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location Switzerland
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Physicist.


Sep
11
comment Are there models/simulations of antigravitational antimatter-galaxies?
I'm pretty sure to remember (but I left QFT almost five years ago) the QED-process of one photon in, one pair out, has a probability amplitude of zero, and at least one other process has to occur. Of course after annihilation that's ok again, but this merely contributes to the photon's mass as a virtual pair... You raise another interesting point btw: Is the inertial mass of antigravitating antimatter plus or minus it's gravitational mass? I know this is pretty hypothetical, but I'm quite positive that your perpetum mobile can be "repaired" (or rather, broken) someway consistent
Sep
11
revised Are there models/simulations of antigravitational antimatter-galaxies?
minor fixes
Sep
11
comment Are there models/simulations of antigravitational antimatter-galaxies?
That's a good point. Though I wonder if there would have to exist an actual interface or whether the antigravitational repulsion might suffice to obtain truly isolated (anti)galaxy(cluster)s with lots of vacuum around. The interface would, after all, have long annihilated...
Sep
11
comment Are there models/simulations of antigravitational antimatter-galaxies?
At least in QED there's only virtual pair production and annihilation if you violate conservation of energy/momentum, but no non-virtual one. The planet would not be entirely passive, it would exchange an (usually neglected, but think $10^{23}$ such events) amount of four-momentum with the pair during both creation and annihilation, and the additional energy in the photons would almost certainly be extracted from the planet (which might e.g. slightly decrease its distance from its star). My point is, just because antigravity is unconventional, it doesn't have to require violating conservation
Sep
11
comment Are there models/simulations of antigravitational antimatter-galaxies?
I'd appreciate the downvoter's feedback on what makes this a bad question - if you merely disagree with they notion of antigravity, please feel free to post an answer instead
Sep
11
comment Are there models/simulations of antigravitational antimatter-galaxies?
Intriguing - though my gut feeling is something is missing: 1. conservation of momentum requires a third particle being present, the involvement of which will probably use up the gained energy to return to the original state, and 2. can one actually direct the photons?
Sep
10
comment How would we tell antimatter galaxies apart?
@JerrySchirmer good point, thanks. I'd appreciate your adding this as comment to my related question, too. though I fail to imagine a Perpetuum mobile that works with antigravity but not with opposite electrical charges
Aug
12
comment Finding the cosmological redshift of a galaxy in the expanding Universe
@user57074 You should ask that as a new question and not in a comment. But in short, the photon has the energy of the "observed" wavelength - when you change the inertial frame, energy is no longer "conserved" the way you'd expect it to be
Aug
11
comment What restores balance to a repulsive electric force on an electroscope?
it's not only about rendering costs, but also about the potential screwing up with the site's search function I'd guess. Anyway, please take no offence in my editing this out
Aug
11
revised What restores balance to a repulsive electric force on an electroscope?
fixed formatting
Aug
7
comment How to measure Entropy?
agreed, let's do this again sometime :D
Aug
7
accepted How to measure Entropy?
Aug
7
comment How to measure Entropy?
@Phonon sounds sadly reasonable :/ one'd have to describe entropy in terms of the standard model to get close to that I guess, and the usual observer-influences-system part of quantum physics destroys my hopes in said omnimeter...
Aug
7
comment How to measure Entropy?
@Phonon interesting analogy :) though ideally one would find a perturbation that yields the same equilibrium as before (what with non-interfering) and observe the evolution itself, no? interesting idea, I wonder if this allows for a sufficiently general device though. I'm still hoping for an entropy-meter...
Aug
7
comment How to measure Entropy?
@Phonon true, but isn't there some kind of boundary condition for entropy that allows one to have a well-defined system that upon contact with the system to be measured can somehow obtain its entropy (or rather, the difference to another point or system) just the way a classical voltmeter works?
Aug
7
comment How to measure Entropy?
Thanks for the input, too bad it ain't that simple... but that means in order to obtain a checkmark on this answer I'm afraid the original question, how to (generally) measure Entropy, needs to be answered. As @celtschk said, temperature is also not "directly" measured yet one does not need to make any assumptions on the system the temperature of which is measured, except for it coupling to the thermometer with little to no interference of course
Aug
2
comment How to measure Entropy?
@Phonon that's an interesting read there, thanks! concerning the vagueness, my question boils down to whether a device similar to a voltmeter or thermometer exists for entropy.
Aug
2
asked How to measure Entropy?
Jul
31
revised Why does $\mathcal L = -\frac14 F^{\mu\nu} F_{\mu\nu}$ imply Photons are massless?
linked to archive version
Jul
22
revised Analytic continuation of imaginary time Greens function in the time domain
improved formula typesetting