Laser-powered nanocraft may test gravity near a black hole

A bold new proposal is straightforward in concept but daunting in execution. The plan is to launch a gram-scale spacecraft, push it with lasers to a hefty fraction of light speed, and aim it at the nearest black hole to run clean physics tests in extreme gravity.

The idea sets a timeline future generations can actually see through: flight on the order of 60 to 75 years. A data return would bring the full mission to roughly 80 to 100 years, assuming the target is a black hole about 20 to 25 light-years away.

Building the gravity mission

Cosimo Bambi of Fudan University in China argues that this is not a fantasy project, but a hard, testable one mapped to specific technologies and targets. Big claims in physics often look out of reach – until they are not.

“Certainly, we do not have the necessary technology today, but it may be available in the next 20 to 30 years,” Bambi said. 

Bambi noted that the first detection of gravitational waves came a full century after Einstein’s prediction. This illustrates how ideas once thought impossible can eventually be achieved.

The same field now boasts images of black hole shadows, beginning with M87 in 2019, which validated predictions of how light behaves near intense gravity.

Hunting nearby black holes

Astronomers have already found a quiet black hole in our neighborhood, known as Gaia BH1, about 1,560 light-years away. They discovered it by tracking the motion of its Sun-like companion. That is still far for a century scale mission, but it proves the detection path works.

Most stellar black holes do not shine, and many are likely solitary, so they hide in plain sight.

The first clear-cut detection of an isolated black hole came through microlensing, in which the object’s gravity bends the light of a background star. Hubble astrometry then nailed down its mass.

New strategies suggest some isolated black holes in the Local Interstellar Cloud could be seen through faint accretion signatures with facilities like ALMA, SKA, and JWST – if we look in the right conditions.

A complementary proposal argues that transient electromagnetic outbursts crossing a black hole’s gravity well can excite spacetime. This can produce a gravitational wave signal detectable in certain cases.

Powering a tiny traveler

The hardware concept builds on light-sail propulsion. Photons push a reflective sheet attached to a tiny payload, yielding acceleration without onboard propellant.

Bambi’s sketch centers on a nanocraft weighing about a gram, paired with a meter-scale sail and driven to roughly one-third light speed by Earth-based lasers.

The laser system scales to the hundred-gigawatt class in several studies. Researchers describe this regime as technically daunting, yet feasible to phase and point with modern photonics.

Probing gravity’s extremes

Once at the target, the mission would split into two small craft and conduct three classes of tests.

The first test would measure whether spacetime around the black hole follows the Kerr metric – the solution of general relativity for rotating black holes.

The team would compare a stable signal near the hole with the reception and timing recorded by another craft farther away. The payoff is precise, cycle-counted frequency tracking that builds sensitivity over many orbits.

Additional tests near the black hole

The second test would check for an event horizon. If a probe fell inward and beamed a steady tone, general relativity predicts the received signal would redshift progressively.

It would dim and shift to lower frequencies, never showing the probe crossing the horizon within the receiver’s finite time. A sharp cutoff or a different evolution would hint at a horizonless alternative.

The third test would look for changes in the fine-structure constant under strong gravity by comparing two atomic transitions with different sensitivities to that constant.

White dwarf studies have already used stellar spectra to set limits on such effects in high-gravity fields, providing a baseline method for the black hole experiment.

Obstacles on the path

Localizing the target to arcminute-scale accuracy before launch is essential, since even a slight pointing error could mean a miss after decades of flight. This requires multi-instrument campaigns to refine positions and distances for any candidate isolated black hole, not just its discovery.

A phased laser array would also push engineering to the edge, from coherent beam combining to atmospheric compensation.

Thermal management would be required at power levels that are high yet still within the limits of realistic materials and control schemes.

Early analyses outline no fundamental physical showstopper at the system level for the power and phasing tasks described above, though the integration challenge remains substantial.

The study is published in the journal iScience.

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