The Physics of Black Holes

What is Black Hole?

Black holes are one of the most popularized ideas of contemporary physics, but are misunderstood by the general public. Colloquially, they’re considered objects shrouded in mystery that have the ability to absorb everything in their wake. While this isn’t totally incorrect, astrophysicists actually understand some of the mathematics behind black holes and know how one might potentially form.

An artist’s rendition of black hole.

So Why are Black Holes so Strange?

For one, a black hole can’t be observed directly! An observation in astrophysics generally means that light has bounced off the object you’re looking at and been received by your eye, or some other instrument like a telescope, but black holes absorb all the light that comes within a certain distance to them. So how do we even know they’re there in the first place?

Even stranger, light can’t escape a black hole? Light, or now generally thought of as ‘photons’, is the fastest moving object in the universe to our knowledge, and travels at a speed:

So what’s the problem? You’d think that a black hole wouldn’t care how fast something was going once it reached a certain size right? The problem here is…..  photons don’t have any mass! The reason that things can’t escape black holes is because of gravitation, attraction between two bodies with mass, so how does this happen?

Universal Gravitation:

Back in the early years of physics, Issac Newton hypothesized a few things from his observations of bodies in nature, ones that you’ve most likely heard before. Among his three laws of motion, Newton hypothesized this:

“Every point mass attracts every single other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them.”

Or in the language of mathematics:

In casual terms this means: The force between two objects with mass depends on how big they are, and how far their separation is.

So putting a photon mass of 0 into this equation makes the whole thing zero. No force, means no acceleration, or ‘change in motion’ by Newton’s second law. So we would expect photons to whiz right by black holes, or reflect off of them, making them observable. So something is wrong here with the way we think about black holes. There are only two possible explanations to remedy this issue.

1.    Black holes simply don’t exist! Because if we can observe them with light, then it isn’t a black hole anymore.

2.    Maybe photons can be affected by gravity for some other reason? Maybe there is a problem with universal gravitation?

Things We Know:

1. Light bends in a gravitational field! This was predicted by Einstein’s General Theory of Relativity, and later confirmed by observation of the ‘Twin Quasar’.

“an intervening mass concentration between Earth and the quasar bends light so that two images of the quasar appear in the sky. This is known as gravitational lensing, and is a consequence of Einsteinian warped space-time.”

2. Theoretically, black holes can exist, and Astrophysicists think they form from the death of ‘Neutron Stars’. The idea is, that when a Neutron star collapses, it reduces to a super dense ‘singularity’, meaning that you condense multiple solar masses (our sun’s mass) into a small space, and the gravitation produced is so large that a black hole forms.

3. There is a point of no return! It’s true, there is a theorized distance from the center of black hole that is the closest any object in the universe can get before getting pulled in with no chance of escape. This is called the ‘Schwarzschild Radius’, and it is associated with the ‘photon energy’ instead of the photon mass. (A problem that we discussed earlier)

This is also known as the ‘event horizon’ or ‘point of no return’.

How Can we Know They Exist?

This is still an issue of observation to be handled with black holes. Since we can’t actually look at them, we have to detect them another way. A potentially convincing example of a black hole is near the blue supergiant Cygnus X-1. This star is about 25 times the mass of the sun, but it seems to be orbiting a non-luminous x-ray source, which is hypothesized to be a neutron star or a black hole. Find out more about Cygnus X-1 on Hyper Physics.

Black holes are a hot topic of interest in modern astrophysics, and there is certainly much more to know about these beasts of the cosmos.