We all know the example where we say that a massless box containing photons has inertia, because the photons exert pressure of the inner walls of the box.

But my question is about a single photon traveling freely. Can it have inertia?

An aspect of this property is the tendency of objects to keep moving in a straight line at a constant speed, when no forces act upon them.


There is another definition of inertia, that is, we need to exert force on an object when we try to remove it from the geodesic it is following.

a photon of energy E confined in massless, perfectly reflecting box has a rest mass because has inertia i.e. it takes force to accelerate the box against the light pressure of the wave reflecting from inside the box: the impulse needed to reach speed v≪c is Ev/c2 so the system could be said to have rest mass and certainly inertial mass E/c2. Photons of energy E always add effective gravitational mass E/c2 to the T00 term in the stress energy tensor "source". So they have gravitational mass E/c2 and indeed there are electrovac solutions of the EFEs where intense light acts on itself through gravity. So inertial mass = gravitational mass

Photon: speed and mass

This answer specifically states that a photon, having stress-energy, contributes to the stress-energy tensor, thus has gravitational mass, and this fact together with the fact that photons do have their own gravitational effects means that inertial mass=gravitational mass for the photon.

Though, many on this site identify inertia with solely massive objects.

Now just like when removing a massive object from its way on a geodesic, we need to use force on it "push it" away from the geodesic, we can do the same with a photon using a mirror.

Now if we have a photon, traveling on a geodesic, and use a mirror to remove it from the geodesic, we use force (constituted by the mirror) to remove the photon from the original geodesic, and the photon will exert pressure on the mirror (opposite force).

Now the photon's pressure (momentum transfer) on the mirror might be miniscule, but it does depend on its frequency, because for photons, energy and frequency and momentum are proportional. This could be interpreted as photons having inertia, proportional to their energy, just like for massive objects, inertia is proportional to their mass (which comes down again to stress-energy).

So ultimately, stress-energy content determines inertia, and that goes for both massive and massless particles.


  1. Do photons have inertia?
  • $\begingroup$ You seem to have already answered your own question: If you define inertia as contributing to the stress-energy tensor, then they do have inertia. What are you asking that is not already contained in your question? $\endgroup$ – ACuriousMind Oct 15 '20 at 16:51
  • $\begingroup$ @ACuriousMind there are a lot on this site, who state that inertia is for massive objects, and a lot of questions and answers using inertia as for only massive particles. My question pertains to establish that ultimately , massless particles can have inertia (photons in the example). $\endgroup$ – Árpád Szendrei Oct 15 '20 at 17:59
  • $\begingroup$ @ACuriousMind Like from wiki :"In common usage, the term "inertia" may refer to an object's "amount of resistance to change in velocity" or for simpler terms, "resistance to a change in motion" (which is quantified by its mass), or sometimes to its momentum, depending on the context." This is an example that uses mass as the definer for inertia itself. $\endgroup$ – Árpád Szendrei Oct 15 '20 at 17:59
  • $\begingroup$ @ACuriousMind this is not as simple as saying, that photons have momentum. I believe having inertia is more then just having momentum (like photons do have momentum). $\endgroup$ – Árpád Szendrei Oct 15 '20 at 18:01

In this link :

Moment of inertia is the name given to rotational inertia, the rotational analog of mass for linear motion. It appears in the relationships for the dynamics of rotational motion. The moment of inertia must be specified with respect to a chosen axis of rotation. For a point mass, the moment of inertia is just the mass times the square of perpendicular distance to the rotation axis, $I = mr^2$.

Massive elementary particles have a moment of intertia, by the definition above, give an axis of rotatio. If the mass is zero the $I$ is zero by definition.

Photons have mass zero so an individual photon must have zero moment of inertia according to the above definition.

where we say that a massless box containing photons has inertia, because the photons exert pressure of the inner walls of the box.

A box containing photons cannot be massless. Mass is the length of the four vector sum of the four vectors of box and photons. Only if all the four vectors of zero mass are collinear in space will the total mass be zero. So since there is a mass there is a moment of inertia according to the definition above.

  • $\begingroup$ Thank you so much, do I understand correctly, that a single photon cannot have inertia? $\endgroup$ – Árpád Szendrei Oct 15 '20 at 21:17
  • $\begingroup$ with the mathematical definition above, yes, a single photon does not have inertia . Light can have. $\endgroup$ – anna v Oct 16 '20 at 4:07

yes they do, and for the reasons you sketched out. In principle, it would be possible to construct a mirror "sail" which, when deployed near a star, could be used to propel a spacecraft via the photon reaction force. However, the reaction force is tiny and to generate useful accelerations, a sail many miles across would be required.

Isaac Asimov may have written a science-fiction short story about "sun sailing" in the 1950's, I'll have to check my library to see if this is true.

  • $\begingroup$ I think you're thinking of the story written in 1964 by Arthur C. Clarke titled Sunjammer. $\endgroup$ – PM 2Ring Oct 15 '20 at 19:15
  • $\begingroup$ @PM2Ring you are exactly right, I read it that year! $\endgroup$ – niels nielsen Oct 16 '20 at 1:42

Somehow questions about inertia are related to these about a photon mass. The discussion about a photon mass can be conducted endlessly. In general:

  • it is clear that a photon has no rest mass. Because it cannot be at rest. It can only exist after its emission until it is not absorbed. In between it moves at the speed of light.
  • the emission of photons (energy) reduces the mass of the emitter Since Einstein, mass and energy have been directly related. The designation of photon energy as mass only makes sense if the calculations are different AND are accompanied by a measurable effect. This does not seem to be the case and apparently will not be the case in the future.

Inertia is the tendency of objects to keep moving in a straight line at a constant speed, when no forces act upon them. In general:

  • for our usual surrounding two photons not interact, no force is exerted on each other. Therefore inertia is not manifested.
  • high-energy photons are capable of annihilating themselves into two or more subatomic particles. I would not call these processes inert, because these photons disappear.

But there is another process in which a photon is deflected. If a photon flies near an edge, it is deflected. That is not surprising; both, photons and the surface electrons of the edge, have magnetic and electric fields, and these interactions are a good reason for the deflection of photons. From the fact of deflection it can be concluded that photons have inertia.


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