Interstellar travels
Author: Tomasz Jędrzejewski
The basis of the dreams about the space is the condition that one day a group of brave men will get on the most advanced vehicle ever built and begin a journey to planets moving round other stars. In the world of books and films, interstellar travelling seems to be extraordinary easy: we just set an enormously high speed and get off. The reality is much more complex, because in fact, the scientists still do not know, whether a construction of suitable engine is possible on Earth. The main enemy of human are cosmic distances. To explain, how big they are, let us show a model. Imagine that Earth is a size of an apple. In this scale, the Moon would lie 3 meters from it. To the Sun, we would have about 1 kilometer, and the nearest star, Alpha Centauri must have been located on the real Moon. Although we see a lot of different object, when we look at the sky, the real „bricks” that form the universe are vacuum, photons and neutrinas. To show the amount of the matter in the space, we will use another model presented by Isaac Asimov: „it is like all the matter in the universe was a seed of a sin in the middle of an empty room with a 30-kilometer edge”. Contrary to such distances, reaching of even our neighbour stars seems to be impossible, if we cover the length of the human life.
Constraints of physics
The major obstacle in the fast universe colonization are the physics laws. The Special Theory of Relativity says that an object cannot move in the space faster than the light speed (300,000 km/s). Although for a typical human it is an enormously huge number, we would have to travel for 4.3 year just to reach the nearest star. On Earth, a 4-year journey is nothing unusual. A lot of well-known explorers have not seen their homes for much more time. However big speeds cause new strange effects. People tend to think that the time goes by constantly, with no changes, and if something is 100 meter long, it is absolutely true. But it is an illusion. The theory mentioned above expects that there is no absolute reference frame. In other words, if we see a motionless box, we can find another reference frame, where the box moves. Moreover, in both of them, the light speed is constant. The time and space are being shortened or lenghtened for the observers in order to keep this condition. The invalid idea survived thousands of years just because the space and time transformations are extremely small (near zero) for slow speeds, like those ones on Earth. But let us imagine a rocket travelling at half the light speed. Inserting this value to the suitable mathematical equations we discover that the time for its crew goes by much slower. The rocket is moving for 5 years, but their families on Earth age 15 years. It is a serious problem for the society. Their results are showed in Stanislaw Lem's novel, "Return from the Stars".
The general conclusion drawn from the Relativity Theory is that the speed cannot be increased to the infinity. There is a kind of limit – the light speed. No object is able to travel faster. Moreover, the higher speed, the greater mass, and near the maximum values, the mass goes to the infinity. We do not have to explain that there is no spaceship able to survive in such situation.
Casual engines
Currently, the problem is to reach even 1/1000 of the light speed. The record achieved by a human-made probe is 252,792 km/h and this is about 70 km/s. However, it belongs to a probe that orbits the Sun. The spaceships with a jet propulsion can give just 40,000 km/h. It is not enough in order to hold the communication even with near planets and we have to design new means of transport to reach stars.
On 24 October 1998, American probe Deep Space 1 has been launched. The mission goal was to test serveral new space technologies, including a new kind of drive, ion thruster, which is approximately ten times efficient than the others used so far. It uses beams of ions, electrically charged atoms or molecules, for propulsion. The ions archieve the speed of 36 km/s in a magnetic field. The mass of the gas is not big, so the ion thruster is not able to speed up rapidly. In the first stage of flight, the probe travels at milimeters per second. The real effects of using ions are visible after a couple of time.
The scientists are interested in antimatter. They discovered in the late twenties of 20. century, that every particle has its own anti-particle. Both of them have the same mass, but the opposite charge and other quantum numbers. For example, the protons are positive, and antiprotons – negative. The fans of Dan Brown who read the book „Anges & Demons” know that mixing of matter and antimatter leads to annihilation, thus giving rise of gamma rays. The annihilation of 1 kg of antimatter emits the energy comparable to the explosion of 43 million tons of TNT. Althogh 50% is „overtaken” by neutrinos, but the rest is still a huge amount, produced with no pollution. The scientists hope that the starships with antimatter propulsion will become a success. We need a tiny piece of it to achieve higher speeds, which is much easier to transport than the fuel for today's chemical engines.
The greatest geographical discoveries on Earth were made with sailing ships. There is a chance that sails will be useful also in the space. Of course, they will not resemble the views and images we know from the seas, because they will use much more sophisticated physical processes. A space sail is developed to use light instead of wind to produce thrust. The scientists say that such a probe is able to travel at 1/10 light speed after some time required to accelerate. What is more, the technology is not a scientific fantasy, because it has been already tested on the orbit by NASA. The experiment showed that it really works.
Between the fiction and reality
Although the technologies described above are being developed nowadays, they will not allow the humans the colonization of whole galaxy because of the problems presented at the beginning of the article. They are useful, if we talk about a single solar system or short distances. The further exploration requires a lot of new discoveries in physics and solving many fundamental questions about the nature of the universe. The idea behind the new engines could be explained in the following sentence: „If your starship cannot reach a neighbour galaxy, make the galaxy comes to your starship”. The word „engine” does not exactly mean, what it should, because the new inventions will not push the vehicle through the vacuum, but they will change the shape of the space directly.
Many people dream about the wormholes, a kind of spacetime tunnels. Let us imagine the space as a piece of paper. To move from the top left to the bottom right corner, we have to cover the whole distance by default. However, we can scroll the paper so that the corners are side by side. Now we can make a simple jump, ignoring the rest of our „space”. The idea behind a wormhole is similar. We scroll the spacetime to connect two distant points with a kind of short tunnel. The starships may now travel through it like through the door. The Relativity Theory does not disallow such wormholes to exist, however the recent discoveries show that they are very unstable structures and are closed immediately. To host the wormhole for longer time, the gate must have a kind of negative energy, or better, a way to produce it. The scientists suppose that it really exists and the conclusion comes from the black holes which emit a radiation.
A wormhole is a big change of the space. Apart from the physical issues, we even do not know, how to „mark” the destination area, so that the tunnel would lead us to the place where we want to be. It is hard to imagine a random space scan, hoping that we find something interesting. Einstein has given us another way to make our dreams come true. We know that two clocks which are motionless for each other, show the same time. We can assume that if we put these clocks in two distant parts of the universe, they will still work. However, the universe is growing up and the clocks get further and further, but the equations do not change and they still show the same time. It turns out that the light speed limit concerns the objects and processes that take place in the space, but does not concern the space itself. It can change at every speed – the light and the objects move with it and the physics laws are not broken. Miguel Alcubierre from the University of Wales has proven that it is possible to obtain such a spacetime configuration, where we can move between every two points in as short time, as we want, without the clock desynchronization. The trick lies in the space transformation. We shrink the space before the starship and widen behind it. The ship is hold by a kind of spacetime capsule and it can even have an ordinary engine. But to produce such a capsule, we also need the negative energy.
Our civilization created a lot of models and is able to prove that they allow to travel faster than light. However, we still do not know whether the physical laws accept them or whether we have enough power to construct suitable devices.
Danger
Our journey to the stars causes not only the technical problems. We have to form a question, how the expansion influences the human race. It is possible that edge colonies would lost their contact to the planet Earth. The discovery of a solution that would prevent us from dividing into serveral races is necessary, if we do not want a kind of horrible star wars. The law also will have to change, so that the group of the strongest powers would not appropriate the whole universe and keep the rest of people on the rocks of Earth.
The end
The first starships will get off in hundreds or even thousands of years. Currently, we have problems with keeping a base on the Moon, located just one lightsecond from Earth. The neighbour planets have been „settled” by mechanical probes, so far. There is a lot to do for the scientists.