Tag Archives: Velocity

Black Holes Explained (Sort of)

Black holes are a confusing topic in astronomy. You’ve heard about them starting from a young age, but whenever you ask someone for more information about them, there’s a whole lot of “I dunno”s. The truth is, black holes have been romanticized by science fiction, when in fact they are nothing more than an oddity of physics (albeit still pretty cool). Once you understand a few basic rules of physics, learning about black holes is easy.

Black Holes

 

Step 1: Gravity

The first step on the road to understanding black holes is understanding how gravity works. An object with mass will cause a bend in spacetime, affecting other objects around itself. This effect is what causes gravity. The more mass something has, the more gravity it produces. The force of gravity gets weaker the farther you get from the center of mass proportional to the inverse of its distance squared. Yeah, I know, that sounds confusing. Maybe it’s easier if you see the equation for calculating the force of gravity.

 

Black Hole Formula

In this equation, M is the mass of the larger object, m is the mass of the smaller object, G is the universal gravitational constant, and r is the distance between the two objects. If you double the distance between two objects, then the force of gravity is ¼ what it was before. If you triple it, then the force of gravity is only ⅑ the original.

Step 2: Escape Velocity

In order to escape the gravitational field of a massive object, you need to attain a specific speed, called its escape velocity. It’s calculated using the force of gravity, taking into account the mass of the large object and how far away you are. Once something is going at minimum the escape velocity, then it is no longer captured by the massive object.

Step 3: Singularity (or, infinite density)

Black holes (NASA)

Density is an easy calculation, as seen below.

Black Holes Singularity Formula

Where m is mass and V is volume. As mass increases or as volume decreases, density goes up. In a black hole, the volume is essentially 0, which causes the density to approach infinity. This creates what is called a singularity.

Putting it all Together

When the most massive stars in the universe go supernova, the force of the explosion causes the core of the star to get smaller and smaller, essentially packing it all into a volume of 0. This point of infinite density creates such a huge bend in spacetime that it creates a singularity. At a certain distance away from this singularity, the force of gravity the escape velocity reaches the speed of light.

As defined by the theory of relativity, nothing in the universe can go faster than the speed of light. So if the escape velocity of this object at a certain distance is higher than that, then not even light can escape the singularity. With no light escaping, the object and the space around it gives off no light.

This is the black hole.

There’s a lot more that goes into defining black holes, such as its mass, spin, etc., but that’s just icing on the cake. There you have it. There’s so much more to learn about black holes, but knowing even this much gives you the tools you need to understand them at a fundamental level.

Written By: Scott Yarbrough

What is Escape Velocity?

Escape Velocity is probably something you’ve heard on a TV show, or maybe you learned about it from NASA talking about their newest spacecraft. It is a commonly discussed term, but it isn’t the easiest thing to understand. Imagine you’re in a strange universe where only the Earth exists. The only gravity comes from its center, and it extends infinitely far away, getting weaker and weaker the further away you get. In order to get to that infinite point before you get pulled back by the Earth’s gravity, you need to be going at least 11.2 km/s (25,000 mph). This speed is the escape velocity!

escape velocity 1
This value depends on both the distance from the gravitational center of the object you’re escaping from and the mass of the object. The closer you are to a heavier object, the faster you need to go to reach escape velocity. For example, escape velocity from the Earth at a distance of the moon’s orbit is only 1.3 km/s (3,000 mph), but to escape from the sun’s gravity at the distance of the Earth is a whopping 44.7 km/s (100,000 mph)!

escape velocity moon

But since there’s no such thing as a universe where only the Earth exists, we have to worry about the gravity of other celestial objects! Once you escape from the Earth’s gravity, you’ll then be captured by the sun. If you escape that, then you’ll be captured by the Milky Way’s gravity! So the hypothetical infinite point is just that: hypothetical! No matter what, there’s always going to be something pulling on you with gravity.

Written By: Scott Yarbrough

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