Was Einstein wrong about the speed of light?

Long believed to be one of the most fundamental constants in all of physics, the speed of light may not have always been the same, and now the scientist who initially raised that possibility in 1998 reports in a new study that he has developed a way to find out for sure.

As Quartz and New Scientist explain, the speed of light in a vacuum has a fixed numerical value of 299,792,458 meters per second, and based on Einstein’s theory of relativity, if an object could travel faster than light, it would break the fundamental laws of physics by being able to observe a stationary electromagnetic wave. Thus, he hypothesized the speed of light must be fixed, he hypothesized.

However, there’s a problem with this – specifically, a phenomenon called the horizon problem, in which cosmologists argue that the universe would have reached a uniform temperature before heat-carrying photons traveling the speed of light would have been able to spread throughout all parts of the universe.

One possible explanation for this is called inflation, a concept that suggests that the temperature of the universe became uniform before it underwent rapid expansion. However, the problem with this idea is that no one can explain how inflation began, or how and why it stopped. In his search for an alternative explanation for the horizon problem, Imperial College London researcher João Magueijo turned to the possibility that the speed of light might not be constant after all.

The problem with Magueijo’s hypothesis is that it could not be tested – until now, that is, as he and co-author Niayesh Afshordi of the Perimeter Institute for Theoretical Physics have published a new study Monday in the journal Physical Review D that argues that in the early universe, light and gravity moved at different speeds – and this time, they can put the hypothesis to the test.

Impact on the laws of physics could be ‘profound’

If light and gravity moved at different speeds – specifically, if photons moved faster than gravity – shortly after the Big Bang, it would have given them enough time to travel to all portions of the universe, allowing temperature equilibrium to be achieved far more quickly, the authors said.

As part of their new study, Magueijo and Afshordi developed a model that allowed them to come up with an exact figure on the spectral index that could be tested by other physicists. Their figure of 0.96478 is close to the most recent estimates of readings of the cosmic microwave background (CMB), which is radiation left over from the early universe, and is within the margin of error.

In short, the study authors’ figure is precise enough that it could soon be tested, either proving or disproving Einstein’s theory that the speed of light is constant. The implications of this, said New Scientist, could be “profound,” potentially providing physicists with a theory that would unite the discrepancies between how the universe behaves at its highest energies and at its smallest scales.

“The theory, which we first proposed in the late-1990s, has now reached a maturity point – it has produced a testable prediction,” Professor Magueijo explained in a statement. “The idea that the speed of light could be variable was radical when first proposed, but with a numerical prediction, it becomes something physicists can actually test.”

“If true, it would mean that the laws of nature were not always the same as they are today… 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,” he added. And if it turns out not to be true? “That would be great – I won’t have to think about these theories again,” Magueijo told New Scientist. “This whole class of theories in which the speed of light varies with respect to the speed of gravity will be ruled out.”

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