28
votes
Accepted
Are photons actually particles at all?
Photons are different from other particles because photons are massless and hence have no meaningful non-relativistic limit as they always move at $c$. I will not rehash the various approaches to ...
- 123k
23
votes
Accepted
Why does light travel in a straight line if the uncertainty principle is true?
If you think about light as a particle, a photon, you can understand that it will continue on its path through space at velocity $c$ unless it scatters off another particle, is absorbed by an atom, ...
- 857
17
votes
Can we understand from basic QED, why is the photon electrically neutral?
Actually I could give a similar answer as given by "Silly Goose" that the gauge transformations of QED form an abelian group, therefore the structure constants of its Lie-algebra are zero, ...
- 9,125
15
votes
Accepted
Limit on how many photons can hit a surface
There is generally no hard limit to the number of photons per unit area in the sense that photons can overlap without issue (i.e., don't think of them as hard billiard balls). This ultimately comes ...
- 7,384
15
votes
Does the light that gets reflected off of us, during our existence, travel through space forever?
In principle, yes. The light reflected from us travels out into the universe and most of it is not intercepted on a timescale of many billions of years.
About 20-30% of visible light is probably ...
- 128k
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♦
- 86.3k
13
votes
Relativistic time dilation and the biological process of aging
I think you misunderstand time dilation. It doesn't mean that clocks slow down. It means that the time between two events is frame dependent. Specifically, the time between two events in a frame where ...
- 25.3k
12
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.
...
- 1,242
11
votes
Accepted
Why is it that, when light travels in a medium, we say it's made of "quasiparticles"?
They're essentially the superposition of photons and the other particles in the medium the photons are interacting with, yes? So clearly they're different from photons.
This is true. In a medium, ...
- 29k
11
votes
Accepted
Does every photon have same momentum because they have same velocity $= c =3\times 10^{8}$ m/s?
The equations you cited are non-relativistic approximations. When dealing with light or other fast objects they do not work. The key equation relating mass energy and momentum for all systems is $$m^2 ...
- 94.6k
10
votes
Accepted
When two gas molecules collide, can they send out an IR photon?
Yes, it is common for colliding molecules to emit IR and microwaves.
When molecules collide some of the kinetic energy may go into rotational and vibrational excitations, so that after the collision ...
- 351k
9
votes
Can we understand from basic QED, why is the photon electrically neutral?
From the wikipedia page for Photon. "The quanta of an Abelian gauge field must be massless, uncharged bosons..." The electromagnetic field is an abelian gauge field. The quanta of the ...
- 1,942
9
votes
Why does light interact with normal matter but not with other light?
Bastam's answer is absolutely correct that in QED photons do interact with other photons.
But you can see why this effect is negligible in every-day scenarios for a couple of reasons:
One is that ...
- 393
8
votes
Why are atoms not being ionized during stimulated emission?
The lifetime of the hydrogen $2p$ state is $1.6$ nanoseconds, so for the atom to be ionised it would have to absorb a second photon within this very short time. This will happen, and some ions will be ...
- 351k
7
votes
Why does light travel in a straight line if the uncertainty principle is true?
The uncertainty principle guarantees that a beam of light will have some minimum divergence in the plane perpendicular to the motion of the beam. If your light beam passes through some aperture with ...
rob♦
- 86.3k
7
votes
Does every photon have same momentum because they have same velocity $= c =3\times 10^{8}$ m/s?
All particles and objects, massless or not, satisfy
$$ E^2 = (pc)^2 + (mc^2)^2 ,$$
where $E$ is the total energy,
While kinetic energy ($T$) satisfies
$$ E = T + mc^2. $$
Note that if $p \ll mc $:
$$ ...
- 31.6k
7
votes
Confusion on Energies of absorbed and emitted photons
Okay so the energy of photons must be EXACTLY EQUAL TO E for the
excitation to take place. Even if the energy of photons is GREATER
than E, the excitation hmmmm, won't take place?
In the first ...
- 57.6k
7
votes
Relativistic time dilation and the biological process of aging
Special Relativity (SR) is now best thought of as giving us 'new' concepts of space and time. For example if two events are separated spatially by distance $\Delta r$ and in time by $\Delta t$ in one ...
- 34.9k
6
votes
Accepted
How do you shoot single photons randomly to a screen?
You take any source of light and create a beam of desired width using solid non-transparent obstacles. Then you put dark half-transparent absorbers in the path of the beam, e.g. darkened glass slabs. ...
- 35.6k
6
votes
Why can't we see in dark where there is a bulb which was once ONN and now it's OFF, the photons released when bulb was ONN should travel to our eye?
When the photon hits a wall, it might be reflected (in a somewhat random direction) or it might be absorbed. If reflected it will encounter another wall and again it may be reflected or it may be ...
- 22.1k
6
votes
What happens to the velocity of a photonic rocket as it approaches $c$?
I we ignore practicalities (eg fuel running out, collisions with dust particles etc), to the crew of the rocket, their ship appears to accelerate at the same rate for ever. In the original rest frame ...
- 25.3k
6
votes
Photon collison with an atom
An atom can take in a photon of one energy and give off a photon of another energy. This is called inelastic scattering or Raman scattering. It is a two-photon process and therefore under typical ...
- 56.8k
6
votes
Accepted
Why is UV radiation below 200nm strongly absorbed by Oxygen?
As a general rule we see discrete energy levels in bound states and continuous energy levels in unbound states. So if a transition is from one bound state to another we will see a sharp line, but if ...
- 351k
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 ...
- 163
5
votes
Where do the photons go in polarized filters?
Common polarizing filters absorb the radiation they don't transmit. They get as hot as any similar object absorbing the same power in the situation they are in (air flow, ambient temperature, ...).
- 18.9k
5
votes
Accepted
How does an electron 'know' if a photon was emitted by a positively or negatively charged particle
There is no real answer to your question, though legitimate because unfortunately it is based on a common misconception driven by pop science. All we can do is "debunk" this idea... Moreover,...
- 2,462
5
votes
Is a photon a single wavelength of monochromatic light?
A photon, as with any quantum system, is described by its wave function. So when you're asking what a photon looks like this is equivalent to asking what the wave function of a photon looks like.
Now ...
- 351k
5
votes
Accepted
Relative speed between two photons
Is it possible to fix an inertial frame to a photon moving in vacuum?
No. In an inertial frame a photon moves at $c$, and in a frame attached to a photon it moves at $0$. That is a contradiction, so ...
- 94.6k
5
votes
Accepted
Why can't photons be localized as wavepackets like massive particles can? What specifically goes wrong in the math if we try to model them this way?
I will only make an answer about the position operator.
The most well-defined things that we may reliably say that we can measure, are Hamiltonian eigenvalues. It should be good on hindsight---you can ...
- 7,432
5
votes
Compton scattering with free electron
The "low energy limit" of Compton scattering is called Thomson scattering, which occurs when the energy of the photon $E_\gamma \ll m_ec^2$. Since $m_e c^2 = 511$ keV, then this condition is ...
- 128k
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