Einstein’s constant speed of light put to the test

Physicists controversial theory that challenges Albert Einstein's fundamental constant for the speed of light ready to to be tested. 

29th November 2016
Einstein’s constant speed of light put to the test (Getty)

Back in 1905, Albert Einstein unleashed his theory of relativity upon the world of physics. The theory is underpinned by the assumption that light travels at a constant speed, which he calculated to be a whopping one billion kilometres per hour. But could Einstein have been wrong?

A group of physicists are challenging Einstein by theorising that the speed of light is not a constant, and in fact travelled much faster just after the Big Bang. The theory put forward by Professor Magueijo of Imperial College and Dr Afshordi from the Perimeter Institute in Canada, is something they’ve had in the pipeline since the 1990s – only now, they think they are ready to put it to the test.

Speed test

All the stars and galaxies in the Universe that we recognise today formed through tiny fluctuations in density after the Big Bang. The spectral index is a measure of these fluctuations the map of the oldest light in the Universe, the cosmic microwave background, and it currently stands at 0.968. Magueijo and Afshordi’s more precise estimate, published in the journal Physical Review D, puts the figure at 0.96478, accurate enough to test their theory of a varied speed of light.

"The theory, which we first proposed in the late-1990s, has now reached a maturity point - it has produced a testable prediction,” says Professor Magueijo. “If observations in the near future do find this number to be accurate, it could lead to a modification of Einstein's theory of gravity.

On the horizon

A variable speed of light could be the answer to a conundrum known as the horizon problem, which has puzzled physicists for decades: how was the Universe created so uniformly after the Big Bang? Magueijo and Afshordi suggest that the uniformity of the Universe could only have come about if light reached every area of the Universe before it expanded out of reach, something that couldn’t have been achieved if light travelled at Einstein’s speed of light.

However, is their research really light years ahead of other theories?

Over thirty years ago, physicists, including Stephen Hawking, formulated a theory called cosmic inflation, which tries to explain this phenomena with the assumption that the speed of light is a constant. They theorised that the Universe underwent a phase of massive expansion straight after Big Bang known as the inflationary epoch. After this phase of inflation the Universe continues to expand to this day, but at a slower rate.

The assumption that light travels at a constant speed is essential to many important theories, so Magueijo and Afshordi’s idea threatens to shake the foundations of physics. However, many cosmologists have their doubts and aren’t quite ready to give up on the theory of inflation quite yet.

“The predictions of inflation developed by Stephen Hawking and others more than 30 years ago have been tested by cosmological observations and faced those tests remarkably well,” says Dr David Marsh, a theoretical physicist at Cambridge University, speaking to The Guardian. “Many scientists regard inflation as a simple and elegant explanation of the origin of galaxies in the universe.”


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