Over a century ago, Albert Einstein found a paradox during a thought experiment involving mirrors moving at the speed of light. Now, a new set of calculations may have unravelled it.
鈥淚f you search the internet for 鈥榳hat would you see in a mirror moving at the speed of light?鈥� you will find intense discussions on physics-related forums. Curiously enough, none of the proposed answers are correct,鈥� says at the ELI Beamlines Facility in the Czech Republic.
Bulanov says that Einstein鈥檚 calculations showed that light reflected from a mirror moving at the speed of light would have infinite intensity and amplitude. But Einstein鈥檚 special theory of relativity 鈥� the study of objects moving at the speed of light 鈥� suggested a different outcome: that any object moving at such high speed must turn transparent and reflect no light at all.
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Bulanov and his colleague at the National Institutes for Quantum and Radiological Science and Technology in Japan think they have now resolved this paradox. They say the problem is that Einstein鈥檚 idea of a mirror was unrealistically perfect.
It would take an infinite amount of energy to make a typical glass mirror move at the speed of light, so the researchers considered a more feasible alternative: mirrors made from plasmas, or mixtures of ions and electrons. Such mirrors have already been produced in experiments using incredibly powerful lasers. When these lasers are directed at certain materials, they create plasmas containing localised disturbances that simultaneously reflect light like a mirror and move through the plasma extremely quickly, at speeds approaching the speed of light.
Crucially, these real mirrors differ from the idealised two-dimensional mirrors Einstein considered because they are three-dimensional 鈥� meaning there is plasma behind the mirror鈥檚 reflective surface as well.
Bulanov and Esirkepov found that the properties of the 3D plasma mirror as a whole 鈥� and the properties of the light striking it 鈥� can vary. In some scenarios, this allows the light to be intensely reflected as Einstein predicted. But in others, the light is transmitted, and in yet other scenarios the light can form a wave whose peaks and valleys don鈥檛 actually travel, oscillating near the mirror without going anywhere.
at Queen鈥檚 University Belfast in the UK says the new work is theoretical, but that it could be tested because of a recent boom in the number of facilities that use extremely strong lasers at the petawatt scale.
Those tests could be valuable, says Dromey, because if plasma mirrors really can be made to reflect very intense light, they could lead to new imaging techniques, akin to more powerful X-rays. The intense light could also advance our understanding of fundamental physics by breaking seemingly empty space into the particle-antiparticle pairs that are thought to make it up, he says.
The assumptions that Bulanov and Esirkepov聽made about the 3D shape of the plasma mirror may still be a simplification, says at the Polytechnic Institute of Paris, but he finds the work valuable as there are few mathematically exact studies of the situation.
Bulanov and Esirkepov are now planning an experimental test of their ideas at a high-powered laser facility in the Czech Republic.
Journal reference:
Physical Review E,
