What if the speed of sound was as fast as the speed of light?

The clouds are hanging low on the horizon; the air is sticky and sizzling with electricity. Suddenly, a silent bolt of lightning cracks open the sky. The boom follows a full four seconds later. 

Compared with light, which moves at a stunning 186,000 miles per second (300,000 kilometers per second), sound waves are downright sluggish, moving through air at 0.2 miles per second (0.3 km per second). That's why you see lightning before you hear the thunder. But what would happen if the speed of sound suddenly were a million times faster — the same as the speed of light?

Of course, thunder would reach you at the precise moment of lightning. But that bolt of lightning would also look pretty eerie. Sound waves are composed of particles, each moving slightly enough to collide into the next. That creates areas of higher and lower density within the wave, said George Gollin, a professor of physics at the University of Illinois at Urbana-Champaign. Just think of a slinky: as the toy moves, the coils continually bunch together and then spread out again. Sound waves are similar. At slow speeds, that change in density is imperceptible. At the speed of light, it's a different story. 

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"What would happen is you have pretty humid air [during a lightning storm], the sound wave comes through and squeezes stuff really hard, and then expands out and the pressure drops a lot," Gollin told Live Science. Because pressure corresponds to temperature, the sudden drop in air pressure after a clap of thunder would cause the humid air to freeze. You'd see the lightning bolt through a dense fog of ice crystals. 

An ultra-fast speed of sound would completely change the way our world sounds. Voices, for instance, would be so high-pitched, they would be inaudible. "Not even your dog could hear you," said William Robertson, a professor in the department of physics and astronomy at Middle Tennessee State University. 

Our vocal cords produce sound by generating standing waves, which behave like those heavy ropes you see tethered to the wall at the gym. When a weight-lifter shakes them fast enough, waves begin oscillating up and down without appearing to travel across the rope. As the ropes are shaken faster and faster, the number of waves — in other words, their frequency — increases. Similarly, when the sound waves produced by our voice box increase in speed, they increase in frequency. 

With sound waves, higher frequency means higher pitch. To get a sense of how much higher our voices would get, here's a fun fact: sound moves three times faster through pure helium than it does through air, and that's why inhaling the gas makes us sound like Mickey Mouse. "And we're talking about making the speed of sound a million times bigger," Robertson said.

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That same effect would wreak havoc on orchestras, Robertson added. Wind instruments act like human vocal cords; sound moves back and forth inside the cavity of an oboe or a trumpet, which produces a standing wave. We would have to design wind instruments to be a million times longer to keep them in tune with the violins and cellos, he said. (A change in the speed of sound as it moves through air wouldn't change the speed of sound along a string, he added.)

Alas, humans wouldn't survive to experience these spectacular changes. Even the soft whistle of a flute would blast anything in its vicinity to smithereens. Light travels in electromagnetic waves, which aren't composed of matter, but sound waves are mechanical — composed of particles colliding into one another. A molecule traveling at the speed of light would have "nearly infinite energy," Gollin said. It would blast through every particle it encountered, sending electrons flying and producing a "spray" of matter and antimatter — particles generated in ultra-high speed collisions that have properties opposite to those of matter. 

"The effects would just be extraordinary," Gollin said. 

Originally published on Live Science.