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May
16
comment How to guess the correct fitting function to some data?
@DavidZ: I had encountered the method used a few times rather popularly in astrophysics, see e.g. Eddington formula. Idea is as follows: If you have an approximate method to solve a certain problem, it might provide you with a complex computationally expensive analytical expression. One might prefer in this case to find another simpler function, which fits the original expression within the same error bounds.
May
15
comment How to guess the correct fitting function to some data?
You are certainly right of course, but you leave out of consideration an interesting mathematical problem, which can be stated intuitively as "find the simplest analytical formula describing the data within a given precision". It actually may be a rather powerful way of compressing data in some cases.
Jan
15
comment Do neutron stars reflect light?
I was having in mind something like this: en.wikipedia.org/wiki/Cloud_albedo . The laser probably would be better reflected if it were tuned at some of the absorption lines. And yes, it is not a black body. But black body approximation is used for getting emission and due to the fact that neutron star's atmosphere is optically thick at the wavelengths it is emitting.
Jan
15
comment Do neutron stars reflect light?
Highly - none, but reflection coefficient of 0.1-0.2 is not that small either. Consider clouds - they are not ionised, granted, but are opaque and rather reflective. Why would dense neutron star atmosphere not be similarly reflective?
Jan
14
comment Do neutron stars reflect light?
Hm, generally, one assumes a general statement first and then argues for a particular one. For example, one would generally expect even optically thick media to be reflective, and indeed why would it not scatter some of the incident light back?
Jan
4
awarded  Yearling
Dec
21
comment Does a charged particle accelerating in a gravitational field radiate?
@sure: No, this is definitely fine, though I thought you meant acceleration in general. Yes, I do agree that an accelerated charge radiates non-locally to infinity, hence feels self-force. However, the radiation as a locally measured quantity is dependent on observer. I guess this settles the discussion.
Dec
21
comment Does a charged particle accelerating in a gravitational field radiate?
@sure: Well, I mean quite precisely, that if you take an observer and consider all the physical quantities (metric, fields, curvature) in his/her vicinity, you will get what observer 'sees'. This way, accelerated observer will 'see' that if it releases a test particle, the particle will get accelerated (cf to what you said). Similarly, accelerated observer can take a detector and measure the flux from a field of an inertial charge. Note, the charge didn't radiate for being inertial, but still can learn that the energy was extracted by a non-inertial observer (again, cf to what you stated).
Dec
21
comment Does a charged particle accelerating in a gravitational field radiate?
@sure, sorry, but it is not absolute. Consider a charge in vacuum - it certainly does not radiate for an observer, for whom the charge is at rest. However, an accelerated observer will see the energy flux. Therefore, the notion "radiate" is dependent on the observer. And therefore, some of your statements need to be restated with observers being specified.
Nov
4
awarded  Notable Question
Jun
4
comment Speed of gravity
For weak gravitational fields you can also introduce the concept of field energy, which indeed does differ in both cases.
Jun
4
comment If a 1kg mass was accelerated close to the speed of light would it turn into a black hole?
@voix, if you rotate a mass, then you have to provide a lot of energy and pressure to keep it rotating (centrifugal forces). Those in turn will gravitate on theire own, and it will mess up the problem a little bit.
Jun
4
comment If a 1kg mass was accelerated close to the speed of light would it turn into a black hole?
@TedBunn: And yet, light bending will become very peculiar for a very fast mass. Special relativistic aberration will come into play in ultrarelativistic regime which will enhance the otherwise weak lensing. Also, the fast body will feel strongly enhanced tidal forces in its frame, and there will be quite a lot of other interesting things happening.
Jun
4
comment Does a charged particle accelerating in a gravitational field radiate?
@JerrySchirmer, by the way, even non-charged point masses may have self-force in GR, similarly to Abraham-Lorenz force.
Jun
4
comment Does a charged particle accelerating in a gravitational field radiate?
@JerrySchirmer: I would say, the answer is also qualitative, it just asks why does the paradox appear. The paradox is resolved by two statements: 1) The charge in its own frame does not radiate only locally, 2) Additionally, static fields may turn into radiation when the frames are switched. So I just didn't go into calculations. And as you show in your answer, it takes an article size derivation to get the field estimates.
Jun
4
comment Does a charged particle accelerating in a gravitational field radiate?
By the way, correct answer, but doesn't expicitly address the question. Indeed, the word 'locally' is the resolution of the paradox. The charge does not emit radiation locally in its comoving frame. And yet does so generally for accelerated or non-comoving observers.
Jun
4
comment Does a charged particle accelerating in a gravitational field radiate?
@JerrySchirmer, and yet, could you please kindly be specific in your statements? Is there any particular argument that you find wrong in my answer? Do you think some aspect of the question was not touched upon? Or in which way could in general the answer be better in your opinion?
Jun
4
comment Does a charged particle accelerating in a gravitational field radiate?
Dear @JerrySchirmer, many thanks for pointing this out!
Mar
9
awarded  Nice Question
Jan
10
comment Does a charged particle accelerating in a gravitational field radiate?
@JánLalinský: It is beacuase when you switch to an accelerated frame as an observer, metric will be constant with your proper time. If you write down Einstein-Maxwell equations, you will have equations for electric and magnetic fields, which do not depend on time. There may possibly be some flux components, though, I don't have a proof at hand.