Under Pressure

Imagine that you're at the bottom of a pool. Feel that pressure?

As a teenager, I had access to a pool that had an 8 foot deep end. You could dive in safely, then cruise to the bottom and hang out for a sec before swimming back up for air. I spent a lot of time cruising around the bottom of the pool, feeling that constant pressure from the water above.

One day, I got to swim at a pool with a deeper deep end, probably about 10 feet. The pressure was noticeably more intense down there. I could feel the pressure in my sinus cavity (basically inside of my face) being crushed, pushing back desperately against this intense inward pressure.

This type of pressure continues as you go down deeper, and I recently wrote about the way creatures survive in our oceans, where the atmospheric pressure can build up to a thousand times what we feel on the surface. That’s where a car would be crushed immediately like an aluminum can, but where magnificent animals live anyway:

That pressure is almost incomprehensibly intense for us, but it’s just business as usual for the likes of the snailfish, who live at the bottom of the Mariana trench and just about everywhere else down there.

Still, that intense pressure is just the beginning of what’s possible. I want to build on that idea of going down into the pool, since I bet you’ve been underwater far enough to feel some of that pressure. You’re familiar with it, and you can probably make the leap to imagining being crushed by that same pressure under five miles of ocean.

The reason the ocean water is so intense down there is intuitively simple: there’s more water being pulled down onto you by gravity. When you’re five miles underwater, you are five miles under water.

It’s not just the water above you, though, because the water beside you is also subject to that same pressure. As a result, water crushes inward from every direction around you.

At the bottom of that 8 foot pool, I could hang out down there, but I felt that pressure trying to crush my face. As I went down to ten feet, the pressure intensified a bit, and so I have a clear correlation in my mind where depth corresponds with pressure (and pain).

Imagining multiplying that pressure by a hundredfold is a nonstarter, but envisioning a car being crushed is helpful, since it’s the same type of pressure I feel at the bottom of the pool. I can hold onto the idea of what that pressure is—it’s just way, way more intense.

Let’s try to keep that incredible crushing pressure in mind as we continue below the ocean, into the mantle. A thousand atmospheres turns into a million near the cusp of the core, and as we finally reach the center, we reach 3.6 million times the pressure we feel on the surface. This number is probably meaningless to imagine, but maybe we can understand what’s happening in a different way: the inner core is solid metal, even though all that pressure causes the temperature to be as hot as the surface of the Sun.

It’s the same sort of crushing pressure, but thousands of times more intense than the pressure that crushes cars. The pressure here smashes liquid into solid. Even more incredibly, this sort of pressure can get much, much more intense.

We need something bigger than the Earth to talk about what happens with much greater pressure still, but we don’t need to leave our Solar System. Our good friend the Sun has all we need for this next thought experiment. If you dive deep into the Earth, the core is crushed into a solid where it would otherwise be liquid. Something even more wild happens at the core of the Sun.

All the pressure of millions of Earths crush downward toward the center, with the core around 230 billion atmospheres. Here, the pressure from the matter doesn’t turn the core into a solid, but way beyond that. The structure of the atoms themselves begins to break down here, as hydrogen fuses into helium, one atom at a time.

The crushing pressure is enough to fuse the atoms themselves together, ultimately transforming one element into another. This fusion is what gives heat and light to the Earth, making it a good place for us to live. It also pushes back out against that gravity, showing us that there is a force powerful enough to hold gravity at bay: the nuclear force.

In fact, the nuclear force is almost unimaginably stronger than gravity, but with so much stuff piling on, the two forces ultimately reach equilibrium in a star like our Sun.

If our Sun was much bigger, there would be enough pressure for something even more mind-boggling. Once the nuclear fuel of a truly big star is exhausted, the fusion can’t stop that bottom-of-the-pool type pressure from crushing the core inward, collapsing it in on itself.

This means that the atoms themselves are collapsing. One outcome here is that the electrons smash into the protons, forming a star core that is made up of only neutrons. Neutron stars are so dense that they can spin ludicrously fast, often creating pulsars that flash light we can see from Earth at regular intervals.

Some pulsars spin more times per second than your kitchen blender. That’s the mass of hundreds of thousands of Earths spinning around hundreds of times per second, but because of their incredible density, they’re only around 12 miles across.

If a star is much bigger still, it does something even more mind-bending. The bottom-of-the-pool pressure, that same force you feel down there, now crushes right past the very structure of matter itself. Even neutrons are unable to withstand this utterly crushing inward force, leaving nothing pushing against gravity’s now dominant force.

With our Sun, the fusion pushes back against gravity, and with a neutron star, the structure of the neutrons keep gravity from smashing into oblivion. Both of these types of phenomena have something keeping gravity at bay, pushing outward so that it can’t turn inward on itself.

Not so with a black hole. The force of collapse is so powerful from all that gravity, that anything and everything trying to push back against it is simply overwhelmed. This inward folding creates an event horizon—a line across which nothing can ever escape, including light, the fastest thing in the universe.

This pressure is no different than the pressure you feel at the bottom of the pool. It’s just unimaginably more intense. It’s tough to imagine physics at the very big or very small scales, but having the same force involved helps my imagination to run wild, connecting the world of everyday experience to the astronomical or quantum worlds.

Have you thought much about this crushing pressure? Are there any other helpful analogies of everyday experience that can help us understand other realms in physics you know of?