640 reputation
716
bio website
location London, United Kingdom
age 30
visits member for 3 years, 10 months
seen Sep 4 at 18:03

Phd student


Apr
19
revised Do Sears Physics and Berkeley Physics Series provide intuition?
added 35 characters in body
Apr
19
answered Do Sears Physics and Berkeley Physics Series provide intuition?
Apr
18
comment What is the difference between 'running' and 'current' quark mass?
It's because there is a 'bare mass' which is not observable, and a 'running mass' that will vary depending on how you actually make the measurement. This wiki page on running couplings (en.wikipedia.org/wiki/Coupling_constant#Running_coupling) doesn't appear to be very good so I'll post a better answer later when I have more time - unless someone else gets there first and saves me the trouble of course! ;)
Apr
15
comment Invariant spacetime - distance - Circular Motion
As a final teaser: what do you think would happen if one were to increase the energy of the proton (and hence also the speed), such that the circumference becomes less than the size of the proton?
Apr
15
comment Invariant spacetime - distance - Circular Motion
+1 for a good, clear answer. I would just like to add that in one of my 4th year particle physics lectures, the Professor calculated the circumference of the circle travelled by a proton circling the LHC in the frame of the proton (as if one were sitting on it.) At about 7TeV, he found that rather than the 27km circumference as measured in the lab frame (i.e. standing next to the LHC,) the proton was in fact circling a circumference of about 3m!! Unfortunately I don't has the calculation to hand but it shouldn't be too difficult to replicate.
Apr
15
comment Why is the contribution of a path in Feynmans path integral formalism $\sim e^{(i/\hbar)S[x(t)]}$
+1 for taking the time to write this up. Whether a direct answer to the OP's question or not, it is certainly related and makes for some interesting reading.
Mar
23
comment If the universe is expanding, what is it expanding into?
In this answer you erroneously suggest that anti-particles have negative energy. This is incorrect. Any third/fourth year undergraduate course on introductory QFT will teach you about how Dirac derived his equation by seeking a relativistic form of the Schrodinger equation. His interpretation of the negative energy solutions is called 'the Dirac sea' but they are now understood as positive energy anti-particles, travelling backwards in time. See the Dirac equation wiki page for more information. (en.wikipedia.org/wiki/Dirac_equation)
Feb
11
comment Does particle indistinguishability and quantised enery levels (in bound states) violate the Pauli Exclusion Principle?
@gigacyan: So just to clarify, are you saying that all the electrons bound in Hydrogen atoms do indeed have slightly different energies, beit due to electromagnetic interaction or wavefunction overlap? Given the effect isn't measurable I'm not too concerned as to whether the prodominant factor is one of the other, just that A: the PEP still applies and B: the energies must therefore be different.
Feb
11
comment Is there a fully quantum-field theoretic treatise of Planck's Law for black-body radiation?
@Moshe I've not yet had much exposure to multi-particle QFT and would be very interested to see this calculation explicitely. I tried a search for a paper proposing to have derived the Planck distribution with QFT but couldn't find it. Does anyone know of a paper that tackles it?
Feb
11
comment Does particle indistinguishability and quantised enery levels (in bound states) violate the Pauli Exclusion Principle?
@gigacyan. Wrt your first paragraph, if indeed the wavefunctions of electrons bound in distinct Hydrogen atoms do indeed overlap - however negligable the effect, I believe it is to this that I was refering. In all honesty I'm now not so sure that the PEP even applies to electrons bound in distinct atoms?? Perhaps it is this part of the question that I remembered incorrectly..
Feb
11
comment Does particle indistinguishability and quantised enery levels (in bound states) violate the Pauli Exclusion Principle?
@Lubosh - Thanks for the comment but this, to me, seems like the clearest attempt to actually answer the question; hence the +1. I agree with what you have said and therefore think I must have remembered the question incorrectly. Perhaps, instead, we could consider all the Universe's fermions in an instant just after the Big Bang. If the available space was sufficiently small (say less than the size of one Hydrogen atom) would this not require all the fermions to adopt minutely differing energies, so as not to violate the PEP?
Feb
10
awarded  Commentator
Feb
10
comment Why does one experience a short pull in the wrong direction when a vehicle stops?
+1 Nicely put, but I feel it prudent to point out that this argument (albeit less formal) has in fact been proposed in other answers.
Feb
10
comment Why does one experience a short pull in the wrong direction when a vehicle stops?
+1 Just read this after having made similar aguments in a comment above. This, IMHO, is the correct explanation.
Feb
10
comment Why does one experience a short pull in the wrong direction when a vehicle stops?
In addition to the tension built up, and released, by the passengers' limbs and torso, there will also be a similar release from the seat, carriage and brakes on the train. Clearly the seat fixings, the carriage-to-chassis fixings and the brakes-to-chassis fixings all have a small amount of 'give' in them (as does the actual plastic comprising the chair and the metal comprising the train). Thus, under braking, the torque due to the deceleration will build up a small, soon-to-be-felt, recoil once the deceleration reduces to zero.
Feb
10
comment Why does one experience a short pull in the wrong direction when a vehicle stops?
I beleive this is the correct answer but offer a slight extension to it:
Feb
9
comment Interference of polarized light
Agreed, +1 for content and style. (tiny typo in the first line though: "not to be unsubstantiated")
Feb
9
awarded  Scholar
Feb
9
accepted How could spacetime become discretised at the Planck scale?
Feb
8
asked Does particle indistinguishability and quantised enery levels (in bound states) violate the Pauli Exclusion Principle?