late at night. (Originally I had written about electrons in electrical wiring, but they I tried to shorten the text…) I hope I didn’t make any similar dumb mistakes in the main electron article, which was written in daylight.

  7. An interesting follow up question would be: do magnetic monopoles exist?

    • But the answer is easy: we don’t know. (Or at least: we are pretty certain there aren’t any lighter than about 100 GeV/c2, but heavier than that we don’t know.)

      • And indeed the plot thickens as condensed matter theorists think of ways to induce image magnetic monopoles at the surfaces of topological insulators. See: arXiv:0811.1303 .

        • But those are not independent monopoles that move independent of a material. It’s not the same issue. We can make all sorts of things inside condensed matter systems that are fascinating, to be sure, but are consequences of the condensed matter system itself, not of the basic properties of our universe.

          A nice example of something you can make in a condensed matter system is a tube, called an Abrikosov vortex, arising in a Type II superconductor. Lattices of these vortices have been observed since the 60′s. Similar things (often called “cosmic strings”) might exist in nature outside of a condensed matter system, but so far, none have been found. Whether they (or magnetic monopoles) exist in nature as independent objects depends on the details of fields not included in the current Standard Model of particle physics.

          • Yes, I agree it’s not the same issue. However, it’s still exciting that axion electrodynamics and the AdS/CFT correspondence have been helpful in the investigation and classification of superconductors and topological insulators. As with the Abrikosov vortex/cosmic string analogy, there might be a bulk/boundary lesson to be learned that sheds light on why it has proven so difficult to observe the independent magnetic monopole.

          • I doubt that there will be such a lesson, but maybe.

  8. /(.) Moreover, it was the failure to describe the behavior of electrons within and outside of atoms that forced physicists to go beyond Newtonian views of physics processes, and introduce the theory of quantum mechanics.
    (.) Readers should demand to know where the knowledge came from./
    Excellent Professor.

  9. Good morning Prof;

    Ah, the shrinking proton …

    http://www.msnbc.msn.com/id/50577959/#.UQKgLvLe-XA

    Quantum gravity? …

    Could the more massive muon be causing shrinkage of the proton space-time curvature, i.e. the energy remains the same except that the density in slightly increase to give this 4% reduction in size?

    I cannot do the math, but it would make sense if you have a system with two more massive particles (proton-muon) as opposed to the proton-electron pair that their space-time curvatures would be more condensed, i.e. higher densities. Because their iteration is of a higher magnitude and hence better defined.

    • a) not quantum gravity — the force of gravity is 10^18 times (i.e. a million million million times) too small to affect the proton and muon. It if is really a new phenomenon, rather than just a subtlety that we all missed somehow, it will be due to a previously unknown force.

      b) however, what this is due to isn’t clear. I will study it more carefully now that it has been confirmed, and will report on what I know once I understand the experimental and theoretical situation.

      • 1. Since the muon is “closer” to the proton than the electron could the Casimir effect be more pronounced and is condensing the proton space-time further (4%). Should be straight forward to deduce the 4% number since it is the difference between a proton-electron and proton-muon iterations?

        2. We know the force of gravity exist and as you mentioned is quite smaller that the electromagnetic, weak, and strong forces. So, is it correct to say that if there is only one particle next in line down, graviton, that the fermions and quarks are not fundamental but, indeed, they are composite.
        I questioned that it maybe quantum gravity because it seems like a gravitational force. You mentioned no since the magnitude of gravity is so small, but is it linear? Could the gravitational force rise exponentially as the two particles (ripples) come closer to each other?

        Which brings me to a question that just doesn’t go away every since I started studying this amazing science. We have been calculating and theorizing in the last few decades using “renormalization” to remove the infinities out of the equations, (those annoying smoke screens which we cannot see through). Well, could this be a case where it has caused us to missing the real phenomenon, as two ripples come closer and closer together the gravitational force rises to infinity, hence the Pauli exclusion principle, that the two cannot share the same space-time because they will never overcome the gravitational barrier?

        • Whatever is up with the proton’s charge-radius, it doesn’t have to do with space-time. Gravitational effects are tiny tiny tiny tiny tiny tiny. 10,000,000,000,000,000 times too tiny. Nowhere near big enough to make a 3-4% effect on the proton’s apparent charge-radius. If gravity grew exponentially at short distances to a degree large enough to affect how muons and protons interact at these distances, it would dramatically alter everything measured at the LHC.

          “So, is it correct to say that if there is only one particle next in line down, graviton, that the fermions and quarks are not fundamental but, indeed, they are composite.” No. Your premise isn’t right, and your conclusion wouldn’t follow even if it were.

          Renormalization isn’t about infinities, and they aren’t smoke screens; this was understood by Kadanoff and by Wilson in the 1960s. You should renormalize even in theories with no infinities, and there’s a good physical reason, which I hope I can figure out how to explain someday to the public, but it’s not easy. In any case, this is a difficult conceptual and technical point not widely appreciated, so I am sure you will find discussions of renormalization for the public which say otherwise.

          • What is c?

            Is it the exist velocity of energy when a spinor is broken, i.e. symmetry breaking?

            Since c is constant would that indicate that the smallest confinement of energy is the same size. Is there any relationship between the Compton wavelength and the smallest possible spinor?

            “… it would dramatically alter everything measured at the LHC. ” would the measurements be different for one proton – one muon system as opposed to millions of proton bombarding each other? You would expect some cancellation of gravity vectors in the LHC, yes? Would the ideal collision (to get the maximum information) be between two protons, millions of times over?

  10. very interesing! but my non-expert question wouldn’t be why you’re sure electrons exist. I would ask why you think quantum mechanics is correct and complete… ?

    • But that’s not a question I can answer — because I don’t know that quantum mechanics is complete. I know it is “correct” in the sense that it does a terrific job predicting the outcomes of experiments. That’s all I really know. Conceptually quantum mechanics remains unclear — but it does work very well. And its successes I could show you in great detail — to the point that you would have to accept, as we do, that there’s no choice but to accept its uncomfortable features for now, and use it.

      By contrast, despite the many confusions about quantum mechanics, I do know electrons and protons really exist (as much as anything does.)

  11. Amplitude carries the physical information—where a physical system follows simultaneously all possible paths with “probability” amplitudes for each path being determined by the action for the path(cause and effect – causality).
    In quantum mechanics, in the absence of “cause(external force), the effect is quantized- means no integral calculations, only differential calculations?
    So the angular momentum(physical effect) is also quantized at quantum level. That means, gauge invariance make polarization(angular momentum) also quantized making, global symmetry(linear momentum) is just a local symmetry(angular momentum) – the problem of of “physical effect” and creation of mass is solved mathematically – but in reality ?

    While an unbound electron does not exhibit quantized energy levels, an electron bound in an atomic orbital has quantized values of angular momentum.

  12. Of course electrons exists — it’s what an electron gun shoots :-)

  13. The oscillation frequency of the standing wave is the quantum state – is the energy of the state – the energy level or times planck’s constant.
    In electron this involve two fields with fliping chirality – one field interact with weak force another not but appear stationary(quantum mixing) ?
    This is called gauge invariance ?
    Energy level and gauge invariance occur only if there is angular momentum ? – so how via Higgs field’s linear momentum mass could be shifted to electron ?

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