Einstein's still right

Einstein may be proven right once again. So may be his famous theory of relativity. It’s not as if the celebrated physicist and his theory are wrong. But there is a section of scientists and other eminent researchers including Nobel laureate Sheldon Glashow which is sceptical about the century-old theory working in high-energy conditions. On that count too, the father of 20th century physics may be proven right, thanks to the efforts of a bunch of Indian scientists.

“The house of science is a very big one and there are many fundamental questions, even in the field of physics. Some of these questions have been with mankind for decades, perhaps even close to a century sometimes. The origin of cosmic rays is one such problem,” said Alak Ray, a physicist from Tata Institute of Fundamental Research, Mumbai, who is part of the team that provided an explanation to save the Einstein theory.

Generally, cosmic rays are high-energy particles, mainly protons and alpha particles, which come from outer space and hit the earth’s atmosphere producing a shower of other particles. But a “long-standing puzzle” for astrophysicists is to explain the origin of ultra high-energy cosmic rays (UHECR), which carries an enormous amount of energy.

The current record holder is a 1994 event detected by the Fly’s Eye in Utah in which cosmic particles with an astounding energy level of 300 EeV hit the earth. An EeV is exa-electron-volt (1 followed by 18 zeros times an eV) which is used to measure extremely high-energy particles. A similar event was later detected by the Japanese scintillation array AGASA.

Many of these cosmic particles are believed to have picked up their energy by interacting with shock waves in the interstellar medium. But the highest-energy ones remain mysterious as nobody knows how they could have acquired such high energies.

The universe, at the moment, is a tame place with the average temperature just three degrees above zero (absolute zero). The average temperature of deep space has gone down to less than -270 degrees celsius, which corresponds to an energy of about 0.0002 electron Volt.

But for fractions of a second after the Big Bang, the universe was hot enough to produce UHECR. There are also explosions in the far-away universe known as GRBs (Gamma Ray Bursts) which can achieve this feat. But they are unable to send ultra high energy particles to the earth because the high-energy particles produced in the Big Bang interacted with the cosmic microwave background radiation (CMBR) – relics of the Big Bang – over hundred million light years or so in the inter-galactic space to lose their energy. The photons in CMBR rips apart the energetic particles.

Origin of cosmic rays

The question before the scientists, therefore, was from where then do UHECR originate? The only logical solution was that the sources must be close in astronomical standards. But in the absence of a source in the nearby universe, physicists like Sydney Coleman and Nobel laureate Sheldon Glashow went on to the extent of suggesting that these particles may be coming from far away sources as Lorentz covariance – a principle in physics, considered as the pillar of relativity – may break down at these very high energies, which is possible only if you consider a distant source for these ultra-high-energy particles.

However, other researchers had a different opinion. They thought of looking harder at the sky rather than discarding the trusted theory of relativity so easily. The first success has come recently when a group comprising young researchers from Tata Institute of Fundamental Research in Mumbai, Harvard University and Royal Military College in Canada reported a new source of UHECR.

“Yes we are proposing a new source of UHECRs. Nobody has suggested this before, simply because this study has been enabled due to our 2010 discovery of the first relativistic supernovae without a GRB,” said Sayan Chakraborti, a doctoral student at TIFR and first author of the study. The findings have been reported in a recent issue of Nature Communications.

A relativistic supernova is different from a regular supernova, which is a gigantic stellar explosion.

“In a supernova explosion, the outer material from which the star is made up of is ejected with a speed of 10,000 to 50,000 km per second. But in a relativistic supernova, the ejected material is moving with speeds, which are at least 10 per cent of speed of light (30,000 km per second) or higher,” explains Poonam Chandra, one of the co-authors from the Royal Military College. The Harvard members in the team are Alicia Soderberg and Abraham Loeb.

“In this work, we have shown that these relativistic supernovae, hiding in our own neighbourhood, might be throwing these particles at us,” said Chakraborti.

Scouring the skies

The researchers observed the sky with two of the world’s most powerful radio telescopes – giant meter wave radio telescope (GMRT) near Pune and Very Large Array in the USA – to find out the first relativistic supernova that does not have a GRB last year. Scientists say if relativistic supernovae are established as the source of ultra-high-energy cosmic particles in subsequent studies, it will open up the window for new physics as almost all physical laws can be tested at extremely high-energy level which is not possible to achieve on  earth, even using the most advanced accelerators.

The mightiest earthly accelerators can reach 7 TeV, whereas these supernovae can accelerate particles to greater than 166 EeV (an EeV is 1 followed by 18 zeros, times an eV). This gives 23 million times more energy. “Hence they can probe physics like no earthly accelerator can to test everything that the physicists hold dear,” Chakraborti said.

“The theory of relativity is one of the best tested theories. But it has been tested in a limited range of energies. Outside this range, it may or may not work. That is why Coleman and Glashow wrote a paper doubting whether it holds at these very high energies.

They did so because no sources of UHECRs were found in the nearby universe. If they had to come from further away, something in physics had to be wrong. Our work on the other hand, shows that there are possible sources hiding in the nearby universe, so it’s not yet time to doubt our theories,” summed up Chakraborti.