Q: If light slows down in different materials, then how can it be a universal speed?

Physicist: This bothers a lot of people.  When you’re learning physics, there are several things that you learn in the first couple years.  Among them are:

1) The speed of light is an absolute.

2) Light slows down when it passes through a medium (like water, glass, air, …).

The first statement is the backbone of all of modern physics (relativity), and the second helps explain things like diffraction and rainbows.  But clearly these statements contradict each other.

Here’s the idea: a medium, whatever it is, is made up of molecules.  When a photon (light particle) hits a molecule it is sometimes absorbed.  Its energy is turned into raised electron-energy-levels, or vibrations and flexing, or movement.  In short order (very short order) the photon is spit out of the other side, none the worse for wear.

When a photon hits a molecule it's sometimes absorbed and re-emitted. That process takes a little bit of time that we interpret as a "slowing".

In between molecules light still travels at light speed.  It’s just that, with all those molecules around, it’s always darting ahead, getting absorbed, pausing for a moment, then being re-emitted.  On the scale we’re used too, this happens so much and so fast that you don’t notice the starting-and-stopping.  Instead you notice an average slowing of the light.

That is, if light always takes about 33% longer to travel through water than air (and it does) due to absorption and re-emission, you’d say “ah, light travels slower through water!”.  The fact that that isn’t quite the case is rarely important.


Answer gravy: This isn’t part of the answer, but it’s interesting: The interaction between light and the medium it moves through is generally “clean”, in the sense that even if the light is in a complicated quantum state before entering the medium, it retains it.  As a result, light continues to point in the same direction (which is good, in terms of seeing stuff), and even keeps its polarization.

What’s really fascinating, is that even more bizarre quantum states, like those involving being spread out over a large area, are also allowed to persist.  If this were not the case it would be impossible to do the double slit experiment (which requires the photon to be in many locations) without a specially prepared vacuum chamber!

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54 Responses to Q: If light slows down in different materials, then how can it be a universal speed?

  1. AJ says:

    If photons had even the slightest mass, no matter how small, couldn’t they ironically be the dark matter we don’t see? We see the light source and the light reflected by different surfaces but we don’t see the light travelling from the source. Space and the air around us must be filled with light. If photons have zero mass why would they be influenced by other materials or gravity?

  2. RJ says:

    That’s interesting: “couldn’t they ironically be the dark matter we don’t see”.
    I sort’a think of that in the same way too..

  3. Doc G says:

    According to general relativity, mass curves spacetime. The curvature affects anything that moves through spacetime, including massless things like light. The surprising observation that even though light has no mass it can still be affected by gravitational fields was seen as extremely convincing evidence that Einstein’s way of thinking about it was correct!

  4. Pablo Jeynes says:

    Interesting paradoxical analyses. Let me try a physicist viewpoint.
    The photon rest mass is known experimentally to be extremely small if not zero. But anyway photon energies are certainly finite, which by the energy-mass equivalence would indeed provide a source of gravitational field. This is well known and would surely ? be accounted for in cosmology if significant. A region of space with a bunch of photons will have an “effective” mass which acts as source of gravitational field (curving spacetime) like any other energy=mass density distribution.
    So light/ photons act as gravitational source and complementarily they also are affected by gravitational attraction. Note even classical infinitesimal small mass particle would fall with the same acceleration as any finite mass , Galileo principle.
    The curved spacetime reality means photons are deflected but they dont accelerate their speed when falling into a gravitational well . Instead of speeding up there can be increase in light/ photon energy via their frequency …. equivalently in classical em wave energy or as quantum photon energy, E=hf, the result is the same.
    If we start changing observer then time dilation in a gravity well is important so everything is consistent.
    I hope this helps clear up the paradoxes. It works for me but I should say I am not an expert in General Relativity, just a “normal” physicist trying to help out. Please anyone feel free to disagree or correct as necessary.

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