Here's an example to describe the issue:
Suppose a high-power laser (e.g., a 100 kW laser, i.e., an electromagnetic weapon) is fired at a target, then it will receive energy and move, (and likely burn or explode, but that's not the point).
My question is, stating that photons have no mass.
Would the laser transmitter get a kickback/recoil like in a "normal" gun?
Dear HDE, The laser beam obviously has energy and momentum so the laser transmitter gets recoiled due to the conservation of energy and momentum. See also:
http://en.wikipedia.org/wiki/Poynting_vector
The property we call mass is expressed by:
$m ~=~ \sqrt{\frac{E^2}{c^4} - \frac{p^2}{c^2}}$
which is zero for photons even though the energy $E$ and momentum $p$ are not zero. The mass m is zero if energy relates to momentum as $E^2=p^2c^2$. This shows you that photons can't be at rest because in that case both $p$ and $E$ are zero.
Regards, Hans
Yes, recoil makes part of such a process. You may consider this as a decay of an excited laser into two parts: a photon beam goes in one direction, a deactivated laser goes in the opposite one. This happens during emitting photons. The beam fate is a quite another story.
I asked a related question recently, Explain how (or if) a box full of photons would weigh more due to massless photons. Both of our questions are concerned with the accounting for physical properties when a photon is in-flight.
The transmitter is pushed back as the photons are emitted. A given force $F$ comes from the laser at all times it is on, which can be calculated from the momentum (addressed in other answers). If the laser is on, pointing in a single direction for a given time $t$, a quantifiable impulse is imparted to the laser apparatus.
Mass-Energy Balance
The challenging part comes when you consider that the laser apparatus (obviously including the energy source) looses mass through this process. If you take the laser apparatus to be at rest, no energy is imparted to it since power is force times the velocity $P=F v$, and $v=0$. That means that all mass that leaves the laser apparatus system ($\Delta m$) is accounted for by energy in the laser beam.
$$E=\Delta m c^2 = N p c$$
$N$ is the number of photons emitted in $t$ time.
Center of Mass (CM) and Net Velocity
Now consider an even more challenging component - the center of mass of the total system. Consider the laser and connected devices to be at some point A, isolated in empty space. Then consider that the laser fires directly toward another heavy isolated system in empty space B for time $t$. Take the distance between them to be $d$ and assume that $d>>c t$. That assumption is to make the beam like a "batch" single emission. The A system is moving away from B after it is fired. Both A and B are at rest before firing, $m_A=m_B$, and the system including both masses, system AB, has a CM at $d/2$.
If you include the photon in your definition of system AB after firing but before absorption by B, the system MUST have a net velocity of zero and the CM must not move. This is an extremely challenging concept, and to the extent of my understanding, you must simply consider the photon's relativistic mass. it has no rest mass, but the location of the photons must have some weighting in calculation of the system CM for the typical principles to still hold, this is $m=\frac{N h}{\lambda c}$. The momentum of the photons balance the recoil momentum of system A, and the position of the photon balances the recoil movement of system A. In this way, the photon is very very similar to if the process were instead a bullet. The main difference is that it has no rest mass and moves at the constant speed of light.
