5,436 reputation
1733
bio website sites.google.com/site/…
location Cambridge, MA
age 26
visits member for 3 years, 2 months
seen 53 mins ago

Physics Graduate student at Harvard University.


1h
answered Equivalent forms of four fermion operators?
1h
comment On four-vectors
Where have you seen $x^\mu$ being treated as a vector?
15h
answered What does it mean by saying the generators of translations transform as vectors under the Lorentz Group?
Jun
26
comment How to write the Lagrangian in terms of a projection
Oh! I see......
Jun
26
comment How to write the Lagrangian in terms of a projection
Just to complete the question, you should write down what is $P_{\mu\nu}^T$ explicitly.
Jun
24
comment Rest Mass and Wigner's Classification
@WillO - If your question was to understand how $m$ tells us about properties of the particle, then it was quite ill-posed. The wording of your problem led me to believe you were asking why $m$ is called rest mass, which has a practically trivial answer. If your question is why does the rest mass characterize a particle at all, that is a more intricate question that is answered by Andrew
Jun
24
comment Rest Mass and Wigner's Classification
@WillO - It is a property of the group representation - it is the eigenvalue of $-P^2$. In the rest frame $P^i = 0$ and therefore $-P^2 = (P^0)^2$. Thus in this frame, $m$ is literally $P^0$. So yes, what I said is true.
Jun
24
comment Rest Mass and Wigner's Classification
@WillO - Because it is literally the "REST MASS", i.e. the mass of the particle when it is at rest. What else should I call it? I am not getting the point of your question.
Jun
24
comment Time reversal in quantum mechanics
@RaulLaasner - He means $p^T$. Also, w.r.t. your comment $p^\dagger = p$ not $-p$.
Jun
24
comment Rest Mass and Wigner's Classification
@WillO - Here's my definition of rest mass - It is the energy of the particle when it is at rest. This is the natural extension of the old idea of mass extended to SR. Anyway, with this definition of rest mass - $m$ is the natural candidate to call rest mass. (Did I understand your question? To some extent, it is a philosophical question. It's just a name. What is important is that it is a Lorentz invariant quantity and therefore can be used to label representations.)
Jun
24
comment Rest Mass and Wigner's Classification
There is in principle a definition of particles independent of nature. What Wigner was calling a particle was simply an "irreducible representation of the Poincare Algebra". Thus, the notion of a particle in this sense is simply a mathematical construct. It is in this sense that @WillO is asking his question. Whether this notion of a particle has anything to do with the usual ones encountered in nature or QFT is a completely separate question.
Jun
24
comment A question about a consequence of symmetry in $\phi^4$ theory
Note that $p^4$ really means $(p^2)^2$. What you would mean by $p^3$ would be $(p^2)^{3/2}$. The latter is a fractional power of $p^2$. Such terms could never appear from Feynman diagrams.
Jun
24
answered Why is cut-off regularization is not Lorentz invariant?
Jun
23
comment Physical motivation for mathematically extending solutions to Einstein's equations
A note - he Schwarzschild black hole has existed forever. It is an eternal black hole.
Jun
20
answered What equation do we use to measure the energy level of a string, to determine it's “particle correlation”
Jun
19
reviewed Close What is the future of gravitional lensing?
Jun
19
reviewed Edit What will happen to a human if he/she falls down into particle accelerator?
Jun
19
revised What will happen to a human if he/she falls down into particle accelerator?
corrected grammar, changed name
Jun
17
reviewed Close Tesla's Egg of Columbus
Jun
17
reviewed Close Feathers and Human Flight