We know time slows down as gravity increases, so is the speed of light in high gravity different from its speed in low gravity?
Eric Kvaalen
Les Essarts-le-Roi, France
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Time doesn’t necessarily slow down as gravity increases. What matters is how deep a point is in a gravitational well, which depends on how much energy it would take to get a 1-kilogram object very far away from the object producing the gravity.
Clocks run slower at the centre of Earth than they do at its surface, even though the gravitational field there is zero. If you stand in a hole in a layer of haematite iron ore or between stacks of gold bars – both of which are made of material that is more than two-thirds the average density of Earth – your feet will experience weaker gravity than your head, but time will still go more slowly for your feet.
The speed of light as measured locally is always the same, around 300,000 kilometres per second. But if it were possible to observe where a photon or a light wave was and the light in question was moving deep in a gravity well, such as near a black hole, then, to us, it would look as though the light was moving slower than 300,000 km/s.
In our frame of reference, a photon emerging from near the event horizon of a black hole takes a very long time to get away from the black hole. By the same token, if we watch an object falling towards a black hole, it appears to us to go slower and slower and, from our point of view, it would never reach the event horizon.
For this reason, the “black holes” that we see in the heavens aren’t yet really black holes, because the material, from our point of view, never actually finishes collapsing to make one.
Ron Dippold
San Diego, California, US
It can be apparently slower if you are far away – it’s all relative!
First, the apparent speed of light varies in different mediums. It is about 30 per cent slower in water than in a vacuum because the light keeps getting absorbed and re-emitted. The gold standard is the speed of light in a vacuum, c, which is conveniently close to 300,000 km/s. So let’s assume we mean a vacuum from here on.
Someone in high gravity measuring the speed of light in a vacuum near them will find it is still c, unchanged. And someone using the same set-up in low gravity will also measure c as unchanged. So the answer is no, right?
Well, let’s say you have someone far away from the event horizon of the black hole in the centre of our galaxy, Sagittarius A*. This is relatively low gravity, so they are our “low-gravity observer”. They are watching an experiment near the event horizon of Sagittarius A*, where a laser is sent all the way around the event horizon, with the help of repeaters, through a tube of thin gas that lights up when the laser goes through. This is definitely high gravity, so someone surviving here (an AI?) is our “high-gravity observer”.
Sagittarius A* is 160 million km in circumference, so our high-gravity observer (near the laser) would indeed see it takes about 10 minutes for light to circle it – c is unchanged. However, the low-gravity observer watching the tube light up from far away would see it take much longer. If the laser and tube were about 15 million km from the event horizon and the low-gravity observer were 200 million km away, the low-gravity observer would see it takes about 15 minutes for the laser to go around. High-gravity light looks much slower to the low -gravity observer.
People argue a lot about what this means. Again, it all depends on what assumptions you make and whose point of view you want to take. Relativity considers time as a fourth dimension (it is “space-time”, after all) and high gravity warps space-time by something known as the Schwarzschild metric. The low-gravity observer can calculate that the speed of light in that warped four-dimensional high-gravity space-time is the same as in their four-dimensional low-gravity space-time when correcting for that warping.
But it is entirely fair to say that for the low-gravity person, the speed of light in high gravity appears to be relatively slower. That is why it is the theory of relativity.
David Hyde
Henley-on-Thames, Oxfordshire, UK
No, light always travels at a constant speed, c. Light has a dual nature, behaving both as a wave and as a particle called a photon, which is massless and therefore isn’t influenced by gravity.
Herman D’Hondt
Sydney, Australia
The fact that the speed of light is constant is one of the cornerstones of Albert Einstein’s theory of special relativity.
In 1887, Albert A. Michelson and Edward W. Morley conducted an experiment in which they tried to measure the difference in the speed of light as Earth moved through the hypothetical “aether”. As our planet moved around the sun, the different directions of its travel through the aether ought to have affected the measured speed of light. They found no difference whatsoever, and this has been confirmed many times since then.
In his 1905 paper on special relativity, Einstein used this fact to tell us why time and length must change depending on the speed of the observer. Ten years later, in his theory of general relativity, he used the same fact to explain why gravity affects our measurement of time. General relativity has been confirmed over and over in the past 100 years. No evidence has ever been found that contradicts it.
By extension, it follows that no evidence has ever been found for any change in the speed of light, whether that is by speed, gravity or any other factor affecting the measurements.
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