By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)

There are a few statements in science that are absolutely true. One such statement is that nothing can travel faster than the speed of light. Such assertions are backed by assumptions that are often not expressed. Read on to find how statements about the speed of light can be confusing.

While nothing can go faster than light in vacuum, it is pretty easy for particles to move faster than light in matter through some medium like glass. Assume a photon beam and high energy muon beam point in the same direction and travel at the same speed as light. Though it has been established that the speed of light would slow down inside glass, the muon does not slow down when it passes through the glass and still maintains the speed of light inside.

Learn more about how relativity is misunderstood.

The Cherenkov Effect

While working with uranium salts dissolved in sulfuric acid, a young Russian physicist named Pavel Cherenkov noticed a glowing light. Initially, the light was attributed directly to radioactivity but when Cherenkov tried to isolate the effect, he realized that radiation made the sulfuric acid glow. Eventually, he was able to show that even water would glow in the presence of a lot of radiation. Named after the student, the Cherenkov radiation occurs when a particle carrying an electric charge travels faster than light through an optically transparent medium such as glass. When these particles travel faster, the glass emits a stunning yet dangerous blue color.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on The Great Courses Plus.

How Does Cherenkov Radiation Work?

Cherenkov shared his findings with his thesis advisor, Sergey Vavilov. Vavilov mentioned the effect to Igor Tamm and Ilya Frank, who figured out what was going on. Cherenkov, Tamm, and Frank went on to win the Nobel Prize in Physics in 1958. The Cherenkov radiation is analogous to a sonic boom that occurs when a jet flies faster than the speed of sound. If the jet flies slower than the speed of sound, then the sound waves propagate ahead of the plane. But when the speed of the jet exceeds the speed of sound, it compresses the air in front of it. As the jet moves ahead faster and faster, the compressed energy also moves ahead. When the compressed air passes over, it results in an extremely loud noise similar to an explosion as all sound arrives at once. When the jet generates a sound from pushing the air out of its way, it creates a series of circles appearing at each point the jet passes. The circles add up and make a cone of sound that surrounds the path of the plane.

Cherenkov Effect in Nuclear Physics

Cherenkov effect is similar to the sonic boom but occurs when a charged particle passes through a transparent material at a speed greater than the speed of light. The charged particle shakes up the molecules of the material and they give off light as it moves. As in the case of sound waves, the light emitted travels outwards in a circular pattern at the speed of light, resulting in a conical pattern called Cherenkov Effect. Physicists use the concept of Cherenkov radiation in nuclear physics or particle physics experiments to distinguish the quickly moving particles from the slower ones. This is because the Cherenkov light is emitted only by electrically charged particles moving faster than light.

Learn more about how quantum mechanics works.

What Can Travel Faster than Light?

Edwin Hubble combined his distance measurements of the galaxies with the velocity measurements of American astronomer Vesto Slipher to draw an interesting graph in 1929. Hubble postulated that there was a linear relationship between the distance of galaxies as measured from Earth and recessional velocity. This correlation was taken as a clear evidence of an expanding universe and the big bang.

However, the expansion of universe has significantly changed over time. From the fast expanding universe about 14 billion years ago to the era of dark energy about 5 billion years and speeding up of expansion now, the changes have been dramatic. We know that Pluto is not expanding away from us at the speed of light nor is the Milky Way or the Andromeda galaxy. According to the Hubble’s hypothesis, the recessional velocity is 70 kilometers per second per mega parsec. So, a galaxy that’s a mega parsec away will be moving away from the Milky Way at a speed of 70 kilometers per second while a galaxy at a distance of 2 mega parsecs will be moving away at a speed of 140 kilometers per second. So, the farther we move away, the faster the objects are moving. Hence, the common theory that the universe is expanding at the speed of light is obviously incorrect and a misconception.

Now let us assume that the universe is indeed expanding at the speed of light and it is calculated that objects 14 billion light years away from us were travelling at the speed of light but objects 28 billion light years away are travelling at twice the speed. So, the entire argument of ‘expanding faster than the speed of light’ is quite mushy. However, the argument that the speed of light is the ultimate speed is within the theory of special relativity.

So, the question whether two galaxies separated by 14 billion light years are moving away from each other at the speed of light can’t be answered objectively. The distance between these galaxies are growing at the speed of light but relative to their little local bit of space, they’re not even moving. Hence, it would be more accurate to say that though objects cannot move through space faster than light, but space itself can move faster than light. The superluminal expansion of the distant universe is really astounding but what we may never get know is the answer to the simple question how big is the universe really?

Common Questions about Can Objects Travel Faster Than Light?

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