Implications for very high-speed/interstellar space travel

“In conclusion, I return to Einstein. If we find a planet in the Alpha Centauri system, its image, captured by a camera travelling at a fifth of light speed, will be slightly distorted due to the effects of special relativity. It would be the first time a spacecraft has flown fast enough to see such effects. In fact, Einstein’s theory is central to the whole mission. Without it, we would have neither lasers nor the ability to perform the calculations necessary for guidance, imaging, and data transmission over twenty-five trillion miles at a fifth of light speed.

We can see a pathway between that sixteen-year-old boy dreaming of riding on a light beam and our own dream, which we are planning to turn into a reality, of riding our own light beam to the stars. We are standing at the threshold of a new era. Human colonisation on other planets is no longer science fiction. It can be science fact. The human race has existed as a separate species for about two million years. Civilisation began about 10,000 years ago, and the rate of development has been steadily increasing. If humanity is to continue for another million years, our future lies in boldly going where no one else has gone before.

I hope for the best. I have to. We have no other option.”

― Stephen Hawking, Brief Answers to the Big Questions

If the considerations in the above sections are correct, they lead to conclusions that are, like special relativity itself, rather surprising, not terribly easy to accept; and for which there are some curious implications. The first would be that the velocity of any object (A) could only be referred to in relation to some other object (B) in the Universe – otherwise the term would be meaningless. The second would be that whether there is a relative velocity, or no velocity, between A and B, would have no effect on A or B, other than to change the way each perceives the other (if they are moving relative to each other). The third would be that either A or B could be made to accelerate, in accordance with Newton’s laws of mechanics, so that their relative velocity increases up to and beyond c. This latter hypothesis would have implications for a subject that was discussed earlier in this book – specifically interstellar travel. We can imagine a spacecraft that employs a developed and efficient form of chemical rocket propulsion, and is able to accelerate at 1g (which is convenient for the travellers on board, since it will provide them with artificial gravity). As this occurs, the situation on board the spacecraft is that a rocket motor is expelling gases from the rear, thereby providing an action that will have a reaction effect of accelerating the craft forward (according to Newton’s laws of motion). These effects will be exactly the same no matter what its distance and velocity is relative to Earth (if we ignore the effects of the Earth’s gravitational field). Therefore, when the craft has attained a velocity of 0.999c relative to Earth, we can imagine there would be no difficulty in accelerating to 1.111c and then continuing to accelerate. In this scenario, the craft would have surpassed the speed limit of c that has traditionally been assumed to exist for human travel. When the craft reaches the halfway point in its travel, it would rotate through 180 degrees and then decelerate at 1g, again providing artificial gravity and enabling it to be stationary once it has reached its destination. It is worth noting that there are some practical problems that would need to be addressed for very high-speed (order of c) space travel. One example problem is the need to carry sufficient fuel to fire the rocket motors for extended periods (days or weeks); while another is heating of the craft exterior that would arise from friction from the very low density of hydrogen atoms that occupy deep space. However, such challenges would need to be addressed through future application of advanced technologies; and do not comprise grounds for believing such travel to be impossible. For example, if the craft were travelling to the red dwarf star Proxima Centauri, it would take just over 4 years to get there using the method outlined in this book – consequently one could make the return journey in less than a decade. This star is particularly interesting, since in August 2016 discovery was announced of Proxima b, which is the exoplanet closest to us, as well as orbiting within Proxima Centauri's circumstellar habitable zone (aka ‘Goldilocks Zone’). Unfortunately, though, if there is any extra- terrestrial life on Proxima b, it would have to be extremely hardy/robust. The reason for this is that in March of 2016, astronomers observed that Proxima Centauri emitted a ‘superflare’.

The flaring red dwarf star Proxima Centauri, with the orbiting exoplanet Proxima b. (Illustration by the author.)

This is rather like the solar flares we observe coming from the Sun, but immensely larger – so large, in fact, that it was bright enough, can you believe it, to be visible to the naked eye! Such superflares would produce high intensities of UV radiation that would, over time, break down any ozone layers in Proxima b’s atmosphere that are needed to protect any organisms on the planet from harmful UV radiation from the star. Consequently, the chances of finding life on this exoplanet may be rather slim.

Although we may not find life on Proxima b there are, of course, multitudes of other star systems in our Galaxy. Observations show that red dwarfs are the most common stars in The Milky Way (comprising around 75%), and it also has to be acknowledged that recent research indicates that about 60% of such stars experience flare activity. Even so, there remain billions of stars that are very likely to also have orbiting exoplanets in the Goldilocks Zone. This is surely an exciting prospect – the question is, how can we get out there and explore this vast potential? Perhaps somewhat surprisingly, the relatively conventional and attainable methods of transportation proposed above might also enable travel to many of these more distant stars; since a continuous acceleration of 1g enables very high relative velocities to be attained in reasonable timescales. For example, using this method a spaceship travelling 50 light years might attain a maximum speed of around 22.5c, meaning that this journey could be achieved in only 5 years. There are 133 stars within 50 light years of earth. Most of these are very similar to the Sun and it is probable that there are many Earth-like exoplanets around these stars. So, the stars might be within reach after all … which is indeed an intriguing prospect. Before we can say the above conjectures are correct, we need more definite proof; but if they are, journeys to exoplanets of sizes similar to the Earth, orbiting stars similar to the Sun, within the habitable zone i.e. POTENTIALLY HABITABLE PLANETS, would be achievable in the not too distant future and it would only take the order of a few years to travel each way. That prospect would be more than intriguing – if you ask me it would be as exciting as hell.