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Name: Bob
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Question:
As a member of the Ask-A-Scientist panel fielding questions on Materials Science and Engineering, it is now my turn to ask a question! My question related to Beta Decay of an atomic nucleus. In Beta Decay, a neutron spontaneously decays to a proton, while emitting an electron and an electron antineutrino. Looking inside the neutron, one of the neutron's Down Quarks changes to an Up Quark (thus changing the neutron to a proton), and in the process emits the electron and electron antineutrino. The question is, where did the electron and electron antineutrino come from?! Quarks, electrons and electron antineutrinos are all fundamental particles, and cannot be broken down further. There are no electrons, for example, hiding in a neutron... only 2 Down Quarks and an Up Quark. How is it possible for a Down Quark (in the process of changing to an Up Quark), to emit two fundamental particles (the electron and the electron antineutrino) that were not in the nucleus in the first place, and which cannot by definition be created from other particles? Thanks for any light you can shed on this.



Replies:
I too have (am) grappling with the arcane "problems" of "fundamental" particles. There is one assumption in your well stated inquiry that I am not sure is known to be true or not true. Specifically, "Quarks, electrons and electron antineutrinos are all fundamental particles, and cannot be broken down further." True at the energies investigated as of now, but maybe they have not been slammed hard enough yet. My search has not been from lack of trying. The best I can do is offer my "reading list" on high energy physics: "Facts and Mysteries in Elementary Particle Physics" by Martinus Veltman; "Supersymmetry" by Gordon Kane; "The Trouble with Physics" by Lee Smolin; and (not yet read, and published in 1994), so it may make it somewhat dated "The Quark and the Jaguar" by Murray Gell-Mann.

Lee Smolin, especially, is critical of the current state of theoretical high energy physics -- and he certainly has the credentials to be critical and credible. What all these books for lay persons have in common is trying to "translate" the conclusions of difficult mathematics into verbal language. The difficulty is that there is no good Math-to-Language --Language-to-Math "dictionary". The only "reality" appears to be in the mathematics which is inaccessible to all but a relatively few very smart specialists. Wish I could answer your question.

Vince Calder


Hi Bob,

I might be able to help in one aspect of your question. Your question seems to imply that there is absolute conservation of fundamental particles -- if so, then this might be where you are getting confused. Fundamental particles are not necessarily conserved; particles can "appear" and "disappear". More precisely stated, matter can emerge from vacuum given correct energy and conservation rules. The electron and anti-neutrino exist in this instance because they fulfill the energy and conservation rules.

I am far from an expert in this field (pun intended), so I will defer to another volunteer to explain the full exchange of particles and conserved properties involved in the weak force (and beta decay). You may benefit from reading about leptons, W and Z bosons and, more generally, the Weak Force. If you like matrix math, this will be very fun for you. :)

> Thanks for any light you can shed on this. Nice pun by you, too!

Hope this helps,

Burr Zimmerman

p.s. special thanks to HLM for helping me formulate this response.


Hi Bob,

Fundamental does not mean 'immutable' - it just means we cannot detect constituents or sub-components. Merely because these particles exchange with energy doesn't make them any less fundamental. Or, if you define fundamental as 'immutable' (e.g. cannot exchange with energy), then no form of matter is fundamental. These particles are still fundamental according to the standard model.

If you want to go outside the standard model, e.g. with string theory, then there is a whole new vocabulary. However, the experimental underpinnings of string theory are ... well, nearly non-existent. So while it is interesting on a theoretical basis, it is far from having the combined theoretical and experimental support that the standard model has. Not passing judgment, just noting that string theory is still in its early development.

Last, the comment on 'illogical'. Something that engineers struggle with in theoretical physics is their 'intuition'. These things just do not work the way we expect. It is not illogical -- quite the opposite in fact. It is supremely logical and mathematically driven and experimentally verified. It is, however, extremely 'counter-intuitive'. Just speaking personally, when I gave up on trying to understand these things intuitively, and just understanding how they *are*, it really helped me. I share this because maybe this realization I came to might help you too.

This is lot of fun too... hope you're enjoying your study as I do!

Burr



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