Speed of Gravitational Waves is very near the speed of light

Proof of gravitational waves came on Sept. 14, 2015, when two giant, L-shaped, 2-mile-long laser instruments, one set up in a swamp in Louisiana and the other in Hanford, Washington, detected a tiny ripple in space, a “chirp” that reached Earth from the gigantic collision of two black holes a billion years ago.

The 2017 Nobel prize in physics was awarded to Rainer Weiss of MIT, 85, Kip Thorne, 77, and Barry Barish, 81, both of Caltech.

“I view this more as a thing that recognizes the work of about 1,000 people, a really dedicated effort,” Weiss said.

Over four decades, the American public has bet about a billion dollars on the LIGO project, which stands for Laser Interferometer Gravitational-Wave Observatory. Five LIGO observatories were set up around the world, based on faith that Einstein was right.

Neil Cornish, Montana State University astrophysicist is working with teams of scientists on other ways to detect gravitational waves, like the Pulsar Timing Array and space-based detectors, to go “hunting monster black holes.”

“This is just the beginning of gravitational wave astronomy,” Cornish said. “It’s like when Galileo first turned his telescope to the heavens.

Bounding the speed of gravity around the speed of light has many significant implications for fundamental physics and cosmology. One of the biggest implications is that the tight bounds provide a more precise test of general relativity and rule out proposed alternatives to general relativity.

“Many alternative theories of gravity, including some that have been invoked to explain the accelerated expansion of the Universe, predict that the speed of gravity is different from the speed of light,” Cornish said. “Several of those theories have now been ruled out, thereby restricting the ways in which Einstein’s theory can sensibly be modified, and making dark energy a more likely explanation for the accelerated expansion.”

The time delay between gravitational wave signals arriving at widely separated detectors can be used to place upper and lower bounds on the speed of gravitational wave propagation. Using a Bayesian approach that combines the first three gravitational wave detections reported by the LIGO Scientific and Virgo Collaborations we constrain the gravitational waves propagation speed c gw to the 90% credible interval 55% to 142% of the speed of light in vacuum. These bounds will improve as more detections are made and as more detectors join the worldwide network. Of order 20 detections by the two LIGO detectors will constrain the speed of gravity to within 20% of the speed of light, while just five detections by the LIGO-Virgo-Kagra network will constrain the speed of gravity to within 1% of the speed of light.

Just two days later (and after the physicists mentioned above wrote their paper), another paper was published in The Astrophysical Journal Letters by the LIGO and Virgo collaborations, whose authors are affiliated with nearly 200 institutions around the world. By using data from the gravitational waves emitted by a binary neutron star merger detected in August, they were able to constrain the difference between the speed of gravity and the speed of light very tightly.

The reason for the huge leap in precision is that the neutron star event did not emit only gravitational waves, but also electromagnetic radiation in the form of gamma rays. The simultaneous emission of both gravitational waves and light from the same source allowed the scientists to set bounds on the speed of gravity that is many orders of magnitude more stringent that what could be set using gravitational wave signals alone.

The scientists measured an arrival delay of just a few seconds between signals that traveled a distance of more than one hundred million light years. Such a small delay across this distance is considered virtually nothing.

Physics Review Letters – Bounding the Speed of Gravity with Gravitational Wave Observations (Neil Cornish, Diego Blas, and Germano Nardini)

Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is 5.0 X 10^ -8. We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of (+  1.74 0.05 s ) between GRB 170817A and GW170817 to:

(i) constrain the difference between the speed of gravity and the speed of light to be between 3X 10^-15 and 7 X 10 ^ -16 times the speed of light,

(ii) place new bounds on the violation of Lorentz invariance,
(iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB 170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1–1.4 per year during the 2018–2019 observing run and 0.3–1.7 per year at design sensitivity.

38 thoughts on “Speed of Gravitational Waves is very near the speed of light”

  1. Are gravitational waves waves or particles? The gravitational wave energy is very strong at the event but very weak 100 M ly away. If the wave spreads out as a wave then what is it that is spreading? A photon does not spread. The same photon created 100 M years ago arrives at the telescope sensor today.

  2. The path of light follows is curved space time.
    – Does the gravitational wave also follow the curve of space time?

    – If a black hole binary merge and propagate gravitational waves in the outward direction, does the event also propagate gravitational waves into the event horizon(s) and are we unable to detect any waves that propagate inward?

    So a black hole binary pair might lose the energy equivalent of a solar mass, which becomes gravitational waves.

    – If gravitational-waves propagate into the event horizon as g-waves, do we observe them as part of the mass of the black hole?
    – Do we observe the energy of the photons falling into an event horizon as mass?
    Since they travel at c it seems that gravitational waves are also massless.

  3. Before preaching that time is a force factor and the energy density of space controls the path of stars … Learn the principles of atomic gravity in the google links below:
    Zero G flight at the Atomic Scale

  4. Uh. …gravity waves go 64 times the speed of light. At least that is what Ive tead about off and on over the years.

  5. i’m sorry to say but Eric van Linde explanation of gravity still stands, despite dark stuff that has never been detected, and most likely will never be detected. There are a few camps in physics with to many to loose if dark stuff would not exist. Go back to the roots of physics and math, dark mater is fairy stories to tax payers and scientist who dont know a better story.

    • And so are a lot of the unproven theories SUSY suspersymmetry, strings, etc, they advance to explain the behavior of hypothetical exotic dark matter. Its becoming obvious that dark matter is ordinary matter, dust clouds, gas, intergalactic plasma etc.

    • Not really. The empirical evidence places an upper bound on the speed of gravity of a few seconds over 100,000,000+ Ly.

      10 (sec) / ( 100,000,000 × 365.25 × 24 × 60 × 60 ) = 3.16×10⁻¹⁵

      So it is now known to be within 3 parts per quadrillion of the speed of light. That’s a pretty hard upper bound.


  6. Proof of gravitational waves came on Sept. 14, 2015, when two giant, L-shaped, 2-mile-long laser instruments, one set up in a swamp in Louisiana and the other in Hanford, Washington, detected a tiny ripple in space, a “chirp” that reached Earth from the gigantic collision of two black holes a billion years ago.

    So proof means, flaky evidence that’s interpreted with confirmation bias.
    So far black holes aren’t even conclusively proven. I have yet to see any pictures of the famed event horizon.

    • Like you aren’t showing confirmation bias by claiming it’s “flakey” evidence.

      ” So far black holes aren’t even conclusively proven ” <– BS.

      " I have yet to see any pictures of the famed event horizon. " <– Proving you don't even understand what you claim to be arguing against, because by definition the event horizon cannot be seen, at most the region just above it away from the singularity could be seen.

      • -1

        tdperk hates reason and hates when you deflate his startrek dreams.

        Defending black holes though… pulsars are close enough for me. They are something very exotic and powerful.

          • I’m intrigued by how many people seem convinced they know more about astrophysics than the rest of the world.

            Not saying that the world physics community is always right, but it seems you’d need a strong case before making the claim that they are wrong.

            • It’s not about denying proven science doc. Too many things fly as fact with only theory behind them and no way to prove them. Asking skeptical people to take physicists at their word about certain astronomical or quantum constructs is tantamount to religion.

  7. “…one [LIGO] set up in a swamp in Louisiana.”
    There are some boggy bits and some open water next to the facility here and there, but I wouldn’t call it a swamp. Mostly piney woods on pretty solid ground, actually.

  8. Does Einstein’s general relativity predict that gravitational waves travel at the speed of light? Does it predict how the Shapiro delay for gravitational waves compares with the Shapiro delay for light?

    The answer to both questions is ‘no’. According to Arthur Eddington, Einstein’s general relativity says nothing about the speed of gravitational waves, let alone their Shapiro time-delay:

    Arthur Eddington: “The statement that in the relativity theory gravitational waves are propagated with the speed of light has, I believe, been based entirely upon the foregoing investigation; but it will be seen that it is only true in a very conventional sense. If coordinates are chosen so as to satisfy a certain condition which has no very clear geometrical importance, the speed is that of light; if the coordinates are slightly different the speed is altogether different from that of light. The result stands or falls by the choice of coordinates and, so far as can be judged, the coordinates here used were purposely introduced in order to obtain the simplification which results from representing the propagation as occurring with the speed of light. The argument thus follows a vicious circle.” The Mathematical Theory of Relativity, pp. 130-131 https://www.amazon.com/Mathematical-Theory-Relativity-S-Eddington/dp/0521091659

    So what is the probability that the gravitational waves (if they exist) travel exactly at the speed of light? Answer: Zero.

    What is the probability that the gravitational waves (if they exist) travel at a speed different from the speed of light? Answer: Unity.

    That is, if LIGO’s fabrication involved different times of arrival, that would have sounded realistic. The claim that the gravitational waves and the optical signal arrived at exactly the same time, which implies that they not only travel at the same speed but also experience the same Shapiro delay, unequivocally proves that LIGO just faked the gravitational wave signals.

    • I love it when other people agree that a Nobel Prize does not legitimize bad science. It means they take flak from the Goat, not me. Without even having a passing interest or any knowledge whatsoever about gravitational wave theory, I was always under the impression that gravity propagated at the speed of light (i.e. as instantaneous as can be). Am I right in saying that if gravitational waves exist, then all orbits should decay due to energy emitted in the form of gravitational waves? Doesn’t sound right.

      • “Am I right in saying that if gravitational waves exist, then all orbits should decay due to energy emitted in the form of gravitational waves?”

        Yes. But the amount of energy lost is tiny. For example, the orbital energy of the Earth is 1.14E36 joules, and it’s estimated that it radiates gravitational waves at about 20 watts. So we’ll be in big trouble in about 1.8E27 years.

        • Assuming that the Sun has no angular momentum. The moon is moving away from the earth because tidal interactions are actually pumping up its orbit.

      • Yah… you are right about inexorable orbital decay. All gravitationally bound systems must emit a gravitational wave signal; it is how we detect the doomed in falling pair of neutron stars. Thing is, that the signal is extraordinarily weak for systems as large as “stars and their planets”.

        Think of electromagnetic waves for a bit. If you have a gigahertz pumped resonator attached to a quarter wave antenna, because of both electric-field and magnetic-tensor oscillation, some fraction of the energy imparted to the antenna will “escape the system”. We can thank Mr. Hertz (or was it Marconi?) for quantifying that in a tidy set of equations. Marconi. Hertz. I don’t remember.

        In any case, the length of the quarter-wave antenna is related to what. The frequency F and the speed of light ‘c’. λ = c/&eta/F. Wavelength = speed of light divided by the index-of-refraction (of the energy) times the frequency of the source.

        Lets keep going: a 1 gigahertz = 1,000,000,000 cycle-per-second source in air (which has an η ≈ 1.000 for microwaves) of 299,792,458 ÷ 1,000,000,000 = 0.299… meters. The whole wave is about 30 cm, the quarter wave 7.5 cm. All we need is a bit of wire 3 inches tall, and it radiates 1 GHz with pretty good efficiency.

        But what about 1 Hz? 299,792,458 ÷ 1 ≈ 300,000 km. A piece of wire most-of-the-way to Moon. Long wire. Hard to emit. Even so, our analogy is loosing steam: a 1 Hz ultra-low-frequency “RF” source’s signal wouldn’t even make it to the end of the wire because of resistance. None-the-less, it shows a number.

        What happens when one’s emission-structure (antenna for RF) scale is appreciably below that quarter wavelength? Well … easy: not as much energy eeks out of the system. In fact, within some wrangling, its somewhat linear. If you use a ⅛ wave antenna, which is ½(¼) then about ½ the energy emits.

        Consider the wavelength of Earth:Sun. 1 year period more or less. A “wavelength” of about 9.5×10¹⁵ meters. Earth:Sol is what, about 150,000,000,000 meters? Something like that. If the “dipole” is 2× that (across the whole system) then the gravitational dipole is about ¹/₇₉₀₀ of a quarter wave. Thus whatever gravitational wave energy is emitted at about ¹/₈₀₀₀ the power that a nicely tuned dipole might.

        Then we can go on to figure out the center of gravity oscillation (which is the equivalent actual radiating dipole). Its way smaller than Earth:Sol distance. Roundly, Sol is about 10³⁰ kg, and Earth is 10²⁴ kg. So, there is 1,000,000 times difference in mass ‘twixt us. Well, that puts the CoG only about ¹/₁₀₀₀₀₀₀ the Earth:Sol nominal distance, so about 150,000 m (150 km) from the center of mass of the Sun itself.

        THAT dipole is WAY smaller than the E:S system orbital radius.

        So the gravitational emission is about one millionth of one eight-thousandth of what a tuned gravitational dipole might emit. One eight-trillionth?

        And we also need to remember that the speed of light itself determines the “precession lag” that is at the core of gravitational energy emission from a close pair of masses. In the case of a neutron:neutron star collapse, the bodies actually being “in front” of their mutual gravitational field effect on the other. That is the part which actually emits energy. Precession. Not the steady-state balanced gravitational attraction (a mind concept, not a physical one).

        So yah… there’s precession (because of ‘c’ and distance) for all gravitationally bound objects. And that precession in turn causes gravitational wave emission. And that energy in turn is derated by the shortness of the dipole-antenna emitting the field oscillations. Net result? We very likely couldn’t build an instrument sensitive enough to measure ANY of the planet’s gravitational emissions from merely a distance of 1 light year. Parts per 10⁻²⁴ against the otherwise quiet gravitational field rumble of interstellar space.

        Its what makes LIGO measurement so darn crazy-fascinating. Only neutron stars and black holes have the scale to emit enough energy to be detected across astrophysical scales.


        • Goat – just picking up on your last sentence – what about detection at non-astrophysical scales? I’m not talking about being right next to a neutron star, but rather what about using atom interferometry to detect gravitational waves from our Moon orbiting around the Earth, or something similar?

          • That’s “near field”; You can’t even say the wave exists at that scale.

            But, no, not possible. Basically the energy would be so low as to be below the universe’s ‘noise floor’. I mean, even the detectable gravity waves are distorting the system by much less than the diameter of a proton.

            • I still dont understand how a distortion less than the width of a proton can be detected by much bigger light waves

        • ok, i’m going to bite..

          exactly how efficient are gravitational systems? I’ve seen arguments that they are unstable, will ultimately decay, but how fast will this happen?

          I’ve seen arguments from people who say that if intelligence *really* wants to stay along for deep, deep time (whilst black holes evaporate, ie: googol’s of years) it will need to learn to ‘hack it’; ie: use very limited amounts of computation per unit time, but do so over the aeons.

          basic idea is that you’d place yourself in a stable orbit around a black hole and harvest hawking radiation and/or tidal energy; ‘subjective’ time in this universe would be very slow; with billions upon billions of years being equal to a second today. speed of light then wouldn’t become a factor in truly universe-scale civilizations, as they would be exchanging data nearly instantaneously compared to their subjective time horizons.

          all this is well and good, but if there is *one* macro effect that is unstable, one way that these computational agents were somehow perturbed in their state, then in a few subjective moments things would come crashing down before any computation would have a chance to occur.

          in my mind, this unstable something would be either tidal forces or gravitational waves, and I’m interested to see the math behind this. Exactly how long would it take, for example for an earth-sized body to have its orbit decay around a black hole due to gravitation? is there a way to balance/harvest tidal energy to somehow stabilize things?

      • Am I right in saying that if gravitational waves exist, then all orbits should decay due to energy emitted in the form of gravitational waves? Doesn’t sound right.

        All orbits decay but only over extremely long timelines. Orbital decay and emission of gravity waves depends on acceleration. Accelerations are very low in most orbits, with orbiting neutron stars and black holes being an exception where this effect is significant.

  9. I want to know if we’ll be able to move beyond LIGO and use something like Atom Lasers to detect gravitational waves or ripples in spacetime. Atom lasers are supposed to be very precise – far more so than optical lasers. Will we then see tabletop versions of LIGO some day?

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