Lecture excerpt: On the subject of momentum and reactionless propulsion
“There is a theory that states that nothing exists. It goes like this. The law of conservation of momentum states that at any one time, the total momentum of the Universe is zero. Since momentum is derived from mass and velocity, both of which are just forms of energy, then all the energy in the Universe cancels out neatly, leaving nothing left behind. Given this is so, one interpretation is that every event, every unit of matter, every jot of work has its exact opposite somewhere out there in the big wide cosmos. This premise formed the basis of a curious sci-fi flick from, I think, the early sixties, in which an astronaut flew to the other side of the Sun and landed on an identical, but back-to-front, copy of the Earth.
“Now, this is all fine and dandy when it comes to science fiction, but when it comes to science fact, the principle of momentum conservation is a fundamental, unshakeable bed of solid rock. Everything that has been observed by mankind, forever, in every circumstance, has obeyed this law. It has been described in countless ways by thousands of great thinkers over hundreds and hundreds of years, although the most famous expression of it you probably recognise as: ‘Every action has an equal and opposite reaction’.
“So, what does it mean? Well, imagine you are in a chair. You are in deepest space and you aren’t moving. Let’s assume you have a spacesuit on or something similar that prevents you from being dead. Now, you decide you want to move your space chair. How would you achieve this? There is nothing for you to ‘push’ against, you can’t ‘swim’ the chair forwards by waggling your arms, there seems to be nothing you can do to move.
“But think carefully about what actually happens when you execute a swimming stroke. You are pushing yourself along obviously, but you are also pushing the water. You go one way, and the water goes the other. An action, and a reaction. Elementary physics, and something we all experience in everyday life.
“So, returning to your interstellar space chair, what if you pushed something away from you? Say the large cup of coffee you are holding that I happened to forget to mention? The coffee mug would move away from you, and you would move away from the coffee cup. And there you have it, you have managed to propel yourself in your space chair! Give yourself a pat on the back for inventing a latte-fuelled spacecraft engine.
“The term we use to describe the product of forces and velocities in these interactions is momentum. The sum of the momentum in a system like this starts out at zero - no force and no velocity. After the event, you have exerted a force on the cup and both you and the cup now have velocity. Performing the calculation again gives the same result - zero momentum. One body will now have a positive momentum, the other a negative momentum, but the total amount of the system adds up to zero.
“Now, I can’t stress enough to you that this is a fundamental property of our universe. Why? Well, in our universe it is believed that position is meaningless - no point is any more significant than any other. This is an axiom that forms the bedrock of cosmology and has endless observational evidence to support it. Wherever you are, whichever direction you look, space looks and acts the same. And if you study this with rigorous mathematics it turns out that the conservation of momentum is an absolute consequence of this.
“So, given that momentum is conserved without condition, what does this mean to us? Well, the most significant repercussion is that we cannot produce an engine for a spacecraft that doesn’t rely on the ejection of a propellant of some form. In order to push a rocket forward, it has to send something out of the back of it. Whether this is rocket exhaust, magnetically accelerated ions or even jets of water or air (or maybe a cappuccino), it doesn’t matter, as long as something is ejected. There are some people out there who are convinced they can build a ‘reactionless’ drive, that is one that doesn’t emit any propellant. This would be a fantastic device as it would mean very little fuel would be required. Unfortunately, no reactionless drive has yet been demonstrated to work, and it is unlikely that such a thing could ever work as it would immediately imply that the law of conservation of momentum does not hold, and therefore that the Universe is in fact somehow inherently ‘unbalanced’.
“The big question that faces us, then, as we move into an uncertain future, is how and if we can spread out amongst the stars. Or is the human race forever confined to the planet Earth, the fates of both inexorably intertwined?”