Wonders of the Universe - Brian Cox [77]
Mercury’s unusual orbit was a real problem, but because of the observational uncertainties it wasn’t until 1859 that the French astronomer Urbain Le Verrier proved that the details of Mercury’s orbit could not be completely explained by Newtonian gravity. To solve the problem, many astronomers reasoned there must be another planet orbiting between the Sun and Mercury. This planet had to be invisible to our telescopes, but it must also exert a gravitational force large enough to disturb Mercury’s orbit. Encouraged by the recent discovery of the planet Neptune, based on a similar anomaly in the orbit of Uranus, they named the ghost planet Vulcan
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For decades astronomers searched and searched for Vulcan, but they never found it. The reason for this is that Vulcan doesn’t exist. The errors in the predictions in fact signalled something far more profound: Newton’s Theory of Universal Gravitation is not correct.
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This image shows an artist’s impression of the hypothetical planet Vulcan, which was once believed to orbit in an asteroid belt closer to the Sun than Mercury. Vulcan was supposedly first sighted by amateur astronomer Lescarbault on 26 March 1859, but further observations were inconclusive and Vulcan was later proved to be a ghost planet.
SCIENCE PHOTO LIBRARY
EINSTEIN’S THEORY OF GENERAL RELATIVITY
German-born physicist Albert Einstein (left) created the famous Theory of General Relativity. British astrophysicist Sir Arthur Eddington (right), later put this theory to the test and confirmed its accuracy.
ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY
Einstein would have loved the Vomit Comet. The fact that the effects of gravity can be completely removed by falling freely in a gravitational field was, for him, the thought experiment that led to his theory of General Relativity. How wonderful it would have been for him to experience it as I did! The reason I say this is that, as I floated next to my little plastic Albert in the Vomit Comet, I understood very deeply why Einstein was so interested in freefall. The point is this; inside the plane, falling towards Earth, it is absolutely impossible to tell that you are moving. It is impossible to tell that you are near a planet. It is impossible to tell that, according to someone stood on the ground, you are accelerating at 9.81 m/s2 towards the ground. You are simply floating, along with everything else in the plane. I let some little drops of water out of a bottle and they floated in front of my face; the cameraman and director floated next to the water droplets, little plastic Albert and me. There was self-evidently no force acting on anything at all, otherwise things would have moved around.
And yet, from the point of view of someone on the ground, we were flying in a parabolic arc, moving forwards through the air at hundreds of miles an hour and accelerating violently towards the ground. The force of gravity is very much present in this description. Einstein’s theory takes the view that the two ways of looking at the Vomit Comet – from inside and outside – should be treated as equivalent. No one inside the plane or out has the right to claim that they are right and the other is wrong! If, inside the plane, there is no experiment you can do to prove that you are accelerating towards the ground, you are well within your rights to claim that you are not. Acceleration has cancelled out gravity. Of course, you could look out of the windows, but even then you could claim that Earth is accelerating up towards you and that you are simply floating. From this perspective, everyone on Earth feels a gravitational force pulling them onto the ground because they are being