Is there anything preventing the following experiment from being done right now?

Imagine that a human ran from point 'a' to point 'b' while light particles that reflected off a clock moved through a special medium from point 'a' to point 'b' as well. Could a human arrive at point 'b' before light? (As for the special medium, I'm imagining something like a complex maze of mirrors submerged in a very dense material.)

If so, if the human waited at the finish line to view light arriving in second place, would they see the time that the race began on the clock?

enter image description here

  • 15
    $\begingroup$ It takes roughly a little over 8 minutes for light to travel from the sun to the earth. So, if the light has to travel through a maze of length equivalent to 150 million kilometres then, obviously, it would be quite plausible to devise a winnable race. $\endgroup$
    – Strawberry
    Jan 11, 2018 at 22:55
  • 4
    $\begingroup$ Note that light has been slowed down to almost human-running speeds: abcnews.go.com/Technology/story?id=99111&page=1. I can't go that fast on a bike, but there are many who can, and it's not too far away from the top speed of 100m sprinters like Usain Bolt. $\endgroup$
    – Arthur
    Jan 12, 2018 at 12:10
  • 44
    $\begingroup$ My first thought in reading this was that of course the human would win. Light doesn't have feet. $\endgroup$ Jan 12, 2018 at 16:28
  • 4
    $\begingroup$ "Scientists ‘freeze’ light for an entire minute" $\endgroup$
    – Richard
    Jan 12, 2018 at 19:50
  • 10
    $\begingroup$ @EngineerToast - If that's the case, then how can someone be "light on their feet"? $\endgroup$
    – Richard
    Jan 13, 2018 at 12:30

7 Answers 7


No physical laws are being broken in this thought experiment. If you are concerned with the relativistic requirement "nothing can go faster than the speed of light", that only applies to the speed light goes in a vacuum: $c = 3 \times 10^8$ m/s. The reference to light in that relativity postulate makes it sound like if you could only find a situation where you slowed light down, you could break the laws of physics; not so. A better statement of the postulate would be "nothing can go faster than $3 \times 10^8$ m/s, which happens to also be the speed light travels at in a vacuum." I don't see anyone going faster than $3 \times 10^8$ m/s in this thought experiment, so no physics violations.

As for what the human at the end of the race sees:

He sees a blinding blue light from the all the Cherenkov Radiation from even the slightest charged particle passing through the medium. And perhaps the time at the start of the race. It's exactly what you would imagine since we are talking non-relativistic speeds. What an anti-climactic answer, eh?

  • 13
    $\begingroup$ How does the Cherenkov radiation come into this experiment? No charged particles are mentioned. Any why would this radiation be blinding? $\endgroup$ Jan 12, 2018 at 1:00
  • 16
    $\begingroup$ @sammygerbil Cherenkov radiation happens when charged particles (which are all over the place) travel faster than the speed of light in the medium. It's a characteristic of nuclear power plants because there are a lot of charged particles there, moving quite close to c, and there are a lot of media that are used to slow these particles. The reason the radiation would be blinding is that the more you lower the speed of light in the medium, the more radiation you're going to get (because more particles "fit"), and lowering the speed of light to ~10 m/s; well, that's lowering it a lot. $\endgroup$ Jan 12, 2018 at 1:49
  • 8
    $\begingroup$ @WillihamTotland, from the diagram and the description, I wasn't imagining at all that the human had to race through the same medium as the light. That seems to be answering a different question. $\endgroup$
    – Wildcard
    Jan 12, 2018 at 3:03
  • 12
    $\begingroup$ @Wildcard where does WillihamTotland imply that the human would also be in the medium? The Cherenkov radiation will be emitted in the medium, eventually reach the edge, and then travel through the air as usual in a big bright flash, reaching any human observers. $\endgroup$
    – OrangeDog
    Jan 12, 2018 at 11:39
  • 1
    $\begingroup$ @cms: Are you sure the light would be visible as blue outside the medium? And would it be that bright? Consider conversation of momentum for this: A particle which causes Cherenkov light to be created, loses some of its energy. If it is very slow, it doesn't have much energy to begin with, so the photons created cannot have much energy, or there are only few of them. In effect, not much will be seen outside. $\endgroup$
    – M.Herzkamp
    Jan 15, 2018 at 11:19

There is a concept of "slow light" which is looking at light pulses whose group velocity propagates very slowly. This is slightly different than your clock example, but close enough that you might be interested in it.

In 1998, Danish physicist Lene Vestergaard Hau led a combined team from Harvard University and the Rowland Institute for Science which succeeded in slowing a beam of light to about 17 meters per second...

Usain Bolt can run roughly 12m/s, so we are not all that far off.

(Of course, we are playing some tremendous games with the light beams while doing these sorts of slow light games. Whether this is actually applicable to your specific thought experiment involving a clock depends on what aspects of the experiment you felt were important)

  • 10
    $\begingroup$ You should have read the rest of that sentence and the next in that Wikipedia article: "researchers at UC Berkeley slowed the speed of light traveling through a semiconductor to 9.7 kilometers per second in 2004. Hau and her colleagues later succeeded in stopping light completely...". Even I can beat light that is standing still. $\endgroup$ Jan 12, 2018 at 15:01
  • 2
    $\begingroup$ @PaulSinclair I chose not to reference stopping light completely. I felt the slowed light experiment was more likely to lead the OP to research what it meant to slow light than stopping light (which was more likely to be deemed "cheating," even if it used the same techniques) $\endgroup$
    – Cort Ammon
    Jan 12, 2018 at 15:21
  • $\begingroup$ 2005 - phys.org/news/2005-04-optical-frozen.html $\endgroup$
    – WernerCD
    Jan 12, 2018 at 15:23

Yes, it is theoretically possible.

For example, you could use two parallel, perfectly reflecting mirrors of length $L$, where $L$ is the distance between point A and point B. Let the distance between the two mirrors be $d$.

Assuming that the ray of light enters the two mirror by hitting one of them close to point A at an angle $\theta$, it will be reflected

$$N \approx \left ( \frac{L}{d \tan \theta} \right)$$

times before arriving at point B, covering a distance

$$l = N \frac d {\cos \theta} \approx \left ( \frac{L}{d \tan \theta} \right) \frac d {\cos \theta} = \frac L {\sin \theta}$$

in the process. Notice that the result does not (somewhat surprisingly) depend on $d$.

Therefore, the time needed to go from point A to point B for the ray of light is

$$t=\frac l c \approx \frac L {c \sin \theta}$$

You can therefore define an "effective speed" $v_e$ for the ray,

$$v_e \equiv c \sin \theta$$

If the speed of a human is $v_h$, the human will be faster than the light ray if

$$v_h > v_e \ \Rightarrow \ \sin \theta < \frac{v_h} c$$

The record speed for a running human (*) is $44.72$ km/h (Usain Bolt, 2009). The speed of light in a vacuum is $1.08 \cdot 10^9$ km/h. You get therefore the condition

$$\sin \theta < 4.14 \cdot 10^{-8}$$

You can see therefore that this is not very easy to realize in practice (and we are neglecting refraction, absorption, scattering, surface roughness etc.).

You also can repeat the calculation assuming that there is a material with refractive index $n$ between the two mirrors.

(*) I want to be generous.

  • $\begingroup$ what if we make a complicated pipe and make light travel in it many times , then it can be done $\endgroup$ Jan 11, 2018 at 18:21
  • 11
    $\begingroup$ A light second that fits in a shoebox has been done with coiled fiber optic cable. $\endgroup$
    – Joshua
    Jan 11, 2018 at 22:47
  • $\begingroup$ @SahilChadha What if we make a wall, so the light can't finish the race? $\endgroup$
    – Chris
    Jan 13, 2018 at 0:10
  • 1
    $\begingroup$ @Joshua Less than a light millisecond, not a full light second: the IEX magic shoe box $\endgroup$
    – Miles
    Jan 14, 2018 at 5:50

It is of course possible not just for a human, but for anything "moving" to beat light moving in a given medium. It is impossible to beat $c=3\cdot10^8\ \mathrm{m/s}$, but not the velocity of light in a given medium.

In the experiment proposed, the man arriving at the end will see time exactly the same way a man out observing, it will not change the notion of Newtonian time, actually.

There is a (natural) misunderstanding about the light velocity (or the velocity of electromagnetic waves): it is always $c=3\cdot10^8\ \mathrm{m/s}$. What happens in other media, is that light propagating around atoms/molecules is constantly absorbed and re-emitted for the electrons given the effect of "slowing down" the net velocity of the light in that medium. But the light velocity, around the medium continues to be $c=3\cdot10^8\ \mathrm{m/s}$, for the effect of movement and relativity theory (except for the absorption and emission moments).

  • 1
    $\begingroup$ Look again. The runner doesn't travel through the same medium as the light does. $\endgroup$
    – Wildcard
    Jan 12, 2018 at 3:04
  • 1
    $\begingroup$ @Wildcard And he never claims that the runner does? $\endgroup$
    – Graipher
    Jan 12, 2018 at 7:30

The dense material would diffuse the light so if you got it dense enough you probably could not read the clock.

In theory if you had enough perfect mirrors you could see the start time.

  • 2
    $\begingroup$ No, refraction is a function of face curvature and/or nonuniform index of refraction (see "GRIN" lens). $\endgroup$ Jan 12, 2018 at 13:46
  • $\begingroup$ One could imagine a complex maze of mirrors simply in air: if the mirrors are ideal, there does not seem to be any fundamental problem with getting a perfect image of the source at the destination. Using a dense material is only "convenient" in that it lets you construct a maze for which the total distance traveled by the light is less than the required distance in air alone. $\endgroup$
    – wchargin
    Jan 15, 2018 at 8:51

To focus on just one aspect of the answer:

If so, if the human waited at the finish line to view light arriving in second place, would they see the time that the race began on the clock?

The medium would have to do the majority of slowing down - after too many mirrors you increase the distance such that the clock appears too small to be legible. If you used slightly convex mirrors to magnify it you'd end up shooting nearly all the light outside the medium, and it would be too dim to see. If you increased the brightness to accommodate that loss, you'd vaporize everything within the initial beam of light.

So while the rest of your thought experiment doesn't pose a problem, there are certain physics limitations which would make it so the runner couldn't see the clock from the past.


I identify 3 questions. The answers are:
First is yes, a human could beat "light" in a timed competition, if the "right" conditions are met. If the light is deflected and sent on a "long enough" path, the light will arrive latter than the human at the finish line.
Second is also yes, if instead of the clock face, a flash of laser light is used, then it is doable (see wikipedia Lunar Laser Ranging experiment).
And Third, if a 1 second "flash" is used (instead of clock face), then the human will see a 1 second flash, some time after getting to the finish line.
For an example, lets use: a 10m distance from start to finish, the human can run 10 m/s, and the laser flash is "bounced" off the moon. The human starts running when he sees the 1 second flash. In 1 second he will get to the finish line (t=d/v) and 1.5 seconds later (2.5 - 1), he will see the "same" 1 second flash again.


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