- When Newton first formulated his theory of gravity, he imagined masses attracting one another, instantaneously, from any great distance across the Universe.
- But it turns out that no signals, not even gravity, can travel faster than the speed of light: the cosmic speed limit for any and all particles and/or forces.
- It took an Einsteinian revolution to restore cosmic order to gravity, and recent observations have confirmed that the speed of gravity really is equal to the speed of light. Here's how we know.
When you look at the Sun, the light you’re seeing isn’t the light that’s being emitted right now. Instead, you’re seeing light that’s a little more than eight minutes old, since the Sun is some 150 million kilometers (93 million miles) away, and light — although it’s fast — can only travel through the Universe at a specific speed: the speed of light. But what about gravitation? Everything on Earth experiences the Sun’s gravitational pull, but is the gravity that the Earth experiences as it orbits the Sun coming from the Sun right now, at this very instant? Or, just like light, are we experiencing gravitation from some time ago?
It’s a puzzling thought experiment for our minds: after all, what dictates how gravity ought to behave? There’s no fundamental connection between gravitation and electromagnetism; the latter is where the speed of light comes from, so why would the speed of gravity have anything to do with the speed of light?
As it turns out, despite whatever intuitive thoughts we might have about how gravitation ought to behave, only experiments, measurements, and observations can provide the answer to a physical question such as this about reality. Those have revealed something fascinating to us, in agreement with what Einstein’s (but not Newton’s) theory of gravity predicted: that the speed of gravity isn’t instantaneous, but rather propagates at exactly the speed of light. Here’s the story of how we know.
Let’s begin with the speed of light. The first person to try to measure it, at least according to legend, was Galileo. He set up an experiment at night, where two people would each be atop adjacent mountain peaks, each one equipped with a lantern, beacons-of-Gondor style. One of them would unveil their lantern, and when the other saw it, they would unveil their own lantern, allowing the first person to measure how much time elapsed. Unfortunately for Galileo, the results appeared instantaneous, limited only by the speed of a human’s reaction time. Unlike all the other quantitative data that Galileo recorded, he could only state that the speed of light was “fast.”
The key advance wouldn’t arrive until 1676, when Ole Rømer had the brilliant idea to observe Jupiter’s innermost large moon, Io, as it passed behind Jupiter and re-emerged from the giant planet’s shadow. Because light has to travel from the Sun to Io, and then from Io back to our eyes, there ought to be an observed delay from when Io leaves Jupiter’s shadow, geometrically, until the arriving light, bouncing off of the moon, becomes observable here on Earth. Although Rømer’s measurement values gave a result for the speed of light that was off by about 30% from the actual value, this was the first measurement of the speed of light, and the first robust demonstration that light traveled at a finite speed after all.
Rømer’s work influenced a number of important scientists of his day, including Christiaan Huygens and Isaac Newton, who came up with the first (but mutually contradictory) scientific descriptions of light, with Huygens developing a wave-like theory and Newton developing a corpuscular (particle-like) theory. About a decade after Rømer, however, Newton turned his attention to gravitation, and all ideas about a finite speed for gravity went out the window. Instead, according to Newton, every massive object in the Universe exerted an attractive force on every other massive object in the Universe, and that interaction was instantaneous, occurring without any delay.
The strength of the gravitational force is always proportional to each of the masses multiplied together, and inversely proportional to the square of the distance between them. Move twice as far away from one another, and the gravitational force becomes just one-quarter as strong. And if you ask which direction the gravitational force points in, it’s always along a straight line connecting those two masses. That’s the way Newton formulated his law of universal gravitation, where the mathematical orbits he derived matched up precisely with the way the planets moved through space.
Of course, we already knew how to describe the way that planets orbited the Sun: Kepler’s laws of planetary motion were many decades old by the time Newton came along. What Newton accomplished that was so remarkable was to put forth a theory of gravity: a mathematical framework that obeyed rules from which all of Kepler’s laws (and many other rules) could be derived. So long as, at every moment in time, the force on any planet always points directly toward where the Sun is at that exact moment, you get the planetary orbits to match up with what we observe.
Newton also realized something profound about what would happen if gravity didn’t act instantaneously on objects, such as between the Sun and the Earth. If you instead allowed the gravitational force to point toward where the Sun was a certain amount of time ago — such as ~8 minutes ago from the perspective of planet Earth — the planetary orbits you’d derive would be all wrong. In order for Newton’s conception of gravity to have a chance at working, the gravitational force was required to be instantaneous. If gravitation is slow, even if “slow” meant that it moved at the speed of light, Newton’s gravity wouldn’t work, after all.
For hundreds of years, Newton’s gravity was able to solve every mechanical problem that nature (and humans) threw at it. When Uranus’s orbit appeared to violate Kepler’s laws, it was a tantalizing clue that perhaps Newton was wrong about something, at long last, but it wasn’t to be. Instead, there was an additional mass out there in the form of the planet Neptune that was causing those unexplained accelerations in Uranus’s orbit. Once Neptune was discovered and its position and mass became known, that puzzle went away.
But Newton’s successes wouldn’t last forever. The first real clue came with the discovery of Special Relativity, and the notion that space and time weren’t actually absolute quantities, but rather how we observe them depends very intricately on our motion and location. In particular, the faster you move through space, the slower clocks appear to run and the shorter distances appear to be. As FitzGerald and Lorentz, working more than a decade before Einstein, described it:
- distances contract
- and time dilates
by ever-greater amounts the closer you move to the speed of light. Unstable particles that decay away with a specific half-life were observed to survive for longer if they moved at high speeds. This led to a revolutionary conclusion: space and time cannot be absolute, but must be relative for each unique observer.
If that’s true, and different observers moving with different velocities and/or at different locations can’t agree on things like distances and times, then how could Newton’s conception of gravity be correct? If someone on Earth and someone on the Sun cannot agree on what time it is or at what instant an “event” occurs, then how can something — even the force exerted by gravity — occur instantly between them? It seems like relativity and the notion of “instantaneous” cannot simultaneously apply to different observers at once; something must be inconsistent here.
One way to think about it is to consider an absurd but useful puzzle: imagine that, somehow, some omnipotent being were able to instantaneously remove the Sun from our Universe. What would someone living on Earth experience from that event?
As far as the light goes, we know that it would continue to arrive for another 8 minutes or so, and the Sun would only appear to go dark — to disappear — once that light stops reaching us. The other planets would only go dark from our perspective only once the sunlight stopped
- reaching them,
- reflecting off of them,
- and at last ceased to arrive at our eyes.
But what about gravitation? Would that cease instantly? Would all of the planets, asteroids, comets, and Kuiper belt objects simply fly off in a straight line all at once? Or would they all continue orbiting for a time, continuing their gravitational dance in blissful ignorance until the effect of gravity finally hit them?
The problem, according to Einstein, is that Newton’s entire picture must be thrown away for the Universe to make sense. In a Universe where space and time aren’t absolute quantities but rather are relative to any observer, gravity can no longer be viewed as a straight-line, instantaneous force connecting any two points in the Universe. As a viable alternative, Einstein put forth a picture where space-and-time are woven together in what he visualized as an inseparable four-dimensional fabric. This fabric would be deformed not only by the presence of masses, but by all forms of matter and energy, wherever and whenever they were located.
And, as an implication of that, instead of the planets orbiting around the Sun because of an invisible force, they simply move along the curved path determined by the curved, distorted fabric of spacetime.
This Einsteinian conception of gravity would lead to a radically different set of equations from Newton’s, and instead would predict that gravity not only propagates at a finite speed, but that speed — the speed of gravity — must be exactly equal to the speed of light. If you were to suddenly “wink” the Sun out of existence, that spacetime fabric would “snap” back to flat the same way a rock falling into a pool of water would cause the water’s surface to snap back. It would come to equilibrium, but the changes in the surface would come in ripples or waves, and they would only propagate at a finite speed: equivalent to the speed of light.
For many years, we’ve had indirect tests of the speed of gravity, but nothing that measured these ripples directly. We measured how the orbits of two pulsing neutron stars changed as they orbited one another, determining that energy was radiating away at a finite speed: the speed of light, to within a 99.8% accuracy. Just as Jupiter’s shadow obscures light, Jupiter’s gravity can bend a background light source, and a 2002 coincidence lined up Earth, Jupiter, and a distant quasar. The gravitational bending of the quasar light due to Jupiter gave us another independent measurement of gravity’s speed: it’s again the speed of light, but comes with a ~20% error.
All of this began to change dramatically in the year 2015, when the first advanced gravitational wave detectors (LIGO) saw their first signals. As the first gravitational waves traveled across the Universe from merging black holes, a journey of more than a billion light-years for our first detection, they arrived at our (then) two gravitational wave detectors just milliseconds apart, a small but significant difference. Because they’re at different points on Earth, we’d expect a slightly different arrival time if gravity propagated at a finite speed, but no difference if it were instantaneous. For every gravitational wave event, the speed of light is consistent with the observed arrival times of the waves.
But in 2017, something spectacular happened that blew all our other constraints — both direct and indirect ones — away. From ~130 million light-years away, a gravitational wave signal arrived in our three detectors that were operating at that time: the two LIGO detectors and the Virgo detector. It started out with a small but detectable amplitude, then increased in power while getting faster in frequency, corresponding to two low-mass objects, likely neutron stars, inspiraling and merging. After only a few seconds, the gravitational wave signal spiked, and then ceased, signaling the merger was complete. And then, a mere 1.7 seconds later, the first sign of light arrived: a high-energy signal that registered as a gamma-ray burst.
It took some 130 million years for both the gravitational waves and the light from this event to travel through the Universe, and they arrived at the exact same time: to within 1.7 seconds of one another. That means, at most, if the speed of light and the speed of gravity are different, then they’re different by no more than about 1 part in a quadrillion (1015), or that those two speeds are 99.9999999999999% identical. In many ways, it’s the most accurate measurement of a cosmic speed ever made. Gravity really does travel at a finite speed, and that speed is identical to the speed of light.
From a modern point of view, this makes sense, as any massless form of radiation — whether particle or wave — must travel at exactly the speed limit for all massless quantities: the speed of light in a vacuum. What started off as an assumption based on the need for self-consistency in our theories has now been directly confirmed observationally. Newton’s original conception of gravitation doesn’t hold up, as gravity isn’t an instantaneous force after all. Instead, the results agree with Einstein: gravitation propagates at a finite speed, and the speed of gravity is exactly equal to the speed of light.
We at last know what would happen if you could somehow make the Sun disappear: the last light from the Sun would continue traveling away from it at the speed of light, and it would only go dark when the light stopped arriving. Similarly, gravity would behave in the same fashion, with the Sun’s gravitational effects continuing to influence the planets, asteroids, and all the other objects in the galaxy until its gravitational signal no longer arrived. Mercury would fly off in a straight line first, followed by Venus, Earth, and all the other orbiting masses in order of their distance from the Sun. The light would stop arriving at exactly the same time the gravitational effects did. As we only now know for certain, gravity and light really do travel at exactly the same speeds.