We all know that many objects (atoms, cats, us) have mass. What you probably don’t know is that there are multiple different types of mass, and this has real physical (and astrophysical) consequences.
Balancing Act
Gravitational potential is a nice mathematical way to describe the effects of gravity on an object. You can get an idea of how gravitational potential is related to gravity by imaging a ball on field of rolling hills.
Matters of Gravity
By the principle of general relativity, free fall under gravity and the absence of gravity feel the same because they are the same. The idea seems ridiculous because we can see the space station orbit the Earth, so something must be pulling it. But remember that space and time are not absolute.
Ugly Theory, Beautiful Data
When we left our story we, had two proposed models: MOND, which posits that for very small forces, the acceleration of an object doesn’t quite go to zero, and dark matter, which introduces an invisible “something” that makes up most of a galaxy’s mass. Neither one of these seem particularly appealing, so why (as I stated yesterday) do I favor dark matter as a solution?
Beautiful Theory, Ugly Data
Both MOND and dark matter were introduced to address a problem with the way galaxies behave. At a basic level, the stars in a galaxy such as ours orbit the galactic center in roughly circular orbits. The speed of a star in its orbit should be governed by Newton’s law of gravity. So, using Newton’s gravitational theory, we can predict a star’s speed given its distance from the center and the distribution of matter in our galaxy. Newton’s beautiful theory doesn’t agree with the experimental data.
A Short Period
In the center of our galaxy, only 27,000 light years away, lies a supermassive black hole. Some of the strongest evidence of this black hole is the observation of stars closely orbiting it. Recently in Science a new star, S0-102, was announced with an period of 11.5 years.
Circling the Drain
It turns out that the rotation of a mass also distorts space and time. For example, as the Earth rotates, it drags the nearby space along with it (an effect known as frame dragging). Similar to water spiraling down a drain, this effect builds up, and as a result, space spirals a bit around the Earth. You have to be a bit careful with this comparison. Spacetime doesn’t “flow” the way water does, but the spiral effect is somewhat similar.
That’s No Moon
Cruithne is a small, rocky body about 5 kilometers across. It was discovered in 1986, and it is sometimes referred to as Earth’s second moon. The argument for calling it a moon of Earth is because it is gravitationally bound by the Earth, just like the Moon. But Cruithne doesn’t orbit the Earth, so what exactly is going on? The answer has to do with some interesting complexities of orbital dynamics.
Aether or…
After the Michelson-Morley experiment, several theorists tried to explain the results by supposing that Earth dragged the aether along with it, or imposed other conditions to explain our inability to observe the aether. But then Einstein proposed special relativity, which eliminated the need for aether altogether.