I'm a chemist not a physicist, so I have a loose relationship with its laws. I read that light (specifically a photon) doesn't have mass, gravity doesn't act on a photon to alter its trajectory. Instead a mass-laden object (planet/blackhole, which inherently produce a gravitational field) acts on the 'fabric of spacetime,' producing relative curvature to the otherwise linear path that a photon would travel along.

So then why is it not logical to conclude that a photon consumes a certain volume or cross sectional area - if a photon has no volume (or area x time-[c]) why would the curvature of the 'fabric of spacetime' affect the photons path?

In relation I wonder - is there a limit to the intensity or # photons per unit area; I'm specifically thinking about concentrated solar energy, is there an upper limit, does Planck have a role here? Maybe a better way to ask this is - can multiple photons continue to combine into one with higher and higher frequency, I suppose up to the frequency length of a Planck-length (material logistics aside)?

On brief research on high-est frequency I was led to the Casimir effect, Alcubierre, and "zero point energy" said to possibly have indirect relation back to gravity - so I know I'm over my head here...your ideas/thoughts/help is appreciated.


I think you may be mixing a Newtonian view of gravity with the modern Einsteinian view. In short anything that creates a stress energy tensor will cause curvature of space-time and all things move "freely" along the geodesics associate with the corresponding metric tensor that describes the geometry created by stress-energy.

As with all things in science and engineering, physics, chemistry, etc, we frequently try to get approximate solutions to the behavior of systems. This is fairly accurate when one component of a system is so much larger that it could be considered a bath, or reservoir. Like in thermodynamics we talk about heat reservoirs, that can emit and/or absorb heat and not change temperature, we have inertial reservoirs in physics. For example, in planetary motion, one considers the sun to be so much more massive than the planets that (1) we can neglect the effect of the sun's motion about the sun+planet center of mass, and (2) we can neglect the effect of the planets on each other. Of course we know this is false and need to do a better job when predicting planetary motion, the tides, etc.

So, in relativity the comparison would not be mass but the equivalent energy of each system. Whatever you read, I would like to know the reference? It seems like a neat way to start a discussion about the paradigm shift of GR but it is not accurate, or even true. The curving of space-time IS seen ad the gravitational force. So, the statement made about photons also applies to planets, they too follow the curvature of space+time rather than being acted on by gravity. But as I stated the bending of space-time IS gravity so the descriptions are really the same. A GR theorist would feel comfortable sating that Gravity bends the trajectories of photons. The trajectories of ALL particles in free motion are geodesics. Photons follow a special class of geodesics call Null, because the have zero length in 4-dim space-time.

As for your other questions, I cannot venture to guess. But I wanted to clarify this point. By the way mass is not the only source of this curvature in relativity, charge, rotation, any and all forms of stress+energy+momentum that produce a non-trivial tensor contribute to gravity, it's not G*M1*M2/R^2.


Photons just move along the straightest path that exists in curved spacetime. There is no reason they would need to have a finite size to do this, any more than a straight line in Euclidean geometry needs to have a finite thickness.

There is no observed limit to the energy a single photon can have, or to the number of photons you can crowd together. As far as we know, photons do not “combine” when they crowd together.

There are some theoretical reasons to believe that there might be limits involving the Planck energy and the Planck volume, but there is no consensus on any such theories, and testing these theories is far beyond our experimental capabilities.

  • $\begingroup$ The same is true for mass too. Massive particles move along the straightest curves possible. $\endgroup$ – ggcg Jan 15 at 3:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.