An imperceptible beat, a tiny yet revolutionary oscillation: al Caltech a very thin membrane vibrated under the pressure of a laser. It might seem like a physics experiment like many others, but behind this dance of light lies an extraordinary discovery. It is a step forward in the photonic propulsion, the principle that could allow space probes to reach other stars without fuel, driven only by the pressure of light.
What is photon propulsion and why is it important?
Photon propulsion uses the pressure exerted by light on a reflective surface. Even though photons have no mass, they transfer a tiny impulse when they hit an object. If you focus a stream of photons on an ultralight sail, it can accelerate into space without the need for fuel.
The idea is not new: solar sails, which use sunlight to propel themselves, have already been successfully tested, as in the case of the Light Sail 2, launched in 2019 by the Planetary Society. However, for interstellar travel, sunlight is not enough. This is where the high power laser, which could push the probes to speeds never achieved before.
The Caltech Experiment: A Trampoline-Sail of Light
The team of the California Institute of Technology tested a microscopic sail made of silicon nitride, an extremely light and resistant material. The study, published in Nature photonics (I link it here), describes a square device of just 40 micrometers on each side and 50 nanometers thick: less than a human hair.
The experiment had one key goal: accurately measure the force exerted by laser light on an ultralight sail. To do this, the researchers suspended the sail with microscopic springs and directed a laser beam at its surface. The observed effect was a vibration similar to that of a diving board. By analyzing these movements, the team was then able to calculate the thrust generated by the laser.
Why is it a step forward?
Until now, photon propulsion has been studied mainly in theory or with experiments on a larger scale, such as solar sails. This test instead:
- Demonstrates the feasibility of extremely thin and light sails for space travel;
- Offers direct measurement of laser thrust, useful for future practical applications;
- Confirms that materials such as silicon nitride can withstand laser radiation without deforming or deteriorating.
This research paves the way for the design of space probes very small mass, capable of reaching extraordinary speeds thanks to laser beams.
To the Stars: Breakthrough Starshot and the Dream of Alpha Centauri
The most ambitious project that focuses on photonic propulsion is Breakthrough Starshot, launched in 2016 by Stephen Hawking and billionaire Yuri Milner. The goal is to send microprobes towards Alpha Centauri, the closest star system to Earth, at approximately 4,37 light years away.
How? With a 100 gigawatt terrestrial laser, which would push a sail weighing a few grams up to the 20% of the speed of lightAt this speed, a microprobe would take just 20 years to reach Alpha Centauri, instead of the thousands of years needed with current technology.
The Caltech experiment is critical because it helps fill one of the project's major gaps: understanding how sails respond to laser radiation. Many challenges remain, including:
- Sail stability: It must maintain the course without deviations.
- Damage Resistance: The laser intensity must not destroy it.
- Laser accuracy: It must be directed over enormous distances without dispersion.
What Now? The Future of Photonic Propulsion
Next steps in the research include the development of advanced materials, capable of absorbing less heat and maintaining structural stability under laser thrust. Furthermore, it will be necessary to improve the techniques of sail auto-correction, so that it can remain aligned with the laser beam for long interstellar journeys.
If these obstacles are overcome, we could see the launch of the first probes within a few decades. outside the Solar System, turning interstellar exploration from dream to reality.
The Caltech experiment shows that the first step has been taken. Light could really become the fuel of the future for unlimited space travel.
