Powerful, twisting lasers could let us create and detect gravitational waves, the ripples in space-time thus far only seen coming from cosmic objects. While the technology to do so is decades away, this could eventually let us communicate via gravitational waves, in the same way we use electromagnetic waves today.
Anything that has mass or energy creates ripples in time and space, known as gravitational waves, as it moves. These waves are almost all so weak that they are impossible to detect, with the exception of those from the most massive objects in the universe, like mergers of supermassive black holes.
To produce a gravitational wave that is potentially measurable on Earth, you would need an incredibly dense energy source, such as a powerful, concentrated beam of light. Though light is made of massless particles called photons, these have energy, so can also produce gravity.
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Now, at the Grenoble Alps University, France, and his colleagues have calculated exactly how a high-powered, twisted laser could create detectable gravitational waves, mapping out how they would move through space and how large the ripples would be from the laser light, which twists around its axis of travel.
“Imagine how much you could learn if you are able to generate controlled gravitational waves in a lab,” says Martineau. “You do not have to wait for signals that come from various distant astrophysical sources. If you have a test to perform on gravitational waves, in order to test different theories of gravity, you can [try it] on gravitational waves in the lab.”
He and his team ran simulations of the gravitational waves that would be produced for different high-powered lasers, many of which are used in nuclear fusion facilities, and the shape of gravitational waves that would be produced.
Unlike massive objects in space, such as rotating black holes, these laser-produced gravitational waves would generate complex patterns of waves in space-time, at frequencies a quadrillion-times higher than those that detectors like the Laser Interferometer Gravitational-Wave Observatory can pick up.
The next step will be a detection system, says Martineau, which could consist of either another high-powered laser nearby, which would wobble in the presence of gravitational waves, or a photon detector, taking advantage of a process called the inverse-Gertsenshtein effect, where gravitational waves are converted into light in the presence of a magnetic field.
The closer you could get to the laser beam, the easier it would be to detect the gravitational wave, which means the highest-powered beams might not necessarily be the best for detection. Martineau and his team found that a twisted laser beam would produce a stronger wave closer to the beam’s direction of travel and so potentially would be easier to measure.
Being able to manipulate and detect gravitational waves could also eventually lead to a new kind of communication system. Gravitational waves aren’t made weaker when they travel through matter, so a signal sent through Earth wouldn’t lose energy. However, sufficiently powerful lasers or detection systems are a long way from being able to generate strong enough signals.
While the detection mechanism still needs to be worked out, our best bet for making gravitational waves will probably be at high frequencies. “If you go to high frequencies, you can create gravitational waves which are a lot stronger than you would expect,” says at the Max Planck Institute for Gravitational Physics in Germany. “Of course, it’s all a bit science fiction.”
Gravitational communication systems are probably many decades off, but Steinhoff points out that the first electromagnetic waves made in a lab, by physicist Heinrich Hertz, were also only over small distances. “It would be really cool if this could be done, even if it’s just over a few centimetres,” says Steinhoff.
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