This is part three of a four part essay. In this part I shall discuss the prospects of interstellar travel and the challenges that face it.
Let us be optimistic and suppose that there are a thousand intelligences in the Milky Way. This should place us within a thousand light years of our nearest neighbor. Let us further suppose that some oracle gave us the precise location of our neighbor. Being good neighbors, we shall desire to say hello.
How shall we do this?
A thousand light years is infinitesimal against the span of the cosmos – it's not even significant on a galactic scale – but it is a daunting gap all the same. For those of us raised on a diet of science fiction, the obvious solution is that we would just hop on a spaceship and go visit them. Unfortunately, there's a number of problems with this.
The most obvious problem is the speed of light. At the very best, it would take a millennium to reach them. Perhaps, instead of just racing off towards them, we'd be better off taking a few hundred years to develop some method of traveling faster than light.
What I am about to say is going to meet with reactions of denial. I must say it never the less: in all probability, there is no way to travel faster than light.
What heresy! What arrogance! Where there's a will, there's a way! Nothing is impossible!
There is a general feeling that, while science may be good at providing solutions to problem, anything we know now can be proven wrong at some point in the future. To a degree, this is true, however there is wrong and there is wrong. The late Isaac Asimov addressed this is a beautiful essay called "The Relativity of Wrong". The point of the essay was to demonstrate that although science works by replacing incorrect theories with newer theories, the general trend of science is cumulative and not revolutionary.
To illustrate what this means, let's take a quick look at Newton's Theory of Universal Gravitation. Newton's theory makes some very specific predictions about the motions of the planets. Unfortunately, it's "wrong". For one thing, it fails to properly predict the orbit of Mercury. Einstein's Theory of Relativity "replaced" Newton's and managed, among other things, to make proper predictions with respect to Mercury's orbit.
Notice the scare quotes around "wrong" and "replaced"? They are there for a reason. Newton's theory was incorrect but the correction that the Theory of Relativity applies to Newtonian predictions is very, very slight. While the Relativistic model does have more explanatory power, its utility is limited to cases of extreme velocities, accelerations, masses and densities. If you were to go into close orbit around a black hole, you'd need to apply relativistic equations to your situation. If you are "merely" launching a probe on a ballistic journey to Pluto, however, all you need is Newton.
Asimov compares this to models of the shape of the Earth. The earliest model of a flat Earth was wrong (although, strictly speaking, it was only off by a few degrees of curvature). The subsequent model of a spherical earth was also wrong because the Earth is not perfectly spherical. Currently, the Earth is described as an oblate spheroid (meaning a slightly flattened sphere). With better topographic surveys, the shape of the Earth will become more and more precisely described with each new description replacing one that is, technically, wrong. Each new description, however, is only a correction to the previous ones. The overall shape of the Earth is well understood. If someone says that the Earth is shaped by a ball, we know that it would be pedantic to insist that they are wrong in saying so.
The Theory of Relativity, which tells us that object or source of information can travel faster than the speed of light, is the most thoroughly tested theory in the history of science. Even the most precise experiments using the most sensitive instruments have failed to detect even the slightest deviation from theory. Given that the theory is approaching its hundredth anniversary, it is an understatement to say that this is impressive.
We do know that the Theory of Relativity is incomplete. We know this because it is incompatible with Quantum Mechanics (which also has an amazing record of experimental verification). The quest to come up with a theory that can unify the quantum and the relativistic realms is the holy grail of physics. The incompatibility of the two theories, however, only shows up in the most extreme possible conditions -- the sort of conditions that you'd find at the hearts' of black holes.
Might an FTL (faster than light) drive pop out of a Unified Equation? It's possible but it's unlikely. It would not be quite like having advanced satellite imagery prove that the Earth is actually a cube and not a spheroid, but it's certainly nearer to that side of the analogy. It would be a delight if the laws of physics did let us, after all, make allowances for FTL, but we shouldn't pin our hopes on it.
Perhaps it's just as well. If FTL did turn out to be possible, it would only make the Fermi Paradox that much more baffling. Rather than having life spread through the galaxy in a cosmic instant, we should expect it to spread through the galaxy in a merely historical instant. The existence of FTL would make it that much more likely that intelligence is truly rare.
Very well, let us suppose that we can't reach our neighbors with some sort of super-science warp drive. Perhaps we can go the long way around. In fact, relativity does offer a kind of solution. As one edges towards the speed of light (never reaching it), time begins to dilate. One second of your personal time could equal a year of time to an Earthbound observer. So long as a daring astronaut was willing to lose all of his loved ones and everything he knew to the dust of history, he could take a one way journey into the future in order to meet our distant neighbors.
Although the Theory of Relativity doesn't forbid such trips, there are two things that do stand in opposition: mass and energy. In order to make a spaceship move, you need reaction mass. Reaction mass is, quite literally, stuff that you throw out in one direction in order to move in the other. The bigger the mass you toss out, and the fast you toss it, the faster you move. Unfortunately, in order to move very far, you need to have a lot of reaction mass. The thing about reaction mass is that it is, in fact, mass. The more mass you have, the slower you accelerate.
It's a diabolical relationship. Let's say you have a ship with a thousand gallons of rocket fuel. If you burn up all your fuel, you can get up to a thousand miles per hour. Although that's pretty fast, you want to go twice as fast, so you pump your ship us with two thousand gallons. Unfortunately, now you're much heavier than you were before (or massive, if you want to be technical). Instead of doubling your speed, you've only gotten part way to your goal. So we add more fuel. But now we have more mass. The more velocity you want, the more fuel you have to add and less you get back from the effort. The mass requirements quickly exponentiate.
I have seen estimates that suggest that using current propellants, you'd have to have a fuel tank the size of the solar system merely to get to the nearest star in a reasonable amount of time. The only way around that is to find some way to get more bang for your buck. That's where energy comes in. Some fuels are more energetic than others. There's a reason that we don't gas up the space shuttle with ethanol. It wouldn't delivery nearly enough energy to get the shuttle into orbit. Instead we use fuels that have a high energy density. You're going to get a lot more energy out of a gallon of liquid oxygen than you would a gallon of gasoline.
In order to reach relativistic velocities (and to decelerate from them – it doesn't do you any good if you can't slow yourself down), rocket fuels aren't going to do the trick. There is, unfortunately, a maximum amount of energy you can get out of a given quantity of reaction mass. This is defined by the famous equation E=mc2. Matter and energy are basically two aspects of the same thing. It is, theoretically, possible to completely convert a given quantity of matter into energy. The equation tells us how much energy we'd get out. It's a lot. Nuclear bombs convert less than 5% of their mass into energy yet twenty kilograms of plutonium can level a city. If we could achieve 100% conversion, there we be no energy crisis – of course, we'd also be able to build truly hellish weapons.
There is a way, however. We could use antimatter. Antimatter is sort of the mirror of ordinary matter. For the purposes of this essay, the important thing about antimatter is what happens when it comes into contact with ordinary matter. The antimatter and the regular matter annihilate with 100% efficiency. Their entire mass is converted into energy. Even better, we can actually create antimatter. Unfortunately, it's expensive and time consuming. Even creating a microgram of antimatter would cost tremendous amounts of money and consume huge amounts of time (we'd also want to be very careful with it so that we didn't accidentally vaporize ourselves in the process).
Let us suppose that these are merely technical hurdles. Suppose that we could generate antimatter in bulk. Is this our ticket to the stars? Yes – sort of. Antimatter, to, falls prey to the diabolic relationship. Even with antimatter, you can only carry so much before you start to hit the limit of diminishing returns. I've seen designs that purport to allow relativistic travel to the nearest stars. The ships described are very small and very light with the majority of the ship being taken up by the reaction mass. They are suitable for carrying only a small crew of astronauts. Even with antimatter, there will be no Star Trek style luxury liners in our future.
Over the years, various brilliant people have tried to find a way around the diabolic equation. One promising idea was known as a Bussard Ram Jet (after the man who conceived it). The idea was that it would be a ship that would scoop up interstellar hydrogen to use as a fuel. Since the ship wouldn't need to carry its reaction mass, it would circumvent the problem. Alas, there's a hitch. The act of scooping up the hydrogen would cause the ship to decelerate. The more you scooped up, the more you'd decelerate.
This is an all too common outcome of interstellar travel proposals. Let us suppose, however, that we can, at least, get past this hurdle. We're still not free and clear. Traveling at relativistic velocities introduces its own set of problems. The biggest problem is that when you're moving near the speed of light (relative to your environment), everything else is moving past you at the same velocity. A grain of interstellar dust might seem a small thing, but when you slam into it at 90% of the speed of light, it's like being hit by a missile packed with high explosives. Even the smallest particles pose a problem because they would zip through your body with all the energy of cosmic rays. It would be like being continually exposed to hard radiation.
The sad fact of the matter is that we don't live in a convenient universe. The very nearest stars are terribly far away. If we do eventually go out to the stars, we'll be going very slowly. Still fast enough to make the Fermi Paradox an issue but not fast enough to fulfill our dreams of imminent alien contact.
Very well, slow but steady wins the race, right? It depends. If there is intelligent life in our galaxy, our distant descendants may one day encounter it. What if, however, life isn't quite that common? After all, since we know that a galaxy can be colonized in a relatively brief span, this may well indicate that if you are an intelligent species in a galaxy, you are almost certainly the first such species. If you weren't, you would never have had a chance to develop in the first place since your world would have long since been colonized or utilized by whichever species did come first. Consider it a special case of the Weak Anthropic Principle.
So what? First in our galaxy doesn't mean first in every galaxy. Traveling between galaxies is even more daunting than traveling between stars but we may suppose that we could find some way to manage (perhaps with ultra-advanced hibernation technologies). Maybe it would take millions or even billions of years to find our neighbors but, in the end, we would not be alone.
Unfortunately, there appears to be a time limit to such explorations and the clock is already ticking.
This concludes part three of this essay. Part four will be published next Sunday.
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