Date: April 17, 2010

Title: Was Einstein Correct?


Podcaster: Stuart Clark

Description: Was Einstein right to explain gravity as a distortion in the fabric of space-time?

Bio: Dr Stuart Clark is an award-winning astronomy author and journalist. His books include The Sun Kings, and the highly illustrated Deep Space, and Galaxy. His next book is Big Questions: Universe, from which this podcast is adapted. Stuart is a Fellow of the Royal Astronomical Society, a Visiting Fellow of the University of Hertfordshire, UK, and senior editor for space science at the European Space Agency. He is also a frequent contributor to newspapers, magazines, radio and television programmes. His website is and his Twitter account is @DrStuClark.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by David Rossetter on behalf of the Mid-Hudson Astronomical Association: A goofy group of geeks who love to observe and share the night sky around New York State’s Mid-Hudson region.



Hello I’m Dr Stuart Clark, astronomy author and journalist. Today I’d like to explore the question: Was Einstein right?

During the nineteenth century, astronomers charting the positions of the planets watched as Mercury and Uranus continually drifted from the orbits predicted by Isaac Newton’s mathematics. They thought that undiscovered planets were pulling Mercury and Uranus off-course and with Uranus they were correct. On 23 September 1846, Neptune was discovered at almost exactly the position calculated by Urbain Le Verrier of the Paris Observatory.

However, the movement of Mercury is due to an unexpected facet of gravity that Newton’s theory doesn’t take into account. Only when Einstein set about explaining the nature of gravity did he come across the remarkable reason for Mercury’s motion.

Einstein’s great insight was the concept of the ‘space-time continuum’. Imagine this as a fabric stretching through all space, in all directions, and including time as the fourth dimension. In Newton’s theory, both space and time were separate rigid frameworks, absolute and invariant. In general relativity, space and time constitute a flexible continuum that can be stretched and warped by the presence of matter and energy. This warping causes gravity.

A related concept central to general relativity is the ‘principle of equivalence’, which states that a gravitational field is indistinguishable from acceleration. Even in the 16th and 17th century, philosophers were beginning to recognize this. Galileo Galilei proved that the speed at which an object falls is independent of its mass. He rolled balls of different masses down inclined slides and noticed that they always took the same time to hit the floor. Apollo astronaut Dave Scott performed a beautiful demonstration on the Moon in 1971 when he dropped a hammer and a feather simultaneously. In the absence of air resistance, both hit the lunar soil at the same time. This proved that not only objects of different mass but also objects of different composition are accelerated equally by a gravitational field. Einstein extended this in 1907, declaring the utter equivalence of gravity and acceleration. He illustrated it in the famous ‘thought experiment’ involving a lift.

Imagine you are in a lift, totally enclosed with no windows. When the lift is stationary, at whatever storey, gravity pulls you down as you stand on the floor. Now if the cables were cut, the lift would fall and you would suddenly feel weightless. Any movement would float you away from the floor because you would be falling at the same speed as the lift. This is what astronauts feel in orbit.

Take the same lift into outer space, well away from any gravitating object: now you feel weightless because no gravitational forces are acting on you and you float through space at the same speed as the lift. This is entirely equivalent to the lift falling on Earth. Trapped inside, you would be unable to distinguish between the two cases.

Now imagine strapping a rocket motor to the base of the lift in space. The lift accelerates but your inertia does not want you to move, so you drop to the floor of the lift, which then pushes against you and this feels like the force of gravity. The faster the acceleration, the greater the force you feel. In fast-moving jets and rockets, it is even called the G-force.

When the gravitational force is weak or, in the language of relativity, when the curvature of space-time is shallow, Einstein’s predictions are identical to those of Newtonian gravity. However, when gravity becomes stronger and the curvature becomes more pronounced, general relativity predicts corrections to the way gravity acts on celestial objects. Unlike the other planets, Mercury is close enough to the massive Sun for the curvature of space to be an important factor and so general relativity was needed to correctly calculate its movement. It proved an early success for Einstein’s theory, but could general relativity predict something new?

Yes, it could. Einstein said that light would have to follow the contours of space-time, just like planets and moons. Calculations showed that the Sun is only object in the Solar System capable of bending starlight by a measurable amount. The only time astronomers can see stars close to the Sun is during a total eclipse when the Moon blocks the glare. In 1919, Arthur Eddington led an expedition to the African island of Principe and, in the sudden darkness, took photographs. He measured the positions of the stars captured by the plates and compared them to similar photographs taken when the Sun was nowhere near. He found that the stars had apparently moved from their expected positions, just as Einstein had said.

Including gravity, there are four fundamental forces of nature. The familiar force of electromagnetism is responsible for all the electrical and magnetic phenomena that hsave been well harnessed by science and technology since the late 19th century. Two other fundamental forces came to light in the early 20th century as physicists succeeded in probing the nucleus of the atom. These are known as the strong nuclear force and the weak nuclear force and only act within atomic nuclei.

Gravity is the weakest of the fundamental forces: watching an iron nail leap upwards to a handheld magnet proves the magnetic force generated by that magnet overwhelms the gravitational force created by the entire Earth. Nevertheless, it is the force of gravity that sculpts the Universe on its largest scales. This is because gravity acts over vast distances, whereas the ranges of other forces are limited.

Physicists can explain the force fields associated with electromagnetism and the two nuclear forces as an exchange of short-lived particles, called virtual particles. Gravity, on the other hand, can only be explained as a large-scale curvature of space. Many physicists hope that gravity will eventually be explained as an exchange of virtual particles called gravitons.

The leading candidate for a theory that can unify gravity with the other forces is known as ‘string theory’. It replaces subatomic particles with minuscule bits of wiggling ‘string’. String theory extends Einstein’s ideas about other dimensions by having the strings wiggle through higher dimensions of space-time. We see point-like particles because we cannot yet perceive these other dimensions. But string theory is not yet proven because it demands violations to Einstein’s predictions. So astronomers and physicists spend a lot of time looking for deviations, chinks in Einstein’s armour.

Take a neutron star the size of a small asteroid – just 10 to 20 kilometres in diameter – and containing several times the mass of the Sun. Radio astronomers call them ‘pulsars’ because, as they spin, they sweep beams of radio emission through space and over the Earth.

In 2003 two pulsars in orbit around each other were found, separated by just 800,000 kilometres. That is almost 90 times closer together than Mercury and the Sun, and speeding around each other in just 2.4 hours. Einstein said orbiting objects would lose some of their energy, radiating it away as a gravitational wave in the space-time continuum. By timing the contraction of the stars’ orbits around each other, astronomers have calculated that the amount of energy, the double pulsar is losing, equals the amount Einstein predicted, to the telescopes’ capability to measure.

The Moon also provides a remarkable gravitational laboratory. Astronomers at the Apache Point Observatory, New Mexico, fire a powerful laser beam targeting suitcase-sized reflectors left on the lunar surface by three of the Apollo missions and two Russian missions. Of the 300 million billion photons of light sent to the Moon, just five find their way back to the telescope. The rest miss the reflectors or are lost to our atmosphere. Using these returned photons, the astronomers have measured the Moon’s movement to an accuracy of a centimetre, and calculate that it is moving as Einstein said it should, to within one part in ten trillion.

Upgraded ground-based equipment can measure to an accuracy of just millimetres and this will place general relativity under even more stringent tests. Many physicists anxiously await for results. But for now, Einstein looks remarkably right; one might even say disappointingly right, because his unwavering success prevents progress to what many feel would be a deeper understanding of the cosmos.

End of podcast:

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