WIREDGOV

UK physicists have for the first time used an extremely powerful laser beam to slow down electrons travelling at near-light speeds – a quantum mechanical phenomenon thought to occur only around objects like black holes. By producing this effect in a lab, scientists hope to provide valuable insight into subatomic processes in the universe’s most extreme environments.

Fast electrons, especially when they travel at near light speeds, are very difficult to stop. Often you require highly dense materials – such as lead – to stop them or slow them down. But now, scientists have shown that they can slow these superfast electrons using a very thin sheet of light; they squeeze trillions of light particles – photons – into a sheet that is a fraction of human hair in thickness.

When light hits an object some of the light bounces back from the surface, often changing its colour (to even X-rays and gamma rays if the object is moving fast) – however, if the object is moving extremely fast and if the light is incredibly intense, strange things can happen.

Electrons, for example, can be shaken so violently that they actually slow down because they radiate so much energy. Quantum physics is required to fully explain this phenomenon. Physicists call this process ‘radiation reaction’, which is thought to occur around objects such as black holes and quasars.

Now, a team of researchers have demonstrated radiation reaction for the first time using the Gemini laser at the Science and Technology Facilities Council’s Central Laser Facility in Oxfordshire.

Gemini scientist Dr Dan Symes said: “Experiments like these are extremely complicated to set up and very difficult to perform. Essentially, you need to focus a laser beam as big as an A4 size paper sheet down to a few microns and hit it with a micron-sized electron bullet that’s travelling very close to the speed of light.”

Gemini Group Leader, Dr Rajeev Pattathil added: “You need two extremely well-synchronised high power laser beams for this: one to produce the high energy electron beam and another to shoot it. Gemini’s dual-beam capability makes it an ideal facility for these types of experiments. Gemini is one of the very few places in the world where such cutting-edge experiments can be performed.”

The research team, led by Imperial College academic Dr Stuart Mangles, were able to observe this radiation reaction by colliding a laser beam that is one quadrillion times brighter than light at the surface of the Sun with a high-energy beam of electrons. All this energy had to be delivered in a very short duration – just 40 femtoseconds long, or 40 quadrillionths of a second.

Senior author of the study Dr Mangles said: “We knew we had been successful in colliding the two beams when we detected very bright high energy gamma-ray radiation.

“The real result then came when we compared this detection with the energy in the electron beam after the collision. We found that these successful collisions had a lower than expected electron energy, which is clear evidence of radiation reaction.”

Study co-author Professor Alec Thomas, from Lancaster University and the University of Michigan, added: “One thing I always find so fascinating about this is that the electrons are stopped as effectively by this sheet of light, a fraction of a hair's breadth thick, as by something like a millimetre of lead. That is extraordinary.”

However more experiments at even higher intensity or with even higher energy electron beams will be needed to confirm if this is true. The team will be carrying out these experiments in the coming year.

Useful web links

Central Laser facility (CLF)

Imperial College London (ICL)