In Our Time: Perpetual Motion

Perpetual motion would be a wonderful thing, if only it were possible – being able to set some machine going and then it would power itself and just carry on & on without end. Free energy from nothing! Which is, of course, why it is impossible – but this wasn’t provable until relatively recently. Discussing the search for, and disproof of, perpetual motion on In Our Time were Ruth Gregory (Durham University), Frank Close (University of Oxford) and Steven Bramwell (University College London).

Before the modern understanding of physics there didn’t seem to be any reason why perpetual motion should necessarily be impossible. In the Aristotelian view of the universe the stars were in perpetual motion in the heavens – so there must surely be some way to replicate this on earth with earthly machinery. This wasn’t (solely) the province of charlatans – people like Leonardo da Vinci, Robert Boyle and Gottfried Leibniz were all involved in attempts to design machines that could power themselves forever. Various approaches were tried – like trying to design a waterwheel that not only ground corn but also pumped the water back uphill so it could flow down again. Or a bottle that siphoned liquid out of itself in order to refill itself. Or some sort of machine that was constantly over-balancing – like an Escher drawing of a waterwheel with buckets labelled 9 travelling down one side, and when they reach the bottom they flip round to now read 6 so they’re lighter. Which works beautifully in the illusory world of Escher’s art but rather less so in our mundane reality. As well as people genuinely trying to investigate the possibility there were also those who claimed to have achieved success – normally with machines that conveniently couldn’t be inspected to expose their charlatanry.

Once physicists started to gain a greater understanding of how the universe worked it became clear that perpetual motion machines were fundamentally impossible. All proposed perpetual motion machines violate either the first or second law of thermodynamics. Before moving on to explain how these laws affect perpetual motion machines they digressed slightly to explain some of the background to the formulation of the laws of thermodynamics. First they gave us the technical meaning of the word “work” in a physics context – important in understanding the rest of the discussion. Work is the energy that is applied to do something. For instance if you want to move something then work = force x distance. Or if you’re heating something up then work refers to the energy you need to cause the temperature change. Experiments by Joule were key to showing a link between heat and energy. Before his work the prevailing theory was that heat was a thing (called calor) that could be transferred between objects – so a fire heated a pot because calor was transferred from flames to pot. But Joule showed that you could generate heat using energy, and later it was realised that heat was a form of energy. Reception of his work at the time (the mid-19th Century) was mixed – the temperature changes he was study were very small and not everyone believed it was possibly to accurately detect them.

The First Law of Thermodynamics is that energy must be conserved in a closed system. I.e. you don’t get something for nothing. When work is done it all turns into motion or heat or some other form of energy. Many perpetual motion machines violate this law, and they are termed “perpetual motion machines of the first kind”. An example of this is a waterwheel that both grinds corn and pumps the water back up to the top to start over again. In order to pump all the water back up you need to use just as much energy as it generated for you on the way down – so there none left over for your corn grinding, even if your machine is perfectly efficient (see below).

The Second Law of Thermodynamics is that entropy always increases or remains the same, it never decreases. Gregory used the example of a room that’s either tidy (a single ordered state) or untidy (many possible disordered states). In order to move from disordered to ordered you need to do work, otherwise over time the random chance will move objects from their positions in the room and it will become more disordered. The Second Law of Thermodynamics is associated with time – it provides directionality to the universe, if things are getting more disordered then they’re moving forwards. Perpetual motion machines which violate this law are categorised as “perpetual motion machines of the second kind”.

Another way that perpetual motion machines can violate the laws of physics is by being too efficient. I touched on that above – in the real world no machine operates without losing some energy (generally in the form of heat due to friction). And so even if you aren’t trying to do anything useful with the energy other than keep the machine moving you’ll still fail to achieve perpetual motion as you won’t have quite enough energy to return to the starting point.

So perpetual motion is impossible, as it would violate the laws of physics. There are some loopholes at the quantum level (aren’t there always?). Implications of the Heisenberg Uncertainty Principle mean that it’s possible to “borrow” energy temporarily from the future, which means the First Law of Thermodynamics doesn’t quite apply. But at the macro level these laws are inviolable and perpetual motion is impossible. They finished by saying that if a way to make a perpetual motion machine work was found then it wouldn’t just be a case of minor tweaks to physics-as-we-understand-it. Instead it would require a re-writing of pretty much all science we’ve ever conceptualised – the laws of thermodynamics are that fundamental to our understanding of the universe.

In Our Time: Relativity

Physics is one of those subjects where I can very clearly see the boundaries of my understanding – as soon as we get to quantum physics or Einstein’s theories of relativity I can follow the surface level explanations & analogies, but I’m always aware I don’t understand it on a deeper level. I assume the same is actually true of all subjects at some point – I’m not a genius, and I spread my self-education widely among many subjects rather than deeply delving into one – but for physics I can see the fence. It’s a peculiar sensation.

The three experts who talked about Einstein’s theories of relativity on In Our Time were Ruth Gregory (Durham University), Martin Rees (Astronomer Royal and University of Cambridge) and Roger Penrose (University of Oxford). The programme started with a bit of context: in 1905 Einstein published four papers, including one on Special Relativity. At the time he was working as a clerk in a patent office & was previously unknown as a physicist. Ten years later he published a paper extending Special Relativity into General Relativity.

Prior to Einstein’s theories of relativity the assumption was that there was some sort of objective measure of time in the universe, the same no matter how it was observed. Einstein theorised that the motion of the observer affected the observation of the passage of time – hence relativity. Apparently he later regretted using that word for his theories because it’s been used since to imply that physics is all just subjective & depends on your point of view, but actually there is still an objective physical reality which can be described mathematically & rigorously it’s just that within the system the point of view of the observer is important for the observations made.

One of the things that Einstein’s theories grew out of was the observation that the speed of light remains constant no matter what direction you’re travelling in or how fast you’re travelling. This seems to be a paradox. Say you think about driving a car towards or away from another car that’s driving towards you – when you’re travelling towards it, it gets closer to you quicker than if you’re travelling away from it. (I hope that makes sense.) But with light if you’re travelling towards it it appears to be travelling the same speed as it travels if you’re travelling away from it. Einstein’s theory explains how this happens by explaining how time is running differently (I think).

Special Relativity implied that time is another dimension like the spatial dimensions, and Minkowski built on this theory to mathematically describe spacetime. Einstein then used this mathematics as part of his theory of General Relativity. One of the key insights of General Relativity is that spacetime is curved by the presence of mass and this curvature explains why gravity exists. Gregory used an analogy I’ve heard before to describe spacetime & its curvature – thinking of spacetime as being like a four-dimensional version of a two-dimensional rubber sheet. If you have your rubber sheet suspended as a flat horizontal plane and then you put something large like a bowling ball on it, the sheet will be distorted & curved where the ball weighs it down. Then if you roll a marble across it it will accelerate down the slope towards the bowling ball – or if you get your angles and speed right you can make it orbit the bowling ball.

There was some discussion of the twin paradox at two different points in the programme. This is a thought experiment where you have twins one of which remains on Earth, and the other one travels away to a different star system at close to the speed of light, and then returns. When the twins meet again the one that stayed on Earth will be older than the one that went to the stars and back. This is a staple of science fiction, and I think the first time I ran into the idea was in “Time for the Stars” by Robert A. Heinlein which I read when I was at middle school. The first time it was discussed on the programme was in the context of Special Relativity as the way of demonstrating what Einstein is talking about. And they mentioned that this has actually been shown experimentally – by getting a very accurate clock (synchronised with a matching clock) and putting it on a plane and flying it to the other side of the world & back. Then when you compare the two clocks the one that travelled has measured less time than the one that stayed put. Gregory pointed out that the observations demonstrate both the effects of relative motion and the effects of distance from a massive object (the maths needs to take into account that the plane is up in the air while the other clock is on the ground). I had no idea prior to this programme that the effects were measurable on such a human scale.

The second time the twin paradox came up was in the context of talking about the geometry of spacetime. Penrose was explaining that with his theories Einstein was trying to explain the universe in geometrical terms. Spacetime is four-dimensional, three dimensions are the familiar spatial ones that can be explained using Euclidean geometry. For the fourth dimension, time, Einstein (and Minkowski?) showed that you could use almost the same geometric rules only needing to reverse a sign – turn a plus to a minus. The way Penrose explained what he meant by this was to use the twin paradox – one twin is moving from event A to event B along a straight line in the time dimension, the other is moving from A to B on a curved line in the time dimension. For the spatial dimensions a curved line is a longer path than a straight line, for the time dimension a curved line is a shorter path than a straight line. (And this is what I mean by being able to see the edge of my understanding – I can write that last sentence as a fact and accept it is true, but I don’t understand why or how.)

I know I’ve missed out various things they discussed but I shall only mention another couple before I finish the post. Firstly there are real world applications of the theories of relativity, it doesn’t just help physicists understand the universe – it’s an important part of the underpinning of how GPS works. The other thing was that Rees was saying that Einstein was in some ways more like an artist than a scientist. By this he meant that for an artist their work is generally unique, if they didn’t exist no-one else would produce the same artworks. But for science generally if one person doesn’t come up with the theory or do the experiment then someone else would not long after. Rees thought (and the other two agreed) that while Special Relativity would probably have been thought of by someone else soon after, General Relativity was such a large jump that if Einstein hadn’t thought of it then we might still have not thought of it.