Monday, October 28, 2013

Opposites: Open and closed

Are they really so opposite?
As Niels Bohr used to say, the opposite of an ordinary truth is a falsehood, but the opposite of a profound truth is another profound truth. I offer this opposite pair:

1) All systems are open.
2) All systems are closed.

A closed system is a set of physical things which can be regarded as isolated from the rest of the universe. An open system, in contrast, is affected by things outside itself, even if those things are not directly observed. So these statements are certainly opposite. How are they both true?

What defines 'the system'?
Experiments try to isolate variables, but we can never achieve perfect isolation. Vacuum chambers are made of steel walls, and over time a few stray gas atoms always percolate in and out of tiny cracks or pores in the steel surface. No laboratory building is perfectly insulated from vibrations. High energy cosmic rays can pierce any barrier; and so on. It may be possible to achieve isolation that is excellent for all practical purposes, but all physical systems are open, strictly speaking. 

If we really want to speak strictly, however, then the very concept of a ‘system’ is inherently an approximation. There is really only one system: the universe. The universe as a whole is closed by definition, so all systems are closed. Of course, it is no less impossible in practice to describe the whole universe than it is to seal off a portion of the universe in perfect isolation from the rest. It is often possible, however, to describe a very large closed system.

And indeed this is precisely what we normally do, to identify the distinctive physical features of an open system: we analyze a large closed system, and then discard all the information that does not refer directly to the small sub-system that represents our ‘open system’.

So any system is open, if we want it to be: it is only a matter of how low we set our threshold for ignoring slight influences from external factors. Conversely, however, any system is closed, if we want it to be: it is only a matter of how large we are willing to make our system, to bring relevant external factors within its frame. The distinction between open and closed systems is an important one, but it is not a distinction between two different ways things can really be. It is a distinction between two different ways of thinking about things. Both ways can be good ways of thinking. Both truths are profound.


An engine would still run inside a large box.
It seems to me that too many physicists today have lost sight of the second truth, however. The most profound mystery that physics still faces is the origin of irreversibility. We don't understand why we can't remember tomorrow. And whatever is going on in quantum measurement, it seems to be an empirical fact that all quantum measurement devices rely crucially on thermodynamically irreversible processes to achieve their extreme amplification. No-one can find a clear explanation of irreversibility within closed-system Hamiltonian mechanics, but few people want to accept that our mighty science is still stumped by such a basic question after a century of breakneck progress, so most people like to think that the open system generalization must be the simple solution.

Open systems can't be the basic explanation of irreversibility, because all systems are also closed. Whether or not a system is open is not a physical fact, but an arbitrary choice of perspective in deciding what to include within the system. So the openness of physical systems cannot make a fundamental difference to anything; anything that can be explained as an open system must also be explicable as a larger closed system. A steam engine would still run, at least for a good long time, inside a big impermeable box.

Thursday, February 14, 2013

Fire Glows

It's not just bright.
Humans discovered fire a long time ago, but for most of that time we only used it for warmth and light and cooking, rather as Bilbo Baggins used his magic ring for years just to avoid unwanted callers. Only in the 18th century did James Watt show up to play Gandalf, and reveal that our curious little trinket was the One Ring to rule them all. Fire has enormous power.

Even after centuries of technological progress since Watt, we still find it very hard to beat combustion as a source of power. Burning a tank of fuel releases enough energy to lift cargo all the way to the Moon, even with the horrible inefficiency of a rocket engine. Combustion provides energy, as one says, to burn. Why is fire such a tremendously greater power source than, say, clockwork springs or a windmill? I’ve never seen a clear answer to this question in any physics text, but I think I have found a succinct one of my own. 

Fire glows.
Light oscillates really fast.
The fact that fire glows demonstrates that fire is releasing energy from motions (of electrons in chemical bonds) with frequencies in the range of visible light. Those are very high frequencies, around 1014 cycles per second. As Planck taught us, energy is proportional to frequency. So if human energy needs are for motion at up to a few thousand RPMS, mere hundreds of cycles per second, combustion lets us tap energy resources on a scale greater by a factor of a million million. Combustion delivers so much energy, because molecular frequencies are so high.

This is what an engine somehow does.
It isn’t easy to gear all that power down by a factor of 1012 so we can use it, though. Electrons whir around in molecules far too fast for our eyes to follow. We can’t just throw a harness over them. Even if we could, they are very light in weight. They bounce off things, rather than dragging them along. To tap them for power, we need some clever way of gently bleeding off their enormous but very rapidly whirring energy, a tiny bit at a time.  There's more to it than just installing an awful lot of tiny gears. 

Getting fire to do work means transferring power across a huge frequency range. That's what thermodynamics is all about. The reason that thermodynamics doesn’t seem very much like the rest of physics is that energy transfer across a huge frequency range is an extreme case, in which certain otherwise obscure aspects of physics become very important. That makes them important in general, though, because high frequencies can deliver so much power. It's well worth learning how thermodynamics really works.

Raising Water by Fire

James Watt dramatically improved the steam engine, but he didn’t invent it. In his time, steam engines were already a practical and economical success. The machines of Thomas Newcomen and Thomas Savery had already begun the new era in human technology. 

Savery had a head for marketing as well as for steam. In 1702 he produced a pamphlet advertising his device as “An Engine to Raise Water By Fire”. His description may have been poetic, but it was literally exact. His engine pumped water by burning coal. Its killer application was draining coal mines. 

Humans may have discovered fire in distant prehistoric times, but the really useful thing about fire was only discovered in the 18th century. Never mind cooking or smelting metal or scaring wolves: fire can raise an awful lot of water. And if you can raise water, you can do pretty much anything, because raising water means you can exert force.

Savery’s and Newcomen’s engines were crude and simple, and by that I don’t mean that they were primitively made, rattling too much or leaking steam. They were just stupid designs, compared to Watt’s machines. They didn’t even use steam pressure to actually do their work, but just let the steam balance atmospheric pressure. Then they condensed the steam, by shooting in cold water, and let the suddenly unbalanced atmospheric pressure do the work. Savery’s engine didn’t even turn any moving parts, but just sucked water through pipes. It wasn’t so much more than a proof of concept, like the aeolipile.

Hindsight is 20/20, of course, and it’s not really fair to call Savery and Newcomen stupid. Watt’s proper steam-pressure engines also needed stronger boilers. The point is that even the crudest engines were such a quantum leap in power technology, compared to wind, water, or animal power, that they rapidly changed the world. In effect they turned lumps of coal into unprecedentedly huge amounts of practical work. Up until 1775, the Russian navy had been using two enormous windmills to drain its dry docks at Kronstadt; each time they drained the docks in order to work on a ship, the draining job took a year. When they installed a single Newcomen engine, it did the job in two weeks.

With coal-fired steam engines, the human capacity to exert physical force suddenly soared. Even today, the biggest problem with changing to power sources other than combustion is that fire can provide so much more power than, say, sunlight or wind. We humans keep thinking wistfully about switching away from combustion, to some form of clean energy, but we really want to maintain our current energetic lifestyle. We're like a big city lawyer who wants to quit the firm and become a social worker, but also wants to keep up the mortgage payments.

Why is fire so very good for raising water? I have some thoughts on this, based on the fact that fire glows.