For the first time, physicists have slowed high-speed electrons using a ‘sheet of light’

Physics


Electrons boosted to near light-speed velocities have been shaken to a virtual crawl after being made to collide with what one physicist describes as ‘a sheet of light’.

Sure, that sheet is an ultra-intense laser briefly brighter than a quadrillion suns … but that just makes it even cooler.

 

Not to mention the fact these kinds of physics are what we might expect on the fringes of a black hole, opening the way to new models for studying quasars and other astronomical phenomena.

An experiment led by researchers from Imperial College London has pushed ordinary physics to its limits, measuring for the first time a process called the radiation reaction.

In simple terms, this describes the force on a charged particle as it jerks in response to emitting a photon of light.

Old-school ‘classical’ models are usually enough to explain what’s going on with the electrons moving inside an electromagnetic field. For most purposes the force of the radiation reaction can be ignored – it’s too small to matter much in these models.

But boost an electron up to super-high speeds and its radiation reaction can no longer be ignored. Quantum physics needs to take over the mathematics under these extreme circumstances.

That’s all great on paper, but until recently physicists haven’t been able to actually observe this force in action.

Now, that’s been changed for the first time – thanks to advances in laser technology.

 

“Testing our theoretical predictions is of central importance for us at Chalmers, especially in new regimes where there is much to learn,” says researcher Mattias Marklund of Chalmers University of Technology, Sweden.

“Paired with theory, these experiments are a foundation for high-intensity laser research in the quantum domain.”

The experiment made use of the Gemini laser at the Science and Technology Facilities Council’s Central Laser Facility in the UK; a device capable of delivering an ultra-intense beam of light in a matter of femtoseconds.

On the other side of this awesome collision was a beam of electrons pushed to high speed using laser-pulses in what’s known as laser wakefield acceleration.

When a ridiculously intense beam of photons meets electrons kicked up to speeds approaching that of light, this whole radiation reaction becomes a serious force.

Or as physicist Alec Thomas from Lancaster University and the University of Michigan put it, “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.”

 

This change in speed becomes apparent in the light the particles emit, which is boosted into the gamma range of wavelengths.

“The real result then came when we compared this detection with the energy in the electron beam after the collision,” says senior author Stuart Mangles from Imperial College London.

“We found that these successful collisions had a lower than expected electron energy, which is clear evidence of radiation reaction.”

Getting a good match between theory and experiment is a useful thing if we’re to understand how charged particles interact with light under some of the more extreme conditions in our Universe.

As matter is whipped into a frenzy on the fringes of quasars – discs of dust and gas surrounding black holes – it’s likely to experience forces such as these.

For now, while the results are solid evidence of radiation reaction, more work needs to be done to further refine the details.

Not that we’re complaining. Bring on the bigger lasers.

This research was published in Physical Review X.

 



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