# Do gravitational waves travel faster than light?

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

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)?

• I would assume that gas in interstellar and even intergalactic space slows light down (but not gravitational waves). Due to the small amount of matter in most of the way the effect will be minute, but since the light travelled 1E9 years, even a difference of 1E-12 gives us a few hours to adjust telescopes etc. – Peter - Reinstate Monica Feb 12 '16 at 10:02
• So basically, Gravity Waves travel at the Speed of Light, Light travels slower. – aslum Feb 12 '16 at 14:42
• Possible duplicate of How fast does gravity propagate? – Kyle Kanos Feb 12 '16 at 15:03
• Also, light needs a lot of time to get from the middle of a star to its surface. Light travels with the speed of light only in vacuum. – vsz Feb 12 '16 at 15:42
• IIRC, there are similar "lots of neutrinos from that direction - look there now" associated with a supernova - there is a lag between the neutrinos getting to Earth and the light. – user20936 Feb 12 '16 at 16:44

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.

• Doesn't it sound counter-intuitive to say that gravitational waves propagate at the speed of light? Light is a particle/wave, while gravity is part of the fabric of reality, so it seems it might make more sense to say that gravitational waves are limited to the speed of the universal constant, which we just happen to correlate with the speed of light. That is to say, if light is affected by gravity as it travels through space-time, wouldn't gravity be the limiting factor for the speed of light, not the other way around? Just an amateur, hope I'm not too far off. – LegendaryDude Feb 12 '16 at 13:34
• Interesting fact about the travel time of the core explosion to the star's outside..-- Btw, the time lag caused by gravitational "signal" travel time to near-by stars would be years though, not hours, so it is probably not what the paper meant. – Peter - Reinstate Monica Feb 12 '16 at 13:52
• @Legendary You're not wrong - we speak of the "speed of light" as a limit because it has no rest mass, not because there's anything inherently special about light. Any other massless particle not only will travel at the speed of light, it must. So you're right to say it's not that gravity is limited by the speed of "light," but c is the velocity limit for information transmission from any point $A$ to any point $B$ in our universe. Any information, including changes in the shape of spacetime, i.e. gravity. – JPattarini Feb 12 '16 at 15:10
• "Speed of causality" is a good way to put it. – JDługosz Feb 12 '16 at 18:08
• "Speed of light" is a phrase that needs to be abolished, replaced with "Fundamental Space-Time Constant" or "Universal Speed Constant" or some such thing. It has nothing special to do with light, but with all massless and near massless things. – DarenW Feb 17 '16 at 6:17

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.

• @J Riverside while technically correct, this answer does not directly address the article that the OP is inquiring about, nor the detection time difference of several hours which is specific to supernovae and is due to an entirely different mechanism. – JPattarini Feb 12 '16 at 12:53
• @ James Patterini That's interesting, do you have a source or link for the delay of hours being specific for supernovae? – J Riverside Feb 12 '16 at 13:28
• @JamesPattarini I saw your valid point about supernova mechanics. I'm still interested in my original idea though -- any ideas about the speed of light in the interstellar and/or intergalactic medium? My googling found mostly SF ;-). JRiver: For the order of events in a supernova cf. universetoday.com/119733/how-quickly-does-a-supernova-happen – Peter - Reinstate Monica Feb 12 '16 at 14:02
• "Absorbed and reemitted" is not an accurate description of light in a transparent medium. – JDługosz Feb 12 '16 at 18:06
• See physics.stackexchange.com/q/90708 -- gravitational waves arrive before light for the same reason neutrinos arrive before light. They all essentially travel at the same speed. However, gravitational waves and neutrinos are generated in the actual collapse, whereas all of the light is generated hours later as the material remnant expands and radiates energy. – user10851 Feb 12 '16 at 19:39

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.

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.

• did you understand the context in which the question has been asked? – Mac164 Feb 12 '16 at 6:13
• in one second , redshift included , the BHs merged and the GW were emited. See the official communication and the simulation they built from the recorded signal – user46925 Feb 12 '16 at 7:31
• @Mac164 : it seems that yes. The OP explains that GW start some hours earlier – user46925 Feb 12 '16 at 7:39