Written in 2019 after some discussions about science fiction.
Recently I’ve had a couple of interesting conversations with people interested in Science Fiction about the speed of light. They’ve been interesting not just because of the physics involved, but also because of where the idea often leads.
The physics is basically as follows. The speed at which light travels seems to be the fastest possible speed. In fact, however fast I move and in whatever direction, any light arriving at my location from any direction always does so at the same fixed speed. This isn’t because of Relativity particularly, it’s a fundamental consequence of Electromagnetism. Maxwell was aware that a changing electric field causes a magnetic field, and vice versa. When he calculated how fast this effect could propagate, he got the speed which had already been measured for light – thus showing what light really was. But the puzzling thing was that his calculation didn’t depend on what coordinates you work in or how fast the observer is moving, so the speed of light can’t depend on those things either. That seems contradictory: surely if I move towards the light source I’ll see light coming to me arriving faster? No. Measurements show that no such thing happens, and Einstein finally explained how this was possible with Relativity. It turns out that, as you increase your speed, space and time prove to be a bit more complicated than first appeared: they adjust size and duration so that light always has the same observed speed.
This means that, according to our current knowledge of physics, travelling faster than light will never be possible. This puts a minimum journey time on a mission to send a spacecraft to another star system. If the distance is in light-years, the minimum time is that number of years. Thus, it will take at least 4 years to send anything to the nearest star or more than 26000 years to send it to the centre of the galaxy. Those are minimum times; allowing for accelerating and decelerating will make them much longer. There is one apparent loophole: time dilation. Because of the behaviour of space and time identified by Einstein, you might not actually experience the passing of 4 years while journeying to the nearest star; it could seem a lot shorter. Anyone watching from Earth by telescope will see your journey take more than 4 years though, and more than 8 years are sure to have elapsed on earth if you return.
My conclusion from all this is that faster-than-light travel is impossible and therefore most of the space even in our own galaxy will never be accessible to us. Here is where things get interesting! I find that other people are reluctant to accept this conclusion even though it’s clearly supported by physics. I’m repeatedly told that “Yes, that’s the state of our knowledge now, but who knows what will happen in the future? Science has turned out to be completely wrong before, and this could happen again”. My reply is that, no, I think our present physics is to be trusted. Actually science is very rarely completely overturned (in fact I can’t think of a really good example in the past), and progress consists of taking into account finer observations and altering theories to suit them. All of our understanding of Electromagnetism (and so also electricity, magnetism and subatomic particles) would have to be badly flawed to change the conclusion about the speed of light; that’s even before bringing Relativity into it. As far as Relativity is concerned, the subtle adjustments to the behaviour of space and time which it predicts have actually been measured. The orbit of Mercury is explained by them, and relativistic time adjustments have been applied to make GPS work (because GPS satellites whizz around at significant speed).
So why don’t people like this conclusion? I get asked to consider all sorts of escape routes: wormholes, warping space, etc. There is often a certain degree of desperation. I always reply that these things are still unknown to physics and remain in the Science Fiction category. And there I think is where the answer lies. The conclusion about travel times flies in the face of popular ideas about humanity’s progress and future. As a culture we’ve bought-in to a far future shown in films like Star Wars in which humanity has gained the freedom of the galaxy and taken its place among alien races. If none of that is possible, what’s in store for our descendants? It’s an indication of how bad the world is now, that the answer “We’ll be staying on this planet” causes such dismay. Even people who firmly reject all forms of religion depend for their world-view on an idea of progress, in which a bright future for humanity offsets the dire situations many humans live in now.
And yet, I think it’s better to be realistic than clutch at straws to believe in an impossible far future. Once I was discussing proposed methods for terraforming Mars with someone. I commented that I would believe that terraforming another world was possible when I saw desertification in North Africa halted and reversed. For some reason, this was considered more difficult than “doing” a whole planet! I think the reality was that it just wasn’t such an exciting idea and the work involved was more evident. Yet, how much more useful it would be.
I have a keen interest in astronomy and the possibilities of future space travel. I’d like to see manned missions to the moon again. However I have to go along with Bill Bryson when he comments that anyone who thinks it will be ever possible to travel to Mars just doesn’t understand how far it is. And I’m convinced that our understanding of physics is complete enough to clearly rule out any possibility of faster-than-light travel; which means we can’t easily reach any other star system and most of even our own galaxy is inacessible. My interest doesn’t motivate me to clutch at straws by believing in a future where the laws of physics can be suspended.
Those who know me know that I enjoy Science Fiction but my future hope depends on God, not human progress. I think relying on Him for the future is better than trusting ideas from Science Fiction. It's paradoxical that many people do trust science fiction so much when it turns out to rely on rejecting what we know about the universe from actual science.