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Electromagnetic radiation will continue to travel until it is absorbed. Some of your wifi signal is escaping to space where it may continue traveling for a very long time. However, the strength of your wifi signal will degrade with distance according to the inverse square law. So if you double the distance between your device and the wifi transmitter, your ...


16

Yes, the atomic hydrogen is probably mostly left over from the Big Bang. [Edited to add: Not sure how much that is true and how much present-day atomic hydrogen is the result of recombination.] And, yes, ${\rm H}_{2}$ does get dissociated by high-energy photons -- and also by cosmic rays, which can penetrate dense, dusty clouds that block most of the high-...


14

Quoting from Nat.Geo. article (which has that same UV image), Then, in 2010 and 2011, Hubble took a close look at the moon. More specifically, it looked at the auroral bands ringing Ganymede’s poles. Hubble studied the auroras in the ultraviolet, but Saur said the shimmering lights would be visible to human eyes. “If somebody could be standing ...


13

Signal-to-noise ratio In addition to what others have said, it is very important to understand the difference between just detecting something and decoding a useful signal from it. The CMB is essentially random noise – in fact, that's how it was discovered in the first place! Still, in normal conditions it is easily drowned by other noise sources and was ...


10

There's higher quantity of atoms in your 20cm wall than there is in the 13.8 billion light-years travelling to the CMB, so the wifi waves hit atoms on their travel. Space has an average density of 5.9 protons per cubic meter, that's 10^-25 g/m3, and there are only 1.22*10^26m to travel to the CMB source. The CMB is an omni directional transmitter source, it'...


9

No. But the reasons are biological, not physical. Your eyes work by the interaction of electromagnetic radiation with certain molecules ( rhodopsin which consists of the protein opsin linked to 11-cis-retinal, a prosthetic group.) These molecules are tuned to detect light of particular wavelengths. But they couldn't be tuned to detect radio-waves, since ...


8

Your question may ulitmately be about the physiology of the eye, which is off-topic here. The spectrum of the Sun seen low on the horizon is quite different to the spectrum of an M-type red dwarf. The reason that a red dwarf is red, is not just that it is cool, but that there are great chunks of the spectrum that are absorbed by molecules in the photosphere ...


7

The first thing you need to recall is that electromagnetic waves do carry momentum as well as energy. This shows up in effects like light pressure. Specifically a photon of wavelength $\lambda$ carries momentum $h/\lambda$. In and of itself, that doesn't answer your question though, since you are asking about the rotation of the pulsar, and changed to its ...


6

The Physics SE answer (or the part quoted) was incorrect. The photon does not have to have "precisely" the right energy to cause a transition. The reality is that there is a non-zero probability of causing a transition at all photon energies, but the probability distribution is sharply peaked at the energy we calculate to be the energy difference ...


5

This is one of those questions that is easy to state but complicated to answer - and this won’t at all be a complete answer, but mostly a quick outline of some important factors to consider and terms you might search for in order to learn more. The question of why the interstellar medium (ISM) has the structure it does is a long-standing one, and one that a ...


5

You cannot have free protons without electrons. Plasmas, in general, are electrically neutral. It is usually electrons that dominate the scattering (note that a point-like charge cannot absorb a photon and conserve energy and momentum) in a plasma at low photon energies. That is basically due to their much lower masses (classically you can think of the ...


4

Surely the sun possesses calcium in its atmosphere, as well as in its bulk volume. This plot, based on the data published in Asplund et al.,(2009), shows what elements can be found in the solar atmosphere: And we can read off that the abundance [Ca]/[Si] = 0.1 for example. Elements in stellar atmospheres can occur both in absorption and emission in stellar ...


4

There are two effects causing this: The relevant quantity for determining whether or not a photon interacts with a particle is the ratio $$ x \equiv \frac{2\pi r}{\lambda}, $$ where $r$ is a size of the scattering agent. Rayleigh scattering When $x\ll 1$, we're in the Rayleigh regime where the wavelength is so long compared to the particle that the ...


3

'Absorption' lines are caused by resonance scattering (scattering the radiation out of the line of sight, see illustration below), and resonance scattering has a very large cross section of roughly $10^{-12} cm^2$. This means that even for a thin layer of 10km ($10^6 cm$) you need only a density of >$10^6 /cm^3$ of an element for the layer to become ...


3

The strength of an absorption feature in the stellar spectrum is dependent on the amount of that element that is in the photosphere but it also depends on the atomic structure of the element and the conditions of temperature and density in the photosphere. For example the CaII lines need there to be singly ionised calcium ions in the photosphere. This ...


3

If we take 1 atmosphere of optical depth to mean looking though the Earth's atmosphere at zenith, then the optical depth to scattering is small - probably of order 0.3 for blue light and much smaller (according to $\lambda^{-4}$) for red light. That means that when the Sun is at zenith, most of the light reaches the ground but some blue light is scattered ...


3

The picture is a mocked-up fake and is not an actual picture of the solar spectrum. You can easily see this because the black "Fraunhofer lines" extend beyond the spectrum and H alpha should have an appreciable width. The table is massively incomplete. It list only a tiny fraction (the strongest) absorption lines in the solar spectrum. There are ...


2

I think there's been a little bit of confusion, both about the passage in Wikipedia and the phrasing in the question. Your post asks two distinct questions: Why an isolated pulsar's rotation slows down over time, and why this slowdown eventually leads to the end of radio emission. The gist of the answer to the first - rotational kinetic energy is transferred ...


1

The core of the star is the seat of nuclear fusion, yes, but by the time this energy reaches the surface (a few hundred thousand years at least in the case of our Sun), it has time to dissipate (from a [comparatively] small core to a huge outer surface). What’s left at the photosphere (the apparent surface of the star) is not nuclear fusion anymore, but ...


1

HII regions or emission nebulae are associated with the presence of massive stars that ionize the gas. The strongest emission line from an HII region comes from H-alpha. What happens in this case is that the hydrogen atom becomes ionized. Next, the electron and proton recombine to form a hydrogen atom again; however, the electron may be at any energy level. ...


1

You basically asked the same question over on Worldbuilding. I'm copying my answer to that question here. This answer does not specifically address the strength of the magnetic pulse because whether or not that strength has any affect is dependent on far too many variables to give you a simple answer (e.g., ground conductivity, ground charge, age of the ...


1

Yes, and the reasons are both physical and biological. Our eyes use molecules that can be excited by electromagnetic visible light waves (wavelength 0.4 to 0.7 microns roughly) and those excitations can then be converted to other molecular signals and eventually depolarization of nerve cell membranes ("neurons firing"). Snake eye-pits use ...


1

I'm uncertain of the answer; there seems to be some uncertainty involved in the mechanism, as if there were some kind of principle involved ;-) I'll never get quantum mechanics, but that's the nature of QM; it simply doesn't work the same way we think the "real world" works. I think the challenge question to this question is "Can a photon even ...


1

While @planetmaker's comment is true if the lines come from the same source, you can have lines emerging from different physical processes which still appear to come from the same location. An example is absorption (or more rarely emission) lines from galactic winds, which are typically blueshifted with respect to the "systemic" redshift, i.e. the &...


1

Reddening (or the fact that blue light is more extincted than red, causing objects to appear more red) is due to the interaction between the light and the dust grains and gas molecules it is going through, and is caused by the relative size of the dust grains and of the wavelength. Indeed, dust grains are very effective at scattering light which has a ...


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