Recent content by cygnet1

Thank you for your reply. Looking into the matter further, I find that in many cases, photons are much easier to work with than electrons. The first experimental verification that Bell's Theorem is violated was done on photons, not electrons. Quoting Wikipedia, When the polarization of both...

Suppose you measure the spin of an electron with a sensor oriented in the +z direction and find that the spin is up (aligned with the sensor). Now if you immediately measure the spin of the electon with a sensor oriented in the -z direction, you are guaranteed that it will be down (oppositely...

Thank you for all your replies, and for the link to Wikipedia about uranium enrichment. I'm wondering whether this is a well-posed problem. Suppose it takes a given amount of time, T, to enrich natural uranium (0.71% U-235) into a fixed amount of reactor-grade uranium (4.5%). Using the same...

Forgive me if this question has been asked before or is a FAQ. Can somebody tell me to what percentage uranium has to be enriched in order to build a bomb? Believe me, I'm not trying to build one! I'm just trying to inject some physics into the current Israel/Iran debate. (Let's confine...

Thanks for your book excerpts, Hans De Vries. It looks like a book I'd like to buy when it comes out. Do you have an estimated publishing date and publisher? I think I understand your point about the Feynman propagators leading to FTL (space-like) interactions, which decay exponentially as...

Are you saying the Feynman way of computing propagators is incorrect because it leads to noncausal virtual interactions? Then how did this method manage to correctly compute the electron magnetic moment to 7 decimal places? Perhaps you're saying that it doesn't matter whether you compute...

The tachyons that paper refers to are virtual, and cannot be directly observed. They have nothing to do with Opera. Still, any tachyons are controversial.

Not everyone agrees that commutators do vanish for space-like separated fields. See the following link, if you have access to AIP Scitation. (Sorry, I don't think copyright restrictions will allow me to attach the article itself.)

That's the crux of my dilemma. Suppose we're studying the behavior of an electron passing through an EM field. It seems exceedingly unlikely that a another electron/positron is spontaneously created out of the vacuum field, the positron takes perfect aim at the original electron and...

Thank you, mfb. That's what I suspected. However, we can directly observe antiparticles. What if the second interaction never occurs? Then the particle keeps going backwards in time indefinitely. To an ordinary observer, this would appear as though an antiparticle appears somewhere, and...

According to the writings of Feynman and others, there is a probability amplitude for a particle to travel forward in time, interact with a field, travel backwards in time to a different spatial position, interact with the field again and resume its path forward in time before it reaches some...

I believe that U(1) symmetry leads to the electromagnetic field. All charged particles have this type of symmetry. SO(3) symmetry is normal rotational symmetry, a property shared by all bosons (having integer spin). SU(2) is the symmetry of fermions (spin 1/2). Finally, SU(3) is the symmetry...

Actually, I think I've answered my own question. The Berkeley lecture notes show a magnetic field in one path but not the other. It is that field that changes the spin and demonstrates the spin-1/2 nature of the fermions.

In the double-slit experiment, without which-path information available, the diffraction pattern is usually shown as an even function with respect to the displacement from the midpoint of the slits: something like sin ay / y. (This is the case in Feynman's lectures, and many others.) The...

Doesn't another definition pertain to how fast the quantum state of a particle rotates in phase, compared to how fast the observation reference frame rotates? So for a fermion, (s=1/2), if your reference frame rotates by 180 degrees, the state function (and hence its "probability waves") will...