It is a widely known fact that the maximum allowed speed for a massless particle is the c – 2.99×108 m/s. One of our kids at Mango Education was curious if there existed particles that could outrun the speed of light. When he brought the question to our AMA channel earlier, we had the opportunity to explain many cool things. This article (as well as all the future editions) will be focused on building upon the answers from our AMA session, with a deeper insight.

Excerpt from the AMA

20/08/16, 1:15 pm – Shubha: Which is faster than light by amruth 8 yrs

20/08/16, 2:30 pm – Prasanth: Speed of light in vacuum is a constant. Till date no particle or wave has been able to break that barrier. But you can have particles travelling faster than the speed of light in different media say water or glass.

20/08/16, 2:32 pm – Obuli Chandran: And expansion of space itself between two objects say between two galaxies can be faster than the speed of light!


Let’s talk about speed of light in general! The speed ‘c’ is the maximum speed allowed in the universe for any massless particle in the vacuum. The well-known massless particle is the photon, and that is the reason why we usually refer c as the speed of light. Technically, c is the maximum speed of all massless particles in vacuum.

Particles travelling faster than the speed of light in different medium

When we introduce light into any medium other than vacuum, then the maximum speed at which it can travel decreases. In layman terms, the velocity appears to be decreased, because when light enters a denser medium, it interacts with the electrons of the medium in a way that the total light beam’s phase velocity changes.

For example, when you shine a laser in water, the photons electromagnetically interact with the electrons of the H2O molecules and excite them to higher energy levels. And by spontaneous emission, these electrons further emit photons to return to its ground state. Due to this effect, the phase velocity of the superimposed waves of all photons (the refracted beam) inside the medium changes.

The magnitude of the phase velocity of a component wave (light) can be obtained with the help of the refractive index of the material. For water, the refractive index is 1.3. Which means the apparent speed of light v in water is 2.99×108/1.3, which is 2.3×108 m/s. So if any particles could travel faster than this ‘apparent’ speed of light inside the water medium, in a sense, it should be travelling faster than light without violating relativity.

Optical boom

To make things more interesting, let’s bring in a pretty picture!

The blue Cherenkov Radiation
Photo – Argonne National Laboratory, CC BY-SA 2.0

The awesome blueish glow you in the photograph above, is called as the Cherenkov radiation. The radiation is more like the sonic boom you hear when a supersonic aircraft passes by, except this is made up of high-energy electrons stacking up together, radiating photons.

If you would like to read about how sonic boom forms, check out my explanation on the topic here.

If you have followed the previous explanation on how particles can travel faster than light in a medium without violating relativity, it’s quite fluid and self-explanatory from here on out. When you accelerate high-energy particles, say electrons, to the phase velocity of the light inside a dense medium, say water, the electromagnetic field interactions of the electrons and photons travel at the phase velocity. When you accelerate the electrons, a wee bit above the phase velocity, like a shockwave on a supersonic aircraft, the high-energy electrons, now travelling at superluminal speed, stack up and radiate photons as they interact with the electrons of the medium. This results in a spectacular blue glow with a high luminance as seen in the picture above.

Here is one in action!

And that’s the reason why Ironman’s Arc Reactor glows too!

Expansion of the universe – faster than the speed of light

The universe is expanding at an accelerated pace, towards a darker future. Literally, a darker future! So many (manymany) years into the future, the acceleration would eventually prevent light travelling between the galaxies. If any life at that point looks up at the sky, they won’t be able to see anything!

Inevitable horror show aside, let’s get back to the science part! How could the galaxies accelerate way from us faster than the speed of light? HOW DARE THE GALAXIES DO THAT!

Technically, the galaxies aren’t the one moving away. The space between the galaxies is the one that’s expanding—if you prefer an alternative word, stretching! So the galaxies are just where they are supposed to be and are not moving. It’s more like dots on an infinitely expanding balloon.

When we observe this apparent motion due to the expansion of space, we interpret the difference between the distances as acceleration. Depending on how far the galaxies are, this interpreted acceleration can go well above the speed of light.

To help you understand better, here is a little analogy! You somehow build a powerful laser that can be seen with the naked eye if you shined it at the moon. Now when you move the laser sideways, you could observe the light spot on the moon, also moving sideways, but much faster than the speed at which you are moving the laser in your hand! With that distance, the observed speed of the spot could reach speeds faster than c.

So in conclusion, you can make a particle go faster than the apparent speed of light inside a medium that’s dense enough to slow down the light. You can also observe astronomical events like the expansion of space, movement of stars, movement of light spots, at greater distances, at an observed speed that’s faster than light.

Please do share your feedback and follow-up questions in the comments below. Subscribe to our email list for more interesting stuff from us. If you would like to participate in upcoming AMA sessions, please register here and we’ll invite you.


Karthikeyan KCProfile | Website

Karthik is an engineer, gamer, author, and a techie who wanders the realms of the interwebs, exploring and learning. He is the creator of the science encyclopedia Swyde. He loves communicating science online and sparking web applications at Mango Education.


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