“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," said professor João Magueijo at the Imperial College London. "If true, it would mean that the laws of nature were not always the same as they are today.”
This past November, 2016 scientists behind a theory that concludes that the speed of light is variable - and not constant as Einstein suggested - have made a prediction that could be tested. Einstein observed that the speed of light remains the same in any situation, and this meant that space and time could be different in different situations.
The assumption that the speed of light is constant, and always has been, underpins many theories in physics, such as Einstein’s theory of general relativity. In particular, it plays a role in models of what happened in the very early universe, seconds after the Big Bang.
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.
But some researchers have suggested that the speed of light could have been much higher in this early universe. Now, one of this theory’s originators, Magueijo from Imperial College London, working with Dr Niayesh Afshordi at the Perimeter Institute in Canada, has made a prediction that could be used to test the theory’s validity.
Structures in the universe, for example galaxies, all formed from fluctuations in the early universe – tiny differences in density from one region to another. A record of these early fluctuations is imprinted on the cosmic microwave background – a map of the oldest light in the universe – in the form of a ‘spectral index’.
Working with their theory that the fluctuations were influenced by a varying speed of light in the early universe, Professor Magueijo and Dr Afshordi have now used a model to put an exact figure on the spectral index. The predicted figure and the model it is based on are published in the journal Physical Review D.
Cosmologists are currently getting ever more precise readings of this figure, so that prediction could soon be tested – either confirming or ruling out the team’s model of the early universe. Their figure is a very precise 0.96478. This is close to the current estimate of readings of the cosmic microwave background, which puts it around 0.968, with some margin of error.
Professor Magueijo said: “The theory, which we first proposed in the late-1990s, has now reached a maturity point – it has produced a testable prediction. 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.
The testability of the varying speed of light theory sets it apart from the more mainstream rival theory: inflation. Inflation says that the early universe went through an extremely rapid expansion phase, much faster than the current rate of expansion of the universe.
These theories are necessary to overcome what physicists call the ‘horizon problem’. The universe as we see it today appears to be everywhere broadly the same, for example it has a relatively homogenous density.
This could only be true if all regions of the universe were able to influence each other. However, if the speed of light has always been the same, then not enough time has passed for light to have travelled to the edge of the universe, and ‘even out’ the energy.
As an analogy, to heat up a room evenly, the warm air from radiators at either end has to travel across the room and mix fully. The problem for the universe is that the ‘room’ – the observed size of the universe – appears to be too large for this to have happened in the time since it was formed.
The varying speed of light theory suggests that the speed of light was much higher in the early universe, allowing the distant edges to be connected as the universe expanded. The speed of light would have then dropped in a predictable way as the density of the universe changed. This variability led the team to the prediction published today.
The alternative theory is inflation, which attempts to solve this problem by saying that the very early universe evened out while incredibly small, and then suddenly expanded, with the uniformity already imprinted on it. While this means the speed of light and the other laws of physics as we know them are preserved, it requires the invention of an ‘inflaton field’ – a set of conditions that only existed at the time.
Flash back to 2012, the CERN's OPERA team had timed neutrinos fired through Earth from the European particle physics laboratory, CERN, near Geneva, Switzerland, and found that they made the 730-kilometer trip to Gran Sasso 60 nanosecond faster than they would traveling at light speed.
Subsequently, five different teams of physicists independently verified that elusive subatomic particles called neutrinos do not travel faster than light. New results contradict those announced in September 2012 by a 170-member crew working with the OPERA particle detector in Italy's subterranean Gran Sasso National Laboratory. The OPERA team made headlines after they suggested neutrinos traveled 0.002% faster than light, thus violating Einstein's theory of special relativity. The OPERA results were debunked months ago, however. So instead of the nail in the coffin of faster-than-light neutrinos, the new suite of results is more like the sod planted atop their grave.
The OPERA team had timed neutrinos fired through Earth from the European particle physics laboratory, CERN, near Geneva, Switzerland, and found that they made the 730-kilometer trip to Gran Sasso 60 nanosecond faster than they would traveling at light speed.
But in February, the OPERA team also discovered that a loose fiber optic cable had introduced a delay in their timing system that explained the effect. A month later, researchers working with the ICARUS particle detector, also housed in Gran Sasso, measured the speed of neutrinos fired from CERN and found that they travel at light speed, as predicted. By that point, most physicists deemed faster-than-light neutrinos really most sincerely dead. Some OPERA team members thought the whole episode had besmirched the collaboration's reputation, and in March, two of the team's elected leaders lost a vote of no confidence and tendered their resignations.
Nevertheless, researchers kept at their efforts to test the result. Gran Sasso houses four particle detectors capable of timing neutrinos fired from CERN: OPERA, ICARUS, BOREXINO, and LVD. All four found that the neutrino's speed is consistent with the speed of light, as Sergio Bertolucci, research director at CERN, reported at the 25th International Conference on Neutrino Physics and Astrophysics in Kyoto, Japan.
The speed of neutrinos was also measured by researchers working with the MINOS experiment, which shoots neutrinos 735 kilometers from Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, to a detector in the Soudan mine in northern Minnesota. The MINOS team has also found that neutrinos travel at light speed, as Fermilab's Phil Adamson reported at the meeting.
So the chorus has sung and the final curtain has fallen on the faster-than-light neutrino saga. "The story captured the public imagination, and has given people the opportunity to see the scientific method in action—an unexpected result was put up for scrutiny, thoroughly investigated and resolved in part thanks to collaboration between normally competing experiments," Bertolucci says in a CERN press release. "That's how science moves forward."
Image credit: Top of page, black hole jet ESA XMM Newton