With UNESCO marking 2015 as the International Year of Light and Light-Based Technologies, will the centenary of Einstein’s presentation of General Relativity inspire advances of current physics and cosmology?
By: Ringo Bones
Back in November 1915, Albert Einstein presented to the world his “Theory of General Relativity” as a way to resolve another contradiction of physics not covered by his “Theory of Special Relativity” ten years before. According to Isaac Newton, gravity travelled instantly through the universe. But according to Einstein’s Theory of Special Relativity, nothing can go faster than light (but there’s an intriguingly convincing work by Thomas Van Flandern of the US Naval Observatory back in the mid 1970s proving otherwise that you can also check out). To overcome these incompatible views, Einstein introduced another, even grander theory in which space and time are not empty but are instead like a fabric that can be curved and stretched. This new picture – in which gravity originates from the bending of sheets of space-time – revolutionized cosmology and gave us the most compelling theory of creation, the Big Bang.
Einstein’s Special Relativity was incomplete because it made no mention of acceleration or gravity. Einstein then made the next key observation: Motion under gravity and motion in an accelerated frame are indistinguishable. Since a light beam will bend in a rocket that is accelerating, a light beam must also bend under gravity.
To show this, Einstein introduced the concept of curved space. In this interpretation, planets move around the sun not because of a gravitational pull but because the sun has warped the space around it, and the curvature of space itself due to the sun pushes the planets. Gravity does not pull you into a chair; space pushes on you, creating the feeling of weight. Space-time has been replaced by a fabric that can stretch and bend.
General relativity can describe the extreme warping of space caused by the gravity of a massive dead star – a black hole. When we apply General Relativity to the universe as a whole, one solution naturally describes an expanding cosmos that originated in a fiery “Big Bang”.
One of the simplest demonstrations using everyday objects to explain Einstein’s General Relativity that even the youngest school-kids can grasp is the bowling ball, marble and bedsheet set-up. Put a bowling ball on a bedsheet and shoot a marble past it. The marble will move in a curved line. A Newtonian physicist would say that the bowling ball exerts a “force” that “pulls” on the marble, making it move in a curved line. A Relativist would say that the ball curves the bedsheet and that the bedsheet “pushes” against the marble. This “simple” demonstration of Einstein’s General Relativity on how gravity shapes the cosmic space-time also explains why the 1919 solar eclipse observation that shows the sun’s gravitational well curving the path of starlight and the advancing perihelion of the planet Mercury that Newtonian physics is at a loss to explain why.
Einstein’s General Relativity also shows that gravitational fields affect the flow of time – making them slow down which was only demonstrated unequivocably just recently – back in the mid 1990s - when atomic clocks were accurate enough to show the difference. Without correcting the effects of General Relativity, the Global Positioning System or GPS signals from the satellites to your receiving unit would have errors of several parts per billion – which is enough to make them useless.
Recently, one of the most grandiose experiments to test the limits of Einstein’s General Relativity was the hunt for gravitational waves. Physicists can’t yet put the entire universe on a lab bench, but experimental tests of Einstein’s theories can now be carried out with subatomic precision. Perhaps the most elusive phenomena predicted by General Relativity – but has yet to be observed – are gravitational waves. In theory, a cataclysmic event such as a spiraling merger of two black holes should produce wavelike ripples in space-time that could still be detectible by the time they reach planet Earth. Two Earth-based observatories, Advanced LIGO and Advanced VIRGO at the University of Pisa in Italy, will look for disturbances as small as a hundred-millionth the diameter of a hydrogen atom.