It is best to start at the very beginning—to explain why this planet is all the human species has. Fully understanding that will help set the stage for all that follows. This is a walk through some science, which you can read by expanding the sections below, or you can just cut to the chase.
The good news is that we don’t really need to understand the special theory of relativity, although as theories go it is more accessible than most. All we need to know is this: Einstein theorized that the speed of light has to be constant, and the theory has been proven correct.
Einstein in Zurich
The story begins with Albert Einstein in the year 1905.
According to historian Milena Wazeck, “By 1900, physics was perceived by many to be an almost completed discipline.” The generally accepted principles of Newtonian physics suggested that space and time were absolute, and any differences observed in their measurement could be attributed to different frames of reference.
However, physicists weren’t exactly sure how light would travel in this system. New theoretical and experimental work didn’t seem to support the Newtonian understanding of light. For example, the equations of James Clerk Maxwell suggested that light was an electromagnetic wave that traveled at a constant speed, and the experiments of A.A. Michelson and Edward Morley found no change in the speed of light no matter what the earth’s motion was. In order to make the Newtonian system work, scientists got twisted in knots trying to prove the existence of aether, a medium through which light waves could travel.
At the time, Einstein was a 26-year-old lowly official in the Zurich patent office. But despite being outside the ranks of an academic institution, he published four papers that year with findings dramatic enough to overthrow the conventional wisdom of Newtonian physics and later win him the Nobel Prize.
Each of the papers of Einstein's annus mirabilis is worthy of study, but the paper that we need to focus on is the one on special relativity. In his book Thinking Fast and Slow, Daniel Kahneman writes, “You know you have made a theoretical advance when you can no longer reconstruct why you failed for so long to see the obvious.” Einstein’s theory of special relativity, now a cornerstone of modern physics, is one such advance.
What the special theory of relativity says
Einstein’s innovation in the special relativity paper, "On the Electrodynamics of Moving Bodies," was to assume that the evidence of light's behavior was accurate, and that the theories that contradicted it were wrong, rather than trying to discredit the evidence because it didn't fit the principles.
His theory of special relativity starts with two essential stipulations. First, the laws of physics are the same for all objects that are not accelerating (known as the principle of relativity). Second, the speed of light in a vacuum is the same for all observers.
Remember, this second principle had been suggested by evidence but contradicted the idea that light would have different speeds in different frames of reference. This was a radical break from accepted knowledge, no matter how simple it might seem in retrospect.
How we know the special theory is correct
While the findings of Michelson and Morley's experiment supported Einstein's theory, two other experiments helped cement the constancy of the speed of light. The first was the Ives-Stilwell experiment, which confirmed the phenomenon of time dilation that special relativity predicts. The second was the Kennedy-Thorndike experiment, which confirmed the contraction of length that corresponds to time dilation. Together, these three experiments form what science writer Brain Koberlein calls "the optical trifecta."
As technology has caught up with Einstein’s theory, it has only strengthened the evidence of its accuracy. The speed of light has held constant even as our ability to measure that speed has become more precise. For example, scientists at the European Organization for Nuclear Research (CERN) in Switzerland have also been able to eject particles at nearly the speed of light that prove the theory of special relativity, and findings of the Fermi telescope in Palo Alto have also shown the constant speed of light.
This may seem incredibly weird. But in the 115 years since Einstein’s paper, it has been proven beyond any reasonable doubt that this is in fact how the universe works. The speed of light is constant, in all places and all times.
Right, so what? The speed of light may be constant, but why should anyone care? Well, we should care for many reasons—including that being aware of the science underpinning the universe we live in has innate value—but there is another reason that is especially relevant to this book and its thesis: namely that the speed of light, or c as it is referred to, being constant at the level that it is means we will never, ever, successfully inhabit another planet.
Einstein showed that the closer an object gets to the speed of light, the more massive (and more difficult to move) it becomes. In other words, we are not going to ever make it to another planet outside our solar system because we just can't get there fast enough. Even at current speeds, interstellar probes headed to our nearest neighbor, Proxima Centauri, four light-years away, would take at least 6,000 years just to reach the star.
We are planet-bound forever
This is not just me being defeatist of negative—Michael Mayor, who was awarded the Nobel Prize for his techniques for detecting exoplanets, has said that it is "completely crazy" to think that humans will colonize any of the planets that his research has uncovered.
How about a less habitable planet closer to home? Some ambitious thinkers have floated the idea of engineering, or "terraforming," Mars to make it hospitable to humans. But this is science fiction, too—we may be able to send astronauts to colonize Mars in the coming decades, but billions of humans will never walk the face of Mars free of spacesuits. Bottom line: it isn't happening.
I can, however, see us mining for minerals on the moon or Mars or within the asteroid belt—although doing so will require technology, energy sources, and wealth that currently seem beyond reach—but there is no scenario whatsoever in which we manage to leave Earth and live in new bio-environments.
This is a good moment for me to make my favorite point about humanity's affinity for complexity—rather than heal the planet we have (and have abused), we dream about getting to other planets!
Again, so what?
"We must take care of our planet, it is very beautiful and still absolutely liveable," said Mayor in 2019.
If our own planet is our only planet, then two things become eminently clear: Earth's biosphere is all we have, and we must nurture it.
We have most definitely not been nurturing our planet. Bear with me—we will have to review the litany of atrocities we have inflicted on Mother Earth before we can talk about how we heal her.
- Video | Crash Course Physics: Special Relativity
- Lecture | World Science Festival: Special Relativity with Brian Greene
- Lecture Notes | University of Cambridge: Lectures on Dynamics and Relativity by David Tong
The limits of humanity