Could light be dark matter?

  • Is it possible that light itself is dark matter? I am speaking of photons (e.g. visible light, infrared, ultraviolet, etc...). I realize light is understood to be massless, but it is obvious it at least contains energy because we can see with it (e.g. it energizes the cells in our retinas). I wonder if light has a very tiny "net mass" (e.g. 0 mass * relativistically infinite speed). I would think that light at least has a little mass, in proportion to its energy. For example, take E=mc^2, then m = E/c^2 would describe how much mass it has. If this is true, light should have a very little gravity too. Although the effect would seem minimal, light is practically everywhere. Gravity from light would be more concentrated inside galaxies, and even more concentrated in the center of galaxies where there are many stars (like dark matter is). It would be interesting to run the calculations, assuming light does have gravity, and see if this matches the gravitational observations of dark matter in the universe. It would be funny and ironic if dark matter really is light.



    Edit: Note that it appears light does have gravity as per the discussions here: How does light affect the universe? If that much is true, I wonder if this is significant enough to account for dark matter?


    This seems a little far-fetched and I would expect that if there was light, we would see it. Also, I believe that all cosmological models account for the effect of light/photons/radiation. However, this is an extremely interesting correlation or a line of thought that I think I am absolutely in love with. Being not very comfortable discussing details about cosmology myself, I would like to see any answers people have. So, a +1. Also, great thinking, keep it up. This is the kind of creativity that leads to great research.

    Light can be detected and Dark matter cannot be detected.

    An interesting question... if you add up all the energy of all the "in transit" photons in the universe (in some definition of "now") and equate it to mass via E=mc^2, is it a meaningful thing to do (does it behave as mass e.g. have gravity), is that mass of any significance, and if so, does it in any way account for the so-called "missing" or dark matter in our universe, even if only partially?

    In this way, you are assuming that light gravitates much more than ordinary matter.

    @AnthonyX It appears the answers and sources to the linked question show that light does gravitate according to the mass it would have by E=mc^2. If this is true, the big question is, is this enough to account for dark matter?

    @Py-ser I would like to see how much of dark matter it accounts for, assuming the mass associated with light (m=E/c^2) gravitates just like the equivalent mass in normal matter.

    Interesting. One common idea has dark matter being a WIMP (Weakly Interacting Massive Particle). While photons don't seem to interact via the weak nuclear force, I wonder if electroweak unification could give the photon dark matter idea an interesting twist.

    This is pretty interesting. Some say, if it were light, we would see it, but would we? We don't see the photons that are coming off the sun, perpendicular to our line of sight. We don't see them as they pass us heading to Jupiter, but yet there they are, reflecting back to us. In a sense we see the effect of light, but not the light it self (like a laser). I always wondered if it was just that instead of admitting the mass of things is more than we thought, we say it has to be something else.

    @fractalspawn I agree with your analysis. We don't see the light unless it enters our retinas (or whatever detection device we use). This means there is a tremendous amount of light we do not see. For example, we don't see a laser beam (light in transit), but we see the point where the beam strikes because light bounces off the object and then enters our eyes.

    Light is massless. Full stop. A better way to think about it is that energy contributes to the spatial curvature that is interpreted as the force of gravity. Either way, the current light content of the universe is a negligible source of energy density.

  • harogaston

    harogaston Correct answer

    8 years ago

    Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain.



    The force of gravity exerted by light is negligibly small yet we have measured the gravitational pull of Dark Matter to be big enough to affect whole galaxies; it is what binds galaxies together.



    Furthermore, we have included everything we can observe (all ordinary matter including photons) when we do the calculations for the amount of gravity there should be. So light is already there. 'Dark matter' is that extra gravity which we cannot account for.


    It appears light may be a gravity source as per the question / answers I linked to at the end of my question.

    Thank you for pointing that article out. As I read it, it is pretty clear that the gravity caused by light is ridiculously small, yet dark matter represents a gravity source even greater than that of ordinary matter. Furthermore, we include everything we can observe (all ordinary matter including photons) when we do the calculations for the amount of gravity. So it is already there. 'Dark matter' is that extra gravity for which we cannot account for.

    I would consider that comment a good answer :-) If you modify your answer with this, if I don't get anything better, I will probably accept that as the answer.

    Edited. Glad you found it explanatory enough.

    This is not correct. It is called "dark matter" both because it cannot be seen, but also because the majority of it needs to be in a form that does not interact electromagnetically. Therefore light cannot be dark matter.

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Content dated before 7/24/2021 11:53 AM