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Gravitational lensing: derivation of magnification

Your definition of magnification as the ratio of the image area to the source area is correct. As you suggested, the magnification is the determinant of the Jacobian matrix for a mapping from the ...
PhysicsProgrammer's user avatar
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Is the radiation from closed boxes blackbody radiation irrespective of the material of the box?

The box is closed so it will have no radiation from outside coming in. Even if the silver box is closed, it is not perfectly reflecting. Radiation of high-enough frequency goes through, and thus ...
Ján Lalinský's user avatar
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What is emission line ratio?

It is as you might expect a measurement of the ratio of the fluxes that are present in the named emission lines. One construct ratios, as opposed to absolute fluxes in the individual lines, because ...
ProfRob's user avatar
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-1 votes

Is the radiation from closed boxes blackbody radiation irrespective of the material of the box?

The system being referenced is a two-state system, described by the Einstein Coefficients as quoted and follows: Einstein A and B Coefficients In 1917, about 9 years before the development of the ...
Stephen Elliott's user avatar
1 vote
Accepted

Is the radiation from closed boxes blackbody radiation irrespective of the material of the box?

As naturallyInconsistent says the answer is yes, but it's interesting to go into why this is. The reason is simply that for any material the emissivity and the absorptance are identical. This is ...
John Rennie's user avatar
0 votes

Is the radiation from closed boxes blackbody radiation irrespective of the material of the box?

The answer to your title question is yes. Even if you used silver, i.e. low emissivity materials, to make the inside of the box, as long as you wait long enough for thermal equilibrium to be ...
naturallyInconsistent's user avatar
-1 votes

Is the radiation from closed boxes blackbody radiation irrespective of the material of the box?

A "black body " is an idealised concept, like a geometric, infinitely thin line. As stated in Wikipedia, "a black body or blackbody is an idealized physical body that absorbs all ...
hdhondt's user avatar
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0 votes

What are black dwarfs?

A black dwarf is what happens when a brown or white dwarf cools beyond the visible spectrum. At present, such objects are only theorized, because of the incredible cooling time (estimated in the ...
Joe Peters's user avatar
1 vote

How can the Cosmic Neutrino Background (CνB) have a temperature? How can any neutrino have a 'temperature'?

Loosely speaking, any system where the degrees of freedom can exchange energy with each other and is in equilibrium has a temperature. A "system" here is a collection of particles, fields, ...
IronWidget's user avatar
7 votes

When do we talk about spaghettification or pancakification in black holes?

Susskind explains it here: the pancakification due to gravitational time dilation is only in the reference frame of Bob. In her own frame Alice gets spaghettified, not pancaked, since the local tidal ...
Yukterez's user avatar
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5 votes
Accepted

When do we talk about spaghettification or pancakification in black holes?

I watched the video, and it's important to note that the term pancakification is not used anywhere else at all. However, it is interesting to see what the video tries to say. Without going too deep ...
Robrecht Keijzer's user avatar
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Kolb and Turner's expresion for number density Eqn. (3.46) and pressure in Eqn. (3.48)

I think relation (2) can be derived the following way. Consider an element of surface $\delta S$ with its unit vector $\vec n$. The particles that will collide with it during a time ${\rm d} t$ lie in ...
Nebul's user avatar
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2 votes

Realistic black holes

Why must we associate real black holes with the mathematical artifacts we call singularities ? The simplest model of the interior (inside the event horizon) of a black hole that is consistent with GR ...
gandalf61's user avatar
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2 votes

Realistic black holes

Why don't we treat theoretical singularity points and realistic black holes separately? Because black holes, as we understand them presently, are the prediction of GR, and the same theory results in ...
S.G's user avatar
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2 votes

Why are planets denser as you approach the center?

TLDR: Every particle of matter in a planet (except for the one particle at the very center) feels net gravitational attraction toward the center. The shell theorem says that particles closer to the ...
Solomon Slow's user avatar
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2 votes
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Why are planets denser as you approach the center?

If a planet formed a spherical shell (which isn't a good model for how planets actually form but we can run with it as a thought experiment), then: it is true that, say, a piece of dust that was ...
Andrew's user avatar
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-1 votes

Explosion in space

You are right! There is no medium in space. That means you cannot hear this explosion. But you can still see it in light and other EM rays like microwave, X-Rays etc. But one interesting thing is that,...
Arnav's user avatar
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2 votes

The magnetic force between the earth and the sun

The magnetic force between the two is minuscule compared to the gravitational force. We can get an order-of-magnitude estimate by treating both the sun and the Earth as ideal dipoles. The force ...
Michael Seifert's user avatar
-1 votes

How hot is the core of a star just before it collapses to form a black hole?

No, they wouldn't be "on the colder side" since the collapsing star would be putting so much pressure on the hydrogen molecules inside of it that the hydrogen molecules would begin to bond ...
Aiden S.'s user avatar
1 vote
Accepted

What is the function that demonstrates a planetary transit light curve given a planet's projected distance from its star?

First, we will need to determine a form for the function $f(r)$ which describes the brightness at a distance $r$ from the center of the star. This is the effect of limb darkening; there are several ...
Riley Scott Jacob's user avatar
1 vote

Can glueballs and bosons survive indefinetely in space (forming structures)?

Can <…> survive indefinitely in space (forming structures)? No, in a universe with positive cosmological constant. Whatever the mechanism is for formation of a composite object (that you called ...
A.V.S.'s user avatar
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2 votes

Using Gravitational Waves as an observation technique

Somewhat YES. Yes in the sense, that theoretically gravitational waves have no upper limit of frequency, given that you can find gravitational source strong enough, you could get these waves scattered ...
Agnius Vasiliauskas's user avatar
0 votes

Relationship between gravitational force and fusion energy in stars

It's actually not much different than a diesel engine. In the engine: The mechanical force of the piston compresses a gas to a much smaller volume, and higher pressure and temperature The high ...
RC_23's user avatar
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3 votes

Relationship between gravitational force and fusion energy in stars

It is not quite the "gravitational force on [its] surface" that has to be balanced by the fusion energy, rather it is teh requirement that, at equilibrium, the gravitational forces on each ...
Penguino's user avatar
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