The idea of interstellar travel via spacecraft propelled by ultrathin sails may seem like a futuristic concept lifted straight from the pages of science fiction novels. Yet, this idea has been the subject of serious scientific exploration since 2016, when the Breakthrough Starshot Initiative was launched. The initiative, spearheaded by theoretical physicist Stephen Hawking and venture capitalist Yuri Milner, is aimed at developing a method to send tiny space probes to Alpha Centauri, the nearest star system to Earth, using light sails powered by lasers.
This ambitious plan to reach Alpha Centauri at unprecedented speeds could eventually open the door to exploring other stars, something that would be impossible with our current propulsion technologies. Caltech is at the forefront of this global effort, contributing research that is laying the groundwork for how these sails will work. The focus of Caltech’s current research is to build and characterize ultrathin membranes that could serve as the light sails for these spacecraft, advancing the possibility of interstellar exploration.
The Concept Behind Lightsail Propulsion
The Breakthrough Starshot Initiative’s vision is to achieve interstellar travel using light sails that are pushed by lasers. The concept is relatively simple: rather than using traditional rocket fuel or nuclear engines, spacecraft would be attached to ultrathin sails, and powerful lasers would be used to propel the sails. The laser beam would exert pressure on the sail, accelerating it to speeds that could eventually reach up to 20% of the speed of light, an unimaginable velocity compared to current spacecraft speeds.
However, the challenges of achieving this goal are substantial. Lightsails must be incredibly lightweight, resilient, and capable of withstanding the immense heat and pressure generated by lasers. The materials used must also be able to maintain their shape and stability while traveling through the harsh conditions of space. Given these immense challenges, the Breakthrough Starshot project is grounded in research on how lasers would interact with such sails and how to design and construct these ultrathin materials.
Caltech’s Role in Advancing Lightsail Research
At the heart of this groundbreaking work is Harry Atwater, the Otis Booth Leadership Chair of the Division of Engineering and Applied Science and the Howard Hughes Professor of Applied Physics and Materials Science at Caltech. Atwater is leading a team of researchers who are developing technologies to characterize and better understand the materials that would be used in lightsails, and the forces at play when lasers interact with them.
Atwater explains, “The lightsail will travel faster than any previous spacecraft, with the potential to eventually enable interstellar exploration that is now only accessible by remote observation.” This bold statement highlights the transformative potential of light sail technology, which could revolutionize space exploration.
To better understand how such sails could be made, Caltech’s researchers have created a novel test platform to study the materials and the propulsive forces involved. This platform will allow them to measure the forces exerted by lasers on miniature light sails, providing crucial data needed to move from theoretical designs to practical, real-world applications.
The Challenge of Creating Lightsails
Before scientists can even begin constructing real lightsails, they must first understand how the materials behave when subjected to laser radiation. The key challenge here lies in the membrane’s response to radiation pressure—the force exerted by light as it strikes the sail. In a spacecraft using traditional propulsion, the engines create force to move the spacecraft. But in the case of a light sail, the force comes from light itself.
To explore this, the Caltech team began with a miniature version of a lightsail. This small test platform, tethered at the corners of a larger membrane, was designed to be a first step toward developing the real thing. The team used cutting-edge facilities like the Kavli Nanoscience Institute at Caltech, as well as electron beam lithography, a technique that allows scientists to pattern structures at extremely small scales—down to the nanometer level.
The researchers patterned a membrane of silicon nitride that was just 50 nanometers thick, resulting in a structure that resembled a microscopic trampoline. This tiny device was only 40 microns wide and long, a size comparable to a speck of dust. The small trampoline-like structure was suspended by silicon nitride springs, which were then subjected to an argon laser beam at a visible wavelength.
The team’s objective was to measure the radiation pressure exerted by the laser on the tiny lightsail. By observing how the tiny structure moved, they could measure how much force the laser was applying to the sail.
Overcoming Challenges in the Experiment
While the setup sounds relatively straightforward, the science behind it is far from simple. A key challenge was the fact that the sail vibrates like a trampoline when hit by light. The vibrations from these forces are primarily driven by heat from the laser beam, which can obscure the measurement of the radiation pressure itself.
To tackle this, the team decided to use the heat-driven vibrations to their advantage. As co-lead author Lior Michaeli explains, the vibration patterns of the device allowed them to measure the force more accurately. The team also discovered that the experimental setup could function as a power meter that measured both the force and the power of the laser beam itself.
The researchers used a technique called common-path interferometry to measure the motion of the lightsail. This method enables the detection of small motions by comparing two laser beams, one of which is directed at the vibrating sample, while the other traces a fixed location. The key advantage of this technique is that it cancels out environmental noise, such as vibrations from nearby equipment or human activity, leaving only the small motion of the sample to be detected.
This setup allowed the team to measure motions as small as picometers (trillionths of a meter), which is essential when studying something as tiny as a lightsail under laser influence. The team also used this technique to measure the stiffness of the membrane by seeing how much the silicon nitride springs deformed when the sail was pushed by the radiation pressure.
Understanding Laser-Sail Dynamics
One of the complexities of this experiment is that the light sail would not always be perfectly aligned with the laser beam. In space, the sail would likely move or rotate, and the laser’s effectiveness would be affected if the sail were angled or tilted. Therefore, the researchers angled the laser beam to mimic the conditions a lightsail might experience in space, measuring how this altered the force exerted on the miniature sail.
The researchers observed that the force exerted by the laser was somewhat lower than expected when the laser beam was angled. One reason for this discrepancy could be that some of the laser beam missed the sail’s surface, especially at the edges, causing part of the light to be scattered in other directions.
Despite these challenges, the researchers were able to make key observations that will aid in the design of future lightsails. By calibrating the results based on the laser power measured by the device, they were able to obtain more precise data on the forces at play.
Looking Toward the Future
This experiment represents only the beginning of what could be a long road toward practical interstellar exploration. The next step, according to co-lead author Ramon Gao, will involve incorporating nanoscience and metamaterials—materials engineered at a microscopic scale to have specific desirable properties—into the design of the lightsail. These materials could help control the side-to-side motion and rotation of a lightsail as it travels through space.
“The goal,” says Gao, “is to use these nanostructured surfaces to impart forces or torques to the lightsail, which could correct any motion or rotation when it moves out of the laser beam’s path.”
By integrating nanotechnology and metamaterials, the team hopes to create sails that can autonomously realign with the laser beam, ensuring a more stable and controlled journey through interstellar space.
Conclusion: A Stepping Stone Toward Interstellar Exploration
The work done by the team at Caltech marks a critical first step toward achieving the goal of sending spacecraft to other star systems. By developing the techniques and tools to measure the forces exerted on miniature lightsails and understanding how materials respond to laser radiation, these researchers are laying the foundation for the next generation of space exploration technologies.
With the continued development of nanostructured materials and metamaterials, the hope is that this research will enable future spacecraft to travel to distant stars at unimaginable speeds, bringing the dream of interstellar exploration closer to reality. Through continued innovation, the vision of sending light-powered probes to Alpha Centauri and beyond may one day be realized.
Reference: Lior Michaeli et al, Direct radiation pressure measurements for lightsail membranes, Nature Photonics (2025). DOI: 10.1038/s41566-024-01605-w