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45 votes

How do photons have temperature?

Your quoted statement is an oversimplification which is often correct enough to be useful, but which has led you astray in this case. Fundamentally, temperature is a relationship between energy and ...
rob's user avatar
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17 votes
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How does a photon "cheat" its way past a neutron?

In Quantum Field Theory, each species of particle is a separate field. There can be a photon in a position, and a neutron in the same position, with no problem - they don't need to "cheat" ...
Nadav Har'El's user avatar
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17 votes
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What fraction of the universe's energy is contained in photons?

See (Fukugita & Peebles 2004), which estimates that $10^{-4.3}$ of the energy density of the universe is in the CMB photons, and $10^{-5.7\pm 0.1}$ in stellar and poststellar radiation.
Anders Sandberg's user avatar
14 votes

Does a running motor generate photons?

It does. There are at least three mechanisms. First, if the moving parts of the motor have electric charge or magnetic dipole moments, those fields are reconfigured as the parts move. The information ...
rob's user avatar
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13 votes

Why does light interact with normal matter but not with other light?

Actually it's not correct that light does not interact with light. Photon-Photon scattering is a recently observed phenomenon that was predicted by QED under the name of Euler-Heisenberg Scattering. ...
Bastam Tajik's user avatar
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12 votes
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How do photons have temperature?

The short answer to your question is "Photons do not have a temperature." The long answer: unfortunately, almost everything you've quoted from the internet is either manifestly untrue or ...
IronWidget's user avatar
11 votes

Is a photon truly massless?

Although everyone has heard of the famous equation: $$ E = mc^2 \tag{1} $$ few realise that this is a special case and applies only in limited circumstances. Specifically it applies only to a massive ...
John Rennie's user avatar
9 votes

Why does light interact with normal matter but not with other light?

Bastam's answer is absolutely correct that in QED photons do interact with other photons. But you can see why this effect is negligible in every-day scenarios for a couple of reasons: One is that ...
KierD's user avatar
  • 391
9 votes

Is a photon truly massless?

The correct version of your syllogism is: $E=mc^2$ for a particle at rest. For a photon, $E>0$. For a photon, $m=0$. The correct conclusion is that a photon can never be at rest.
WillO's user avatar
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8 votes

If matter cannot be created or destroyed, how were scientists able to "create" matter out of light?

Matter can be created and destroyed. Matter is not a conserved quantity. Conserved quantities include energy, momentum, charge, angular momentum, etc. Those cannot be created or destroyed, at least ...
Dale's user avatar
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8 votes
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Physical meaning of the multipolarity of gamma radiation

Think of the nucleus as a distribution of charges, which come from the protons, and a distribution of currents, which result from the motion of protons inside the nucleus. The oscillation of these ...
cconsta1's user avatar
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8 votes

How do photons have temperature?

There have been a lot of questions about momentum, energy and gravitation of massless objects lately, so I'll make a blanket statement: Newtonian equations ($p=mv$, $KE=\frac 1 2 m v^2$, $\Phi=GM/r$) ...
JEB's user avatar
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6 votes

Why does light interact with normal matter but not with other light?

While it is true that in a QED, photons do interact with each other, if we consider light as a wave for our present purposes, light waves do interact with each other, such as in constructive or ...
Robo's user avatar
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6 votes

Does the number of photoelectrons depend on the area of the beam for a given beam power?

This answer was for version 1 of the question. Experimentally, how did they measure the amount of ejected electrons ? Typically you just measure the current produced by the ejected electrons, which ...
BioPhysicist's user avatar
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6 votes

Is a photon truly massless?

Light has inertia: it takes a force to change the direction in which they travel, and if you have a box with light bouncing around inside of it, it takes more force to change the speed of the box than ...
g s's user avatar
  • 14k
6 votes

Could a single gamma ray photon break the Schwinger limit? If so, at what energy?

Consider an electorn-positron collision, and the subsequent production if photons. It is known that there must be at least two photons produced. This is because momentum must be conserved. In the ...
CompassBearer's user avatar
5 votes
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Photon Doppler shift and Lorentz invariance

The expression (form) $p^\mu q_\mu$ is Lorentz-invariant (has the same value in all frames), but the expression $m_p E_q$ is not Lorentz-invariant (it does not have the same value in all frames). How ...
Ján Lalinský's user avatar
5 votes

Is a photon truly massless?

I like pictures, and the relevant picture is: You can ignore the formulae, they are just high school trig, tho, so nothing prohibitive. (The do look bad, though, I think it's because they are crammed ...
JEB's user avatar
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5 votes
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Does photocurrent depend on intensity or on number of photons?

You have it right. You can idealize it as each photon kicks off one electron. (It might be really that each photon kicks off one avalanche.) So current is proportional to number of photons. If the ...
mmesser314's user avatar
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4 votes

How is a photon being consumed by electron in QM?

a free-standing unbound electron. It has it's wave-function describes probabilities to find it in particular position. Also there is incoming photon with a proper wave-length to be consumed. This ...
Dale's user avatar
  • 103k
4 votes
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What photon energy is needed to photodissociate large elements?

You can calculate the difference in binding energies between a $^{56}$Fe nucleus and the sum of binding energies for an alpha particle and $^{52}$Cr. It is 7.61 MeV. The difference between the binding ...
ProfRob's user avatar
  • 133k
4 votes

Connection between photons and EM waves

I am not sure that my answer will really answer your question. But I hope it helps. You might also check Anna V's answer to How do photons induce current in an antenna?. Or Relation between radio ...
mmesser314's user avatar
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4 votes

Physical interpretation of photon propagator

The two free indices on a non-scalar propagator simply label the spin- or polarization-state of the particle. So naively, $D_{\mu\nu}(x,y)$ is about a photon with polarization vector parallel to the $\...
ACuriousMind's user avatar
  • 126k
4 votes

Physical meaning of the multipolarity of gamma radiation

The classification into electric and magnetic multipole radiation is a general feature of the multipole expansion of the far field of a radiator, it can even be discussed purely classical. The ...
Sebastian Riese's user avatar
4 votes

Is reflected photon the same?

at this scale how do you define original? because in the end it's just energy. a wave reflecting back or a photon reflecting back obeying their characteristic nature.
Lakshya Dubey's user avatar
4 votes

Does the number of photoelectrons depend on the area of the beam for a given beam power?

Classically, it is expected that the EM field will deliver energy into a spherical volume at a rate proportional to its intensity. This in turn implies some delay in emission of particles absorbing ...
R. Romero's user avatar
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4 votes

In a universe with no photons, will everything necessarily be at absolute zero temperature?

A universe filled with only dark matter would contain no photons at non-zero temperature. It requires matter that is coupled to the electromagnetic field to generate radiation.
my2cts's user avatar
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4 votes
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Why can photon be treated like gas?

So, usually, any group of particles which have very weak interaction between them can be compared to an ideal gas. If the interaction is slightly more, then it is compared to a liquid and if the inter ...
SchrodingersCat's user avatar
4 votes

Does photocurrent depend on intensity or on number of photons?

The current depends on the number of photons with high enough frequency(energy). 1 radio photon and 1 gamma photon will give you 1 electron, because gamma photon has enough energy to expell an ...
Alien from future's user avatar
3 votes

If photons are travelling so fast that time stands still then how does the energy of a photon change when this is tied to the frequency of the photon

The real meaning of a sentence like ...no time will pass for the photon as it travels. Is simply that there is no reference frame where the photon is at rest. It may sound weird for people used to a ...
GiorgioP-DoomsdayClockIsAt-90's user avatar

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