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, corresponding to the amount of time it takes a gravitational signal to traverse that cosmic distance? It’s a puzzling thought experiment for our minds.
After all, what is it that 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. When a gravitational microlensing event occurs, the background light from a star gets distorted and magnified as an intervening mass travels across or near the line-of-sight to the star.
The effect of the intervening gravity bends the space between the light and our eyes, creating a specific signal that reveals the mass and speed of the object in question. Only in the event of a perfect alignment of the observer, the foreground mass, and the background light source is a ring temporarily created: the same as an Einstein ring under an ideal, general gravitational lensing configuration. vatory, University of Warsaw 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.
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