Does anything other than light travel at the speed of light? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.
Anything massless would travel in a vacuum at the invariant speed, which is known as the vacuum speed of light. Light itself consists of massless photons, which travel at this speed in a vacuum. The only other thing we know that is massless and stable when free, would be the gravitational field (and its presumed quanta, the gravitons.) So like light (propagating waves of the electromagnetic field), gravitational radiation (propagating waves of the gravitational field) also travel at the vacuum speed of light.
Neutrinos are not massless but they are very light. Their rest mass is unknown but tiny, so most neutrinos produced in nuclear interactions travel at a speed very close to the vacuum speed of light.
Back in 1987, a supernova (SN1987A) in the Large Magellanic Cloud, some 170,000 light years from here, was seen in visible light, but also in the form of a brief burst neutrinos, which were detected by several neutrino observatories. The neutrinos arrived, in fact, several hours before light; the delay is due to the time it takes for the collapse shock wave to reach the progenitor star’s surface, whereas neutrinos leave the collapse region mostly unimpeded.
And in August last year, the gravitational wave event GW170817 was also observed in multiple electromagnetic wavelengths at the same time, with no noticeable difference in the arrival times of gravitational vs. electromagnetic waves after traveling a distance of some 130 million light years.
Last but not least, I should mention that when light travels in a medium, it can slow down. In water, it slows down to about 75% of the vacuum speed of light. Thus it is not uncommon for high energy particles to actually travel faster than light in such a medium. If the particle is charged, it produces a optical shockwave not unlike, in principle, the acoustic shockwave of a supersonic aircraft. This shockwave is often in the form of visible light, when it is called Cherenkov radiation. This radiation can be seen, e.g., in nuclear power plants in spent fuel pools, and it is also used as a means to detect high energy neutrinos in very large tanks of very pure water (if, in a rare interaction, a neutrino knocks out an electron with enough kinetic energy, that electron will produce a flash of Cherenkov radiation.)
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