Interstellar and Intergalactic Travel Help (page 3)
Water, Air, And Food—again
All the challenges of interplanetary travel will exist in interstellar space on an exaggerated scale. In addition, there will be new problems and inspirations.
The essentials of life will be harder to come by on interstellar missions than on trips to other worlds in the Solar System. This will open up new avenues of technology. There should be no lack of work for people who want to design star-wandering ships.
Hydrogen exists in the voids between the stars, although it is at extremely low pressure, comparable with the best laboratory vacuum ever obtained on Earth. This hydrogen (H 2 ) can be combined with O 2 to provide H 2 O. But where will the O 2 , necessary not only for H 2 O but for a breathable atmosphere, come from? Some scientists think it exists bound up in icy rocks floating among the stars. The distant Oort Cloud, the belt of comets that surrounds our Solar System and hopefully other star systems, can be expected to provide a source of H 2 O, and therefore of O 2 as well. The challenge will be snaring the comets while traveling through the cloud at speeds of many kilometers per second!
Another way of getting breathable air is to split apart the CO 2 exhaled by the astronauts, mixing the O 2 with the ever-present nitrogen gas, and setting the carbon residue aside. No one has yet figured out an easy way to do this, but optimists believe that this problem eventually will be solved.
Food on interstellar voyages will be grown in the form of plant life, as well as carried along in the form of protein, vitamin, and mineral supplements. Exhaled human CO 2 will be a blessing here. Plants convert CO 2 into O 2 , which is released into the air; the residual carbon is used by the plants to build their own living matter. Special gardens will be provided for the dual purpose of supplementing the O 2 stores and obtaining food. The gardens also will serve an aesthetic purpose. Astronauts will find psychological and emotional respite from the rigors of their artificial environment by sitting or strolling on the “garden deck” among the plants.
The Sun is not the only source of high-speed subatomic particles. All the stars in the Universe emit them. The cores of galaxies are intense sources, some more than others. Supernovae can produce great quantities of radiation. There are x-ray and gamma-ray objects scattered throughout the galaxies. During a long interstellar or intergalactic voyage, astronauts will be exposed to unknown quantities of this radiation.
In the long term, we should not be surprised if long-distance space travelers have an above-average incidence of cancer. If multigeneration space voyages are carried out, the later generations will be subject to more-often-than-usual occurrences of birth defects. In the extreme, high-intensity cosmic radiation will shorten the life spans of space travelers, and they will be sick for much of their lives.
Hopefully, some scheme will be found to protect interstellar astronauts from cosmic radiation. Radiation shelters, mentioned earlier in this chapter as a means of staying safe from the perils of solar flares, may serve as sleeping quarters to minimize long-term exposure. The deadly cosmic particles may be deflected away from interstellar spacecraft by devices yet to be invented and perfected.
It would be naive and arrogant of us to suppose that we are aware of all the dangers and challenges interstellar and intergalactic travelers will face. What about dark matter? We know it exists, but in what form? Billions upon billions of tiny black holes? Quadrillions of meteoroids and dormant comets? These are not the sorts of things a starship captain will want to encounter at 99 percent of the speed of light (or at any speed).
The large-scale structure—the “shape”—of the Universe is not known with certainty. One theory holds that the Universe is a gigantic four-dimensional hypersphere , with our space continuum comprising the three-dimensional curved surface. Other theories give the Universe different shapes such as a hypersaddle, or a hyperfunnel. Curvature of space, according to Einstein’s general theory of relativity, is the inevitable companion of gravitation. What if gravitation, working over distances of billions of light-years, has different effects than those with which we are familiar on a local scale?
Historically, scientists have assumed that the laws of physics are the same everywhere in the Universe. By observing the structures and spectral emissions of distant galaxies, we can see that they appear to behave in a manner similar to those closer to us. But are the physical constants the same? We would like to think so, but we do not know with absolute certainty. If the distant history of our Universe, according to the Big Bang theory, is any indicator, we have reason to suspect that physical constants change with time. Because time and distance are inextricably linked on a cosmic scale, the very foundations of reality may not be the same at the end of a long intergalactic journey as they were at the beginning. How might this affect human beings making such a trip?
If a spacecraft could be accelerated to high enough speed, time would flow more slowly for the occupants of the vessel than for everything in the surrounding Universe. This is the result of relativistic time dilation, which you learned about in Chapter 16. This could work to the advantage of interstellar and intergalactic travelers in some ways, although it would be a mixed blessing.
Suppose that we have a spacecraft with powerful engines that can boost it to almost the speed of light. We’re going to visit a star system 100 light-years away (Fig. 19-6). We won’t spend any time at the distant system once we get there; this is just a trial run to see if we can do it. Our space ship has a propulsion system so advanced that it averages 99.995 percent of the speed of light during the outbound and return journeys! This produces a time dilation factor of 100. For every second that passes in the outside Cosmos, only 0.01 s passes for us in the ship.
The round-trip distance is 200 light-years. We arrive back in our Solar System, according to Earth-based timekeepers, slightly more than 200 years after we departed. This is longer than a lifetime. When we return, we do not know a single person on our home planet. Even our children are deceased (assuming medical technology has not extended the average human lifespan severalfold). However, according to our own sense of time, the whole round trip has taken only a little more than 2 years.
This time dilation effect is no illusion. It is real. We only need to provide for a 2-year journey, even though it takes 200 years according to time-keepers in the greater Cosmos. If we could get even closer to the speed of light, we could travel to stars 1,000 light-years, 10,000 light-years, or even 100,000 light-years distant in the same 2 years. Our Milky Way galaxy is 100,000 light-years across.
Extrapolating still further, there is no limit to the distance we might travel within the span of 2 years according to our sense of time, provided that we could accelerate to a speed arbitrarily close to the speed of light in free space. However, if we dare to travel over intergalactic distances, we must realize that we are leaving home forever. Even if we return to Earth, millions or billions of years will have passed. Will human beings even exist any more? If so, what will they be like? What about the climate? If we are gone more than about 4 billion years, the Sun will be in its red-giant phase when we return, and Earth will not be the sort of planet we would want to visit, if it exists at all.
Will we ever make journeys like this if the technology becomes available? In his book Journey to the Stars , Robert Jastrow expresses the belief that we will. Journeys of this sort have been undertaken before. South Pacific natives searched for, found, and settled mysterious islands north of the equator (now known as Hawai‘i) by embarking into the unknown in outrigger canoes. European adventurers discovered lands in a new hemisphere (the Americas) after sailing across stormy seas. These people placed little importance on “returning home.” To them, “home” was wherever they were bound. The most daring of our descendants, if and when we build ships that can travel among the stars, will have the same attitude. They will venture into the Cosmos because the alternative— not to—will be unthinkable.
Practice problems of this concept can be found at: Traveling and Living in Space Practice Problems
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