For a moment, imagine standing on a spaceship, in deep space. To the rear of the ship — through a shaded, dusky window — is a breathtaking view of an expanding, suicidally-bright blue ball of light. If it weren't for the thick, UV-blocking glass of the viewport, every open eye would go blind from what's in store. Razor-thin light sails the size of skyscrapers slowly unfurl from the hull, one to each side, like wings of a monstrous, mechanical moth. As they stretch out, the stars ahead give way, and appear to move — slowly at first, like satellites in the midnight sky — as a new and indescribable feeling of motion takes hold. Unfamiliar constellations seem to twist, and accelerate, faster and faster. Way out ahead, beyond the deep black of empty space, is a pale blue dot. Ten million years in the making, Betelgeuse has finally gone supernova, and we're surfing the crest at the speed of light, on our way to Earth.
Surfing a supernova with Harvard's Avi Loeb
This scene may sound like science fiction, but — because of the time it takes for light to cross the galaxy — another civilization could be surfing the wave of Betelgeuse going supernova, more than 600 light-years away.
In a Scientific American Op-Ed titled "Surfing a Supernova," Professor Avi Loeb, chair of the astronomy department at Harvard University, wrote that light sails weighing less than half-a-gram per square meter can actually achieve lightspeed — even if the vessel attached to them is located one hundred times farther away from the exploding supernova than Earth is from the Sun.
Our Sun can barely push light sails — with much effort — to one thousandth of the speed of light. But supernovae have a luminosity equivalent to one billion Suns shining for an entire month.
Of course, there are a few ways around the Sun's relatively weak push. Powerful lasers can apply a force far more efficiently than the Sun. Breakthrough Starshot, a project scientists and investors hope can reach a few tenths of the speed of light by shooting a laser beam at a lightweight sail for a few minutes, might achieve 10 gigawatts of power transfer per square meter — 10 million times brighter than the clear blue sky of Earth.
But it's difficult to secure the major investments necessary to support the massive infrastructure required to generate light waves of this magnitude. Additionally, we'd have to ensure that the light waves are all adjusted to the same, parallel direction (to maximize force).
Timing and position are key
If there is another civilization in the neighborhood of Betelgeuse or Eta Carinae, nothing is stopping them from setting up in optimal position, opening their light sails, and waiting for the mind-sweatingly powerful explosion to launch them at the speed of light, at a low, low cost.
It's strange to imagine the preparation behind a feat like this. Supernovae don't come around every day, or even during the lifetime of an entire civilization. The gigantic stars that produce a supernova live for millions of years, and it's extremely hard to predict when exactly they will blow. Eta Carinae — another giant star nearing death — has a lifetime of several million years. China has the longest continuous recorded history in the world, but it's only 3,500 years old. For supernovae, the scale for years is millions.
The death of stars and civilizations are both difficult to predict with perfect accuracy, but only one has a definitive, final conclusion. Until the star goes out with a bang, light sails can be moved into launch position with ordinary chemical rockets. But with conventional propellant, it would take millions of years to cross the molecular cloud that gave birth to the dying star.
Additionally, since a light sail's orientation relative to the star will determine the sail's trajectory — directly away from the center of the explosion — the crew of the vessel needs to make sure they're on the correct side of the doomed star, or they will find themselves moving very fast in the wrong direction.
The trick in tacking into the wave
Open the sails too early, and the crew risks being pushed away by bright starlight before the actual explosion. This would move them away from the star before their ship has a chance to absorb the full acceleration. Additionally, the sails also need to be highly reflective, else they would absorb too much heat, and endanger the crew.
Once the sails catch the cosmic wave, the crew better hope they've charted a course clear of debris because, at the speed of light, a collision with even a dime would make an advanced rail gun seem like a slingshot, in comparison.
Stars like Eta Carinae and Betelgeuse — both giant, cosmic powder-kegs — could also collapse into black holes, creating powerful beams of deadly radiation, which astronomers observe from afar as gamma-ray bursts. If a light sail were in the path of such a beam at the time of the explosion, it would receive a substantial boost in velocity, high enough to achieve a relativistic Lorentz factor of one thousand. At this speed, a light sail can cross the entire Milky Way galaxy in less than a single human lifetime, measured from their time frame.
Of course, to those on the spacecraft, the relative passage of time on Earth would accelerate drastically, leaving everyone the crew could tell about their amazing journey long dead, and ancient history, by the time they returned from Betelgeuse.
Since supernovae have an amazing potential utility for propulsion, massive stars like Betelgeuse — and their stunning, nebulous remnants — could be great candidates for the Search for Extraterrestrial Intelligence (SETI). A singular light sail near an imminent supernova explosion would be too faint to track. But according to Loeb, the aggregate bow shocks of many sails, and the communication signals between whoever pilots them, might be detectable with current telescope technology.
Betelgeuse is more than 600 light-years away, which means it could explode at any moment, and send its brilliant light our way. But one question seemed to elude Loeb's Op-Ed: if it takes a supernova to successfully accelerate a light sail to the speed of light, one wonders what could safely slow our hypothetical craft in time for a gentle splash-down, back on Earth.