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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

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
  • 15.9k
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
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
  • 35.2k
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
4 votes
Accepted

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
  • 40.7k
3 votes

Can photons interact with one another?

Photons can interact with each other. However, the interaction is very weak in the vacuum. I'm not sure this can be explained using a simple explanation from wave-particle duality. The full ...
Josh Newey's user avatar
3 votes

How many photons pass through us every second?

Say we have a beam of ordinary electromagnetic waves, with power $P$. Then the energy passing a plane, each second, is $P$. If it is monochromatic with frequency $\nu$ then the number of photons per ...
Andrew Steane's user avatar
3 votes

Photonic black holes

For the sake of argument, let us simplistically take the photon energy $E=hf$, and see what kind of black hole we get. The Schwarzschild radius of a given parcel of energy $E$, which has an equivalent ...
RC_23's user avatar
  • 9,463
2 votes

How is light interference explained with photons?

Now, if we model light as collections of photons, how is light interference explained? It is explained the same as classically. Classically there is an electromagnetic field that obeys Maxwell’s ...
Dale's user avatar
  • 103k
2 votes
Accepted

Why does the up and anti-up quark combine into a pion and not a photon?

Particle annihilation occurs when a particle meets its corresponding antiparticle, converting their mass-energy into two photons. This is a useful oversimplification. Even electron-positron pairs, ...
rob's user avatar
  • 91.4k
1 vote

Is there an actual difference between the scattering and absorption/emission of a photon?

I'm unhappy that the existing answers came close to the essence of your question but missed, so here goes: Consider an isolated atom in an empty universe. Then, as is popularly stated in introductory ...
naturallyInconsistent's user avatar
1 vote

Is there an actual difference between the scattering and absorption/emission of a photon?

So, what is the actual difference between the phenomena of absorption and then emission of light (or even the absorption and transferal into vibrational atomic motion) and the various scattering ...
Ján Lalinský's user avatar

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