Science
& Culture Corner
Reaching for the Stars
A disproportionate percent of our sci-fi books and movies involve travel from solar system to solar system where aliens are encountered. Many involve aliens reaching earth from very far off. As tantalizing as it is for a Star Trek ship warping space to travel at gigantic speeds, it is actually much more difficult than the stories make it seem.
Newton reasoned out much about the mechanics of space for us. We certainly owe him for this a debt. Yet exactly that for which he is so famous- calculating planet orbits- is that which his laws could not fully explain. Of all known planets and moons, only one, Mercury, refused to follow his rules. Using his math, anyone can predict the orbits of the planets except for Mercury. Why should one operate by different rules?
Only Einstein's formulas accounted for the fact that Mercury revolves under, by its proximity to our sun, an immense gravitational force. Mass and speed, it turned out, do unexpected and counter-intuitive things to objects. If an insect strikes the windshield of your moving automobile, you would expect it to splatter. However, if the car (or the insect) is moving fast enough, the car's windshield will be destroyed. Why is this so? Because speed represents energy since a mass attaining speed requires the expense of energy to get to some speed. The greater that mass, the more energy is required to accelerate it. Thus, the more energy that acquired speed represents.
Things must balance. It is the law of conservation. When mass is converted into energy, that energy must be proportional to how much mass was converted, and vice versa. If any mass is accelerated, that increased speed must represent more energy proportional to the mass amount and the speed, etc. Putting it the other way around- and these things always work in both directions- since a speeding bicycle would crash with less force than a speeding truck (at the same rate of speed), it must take more energy to accelerate a truck to some speed than a bicycle. (Just try pushing a truck up a hill.)
To achieve long distance space travel, a high speed is needed. To get to that speed, acceleration is necessary. As we have seen, the higher the speed, the more energy is needed. The ship's inertial resistance- resistance to changing speed- must be overcome for any and every increase in speed.
Earth-made ships generally use the burning of chemical fuels to increase their velocities. This works fairly well for lower speeds, but comes with an inherent problem. To go even faster, even more fuel must be combusted. That extra fuel has weight (mass) which must also be accelerated, thus requiring even more fuel, which has even more weight, ad infinitum. The consequence is that chemical rockets reach a point at which speed is effectively limited. The common procedure of ejecting empty fuel tanks in flight helps, but only slightly. Chemical fuel systems limit the speed, and thus practical range, of rocket flight. For example, at a continuous speed of 50,000 miles per hour, the highest speed attainable from anything within our current engineering ability (even using nuclear fuel), a trip to a star system 20 light years distant would take 268,000 years each way. This is obviously an impractical length of time. Travel between star systems is thus simply not doable with chemical rocket technology.
Another challenge to high-speed travel is inertia's effect upon the traveler inside the ship and on ship components. The force that jerks you backward when an automobile or train suddenly accelerates is very serious. Even if a solution to the fuel problem (above) were at hand, in order to reach a very high speed in a reasonable span of time, the acceleration would tend to squash a pilot against his seat to where his whole body is crushed paper-thin. Some science fiction writers have noticed this problem and invented, conveniently, "inertial dampening systems" that have the ability to evade this effect. There are however, outside of fiction, not even theories suggesting that this might be actually be possible. The reality is that high acceleration is deadly.
What then if acceleration were to proceed at a conservative rate? Although a human can survive at up to 12 g's (about thrice the force of a powerful racecar starting out), it is doubtful that any creature of complex biology could survive more than 2 g's for an extended time. Travel to other star systems must therefore require the extended time to gradually reach very high speed.
Suppose then (with some solution to the problem of fuel) we wanted to fly to a planet 20 light years distant. Couldn't we try it by limiting our rate of acceleration to something manageable? Yes, but the time in flight would be long. Before we could reach any sort of effective high speed we'd have to be slowly accelerating. Before even reaching 1/4 the speed of light, we'd reach the halfway mark to the new planet (these rules work in reverse as well), we'd have to begin decelerating at an equally slow rate. Fuel permitting, we might arrive with an average overall speed of .2 light speed. Our trip takes some 200 years (each way). Obviously, this is not practical.
These same problems necessarily affect any other species that might wish to cross a part of our galaxy. But what if we solved the fuel problem and built an unmanned ship with components that could survive tremendous forces of inertia? We'd still have troubles. Even if we ignored the inertia problems with acceleration and deceleration and could fly at 8/10 light speed, we still couldn't get to another star system.
As it turns out, the faster something travels, the more its mass increases. (Its increase in mass is a direct function of its speed.) We don't notice this at ordinary speeds because the increase is exponential, that is, it is insignificant below half the speed of light. By .8 light speed it is an enormous problem. As we reach the speed of light, the weight of our ship approaches infinity. To increase from .799 light speed to .8 light speed would be more difficult (due to increased mass) than for an ant to push a skyscraper up a hill. Increasing acceleration depends upon the mass to be accelerated. When the mass grows to say, that of one million earths, what sort of engines could ever affect it? Einstein's equations representing this phenomenon thus preclude travel at or near light speed.
Were that not enough, there are the natural risks of long, high-speed travel. For example, the high speed represents high energy (remember the insect against the windshield?), suppose our ship impacts a stray micro-meteor somewhere after takeoff? The energy of that impact at high speed would be more than a hydrogen bomb. It doesn't matter if it's the meteor or the ship going at a fast speed. The energy is relative to the difference in speed between the two objects. For this reason, it is called relativity. Any such collision with any object would destroy even the powerful ships of science fiction lore.
All this does not mean that very high-speed travel is impossible absolutely. Perhaps in a few centuries we might engineer a method of compressing or "warping" space before we travel through it (as they do in Star Trek and Star Wars). Perhaps other species have managed this already. Perhaps a ship's mass could somehow be converted into energy and travel massless. Perhaps not.
Far more realistic is the idea that traveling vast distances requires a vast outlay of time. There is simply no way around this. That our technology has its present limits does not have to mean that the technology of another civilization is at these same limits. But the same laws of physics are equally imposed upon other civilizations.
And maybe this is actually a good thing. Our radio/TV signals travel in all directions from earth at the speed of light- essentially, advertising our location. There is no limit on the signal range (although signals weaken by the square of the distance traveled). Since radio has been in operation for about a century, that means that our signals are now at a distance from earth of about a 200 light year diameter. Since only about 75 years passed between our civilization's discovery of radio and launching a ship (Voyager) out of our solar system, we may predict that other civilizations, if they exist, could also achieve space travel shortly after becoming able to receive radio signals. Were travel near, at, or beyond light speed actually possible, we might have all manner of potential conquerors, with technologies centuries beyond ours, arriving by surprise.
by Angel
Stephens
angel_stephens@hotmail.com
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