Speed of Gravity

Einstein and Relativity
Albert Einstein

What is the speed of gravity? It’s 299,792,458 m/s. Seem familiar? Yep, it’s the speed of light (in a vacuum)!

How do we know that that’s the speed of gravity? Not by direct measurement, yet, but by the great success of Einstein’s theory of general relativity (GR).

In general, because it is so successful, and because the speed of gravity in GR is the same as the speed of light, we can say we know how fast gravity propagates.

In particular, observations of the Hulse-Taylor binary pulsar (and other binary pulsars) show the mutual orbit is decaying (the stars are slowly spiraling in, and will one day collide). The rate of decay is exactly as predicted by GR, and is due to the system radiating gravitational waves. The rate at which the system is losing energy tells us how fast that gravitational wave radiation is travelling … and it’s c, the speed of light, to within 1%!

Working out how gravity, as geometry in GR, makes planets in our solar system orbit the Sun is somewhat tricky, and misunderstanding of the details is what’s behind an erroneous claim you might come across on many websites (that the speed of gravity is many millions of times c, or even infinite).

A very long baseline radio interferometric observation of a quasar as it passed near Jupiter, in 2002, lead two researchers to claim to have directly measured the speed of gravity (they found it to be c, plus or minus about 20%). However, this claim is controversial, with several GR experts claiming the analysis contains subtle flaws, and that what was actually measured is the speed of light. The method Fomalont and Kopeikin used might allow a direct estimate of the speed of gravity to be made in future, in the view of their critics, with big improvements in precision.

More to explore: Speed of Gravity (NASA), What is the speed of gravity? (Cornell University), and Does Gravity Travel at the Speed of Light? (University of California Riverside).

Gravity Moves at the Speed of Light is an interesting Universe Today story on the speed of gravity; Warp Drives Probably Impossible After All is a very different take!

And check out the Astronomy Cast September 18th, 2008 Questions Show episode for more on the speed of gravity.

Nobel Prize Press Release
Living Reviews in Relativity
Cornell Astronomy

How Fast is the Speed of Light?

You may think that a lot of things are fast, like speeding bullets and Superman and the passage of time when you are having fun. But all of these things are nothing compared to the speed of light, which is the fastest that something can travel through the Universe. The speed of light is sometimes referred to as the “cosmic speed limit”. Light travels in a vacuum at 186,282.4 miles per second or 299,792,458 meters/second. For simplicity, it is often said that these numbers are 186,000 miles per second, and 3.00 x 10^8 meters per second.

How fast is this in normal terms? Well, the record for the fastest aircraft is held by the Boeing x-43 scramjet. Scramjets are single-use unmanned aircraft designed to go at hypersonic speeds. The x-43 traveled at  12,144 km/h (7,546 mph), or Mach 9.8, on November 16th, 2004. That is .000405% of the speed of light. And this is a jet that can travel from New York to Los Angeles in 20 minutes. While it takes photons about 8 minutes to travel the distance from the Sun to the Earth – at its furthest, 152 million km (94.4 million miles) – this scramjet traveling at its maximum speed would take about 522 days!

The speed of light is really fast, and at this speed some bizarre things start to happen. First off, photons can only travel this speed because they have zero rest mass, meaning that if you were to somehow trap a photon and put it on a scale, it would have no mass. It’s virtually impossible for something with mass to travel this speed, because as you get faster and faster, it takes more and more energy to get you to the speed of light, which makes you heavier, which requires more energy, etc. Time also changes when you get to these speeds. If you left the Earth going the speed of light, then came back around and landed, you would perceive time as moving normally, but when you returned it would seem as if time sped up for everybody on the Earth, and all of your friends and family would be much, much older.

The speed of light is not constant in all materials, though, and can be slowed down. Here’s an excellent article on how researchers can slow down the speed of light by passing it through different materials, with the slowest speed being 38 miles per hour!

To learn more about the speed of light – and there is a lot, lot more to learn, check out the Astronomy Cast questions shows from October 26, 2008June 4, 2009 and September 26, 2008, or the Physics section in the Guide to Space.