But what if we switched the rules a little?

Imagine a world, where the speed of light was slower -

about the speed that sound travels at.

In turn, the sound in that hypothetical world

would be accelerated

until it reached the speed of light.

This is WHAT IF,

and here's what would happen

if the speed of light and sound switched.

Light travels at a constant speed of 300,000 km/s (186,000 mi/s).

If you, personally, were able to travel at that kind of speed,

you could zip around the Earth's equator 7.5 times

in just one second.

If sound could travel that fast...

well, we'll get to that part in a moment.

First, let me explain something.

There's one big difference between the speed of light

and the speed of sound.

Light speed is a universal constant.

It's always the same - for every observer.

For sound, it's different.

The speed of sound depends on the density of the medium it travels through.

What we usually refer to as the speed of sound

is the speed of sound waves passing through dry air at 20 °C (68 °F).

That speed is 343 m/s (1125 ft/s).

And that's almost a million times slower than the speed of light.

If light traveled that slow,

how different would our Universe have turned out to be?

Since light speed is a universal speed limit,

changing it would completely change how the Universe works.

With the new speed limit of 343 m/s (1125 ft/s),

everything would be so slow.

You'd find that moving at 0.2 km/h (0.12 mi/h)

would only be possible with a rocket.

Or at least a nuclear-powered car.

Driving it would be a challenge,

as you wouldn't be able to see far ahead.

But you would experience a cool Doppler effect around you.

Things in front of your car would seem blue.

In your rear-view mirror, everything would look red.

In that world, it would take hours to send a message

to someone on the other side of the planet.

You know what would be even more annoying?

Talking to someone who's further than a hand's distance from you.

The delays in visuals would make it look

as if you were video calling someone with a bad signal,

only in-person.

We would never be able to make it to space.

Because to do that, we'd need to escape from the gravitational influence of Earth.

That requires the speed greater than the speed of light

in our hypothetical world.

We'd never reach it to begin with.

There would be no Moon landing,

no Solar System exploration,

not even the International Space Station.

On a more disturbing note,

the chemical reactions in your body would slow down.

That can't be good.

Essential functions, like your metabolism, depend on those reactions.

I can't tell you exactly how this would end,

but it sure wouldn't take long.

All that and we haven't even got to the speed of sound yet.

For the sake of your survival,

let's say your body adjusted to the slow motion inside it.

Now, sound enters the game.

It's faster than light now,

so you would hear everything before you got to see it.

Because sound relies on the environment,

in order for this to happen,

air would have to turn incredibly dense.

We're talking as dense as neutron stars.

Not a single living organism would survive under such strong pressure.

Not to mention that all this breaks the rules of physics.

Nothing can exceed the universal speed limit.

And if sound did travel faster,

that's not good news for anyone.

The Universe would collapse upon itself

long before we got to experience that cool Doppler effect.

Next time, try increasing the speed of light instead of decreasing it.

That's a story for another WHAT IF.