x Abu Dhabi, UAEThursday 18 January 2018

Why E may not equal MC squared

Every so often, nature reminds scientists that they're not as smart as they think.

Every so often, nature reminds scientists that they're not as smart as they think.

For astrophysicists, these reminders have an irksome habit of coinciding with celebrations of one of their most spectacular achievements: the discovery of cosmic rays.

Their latest lesson in humility has come as they mark the centenary of the identification of these ultra-fast particles that bombard our planet from deep space.

Forget the large Hadron collider (LHC); as the Austrian physicist Victor Hess discovered in 1912, our universe harbours natural accelerators reaching energies far beyond anything humans could wield.

Yet the whereabouts and nature of these awesome "machines" remains a mystery.

Hess initially suspected the sun, but demolished that idea by taking high-altitude balloon flights at night, which revealed that the rays still arrived even in the dark.

He faced incredulity over his cosmic claims but by the mid 1930s his findings had been confirmed by others and won him a Nobel Prize. Astrophysicists then felt compelled to explain how cosmic rays acquired such astonishing amounts of energy.

By the time preparations were being made for the 50th anniversary of Hess's discovery, theorists had fleshed out a few ideas.

Analysis of the paths of cosmic rays suggested they came from within our own galaxy, perhaps from exploding stars.

Then in February 1962, just weeks before the anniversary, nature fired a shot across the bow of science.

Instruments set up in the New Mexico desert detected a single subatomic particle smashing into the Earth with an energy 10 billion times higher than any man-made accelerator could then achieve (and 10 million times what the LHC can reach, even now).

Suddenly, the theories claiming to explain cosmic rays were found to be wanting. Now an explanation was needed for how nature could accelerate particles to even greater energies than anyone thought possible.

Again, astrophysicists set to work, now believing they would be marking the centenary of Hess's discovery with some final resolution of the mystery.

A big clue was handed to them by, oddly enough, the US Pentagon, which had accidentally uncovered an excellent candidate for the powerhouse that spawns cosmic rays.

Spy satellites designed to detect nuclear tests by the Soviets had picked up bursts of gamma radiation coming from deep space.

Astrophysicists realised these incredibly violent events - equal to our sun releasing its lifetime energy output in a few seconds - were probably due to the collapse of gigantic stars into black holes, or the collision between the remnants of such stars.

Calculations suggested the resulting gamma-ray bursts (GRBs) could also give cosmic rays their energy. Theorists even proposed a way of confirming the link, by looking for ghostly particles called neutrinos streaming out of GRBs, as these should be produced along with the cosmic rays.

But last month, right on schedule, nature yet again rained on the astrophysicists' parade. Just in time for the centenary of Hess's discovery, the science journal Nature reported new results blowing a hole in the GRB theory.

Researchers at the IceCube neutrino observatory in Antarctica looked for bursts of neutrinos coinciding with the 300 GRBs spotted by satellites between May 2008 and April 2010 - and failed to find a single one.

Naturally they're trying to put a brave face on things.

Maybe their theories just need tweaking to weaken the link between cosmic rays and neutrinos. Or maybe the GRB theory is just plain wrong - in which case, it's time to look for an even more violent cosmic event.

Short of the Big Bang, the most powerful objects in the universe are the huge black holes lurking at the centre of most galaxies, including our own.

Their titanic gravity tears apart whole stars, forming a searingly hot disc of swirling debris. Could these be the origin of cosmic rays?

A clue emerged in November 2008, when researchers at the Pierre Auger Observatory in Argentina announced the detection of ultra-high-energy cosmic rays streaming out of the erupting hearts of nearby galaxies. So now astrophysicists are focusing their attention on these.

If they feel victimised by nature, they shouldn't: they are not alone in being given unwelcome lessons in humility.

Cosmologists are marking the 20th anniversary of one of their biggest advances in centuries: the first "map" of the heat left over from the Big Bang 14 billion years ago.

It was compiled using a pioneering Nasa satellite called Cobe, whose designers, like Hess, won a Nobel Prize. Yet the measurements sent back by Cobe and its successors have raised far more questions than they answer.

Notoriously, they have revealed that just 4 per cent of the universe is made from stuff we understand, such as gas and dust. The other 96 per cent is made from dark matter, made from particles that have never been detected, and dark energy, a kind of anti-gravitational force that no one understands.

Maybe astrophysicists and cosmologists will have resolved their cosmic problems by 2062. But there are intriguing hints that their fates may be intertwined - and that they may help each other create the next scientific revolution.

That's because cosmic rays interact with the heat left over from the Big Bang, hugely reducing the number of rays travelling at ultra-high energies.

During the 1990s, cosmic ray observatories in Japan and the US reported finding far more high-energy cosmic rays than expected.

And the simplest explanation is that there is something wrong with our understanding of mass, velocity and energy - in other words, with Einstein's theory of relativity.

The initial hints of this revolution have since fallen out of favour. But it's just possible that long before 2062 scientists will have something else to celebrate: the birth of a successor to Einstein's brainchild.

Robert Matthews is visiting reader in science at Aston University, Birmingham, England