Thursday, 29 September 2011

Breaking the speed of light

This article was written for the Hamilton Astronomical Society monthly bulletin, October 2011.

Unless you live in a cave on Titan you’ll already be aware of the biggest news in science this month - the apparent violation of Einstein’s speed of light limit. As is so often the case with breaking science news, the media coverage is variable in its quality and accuracy. Here’s a quick, simplified overview of the story...

What exactly is the claim?

Scientists working on an experiment in Italy have noticed that sub-atomic particles called neutrinos appear to travel faster than the speed of light. According to Einstein’s special theory of relativity, this is impossible.

The experiment is called Oscillation Project with Emulsion-tRacking Apparatus (OPERA). It involved two physics laboratories separated by 730 km: The CERN laboratory in Switzerland and the LNGS laboratory in Italy.

The CERN lab generated neutrinos which travelled through the Earth’s crust to the LNGS lab (there is no physical tunnel, the particles travelled straight through the Earth). Very precise measurements were made of the distance and time taken, and the results showed the neutrinos arriving 60 nanoseconds earlier than they should if they were travelling at the speed of light.

It’s very important to note that the researchers are not claiming to have “broken the speed limit of light” or “disproved Einstein’s theories”. Despite many headlines in mainstream media, the researchers are not claiming proof of anything, they are opening up their research in the hope that other scientists will be able to help solve the mystery.

What are neutrinos?

Fundamental particles, electrically neutral, with negligible mass. They rarely interact with "normal" matter, so they pass through most things without any effect and are therefore difficult to detect. They are produced in nuclear reactions such as the fusion inside our Sun. They are very common - billions are passing through your body right now.

Why do we think the speed of light can’t be broken?

In Einstein’s famous and spectacularly successful equation E=mc2, "c" is the speed of light in a vacuum and it’s a constant. It never changes. Any object with mass is constrained to this limit.

It’s not just that we’ve never been able to find anything that breaks this speed limit. According to the well-tested maths as well as physical experiments, this limit is an inescapable property of the universe. For example, as you approach the speed of the light the energy required to increase your velocity increases dramatically and would become infinite at the speed of light.

Has anything like this happened before?

Yes, superluminal (faster than light) speed has been reported in some previous experiments. For example, a 2007 experiment in Minnesota USA called the Main Injector Neutrino Oscillation Search (MINOS) noticed the same effect but their measurements weren’t accurate enough to give any confidence.

The OPERA experiment is the first time such a high degree of confidence has been reported.

On the other hand, many more previous experiments and observations show neutrinos behaving exactly as Einstein’s laws dictate. For example, when supernova 1987a was observed, neutrinos created in the blast arrived at Earth as expected - within hours of the visible light. If they had travelled at the superluminal speeds claimed by OPERA they would have arrived several years earlier.

How reliable are the observations?

The effect has been measured over 15,000 times. The margin of error in distance measurement is 20 cm. The margin of error in time measurement is 10 nanoseconds. The researchers give the result a statistical significance of six-sigma, which basically means it’s a statistical certainty.

How significant is this claim?

It’s huge. The fact that you own a computer is down to our understanding of Einstein’s theories. A century of scientific research has held up the idea that superluminal speed is impossible for objects with mass.

Claiming to have broken the superluminal speed limit is almost like claiming to have broken the law of gravity. Imagine if someone claimed to have seen objects falling up instead of down - would you believe them or would you be looking for some other explanation?

You can see why other scientists are skeptical. It would be a brave person to accept the new results without a lot more investigation.

What are the possible explanations?

There could be some form of systematic error in the OPERA experiment, for example, the distance may not have been measured correctly. Although no-one has yet been able to spot any such errors, this is still considered the most likely explanation.

Einstein could be wrong, or at least not completely right. Perhaps his theories need tweaking. This is widely considered to be unlikely.

In some theoretical models such as string theory, the universe consists of more dimensions than the four we’re familiar with (3 space + 1 time). Neutrinos could be slipping in and out of these extra dimensions, creating shortcuts in space-time. In this scenario the speed of light would be maintained but the distance covered would be reduced, giving the illusion of superluminal speed.

There are many other possibilities being discussed with varying levels of support, from the speed of light needing another correction to photons having mass.

What would the implications be if it’s true?

Sadly superluminal neutrinos will not lead directly to spaceships with warp drive. It’s hard to imagine what the real implications would be, but we do know that it would rock the scientific world and we’d need to completely re-examine our understanding of the laws of physics. Now what?

Other scientists need to examine the paper and look for possible errors. At the same time other laboratories will attempt to replicate the result. This could take up to two years. We’ll all need some patience.

Read the paper at static.arxiv.org/pdf/1109.4897.pdf

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