What elements travel at the speed of light. I was under the impression it was light and gravity but apparently electromagnetic waves (radio waves) travel that fast as well. And is element the right word? Is gravity an element?

FYI Feel free to update the tags. Light, photons and speed were not listed


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    $\begingroup$ Light is an electromagnetic wave. Anything that is massless will travel at the speed of light. $\endgroup$ – Phiteros Oct 5 '17 at 5:04

Firstly "element" is probably the wrong word to use here since in scientific language that would refer to a chemical element (think hydrogen, helium, etc).

As for what travels at the speed of light, all electromagnetic waves (including light itself) will travel at the same speed with the exact value dependent upon the medium through which they are traveling. In a vacuum, this is approximately 300,000km per second and is often denoted by the term c (of E=mc² fame!).

Gravity is a tiny bit more complicated… according to Einstein's theory of gravity what objects experience as gravity is the distortion to the "normal" curvature of spacetime caused by the presence of a large amount of energy (typically a mass). It's a bit of a simplified metaphor but if you think of spacetime as like a trampoline then put a bowling ball on the trampoline it causes the trampoline's surface to distort downwards around the weight of the bowling ball, and the extent of this distortion decreases the further you get from the bowling ball.

That's kind of what gravity is, so it's not really traveling per se. What travels is any changes to this distortion. If the source of that distortion were to be moved (you come along and pick the bowling ball up off the trampoline or roll it around to a different section etc) then the distortion to the surface changes accordingly and these changes aren't instantaneous.

These changes to the curvature propagate in a wave-like manner and are referred to as "gravity waves" or "gravitational radiation". We don't know that much about gravity waves yet, but it's theorized that they are carried by a hypothetical particle called a "graviton" which is the gravitational version of a photon.

Lots and lots of complicated maths later and the headline is that according to Einstein the propagation of changes to this distortion (i.e. the speed of gravity) is expected to be the same as the speed of light and recent experiments seem to show that this is indeed the case.

So anything that has no mass when it's not moving will travel at the speed of light and basically, everything with a resting mass > 0 is slower (yes neutrinos, I'm looking at you!)

  • $\begingroup$ But I read somewhere that universe is traveling with speed greater than light. But according to you everything which have mass shall be moving slower than light ?? $\endgroup$ – Sudhanshu Gaur Oct 7 '17 at 23:40

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