The search for extraterrestrial intelligence dates back at least a century, a period in which humans have devised numerous ways to look for intelligent life elsewhere in the universe. These attempts generally rely on the idea that alien civilizations must be broadcasting signals into space, perhaps in a deliberate attempt to contact us but also for other reasons such as the leakage of their own terrestrial radio and TV signals or just the general energy pollution associated with advanced living.

These ideas have been extensively explored by scientists, science fiction writers, and futurists of one kind or another. So it is unusual for an entirely new potential source of alien signals to come to light.

Today, James Guillochon and Abraham Loeb at Harvard University in Cambridge, Massachusetts, describe an entirely new rationale for alien civilizations to broadcast signals into space that would be powerful enough for us to spot here on Earth. Their calculations suggest that looking for these signals could be an important new strategy in the search for extraterrestrial intelligence.

The new source of signals is a side-effect of any civilization’s natural desire to explore its system of planets. It is a goal humanity shares. One of NASA’s aims is for humans to visit Mars in the next 20 years or so. The only way to do that with current technology is with chemical rockets, which are significantly limited in speed by the amount of fuel they can carry.

An alternative approach is to use a light sail propelled by microwave radiation beamed from Earth. In this way, the spacecraft which would not need any fuel for propulsion. This has the potential to accelerate a one-ton spacecraft to around 60 kilometers per second during a trip to Mars, that’s triple the speed of the fastest chemical rockets.

Although expensive to build, such a system would be extremely cheap to operate. Guillochon and Loeb estimate that such a system might cost $30 billion to construct but only $40 million per mission to propel.

If the same economics hold in other planetary systems, Guillochon and Loeb argue that other civilizations may be currently exploring their planetary systems using light sail technologies. If so, this provides a new way to spot the tell-tale signs of the technology at work.

Their argument goes like this. If such a light sail system was in operation on Earth, then engineers would design a microwave array to beam energy to the sail. However, as the sail moves further away some of this energy would spill over the edges and be lost into space.

This leaked energy would be hugely intense and so creates a tell-tale signature that can be spotted from other star systems. So an interesting question is how easy would it be to spot such a signature?

Guillochon and Loeb provide the answer. They calculate that because the beam width is so small, the energy would be concentrated in a way that produces an equivalent luminosity around 1 million times larger than the total radio output of the Sun. That should make such a propulsion system around a distant star clearly visible here on Earth.

But there is an additional problem. The beam leakage is only visible when pointing directly toward us. Since any light sail would have to be tracked, the beam would sweep across the sky, which increases the probability of detection.

However, it would only point toward Earth when the planets are aligned with the sail traveling between them and towards us.

Any observer on Earth would need to be vigilant. This motion would ensure that the beam spends only a few seconds pointing in any given direction.

Such a signal would have a characteristic shape since the beam is missing the central region which is the shadow of the sale.

Guillochon and Loeb go on to calculate the optimal observing strategy given that any radio telescope but only be able to listen for these kinds of SETI signals for, say, 25 percent of its observing time. If there are inhabited systems out there travelling in this way, Guillochon and Loeb calculate that the chances of finding them are about 1 in 100 for a five-year survey per star system.

There are caveats, of course. Guillochon and Loeb have to make some significant assumptions about the way these propulsion systems would work.

In particular, they assume that any microwave array would not be designed to focus light onto a sail as it moves further away and appears smaller. That’s why up to 10 percent of the microwaves leak out. Given the huge amounts of energy that such a microwave array would be designed to produce, is it likely that an intelligent civilization would allow 10 percent of it to leak into space?

Perhaps. Either way, it’s an interesting idea that raises the possibility that extra terrestrial civilizations could be producing entirely new signatures for us to search for and one that is likely to send a ripple of excitement through the SETI community.

Ref: arxiv.org/abs/1508.03043: Seti Via Leakage from Light Sails In Exoplanetary Systems