Life's Great Mysteries... If two joggers run side-by-side at the speed of sound, can they hear each other talking?

This is one of those silly questions people ask as a joke, such as “Can you daydream at night?” or “Can a short person talk down to a taller person?” or “Can crop circles be square?” But I think this question is worth considering. Seriously, what happens to sound when you travel at the speed of sound?

First, what exactly is the speed of sound? This is how fast sound waves—vibrations, in other words—travel through a particular medium. The speed is different depending on the medium. For example, sound travels 4.4 times faster through water than through air. And with air, the speed depends on the air’s temperature and composition. At 68º F (20º C), sound travels at about 767 miles per hour (1,235 km/hr). So, we’ll use 767 mph as a standard for this discussion.

When we hear something, our brain is interpreting compressive waves that hit our eardrums. If you were capable of running away from those waves faster than the waves travel, then you wouldn’t be able to hear any sounds coming from behind you because you would be outrunning them. If you were running into waves coming at you from in front, you would be able to hear those. The thing is, no one can run at 767 mph. In fact, if you were in a jet going that fast and you stuck your head out the window, the effects of the air hitting your head would be devastating. You simply cannot move that fast in the atmosphere without being inside a protective environment like a jet. A streamlined, reinforced jet can withstand the constant impact of all that air. Humans cannot.

The air inside of a jet is moving with you, so you can speak to the people around you and hear them normally. This is also true when you are standing in your yard talking to someone—the Earth is orbiting the sun at a speed of 67,000 mph, but you are safely within its bubble of atmosphere, like being in a jet. Interestingly, if your jet is moving faster than the speed of sound, the air in the cabin is moving with the jet, so when you speak toward the front of the jet, your sound waves are moving faster than the speed of sound (but the sound is still only moving at 767 mph relative to the air in the cabin). Weird.

If a jet is traveling toward you faster than 767 mph, you cannot hear it approaching because it is outrunning its own sound waves. However, when the jet gets parallel to you, the sound waves will eventually reach your ears. If the jet is high above, it may already be past and out of sight before you hear it because the sound waves also have to travel from the jet’s height all the way to your position on the ground. If the jet is 40,000 feet (7.6 miles) high, the sound takes about 35 seconds to reach you.

To answer the original question: a jogger cannot run anywhere near the speed of light. Let’s imagine that you and your jogging partner get in a car that goes faster than sound. No problem—you both are in an enclosed space (like in a jet) and the air inside is moving with you. Okay, let’s imagine that you and your jogging partner get on two motorcycles that can go faster than sound, and you ride beside each other. You would need helmets to protect your heads, of course, and if you tried to yell at your partner, the sound waves would be left behind and your partner would not hear you. However, you could talk using radio headsets inside your helmets because radio waves travel at the speed of light—waaaay faster than 767 mph!

Photo Credit:
- Two fast joggers - Midjourney 7