Could the strength of gravity have been different in the past? How can we tell that gravity has never changed?
In an earlier post I wrote about how light moves at a single universal speed. Not only has this been well observed experimentally, it forms the foundation for the theories of special and general relativity, which are also well supported by experiment. Often, relativity is summarized as “nothing can travel faster than light.” Which raises the interesting question, what about gravity?
Newton’s law of gravity takes a bit of calculus to really wrap your head around it, but the basic relation is very simple. Every pair of masses in the universe experiences a mutual gravitational attraction, each feeling the pull of the other’s gravitational field. The force of attraction is mutual, which leads to some interesting consequences. For example, when you …
Yesterday’s post on testing the assumption that photons are massless raised a few questions for readers. One of the most common was the idea that the gravitational lensing of light must mean that photons have mass. After all, if a star or galaxy can deflect light gravitationally, doesn’t that mean the light is gravitationally attracted to it? If that is the case, doesn’t that mean that light has mass?
We often refer to gravity as a gravitational field because it can be described mathematically as a field. That is, each each point in space and time has certain characteristics, and they are related to each other through field equations. For gravity, the field equations are the equations of general relativity. Lots of physical quantities can be described as fields, including electromagnetism and quantum objects. Fluids are also described mathematically as fields. With a flow of water or air, for example, every point has properties such as pressure and speed, and they are related by equations of fluid dynamics.
Newton’s law of gravity states that between any two masses there is a gravitational force. The strength of that force depends not only on the masses, but on the distance between those masses, following what is known as an inverse square relation. Newton felt that this inverse square relation was exact, but is it?
The popular press has been abuzz about a new proposal to use the solar system to test string theory. Turns out that is a bit over-hyped. The actual paper, published in Classical and Quantum Gravity outlines an idea to use the motion of planets and moons to test what is known as the equivalence principle. The reason string theory gets mentioned …
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.
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.