Do gravitational waves travel faster than light?

Question

In Feb 12, 2016 edition of Times of India, an article read

[with the discovery of gravitational waves, we will be able to] Track Supernovas hours before they're visible to any telescope because the waves arrive Earth long before any light does, giving astronomers time to point telescopes like Hubble in that direction

See also page 13 of the paper.

Does this mean that gravitational waves reach us before light from a source? Can this be some printing mistake or am I interpreting it wrongly?

Edit: Can there be special cases (as explained in some answers) where gravitational waves seem to reach before light waves from a source (though not violating the speed limit)?

Answer 1

It's an incredibly misleading statement, so it's not you.

Gravitational waves propagate at the speed of light, so their detection by Earth-bound detectors is expected to correlate with the arrival of light from distant events assuming the source of light generation is identical (not spatially or temporally separated) to the source of the gravitational disturbance.

In the case of a supernova, it's actually a dynamic process instead of a flip of a switch, and so the change in the magnitude of light emission can indeed lag behind by several hours from the start of collapse of the star's core - the detection of gravitational waves could allow us to "buy back" that several hour window by detecting the gravitational waves produced by core collapse instead of having to wait for the light magnitude increase. There's no disconnect here, just sloppy reporting.

In many cases however, we infer gravitational events or influences have occurred or exist by witnessing a change in motion of light emitting (or reflecting) objects that are directly affected by the event/influence - think of a supermassive black hole at a galactic center that we can't observe directly, but infer its existence by the motion of stars in its vicinity. Or the orbital behavior of Neptune that suggested other massive objects yet to be found in our solar system.

Depending on the nature of the event, we may have to infer that a black hole merger, for example, has happened by observing the changes in motion of objects we can see with traditional telescopes. This introduces a time-lag in addition to the normal speed-of-light timelag we're bound by whenever we look up at the night sky:

Gravitational influence must travel at the speed of light from the site of the event to the light-emitting object that we can observe, and then the light from that object must travel to our telescopes, again at the speed of light. At the moment that the event happened, the light from the object we're observing with our telescopes had not yet felt the disturbance, so there's an additional lag in detection time that must be accounted for - we're not really observing the black hole in this example, we're observing a surrogate object.

The ability to detect gravitational waves may allow us to "buy back" this additional lag by now 'directly' observing the inciting events... bound by the speed of light, of course.

Answer 2

Peter A. Schneider already gave the correct answer in the comments.

Do gravitational waves travel faster than light? No, gravitational waves also travel at the speed of light in vacuum.

However, the interstellar medium is not perfectly empty but filled with plasmas which slow electromagnetic waves (light, radio) down by a factor n, the refractive index. The slowing occurs because the photons are absorbed and reemitted, which takes some time. As far as I know, gravitational waves are not absorbed & reemitted and therefore travel with the speed of light in vacuum c as opposed to EM-waves which travel at a speed c/n.

At the bottom of this link is an example of how you could calculate the refractive index in space for radio waves: link (Edit: Please note that the link uses a different definition of refractive index, $\mu$ = 1/n).

So, does this mean that gravitational waves reach us before light from a source? Yes.

Answer 3

GW will give some advanced notice due to the reasons mentioned in other answers. However, the actual benefit will be only realized if the direction of GW is sufficiently pin pointed. Otherwise, space is so vast that a broad direction will not be much helpful in terms of observing the luminous events even if we get hours of notice.

Would it be better than keeping the telescope pointed in a single direction for long time and wait for such events to happen in its view, as they do it today? Will depend upon the accuracy of detection of direction of GW.

Then there will be another phenomena that will muddy the water - Gravitational lensing.

Answer 4

It is not a mistake. Gravitational waves travel at the speed of light.

Scientists can't directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby.

black-holes by science.nasa.gov

To directly observe the merging of blackholes with telescopes, one would need to observe the changes in the nearby matter that emits light. Because it takes time for the change in the gravitational field to act on the surrounding objects, these changes will have a time delay from the moment of the merging.

The same can be said for a supernova. A major activity in the core of a supernova could generate gravitational waves that are detectable on earth. It takes time for these newly generated waves to induce motions of the surrounding objects that emit light. No to mention that the motions of the surrounding objects need to be large enough to be noticeable on earth. In other words, there is a lag between the generation of gravitational waves and the movements of the surrounding bright objects.