We all imagine that advanced civilizations in our Milky Way Galaxy will build spacecraft to travel among the stars, à la Star Wars and Star Trek. Unfortunately, the laws of physics conspire against interstellar travel. The two problems are time and energy.
Let’s consider a trip in a car-sized spacecraft from Earth to Alpha Centauri, traveling at 50% the the speed of light. Sad to say, the trip will take 9 years, and 18 years round trip. Along the journey you’ll find no rest stops, Starbucks, nor pizzerias, never mind a pharmacy or Apple repair store.
Worse, the needed propulsion energy of your craft is gigantic. Kinetic energy is given by the equation, 1/2 the mass of the craft multiplied by its velocity squared (Energy = 1/2 mass vel^2). This basic physics shows that the needed propulsion energy is 3 trillion kilowatt-hours. This is 1000x the total electricity energy used in New York city during a full year – a daunting amount of energy to acquire and control. Carrying the required propellant mass makes matters even worse.
To save energy, you might propel your ship to only 1/10 light speed toward Alpha Centauri. At this snails pace, the two-way trip now takes 90 years. Few people want to spend a century in the cold, darkness of space, confined to a VW Golf. Oh, and an additional problem is that interstellar dust particles and cosmic rays will hit your craft at more than 1/10 the speed of light, puncturing holes, causing cancer, and damaging equipment. (See, you’ll need that Apple genius bar.) Advanced civilizations might do a few trips like this, but the daunting energy and time becomes unappealing.
There’s a better way to explore the Galaxy. You can build small, inexpensive robotic probes, similar to our Pioneer and Voyager spacecraft. Low mass and traveling at a slow speed of, let’s say, 1/100 the speed of light, they will require little energy. The long travel time to the nearest star is not a problem for a machine. It simply takes a few centuries, which is a blink in Galactic time and brief compared to the half-life of nuclear fuel. Civilizations can easily send probes to thousands of stars in their neighborhood, equipped with cameras and transceivers that transmit pics (including selfies) back home.
Our home Milky Way Galaxy may be populated by millions of these robotic probes, sent to many stars for in situ reconnaissance. Our home Solar System may contain one. A network of such probes may be optimized by spacing some “repeater nodes” by less than a light year, minimizing the attenuation of signals by the inverse square law. Communication beams may enjoy privacy, high bit rate, and minimal energy usage by using the tight beams of lasers – ultraviolet, optical, or infrared. Indeed, laser beams and repeaters are already widely used by human-made satellites orbiting Earth.
We may detect extraterrestrial probes in the Solar System two ways. They will reflect sunlight, as do all satellites. Glints caused by specular reflection off flat, metallic surfaces will make especially bright glints. We should search for glints coming from probes orbiting the Earth or the Sun. Also, probes will be receiving and emitting transmission beams that broaden like flashlight beams, overfilling their intended target receiver, allowing us to detect the spill-over light that travels onward (forever). Our telescopes can search for this laser spill-over light.
We have a new opportunity to search for advanced technology in the Galaxy. Rather than hunting for the inhabitants of civilizations, we should hunt for their “Voyagers”, by their glints and lasers.
Thanks to Beatriz Villarroel, John Gertz, Ben Zuckerman, and Lawrence Krauss for many of the ideas here.
VASCO and Director of Space Laser Awareness