Always interested in science, I’ve been fascinated by the consequences of the speed of light, and how the passage of time is relative to the speed of the observer. Imagine the effects if we could invent a mirror that slowed down the speed of light reflecting in it!

In other words, could we invent a mirror that slowed down light so much that you saw an image of you when you were years younger in the mirror?

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    $\begingroup$ You are basically describing a camera: i.e. i) collect light from your image now (e.g. with a camera), ii) store it for a while (e.g. on a memory card) iii) show it later (e.g. on a computer screen). iv) Remark: gosh don't I look young there! $\endgroup$
    – innisfree
    Commented Feb 15, 2017 at 5:35
  • $\begingroup$ How about using glass? It slows the down the speed of light by 33%. $\endgroup$
    – Yashas
    Commented Feb 15, 2017 at 10:37
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    $\begingroup$ Slowing down light at a mirror couldn't make you look years younger. Making light slow couldn't make it come from years in the past. It doesn't really make any sense. $\endgroup$
    – JMac
    Commented Feb 15, 2017 at 10:53
  • $\begingroup$ @JMac's comment is worth a hightlight. We shouldn't confuse "running slower" with "running backwards". $\endgroup$
    – Steeven
    Commented Feb 15, 2017 at 13:16
  • $\begingroup$ Thank you all for the enlightening (Sorry - I'm punning again!) responses. While it was a bit tongue in cheek, there was a real, if naïve, science question behind it. Acknowledging JMac's comment, isn't it true that when scientists view the "big bang" area of the universe, they see what was actually there billions of years ago. So is it such a big ask to get those photons to slow down and show me as I was fifty years ago? We all thought a "cloak of invisibility" was impossible only a few years ago, but I presume that's been achieved by bending the photons, rather than slowing them down. :) $\endgroup$ Commented Feb 16, 2017 at 4:15

2 Answers 2


Light is slowed when it is travelling though any substance (i.e. not in vacuum), and the ratio of the speed of light in vacuum $c$ to the speed of light in the substance $v$ (more precisely its phase velocity) is called its refractive index $$n=\frac{c}{v}$$ For example, water slows light by around 25%, window glass slows it by around 35%, and the air around you slows it by around 3 hundredths of a percent. This slowing comes about on the molecular level by the light interacting with the electrons in a material. By engineering the way in which light interacts with a material physicists have slowed light down and even stopped it completely. Now if the light beam is stopped then where did the energy go? Well, now it is stored not as photons but in the material itself. E.g. some electrons have been excited to a higher energy level in a way such that it can all be undone and the light can be re-emitted again (with the same direction and coherently with respect to the input beam of light).

By using this kind of a material, you could conceivably stand in front of a mirror and 'save' your reflection, then come back and look at it years later. However it would be much easier to simply take a photo. The real advantage of using this technique is in the phase and polarisation information stored in the "stopped" light, which we don't need to see an image of ourselves, but we do need in order to build a storage device for quantum computers based on light.

  • $\begingroup$ The OP asks how to slow photons down, not light. They are not the same. $\endgroup$
    – Yashas
    Commented Feb 16, 2017 at 8:44
  • $\begingroup$ @YashasSamaga "Imagine the effects if we could invent a mirror that slowed down the speed of light reflecting in it!" - OP in original question. And yes photons and light are different just like grains of sand and a beach are different, but they are also clearly related concepts. $\endgroup$ Commented Feb 16, 2017 at 8:57

We have the following equations from Einstein which hold good in all frames of reference: $$E^2 = (m_oc^2)^2 + (pc)^2 \space \space - (1)$$

$$p = m_ov\gamma = m_ov\frac{1}{\sqrt{1-\frac{v^2}{c^2}}} - (2)$$

where $E$ is the energy associated with the particle, $m_o$ is the rest mass of the particle, $c$ is the speed of light, $p$ is the momentum of the particle and $v$ is the velocity of the particle.

I'll make a claim that all massless particles must travel at the speed of light. If they don't, they aren't of any interest to us.

We all agree that photons have a zero rest mass. So what will happen if a photon was slowed down? Let's find out.

The equation $(1)$ reduces to the following as the rest mass is zero: $$E^2 = (pc)^2 - (3)$$

The interesting bit happens in the second equation. Is the momentum of the photon zero if its rest mass is zero? Not necessarily, the $\gamma$ can go to infinity if $v$ is equal to $c$. This would allow $p$ to take any value or in other words, you cannot obtain $p$ from equation $(2)$. But we have assumed that the photon was slowed down, so it is traveling at a speed lesser than $c$. This would mean that $\gamma$ is finite which in turn implies that the momentum of the photon is zero as there is no infinite which could fight the zero.

$$p = m_ov\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$$

As $m_o$ is zero, $v$ is finite, $\gamma$ is finite, $p$ turns out to be zero.

If $p$ is zero, then from equation $(3)$, we get $E$ to be zero.

If a particle has zero energy and zero momentum, how are we going to detect it? It will be a ghost particle.

Therefore, a device which slows photons down will never exist. If it manages to slow the photons down, we'll never see those photons ever again. The energy and momentum which the photons had will disappear.

The only way out is to hope that physics is wrong so that we might be able to slow down photons in the future.


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