It would be much more promising, however, if the stars outside of our solar system could also be considered in this context, because the number is enormous even of only those celestial bodies that, since they are in a white-hot state, are visible and, therefore, are known to us as fixed stars. Many of these are similar to our sun and, as powerful centers of gravity, are probably orbited exactly like the sun by a number of small and large bodies of varying types.
Shouldn't we expect to find among these bodies also some that are similar to our planets? Of course, they are too far away for us to perceive them; however, probability speaks strongly for their existence. In fact, the most recent scientific research as one of its most wonderful results has been able to show that the entire universe, even in its most distant parts, is both controlled by the same natural laws and structured from the same material as the Earth and our solar system! At other locations within the universe, wouldn't something similar, in many cases almost the same thing, have to materialize under the same conditions (from the same matter and under the effect of the same laws) as in our case?
It is certainly not unjustified to assume that there would be other solar systems more or less similar to ours in the universe. And among their numerous planets, there surely would be some that are almost similar to the Earth in their physical and other conditions and, therefore, could be inhabited or populated by people from Earth, or perhaps they may already be populated by some living beings, even intelligent ones. At least the probability that this may be the case is significantly greater than if we only consider the relatively few planetary bodies of our solar system.
Yet, would it really be conceivable that those immeasurable distances still separating us even from the closest fixed stars could be traveled by humans, even taking into account the limit that is set by the average life span of a person, completely ignoring the related necessary technical performance of the vehicle?
Let's assume that a goal, which appears enormous even for today's concepts, has been achieved: perfecting the rocket propulsion system to such an extent that an acceleration of approximately 15 m/sec2 could continually be imparted to the space ship over a very long time, even through years. Humans would probably be able to tolerate this acceleration over long periods of time through a gradual accommodation. To travel a given distance in space, it would then be possible to accelerate the vehicle continually and uniformly over the entire first half of its trip, that is, to give it more and more velocity, and to decelerate it in the same way over the second half and consequently to brake it gradually again (Figure 100). With this method, a given distance will be covered in the shortest possibly achievable time with given constant acceleration and deceleration.
Figure 100. Covering a distance when the vehicle is uniformly accelerated over the entire first half of the distance, and similarly the vehicle is decelerated over the second half. The highest velocity of motion resulting from this method is reached at the midpoint.
Key: 1. Midpoint; 2. Accelerated over this distance; 3. Decelerated over this distance; 4. Distance to be traveled; 5. Direction of motion; 6. Velocity curve; 7. Instantaneous velocity.
If the trip now took place to neighboring fixed stars in this manner, then the following travel time would result for the entire round trip (the first visit would have to be a round trip) based on mathematical calculations: 7 years to Alpha Centauri, the star known to be the closest to us, and 10 years to the four fixed stars next in distance; numerous fixed stars could be reached in a total round trip travel time of 12 years.
However, it is quietly assumed here that any velocity, without limitation, is possible in empty ether space. In accordance with the theory of relativity, a velocity greater than the speed of light of 300,000 km per second can never be attained in nature.
If this is taken into account and if it is further assumed that no other obstacle (currently unknown to us, perhaps inherent in the nature of universal world ether) would prevent us from attaining travel velocities approaching the speed of light, then we could, nevertheless, reach the fixed star Alpha Centauri in around 10 years, the four farther stars in 20 years, and a considerable number of neighboring fixed stars presently known to us in 30 years; these figures represent total round trip travel times.
For the one-way trip, which would be of interest for continual traffic, half as much time would suffice. No doubt, trips of such duration would be fairly close to the limit of human endurance; however, they cannot yet be discarded as completely non- implementable, since no fundamental obstacle can, in fact, be seen for reaching the closest fixed stars.
Meanwhile, the question still remains open as to whether vehicles could ever be built having the technical perfection necessary for such performances. However, even this question cannot be answered with an unequivocal no because, as has been pointed out previously, natural phenomena exist that could provide possibilities, such as exploiting the energy bound up in matter by smashing atoms, or utilizing the decay of radium, or cathode radiation, etc.
Admittedly, we are far away today from that goal of completely mastering such natural phenomena to such an extent that we would be able to use them in an engineering sense for space travel purposes! And, we don't know whether this will ever be successful at all.
As far as is humanly possible to predict, the sons of our time will hardly achieve this. Therefore, the fixed stars, which conceal the great secrets of the universe in their immensity, will no doubt remain unreachable for them. Who can say what scientific triumphs and technical potentials future times will bring! Since mankind has now become confident with scientific reasoning, what tremendous progress is achieved today in only a few decades; and what are a hundred, even a thousand years in those eons of human development still ahead of us.
Conquering space! It would be the most grandiose of all achievements ever dreamed of, a fulfillment of the highest purpose: to save the intellectual accomplishments of mankind for eternity before the final plunge into oblivion. Only when we succeed in transplanting our civilization to other celestial bodies, thus spreading it over the entire universe, only when mankind with all its efforts and work and hopes and with what it has achieved in many thousands of years of striving, only when all of this is no longer just a whim of cosmic events, a result of random incidents in eternal nature's game that arise and die down with our little Earth so large for us and yet so tiny in the universe will we be justified to feel as if we were sent by God as an agent for a higher purpose, although the means to fulfill this purpose were created by man himself through his own actions.