Reflecting Gravitons: The Graviton Laser and the Gertsenshtein Effect ✦
The feasibility of a laboratory graviton laser is explored. The main hurdle is the lack of reflectors for gravitons. A proposed solution leverages the Gertsenshtein effect: converting gravitons into photons in an external magnetic field. The setup involves turning gravitons into photons, reflecting them with standard mirrors, and converting them back into gravitons. An identical arrangement on the other side allows arbitrary extension of the effective path length through the amplifying medium. Various gravitating systems can serve as the active medium (three examples are given). This approach demonstrates for the first time the fundamental feasibility of a graviton laser under terrestrial conditions.
Context
Building a graviton laser faces a fundamental obstacle: gravitons—quanta of gravitational waves—barely reflect off any substance. For a regular laser, the radiation must bounce back and forth through the amplifying medium many times, impossible without mirrors. The proposed scheme skirts this limitation by converting gravitons into photons, which are easily reflected, and then back into gravitons. This opens a path to the first laboratory amplification of gravitational waves and direct exploration of the quantum nature of gravity.
Methods
The researchers applied the Gertsenshtein effect—predicted in 1962—the conversion of gravitons into photons in an external electromagnetic field. In the quantum field description, the interaction Lagrangian of quantized fields contains cross-terms that mix the graviton and photon states when they move at the same speed. Upon entering a region of constant magnetic field, the particle's wave function turns into a superposition of photon and graviton components, resulting in partial conversion. The outgoing photons can be bounced off an ordinary mirror and then sent back through a reverse converter, regenerating gravitons. As the amplifying medium, ultracold neutrons in a gravity field, ultralight dark matter orbiting black holes, or macroscopic quantum oscillators like the mirrors in LIGO could serve. The cross section for stimulated emission of a graviton turns out to be universal—on the order of the Planck area with a dimensionless form factor.
Results
The analysis showed that the conversion amplitude depends on the parameter αL/c, where α is proportional to the product of the gravitational constant, the magnetic field, the wave vector, and the square root of the number of gravitons and photons in the beam. Under earthly lab conditions (100 Tesla magnetic field, 1 meter long region, 500 nm wavelength), αL/c ≈ 10⁻⁴⁵—vanishingly small. However, for enormous graviton fluxes, such as from merging black holes (number of gravitons ~10⁷⁸), the suppression factor disappears. Moreover, in the extreme fields of magnetars (~10¹¹ Tesla), the effect is amplified by many orders of magnitude. This means that with a sufficiently powerful initial gravitational signal, nearly complete conversion of gravitons into photons and back can be achieved, enabling multi-pass amplification in a graviton laser.
Implications
Realizing a graviton laser would be a breakthrough in quantum gravity, akin to the invention of the maser by Charles Townes. Understanding the conversion mechanism deepens the connection between gravity and electromagnetism, rooted in the theories of Einstein and Maxwell. Laboratory amplification of gravitons would allow direct investigation of quantum properties of spacetime.
Future development
In the future, detailed calculations of losses in realistic setups and identification of optimal amplifying media are needed. Harnessing the ultra-strong fields of magnetars or building compact powerful magnets could be possible. Astrophysical sources like merging black holes could serve as natural 'seeds'. Accounting for quantum fluctuations and decoherence will also be necessary, potentially yielding new predictions in quantum gravity.
Impact
A successful graviton laser would revolutionize gravitational-wave astronomy and quantum field theory.
Next steps
First and foremost, an analysis of realistic photon losses in the mirror and magnetic field system, as well as quantum noise in the amplifying medium, is needed to determine the lasing threshold.
Key open problems
This work directly ties into the unsolved problem of quantizing gravity and the nature of spacetime at the microlevel. Laboratory amplification of gravitons would provide an experimental foundation for testing theories of quantum gravity and the role of gravitational waves in particle physics.
🎯 The Planck area (~10⁻⁷⁰ m²) is the surface area of a sphere with the Planck length as its radius. For comparison, if you blew an atom up to the size of Earth, the Planck length would be smaller than a millimeter. This minuscule scale sets the probability of graviton interactions, but astronomical numbers of particles can compensate for it.
🎬 The idea of controlling gravity has long excited sci-fi imaginations. For instance, in the 'Star Trek' series, graviton beams are used to tow objects, and in Greg Bear's novel 'Anvil of Stars', aliens create gravitational weapons. A lab-based graviton laser is a first step toward that dream.
Key numbers
- Planck area: 10⁻⁷⁰ m²
- number of gravitons in a black-hole merger: 10⁷⁸
- wavelength of gravitons (visible range): 500 nm (q ≈ 2×10⁻⁸ GeV)
- magnetic field of 100 Tesla: 2×10⁻¹⁴ GeV² in natural units
- length of magnetic region 1 meter: 5×10¹⁵ GeV⁻¹