22,420 reputation
150115
bio website lightandmatter.com
location Fullerton, California
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visits member for 3 years, 2 months
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I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.


Oct
7
answered What axiomatizations exist for special relativity?
Oct
7
comment Is special relativity axiomatic?
But it doesn't make sense to add to a set of axioms unless the axioms being added are logically independent of the ones you already have.
Oct
7
comment Is special relativity axiomatic?
to derive something like $E = mc^2$ some physics is needed (they cannot be obtained by pure deduction from the postulates). What do you consider to be the additional ingredients needed? Einstein didn't introduce any new physical assumptions in his $E=mc^2$ paper that were not already present in the original paper on SR. He merely combined the postulates from the original SR paper with known physical principles.
Oct
7
comment Is special relativity axiomatic?
These aren't logically independent of the axiom in the question. Your four axioms suffice to prove the axiom given in the question, so your axioms and the one in the question would belong to two different axiomatizations.
Oct
7
answered Is special relativity axiomatic?
Oct
7
answered Why is the binding energy per nucleon of helium-3 less than that of helium-4?
Oct
6
comment Is it possible for the entropy in an isolated system to decrease?
@eJunior: Sure, in that trivial sense it's arbitrary. It's arbitrary up to a multiplicative constant, and the constant can be negative if you switch the words around in the 2nd law.
Oct
6
comment Is it possible for the entropy in an isolated system to decrease?
@eJunior: Sorry, I don't understand your comment. Did you write "decreasing" and "increases" when you meant "increasing" and "never decreases?" Or do you mean that we could arbitrarily change the definition of entropy from $\ln\Omega$ to $-\ln\Omega$, in which case it would always decrease? In essence it only means one state goes towards another and dont ever go back to its previous state (in isolation). This is sort of true, see Lieb and Yngvason, arxiv.org/abs/math-ph/0003028 . But that doesn't mean that the definition of entropy is arbitrary. In fact, L&Y prove that it's unique.
Oct
6
comment Is a world with constant/decreasing entropy theoretically impossible?
The voting decides how clear, unclear or wrong the answers are. No, voting does not decide whether an answer is wrong. Logic and evidence decide whether it's wrong. "If I were wrong, then one would have been enough!" -- en.wikipedia.org/wiki/…
Oct
6
comment Is a world with constant/decreasing entropy theoretically impossible?
Also, this answer I just found (by Nathaniel, unsurprisingly) is basically what I would write. Nathaniel's answer is great, and I just gave it a +1. However, I don't see any connection between it and this question, or between it and annav's answer.
Oct
6
comment Is a world with constant/decreasing entropy theoretically impossible?
@MarkMitchison: Do you disagree with my point of view? I don't know what your point of view is. Would you like to express it in an answer? I appreciate that such a long discussion is hardly ideal but I think the answer + comments is worthy of an upvote. I disagree. I would nominate this answer as one of the worst ever to receive an upvote on physics.SE.
Oct
6
answered Is it possible for the entropy in an isolated system to decrease?
Oct
6
comment Is it possible for the entropy in an isolated system to decrease?
related: physics.stackexchange.com/q/20401
Oct
6
comment Is a world with constant/decreasing entropy theoretically impossible?
The long dialog in comments fails to convince me that anything has been clarified. This answer is simply wrong. @annav, if you really believe that there is a valid argument here, which could be expressed clearly, then the responsible thing to do would be to go back and edit the answer to express it clearly with the benefit of Mark Mitchison's comments. All I see is a jumbled mess of non sequiturs, assumptions invoked without justification, and garbled use of terminology that sounds impressive.
Oct
6
comment Do spin-spin interactions break time reversal symmetry?
From a quick look at the Lunde paper, I think what they're saying is that if you have a spin-spin interaction, then the mean field experienced by single particles violates time-reversal symmetry. The mean field always violates lots of symmetries that are present in the underlying two-body interaction. For example, in a nucleus, we have a mean field that is an attractive potential with a certain shape, centered on a certain point in space. This clearly violates translational invariance, even though the two-body interaction is symmetric under translational invariance.
Oct
5
comment Is a world with constant/decreasing entropy theoretically impossible?
I'm sorry, but I just don't see an argument in what you've written. Are you arguing that it's self-evident that the number of microstates must increase over time? That would be wrong, but I'm just guessing whether that's what you have in mind, because you haven't laid out what your position is. It is a continually increasing number due to the bosonic nature of photons. This doesn't make any sense. Ritz's argument is a classical one based on Maxwell's equations. If you have some quantum-mechanical version of Ritz's argument, which you believe fixes it up, you haven't explained what it is.
Oct
5
revised Is a world with constant/decreasing entropy theoretically impossible?
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Oct
5
comment Is a world with constant/decreasing entropy theoretically impossible?
The position you seem to be arguing is the one taken by Ritz in a 1909 debate with Einstein. However, it's very hard to tell what your argument is, since your answer is very brief and, at least to me, doesn't seem to have a coherent logical thread. In the year 2013, there is a consensus that Ritz was wrong. See section 2.2 of this paper plato.stanford.edu/archives/fall2011/entries/time-thermo, and references therein. If you want to argue that the modern consensus is incorrect after all, then the burden of proof is on you to put together something more compelling than this.
Oct
5
revised Is a world with constant/decreasing entropy theoretically impossible?
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Oct
5
answered Is a world with constant/decreasing entropy theoretically impossible?