Quantum theory has predicted many phenomena that are difficult, if not impossible, to observe in practice. One particularly tricky example is the Unruh effect, which would take longer than the age of the universe to reveal itself in straightforward experiments. However, a team of physicists have argued it is theoretically possible to shorten this process to a few hours. They’re now working on ways to actually carry the idea out, hopefully catching a thermal glow that will confirm one part of our understanding of the basic laws of the universe.
The Unruh (or Fulling-Davies-Unruh) effect is thought to cause accelerating objects to be bathed in a “thermal bath” of electromagnetic radiation. If some immense power allowed a spacecraft to rapidly approach light speed, passengers not squashed by the extreme g forces would witness a warm glow around them. As envisaged, it’s a counterpart to Hawking radiation produced by black holes, and observing either would help confirm the other. The problem for experimentalists is the amount of radiation produced under most circumstances is so low as to be effectively undetectable.
However, in Physical Review Letters physicists note you can stimulate the Unruh effect by accelerating your object in the presence of electromagnetic radiation. Although this light would normally induce other effects that would once again make the Unruh radiation undetectable, they claim to have found ways around this.
One of the mind-bending consequences of quantum theory is that there are no true vacuums – pairs of subatomic virtual particles are constantly fluctuating into existence before almost immediately annihilating each other. Unruh’s theory postulates objects with mass amplify these quantum fluctuations when accelerating, warming themselves and creating a thermal glow that others should be able to see.
Most acceleration simply isn’t large enough to produce anything noticeable, however, and even when we apply all the power we can muster in a particle accelerator we’re unlikely to witness anything. However, every photon of light passing through a vacuum increases the density of quantum fluctuations, making it more likely an accelerated particle will experience a noticeable Unruh effect.
However, an atom can also absorb the light used to stimulate Unruh radiation, raising its energy level enough to overwhelm something so subtle. This is just one of three “resonant effects” light can have on an atom. Observing the effect becomes a little like trying to spot a planet by the reflected light of its star. Extra starlight makes the planet brighter, but also makes it harder to see in the star’s glare.
Just as astronomers mask stars to let us see their planets, University of Waterloo PhD student Barbara Soda argues it is possible to make the atom invisible to the light so it cannot absorb any of the photons. This would prevent the absorption from obscuring our view of the Unruh radiation. Soda and co-authors call this acceleration-induced transparency.
Provided the accelerating object’s path through a field of photons is right, the authors conclude we can get the Unruh effect without the absorption. “We show that by engineering the trajectory of the particle, we can essentially turn off [the resonant] effects,” Soda said in a statement.
Co-author Dr Vivishek Sudhir of MIT is working on designing a practical experiment to implement the idea by firing electrons at close to the speed of light through a microwave laser at the appropriate angle.
“Now we have this mechanism that seems to statistically amplify this effect via stimulation,” Sudhir said. “Given the 40-year history of this problem, we’ve now in theory fixed the biggest bottleneck.”
Unexpected acceleration of certain spacecraft as they flew by Earth has been attributed to the Unruh effect, but competing explanations exist. If the Unruh effect actually is the cause it would reveal a real-world influence, one we might even be able to harness.