#speed of gravity

[webeducationsciences]

Scientists Finally Calculated The Speed of Gravity - Web Education

Scientists Finally Calculated The Speed of Gravity Scientists Finally Calculated The Speed of Gravity In classical theories of gravitation, the changes in a gravitational field propagate. A change in the distribution of energy and momentum of matter results in subsequent alteration, at a distance, of the gravitational field which it produces. In the relativistic sense, the « speed of gravity » refers to the speed of...

[startswithabang]

Ask Ethan: Why Doesn’t Gravity Happen Instantly?

"the relationship of gravitational wave speed to that of light... At first I saw no connection, since gravity derives from mass and is a totally separate effect versus electromagnetics. One might assume [this] would cause gravitational effects to be slower than light [in terms of] propagation time.”

If you take a look at the Sun with your eyes, you’re not seeing the light that’s being emitted by the Sun right now, but rather the light that was emitted a little bit more than 8 minutes ago: the light that’s arriving at our eyes right now after propagating at the speed of light. Well, similarly, what about the gravity we feel from the Sun here on planet Earth? Is that instantaneous, from where the Sun is right now, or is that from some finite amount of time ago, too?

According to Newton, gravity has to be instantaneous, or it won’t give you the right orbits. Einstein had a different prediction: it must travel at the speed of light, with gravitation working differently from how Newton predicted. Although we had lots of indirect evidence for many years, and we fully anticipated that Einstein would be right, this was only tested directly for the first time 5 years ago, and the best constraints didn’t happen until just 3 years ago: in 2017, when we saw two neutron stars merge in both gravitational waves and light for the first time.

Gravity isn’t instantaneous, and propagates through the Universe at a very familiar speed: the speed of light. Here’s the science behind it.

[startswithabang]

This Is Why The Speed Of Gravity Must Equal The Speed Of Light

“In order to get different observers to agree on how gravitation works, there can be no such thing as absolute space, absolute time, or a signal that propagates at infinite speed. Instead, space and time must both be relative for different observers, and signals can only propagate at speeds that exactly equal the speed of light (if the propagating particle is massless) or at speeds that are blow the speed of light (if the particle has mass).

In order for this to work out, though, there has to be an additional effect to cancel out the problem of a non-zero tangential acceleration, which is induced by a finite speed of gravity. This phenomenon, known as gravitational aberration, is almost exactly cancelled by the fact that General Relativity also has velocity-dependent interactions. As the Earth moves through space, for example, it feels the force from the Sun change as it changes its position, the same way a boat traveling through the ocean will come down in a different position as it gets lifted up and lowered again by a passing wave.”

According to Newtonian gravity, space and time are absolute, and the gravitational force between any two objects is defined by the distance between them. In relativity, though, different observers don’t agree on distances, which means they won’t agree on forces, accelerations, or other properties of motion from a relativistic perspective. And yet, if you use Newton’s law of gravitation to compute the orbits of Solar System objects, it gets the right answer. If you instead tried to use Newton’s laws but allowed planets to be attracted to where the Sun was in the past, you’d get the wrong answer! Does this mean that the speed of gravity is infinite?

Hardly, but you have to dive deep into relativity to understand what else is up. Thankfully, we’ve done this, and you can enjoy the answer for yourself! Here’s why the speed of gravity must equal the speed of light.

[startswithabang]

Ask Ethan: Why Do Gravitational Waves Travel Exactly At The Speed Of Light?

We know that the speed of electromagnetic radiation can be derived from Maxwell’s equation[s] in a vacuum. What equations (similar to Maxwell’s - perhaps?) offer a mathematical proof that Gravity Waves must travel [at the] speed of light?

If you were to somehow make the Sun disappear, you would still see its emitted light for 8 minutes and 20 seconds: the amount of time it takes light to travel from the Sun to the Earth across 150,000,000 km of space. But what about gravitation? Would the Earth continue to orbit where the Sun was for that same 8 minutes and 20 seconds, or would it fly off in a straight line immediately?

There are two ways to look at this puzzle: theoretically and experimentally/observationally. From a theoretical point of view, this represents one of the most profound differences from Newton’s gravitation to Einstein’s, and demonstrates what a revolutionary leap General Relativity was. Observationally, we only had indirect measurements until 2017, where we determined the speed of gravity and the speed of light were equal to 15 significant digits!

Gravitational waves do travel at the speed of light, which equals the speed of gravity to a better precision than ever. Here’s how we know.