When a photon is emitted, does the rest mass of the emitting body decrease by $E/c^2$ wherein E =photon energy? If yes, does the photon have a rest mass equal to $E/c^2$?
28403
Photons are massless, but if $m = 0$ and , then $E = 0c^2 = 0$. This would say that photons have no energy, which is not true.  However, given the formula $E = ℎf$, a photon does have energy as $ℎ$ is a non-zero constant and photons with a zero frequency do not exist.  What am I missing here, or does the famous formula not apply to photons?
7852
This might be a stupid question, but nonetheless, it has been bothering me.  If you take a photon, make it go through some atoms in a solid, liquid or whatever, then you have the chance of this photon being absorbed by an electron, and thereby exciting the electron. This requires the photon to have enough energy to actually excite the electron to another energy level.  My question is then: How does the photon know if it has enough energy or not? Do they interact very quickly to determine if it's okay or not, or is it just something it "knows?"
14407
I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension of space-time at 1 second per second as time goes forward.  What troubles me here, is the fact that it is traveling through space-time at the speed of light. Why is it at the speed of light?
2102
So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light having a lower frequency which equates to lowering its energy.  My question is, where does the energy of the light go? Energy must go somewhere!   Does the energy the light had before go into the mechanism that's expanding the space? I'm imagining that light is being stretched out when its being red-shifted. So would this mean that the energy is still there and that it is just spread out over more space?
31656
Somebody mentioned on Freenode chatroom for physics that All  are delivered in form of Photons not just light.   Is it true? Does that mean if we get a THF electrical oscillator to emit the frequencies that are visible to the eye, do we see light coming out of the circuit?
5219
If the Higgs field gives mass to particles, and the Higgs boson itself has mass, does this mean there is some kind of self-interaction?  Also, does the Higgs Boson have zero rest mass and so move at light-speed?
46120
I've been thinking about this for quite some time, and from looking online I haven't found a satisfying answer.  Lots of photons, such as visible-light photons have very small wavelength (which from my understanding of basic physics is the distance between two crests/troughs), but I also know that some EM waves have wavelengths a few metres or even kilometres long e.g radio waves.   What keeps me up at night is the question "How can a photon have a wavelength of a few kilometres and yet still be thought of as a particle?"  Does this mean that one individual photon is several kilometres long? If so, wouldn't it be subject to so many variations between the beginning of the wave and its end?  I realise that matter is also wave-like, where it's uncertainty in position is given by its De Broglie wavelength. Does this apply to the photon?  In other words, is the wavelength of a photon simply the uncertainty in its position?
109857
Is it theoretically possible to have a photon of any energy/wavelength? I have a vague memory in my mind of something like there being a minimum energy level for a photon and then the possible energy levels jump up in discrete but ever decreasing amounts, so that near e.g. the visible spectrum photon energies are for practical purposes continuous.
25139
I know that there's a difference between relativistic and rest mass. Relativistic mass is "acquired" when an object is moving at speeds comparable to the speed of light. Rest mass is the inherent mass that something has regardless of the speed its moving at.  When fusion happens, a certain percentage of the rest mass of two hydrogen atoms is converted into energy. I understand that $E = m_{rest}c^2$.   How does the rest mass of an object turn into energy? Does this involve particle physics? Doesn't this suggest that rest mass can be viewed as some sort of potential energy? It seems to me that it's a sort of locked energy that has to undergo a certain process to expel its energy to surrounding particles.  Also, Why does only a small percentage of rest mass turn into energy? Why not all of it. What dictates how much rest mass gets converted into energy?
1740
The simplest equation expressing mass–energy equivalence is the famous $E=mc^2$ where $c$ represents the speed of light. Compare this with $E_K = \frac{1}{2}mv^2$.   Since $E=mc^2$ can be applied to rest mass ($m_0$) and rest energy ($E_0$) to show their proportionality as  $E_0=m_0c^2$, I ask whether this resemblance is just a coincidence created by the need of any equation to be homogeneous for units, or are the two equations fundamentally related?  I know about  but I could not find the answer I'm looking for there.
90933
So I was taught that:  Kinetic Energy (of electron) = Energy (of photon) - Ionization Energy  If E(photon) = IE, then KE=0 of the electron.   What does this physically/theoretically mean?   My current thoughts/interpretation is that enough energy/force is applied to ionize the electron so it is 'sufficiently far' from the atom, and then I guess it just moves with whatever speed it is moving at with natural kinetic laws, since no more energy/force is being applied....?  Any clarifications would be greatly appreciated, thanks in advance.
69880
As a student in a highschool physics class, my teacher has repeatedly told me that photons are massless. Yet, I have also heard from other sources that photons have momentum. If photons were to have momementum, that would mean that they have mass as according to p = mv.  Do photons really have mass?  Also, how would this mass be calculated?
38022
If I am standing on a platform moving along the X-axis and I throw a basketball vertically upward along the z-axis, the ball also travels along the x-axis. Does the same apply to a photon? According to a special relativity postulate  speed of light is not affected by the velocity of the inertial frame or the velocity of the emitter. If this postulate is true, shouldn't a photon fired vertically as above should travel only along the z-axis but not along the x-axis?
120728
I was wondering if a photon is divisible.   If you look at a photon as a particle, then you may be able to split it (in theory).  Is it possible and how do you split it?
113265
I know that the rest mass of a photon is zero. but the photon can be bent by gravity (which can also be explained by the curvature of space-time due to the effect of mass), this implies that it must have some effective mass, while in motion, therefore does it also bend space-time? can the mass of the photon be defined in common(SI) units, how?
18685
In the derivation of the , there is nothing whatsoever that relates to a particular/specific material. But we nonetheless use this formula for several distinct sources in nature, such as the Sun and the CMB. How can it be applicable to so many different things? As far as I understand, the emitted photons come from the rotational and vibrational levels. But shouldn't these levels depend on the particular material/elements that the object is made of?   Summarising:   Is the black body radiation formula applicable to an macroscopic object composed of different elements only?  How can it describe so different things (eg Sun spectrum, CMB) if there is nothing in the formula that relates to a specific material/element? If the photons come from vibrational/rotational levels transitions, shouldn't these depend on the object in question? Also, what's the actual reason why there are no contributions from translational motion? And what about excited electrons?
66996
Do x-rays and gamma rays also contain photons like visible light does? If so, then what makes photons of visible light and other waves different?  The rest mass of a photon is zero, but as it moves at the speed of light, its mass is infinite. By $E=mc^2$, its energy is infinite, but using formula $ E=h\nu$, we get finite energy. How is this happening?