Why Does Gravity Travel at the Speed of Light?

By Eric Betz | December 8, 2017 12:04 pm
Two neutron stars collide; the resulting gravitational wave spread at the speed of light. (Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonne)

Two neutron stars collide; the resulting gravitational wave spread at the speed of light. (Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonne)

The dead cores of two stars collided 130 million years ago in a galaxy somewhat far away.

The collision was so extreme that it caused a wrinkle in space-time — a gravitational wave. That gravitational wave and the light from the stellar explosion traveled together across the cosmos. They arrived at Earth simultaneously at 6:41 a.m. Eastern on August 17.

The event prompted worldwide headlines as the dawn of “multimessenger astronomy.” Astronomers had waited a generation for this moment. But it was also the first-ever direct confirmation that gravity travels at the speed of light.

The Speed of Gravity

We all know light obeys a speed limit — roughly 186,000 miles per second. Nothing travels faster. But why should gravity travel at the same speed?

That question requires a quick dive into Albert Einstein’s general relativity, or theory of gravity — the same theory that predicted gravitational waves a century ago.

Einstein overthrew Isaac Newton’s idea of “absolute time.” Newton thought time marched onward everywhere at an identical pace — regardless of how we mortals perceived it. It was unflinching. By that line of thinking, one second on Earth is one second near a black hole (which he didn’t know existed).

Newton also thought gravity acted instantaneously. Distance didn’t matter.

It’s All Relative

But then Einstein showed that time is relative. It changes with speed and in the presence of gravity. One of the ramifications of that is that you can’t have simultaneous actions at a distance. So information of any kind has a finite speed, whether it’s a photon — the light-carrying particle — or a graviton, which carries the force of gravity.

“In relativity, there is a ‘speed of information’ — the maximum speed that you can send information from one point to another,” says University of Wisconsin-Milwaukee physicist Jolien Creighton, an expert on general relativity and member of the LIGO team that first spotted gravitational waves.

Creighton explains that in electromagnetism, when you shake an electron, it creates a change in the electric field that spreads out at the speed of light. Gravity works the same way. Shake a mass and the change in the gravitational field — the gravitational wave — propagates at that same speed.

“So the fact that the speed of gravitational waves is equal to the speed of electromagnetic waves is simply because they both travel at the speed of information,” Creighton says.

There’s an easy way to picture this, too. Imagine the sun vanished right now. Earth wouldn’t just drift into space instantly. After eight minutes, Earth would go dark and simultaneously push off in a straight line.

  • Ld_Elon_Postman_RoyalMale


    Light is the fires of ‘hades’, WHICH IS BLACK as coal setting alight of its own heat and combustion and where it be cold, that be hot as plasma.

    You see life on earth is = to black hole, where life/light arises and falls back into, and from the darkness of the void.

    One instantly switches polarity and thus spaces {light years across}.

    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      “LISTEN UP, pus-for-brains! You snipe jocks WILL have those zube distorter modules ratcheted by 0700 hours. Any of you vat-grown industrial rejects miscalibrates a D*, your chromosomes gonna be strung across reactor containment!”

      Youtube v=Ac7G7xOG2Ag

    • Oliver Sol

      I won’t pretend to understand what you’re talking about or referencing, but, if I’ve inferred correctly, it seems that last bit is challenging the statement that information can not travel faster than light due to the entanglement properties in quantum mechanics.
      Does not the no-communication theorem of quantum mechanics say that the results of quantum entanglement do not constitute information exchange?

  • Ld_Elon_Postman_RoyalMale

    Stargate likely represents,~ colour chart correlation syncs, yin yang, represents polaric teleportations, too. Black and white represents the two pure states [Of colour]. polarity duplex.

    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      Comments: 1043
      Upvotes: 92

  • Noah Ibrahim

    Isn’t a graviton a hypothetical particle? You should have stated it “in theory”.

    • Ld_Elon_Postman_RoyalMale

      Interesting, especially as one thunk plasma has a bend and thus light has a mass {force} enough for its ‘gravity’ to pull it in on itself and create the {bonded} plasma ‘geometric’ curvature,.

      • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

        Behold a null comment about a null geodesic path.

  • Matthew Crochet

    What? This is not a complete argument. The ‘speed of information’ or ‘speed of causality’ is not WHY gravity travels at the speed of light. It simply is the maximum speed that light and gravity travel at. Usually is travels slower when observed traveling throigh a medium. Nobody has answered what makes that the speed limit, and don’t say ‘action at a distance’ is a scientific concept- it’s a scientific mystery. Not one of the industry scientists todag have an explaination of spooky action at a distance, or gravity. Good luck explaining it without first understanding what makes up light. Whoever researches these should probably do more general research- then study focused experiments and their results. This is just coloring data.

  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    “graviton” There is zero empirical evidence that gravitation is quantized. To 15 decimal places (MICROSCOPE), the only validated gravitation theory, general relativity, is wholly incompatible with quantum mechanics (separability versus entanglement, and no uncertainty in measurement).

    That gravitational wave and light signals from GW170817 neutron star merger arrived within 2.7 seconds (about the sun’s radius) window over a 140 million lightyear path implies spacetime is massless. Disturbances propagate at lightspeed. Dirac sea onward…oopsie!

  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Shake a mass” Be very careful here, gravitation versus gravitational radiation. GR demands ligthspeed is finite. Its value is observed not calculated. Group vs. phase velocities and negative refractive index in meta-materials
    …DOI:10.1103/PhysRevLett.110.013902, …DOI:10.1021/acsphotonics.7b00760).

    Dipole radiation is sourced by a changing dipole moment. (Punctiliously, the second time derivative of the dipole moment, acceleration) For a pair of charges
    …d = qr + q’r’
    There’s nothing special about the derivatives. For a pair of masses, the gravitational dipole moment and its time derivative are
    …d = mr + m’r’
    …mv + m’v’ = p + p’
    By conservation of momentum the second time derivative of the gravitational dipole moment is zero. Go to a center of momentum frame and set the first derivative to zero as well. There is no gravitational “electric dipole” radiation. Consider the analog of “magnetic dipole” radiation. The gravitational equivalent of the magnetic dipole moment for a pair of charges is
    …M = mv x r + m’v’ x r’ (“x” is the cross product, “mv” is the “mass current”)
    But M is the total angular momentum, which is also conserved. There is no gravitational “magnetic dipole” radiation.

    The next moment up is quadrupole, with no relevant conservation laws, so gravitational quadrupole radiation is permitted. Thus gravity must be a tensorial (spin-2) interaction and uniformly attractive.
    Electromagnetism is mediated by spin-1 photons and can be repulsive.

  • Dr. Manos PhD

    “In relativity, there is a ‘speed of information’ — the maximum speed that you can send information from one point to another,”
    So, what about the ‘instant’ transfer of information through Quantum entanglement, no matter the distance.?
    By counting that, there’s absolutely no maximum speed that you can send information from one point to another. There are no limits in relativity.

    • Oliver Sol

      The No-communication Theorem states that classical bits of information can not be transmitted thru quantum entanglement. I can’t say I understand the principle completely, only that there is an answer to your challenge that states there is no transfer of information thru entanglement, & that the information speed limit holds up due to this.
      Honestly, in my very limited picture of quantum entanglement, I would’ve had the same question, but apparently there’s an answer.
      If you look up the not communication theorem, you will find that answer, & perhaps you will have better luck at understanding it than I. Hope this helps!

      • Dr. Manos PhD

        That’s a very interesting theorem. It cleaned my view on the subject. Although, I believe that it’s very limited (except if I didn’t get it completely). I believe there is a way to overcome that problem, by using multi way entanglement apparatuses. For example, with binary communication, a separate apparatus for 1 and a separate for 0, = 4 total apparatuses at both ends of communication (any distance) for 0 & 1. All other variables are solvable.

    • OWilson

      The big problem with Quantm Entanglement is that there is no way to cofirm that the “spooky action at a distance” is valid in any experiment except the reciever reporting back to the lab by the conventional (space?) telephone, using traditional EMFs at the limited speed of light!

      It’s the confirmation (comparison of the dead cat, or the left glove) that is actually “information”. :)

      • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

        The Einstein-Podolsky-Rosen argument and the Bell Inequalities are realized through a satellite up-link and down-link. Entanglement works by the book with no hidden variables.

    • andrewp111

      There is no “speed of information” in relativity. The speed of light is the only special speed, since it describes the relation between space and time. The equations of relativity describe the behavior of things that travel slower than light (all ordinary things) and things that travel faster than light (tachyons) as well. Experimentally, no faster than light particle or communicative influence has ever been observed. However, absence of evidence is not proof of absence. There could be faster than light phenomena that we don’t know how to observe.

  • Martolt

    An alternate theory is that what we call the independent force of gravity is actually just a subset of the all-pervasive electric force. In the Electric Universe model, it is electricity that is the driving force behind the Universe. We observe this everywhere we look – vast plasma filamentary structures feeding electric current throughout and forming metallic Hydrogen electrodes, the stars, at plasma Z-pinch points in the circuit.

    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      Electromagnetism is dipolar and can attract or repel. Gravitation is quadrupolar and strictly attractive.

      ” metallic Hydrogen electrodes” Dark matter Zambonies.

  • Erkan Ermiş

    You are wrong. According to the theory of general relativity, sudden removal of the sun creates a space-time disturbance by gravitational shock waves with infinite peak that would evaporate all celestial objects on its way.

    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      Gravitational waves are quadrupolar disturbances. Suddenly vanishing the sun would do literal nothing. You need to instantly implode it. The resultant neutrino blast would overshadow any spacetime distortion consequences,

      37Ar 35 day half-life decays by electron capture to 37Cl, then Auger electron cascade x-ray emissions.

    • Russ Hamilton

      That is NOT the point…

  • Antifaarecommies

    If gravity travels at the speed of light does that mean we’re constantly eight minutes behind the sun. I believe in some do that gravity is almost instantaneous no matter the distance.

    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      Gravitation propagates at lightspeed . Beleve what you want – gods to open borders. Nothing propagates faster than a massless null geodesic. Nothing “travels” during entanglement decoherence (1203 km Earth separation, 1600 km to 2400 km pathlength, arXiv:1707.01339). Hindus have 30 crores of gods; ask an American Indian about “open borders.” What looks good through a pinhole must still obtain in a ten ton load.

      • Purgedoj

        Obviously, I can’t participate in the physics side of the argument. If entanglement prevents travel, then we must be entangled throughout the universe….since there appears to be order. Since gravity is a very weak force, entanglement must be the product of electromagnetism, which is 39 ( to the 10th power) times more powerful than gravity. Therefore, gravity does not determine much of anything.

    • Russ Hamilton

      I always wondered about how you could maintain a stable orbit if gravity was not infinite or at least much speedier than light. Supposedly angular momentum cancels out or balances the fact that it does propagate only at light speed…fwiw…

  • Jon Johnson

    Do not try to explain what you do not understand, can not prove physically or demonstrate in your control.
    Show me this thing you call gravity.
    There is no gravity, only divergent/convergent magnetism.

    • ECarpenter

      You don’t seem to understand what magnetism is. And you certainly can’t show me the thing you think is magnetism.

      Gravity waves, on the other hand, have been physically measured now.

      It’s a real problem for you flat-earthers.

      • OWilson

        We don’t know “what” gravity or EM actually IS, we only know how to measure and observe some of it’s “properties”.

        The very word gravity was given for “the force that pulls”.

        Until liberal education starts producing Einsteins instead of Bill Nyes, Madam Curies, instead of Kardashians, it will remain that way! :)

  • Kurt Stocklmeir

    around 1990 I put my theory on the internet using sci physics – all forces do not have any Doppler effect all forces move at an almost infinite speed – there is not any Doppler effect for any force – how strong gravity is does not change with direction of speed – it is probably true I was the first person to say this – these are my theories – gravity waves do not have any Doppler effect gravity waves move at an almost infinite speed – if gravity waves move at the speed of light particles like photons and neutrinos would see some gravity waves with infinite energy – people can not see gravity waves – there can not be any experiments that see gravity waves – space and time around mass makes a resonator for standing waves of gravity waves – it could be true in the future people will see standing waves of gravity waves Kurt Stocklmeir

    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      all forces move at an almost infinite speed ” ALMOST infinite? “ACK! THBBFT!” so sad

      • Russ Hamilton

        Yes lets suppose we are travelling at 1/2 the speed of infinity or 1 percent of infinity….wait, which is faster?

    • John Thompson

      One thing I find interesting is at the speed of light time stops for the thing moving at the speed of light.
      So a photon doesn’t “age”.
      But that also means the photon moves at infinite speed – from it’s own perspective.
      Because no matter what distance it covers, it covers it instantaneously from the perspective of the photon itself.
      Correct me if I’m wrong, but isn’t the definition of infinite speed the ability to go infinite distance in no time?
      Oh, yeah, that’s hurting my head just trying to conceive of it.

      • Kurt Stocklmeir

        John time is a lot space – if any particle does not have any size associated with a dimension of space the particle will have infinite energy – that is why Einstein said if a particle like an electron is moving at the speed of light the particle will have infinite energy and infinite mass – this is true for time – any particle is spread through time a lot like space – a particle can not be at 1 piece of space and a particle can not be at 1 time – if a photon does not see any time that means the photon is not spread through any time and it will have infinite energy – if a particle like an electron is moving at the speed of light because of relativity it will not see any distance a lot like it will not see any time

  • http://calystarose.tumblr.com/ cirose

    After eight minutes, Earth would go dark and simultaneously push off in a straight line. — that’s quality nightmare material. :)

  • Paul Wilson

    Ok, all true. Then we have “spooky action at a distance” a la quantum entaglement. Physicists are at pains to insist that no “information” can be instantaneously transmitted. Only quantum states. But even they admit that something spooky is going on there.

  • Alberto Carvalhal Campos

    Hold on, this is waiting to be approved by D-brief.
    The speed of gravity is really instantaneous. Newton was right. Light falls from one point to another and can take billions of years to arrive. Gravity is different. all bodies attract each other. To better understand only two bodies A and B. If A attracts B, B also attracts A. the mutual attraction from which these bodies were created (a tiny grain of matter). When they grow they already grow with greater gravity. The wine is not particle. She is a force. The graviton does not exist. It was never detected. It is hypothetical (it would be like a deity). When gravity ends in A (desitegrado), it also ends in B, because it does not matter if B has gravity if it does not have A to attract. Attraction is a mutual property. There is no solitude.
    Gravitational waves may also not exist. See details at: http://www.olhandoouniverso.blogsp...

  • nihonpro Gerard

    Ahmmm… to be honest here. Gravity travels faster then light. Its absolute speed can not be really calculated, BUT, big but, a recent discovery at CERN hints, that we are wrong about the speed of gravitational waves and that again leads us to a HUGE puzzle. It states by the first calculations and the first tests, junks of Data that gravitational waves travel at a speed of 10 000 0000 m/s. There was a huge suprise, even after recalculating and testing again, this still stands up. Soo stay up tuned, its the latest.

  • http://twitter.com/c_branderiz Carlos Branderiz

    The earth planet have a natural security protección, don’t burry gays!!!, to much tehorias arround the whorld in all lenguish..

  • Eugene McCreary

    Isn’t gravity a term for the curvature of space-time,not a force, as Newton perceived it? In that sense gravity is everywhere there is mass, all at the same time. In that sense gravity doesn’t travel or go anywhere. It just is, though the nature of the curvature changes as mass moves. Help me out here, please.

    • Francis Smith

      I have the same issue. Gravity is a word to describe a bend/curvature/distortion in space because of mass. So maybe that bend can ripple also and if the disturbance is violent enough we can measure it? I’m thinking it is like trampoline fabric that is warping around my feet all the way out to the springs, the further away the less distortion there is. But now, I am thinking it is more like the distortion on the surface of water made by a Jesus bug. When the Jesus bug is still there is a slight dimple on the surface but when it starts to move it also lets out ripples.

  • Steve Humphries

    Does the finite speed of the photon and graviton reveal anything about the structure of the space-time medium it is traveling in?

  • Russ Hamilton

    Since time is a factor of “speed” and time is standing still at the speed of light there is no way to go beyond it…



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