As a scientist who has researched the laser technology for Breakthrough Starshot, I appreciate the Physics Today Online article by Howard Milchberg on the feasibility of the concept. I agree that Starshot is a difficult endeavor, but I am more optimistic about overcoming the numerous obstacles—largely because of what has been achieved in recent history.
Suppose that in 1981 an ambitious (or perhaps just crazy) team of scientists had proposed Project Tera, whose goal was to produce the following within 35 years:
- A teraflop computer that costs $50 (a 109 reduction from 1981 technology), fits in your hand (a 109 reduction in size), and uses 10 W (108 less power per flop).
- A terabyte of gigahertz memory for $400 (a 106 reduction in cost and size from 1981 tech and a 103 increase in performance).
- A 1 W blue laser with 50% efficiency that costs less than $0.40.
- A 30 m diffraction-limited telescope.
- A self-driving electric car that has a 300 km range and costs less than $10 000.
Look at each of those goals and ask yourself what you would have thought of Project Tera in 1981. Nuts, huh? The last three weren’t even feasible at the time.
Yet every one of those goals has already been accomplished or is about to be.
Project Starshot aims to produce, in less than 35 years, a 100 GW laser phased array that uses 100 million currently existing 1-kW-class ytterbium amplifiers. The plan assumes no increase in efficiency, a very modest increase in coherence length, and a 100-fold reduction in laser amplifier costs to about $0.10/W (which is roughly the current cost of high-power LEDs). Note that there’s already been a two-order-of-magnitude drop in the cost of such laser amplifiers over the past decade. By comparison with Project Tera, the Starshot goals look extremely conservative.
In work sponsored by NASA, Philip Lubin is researching directed-energy propulsion, illustrated here. (Image credit: Philip Lubin, University of California)
The 1 km Starshot laser array is approximately 30 times as large as the 30 m telescopes currently under construction, each of which has adaptive optics and is fully expected to achieve near-diffraction-limit performance at a wavelength around 1 µm. Starshot will have 100 million individually phase-controlled subelements that are each approximately 10 cm in size. The Starshot array would offer vastly superior adaptive optics, with numerous coherent beacons (the analogs of artificial stars in astronomy applications) both inside and outside the atmosphere to enable phase reconstruction, beam formation, pointing, and atmospheric mitigation. If the atmosphere proves to be the one insurmountable obstacle, we have the fallback, albeit expensive, of putting the laser array in space. Similar alternatives exist for every step of Starshot should technical difficulties occur (and they will).
As the hypothetical Project Tera illustrates, we tend to underestimate technological innovations in the future. Hindsight is easy; foresight is harder. In its roadmap, the semiconductor industry builds in the assumption that 10 years hence innovation will provide vastly superior consumer- and industrial-level electronic advances. The industry knows there will be huge performance increases and cost reductions. We are in the midst of amazing revolutions in photonics and electronics: The screen on which you are reading this article is evidence of that. Starshot leverages both of those revolutions.
Yes, Starshot is a complex project with many technological barriers. But it is absolutely worth doing, for the results will be truly revolutionary.
I encourage you to visit our University of California, Santa Barbara, website for details of our NASA and related directed-energy programs. You can also visit the Breakthrough website and read my detailed technical paper on achieving interstellar flight.
Philip Lubin, a professor of physics at the University of California, Santa Barbara, was funded by NASA in 2015 to work on Directed Energy Propulsion for Interstellar Exploration (DEEP IN), which was the instigation for Breakthrough Starshot. He is on the Starshot management and scientific advisory committee.
