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7月30日 Riding a SunbeamTowards the end of my second semester physics classes, we show that light can push on things. Yes, you heard that right. Light can push on things. Light shining on an object exerts a force on that object. Of course, Newton's third law also is at work here, so the think shining the light is also pushed by shining light. Now, it isn't much force involved. You aren't going to have to worry about a flashlight leaping out of your hand due to its recoil. But there are a few really powerful lasers that exist which do experience significant recoil. So, how much does light push on things? Well, normally we derive the pressure, of the force per area in terms of light intensity. And, it is really simple. To find the pressure, just divide the intensity (measured in power per area) by the speed of light. Now, the speed of light is a huge number, so unless you've got a ridiculously large intensity, then you get a really small number when you divide. So, light has very low pressure. Low pressure normally means low force. This is for light being absorbed. If light is reflected, then you get double the pressure, and hence double the force. Now, another interesting thing is that there are two ways to derive the light pressure. For my calculus based class (physics and engineering majors, mostly) you can derive the light pressure quite easily using Maxwell's Equations. James Clerk Maxwell put four equations together (that other people came up with) and got a set of equations that can then be solved for the wave properties of light. But, Maxwell's equations are calculus equations. And manipulating them to yield light pressure involves calculus. That is perfectly appropriate for that class. However, I also sometimes teach a non-calculus based class (life science majors, architecture majors, and pretty much all the rest that need physics). In this class, we can't really do Maxwell's Equations right. But, interestingly there is an entirely different way to derive light pressure. Special Relativity only requires high school algebra to derive the basic equations. And, from special relativity, you can show that light must carry momentum. A change in momentum is associated with a force, so when light is absorbed or reflected, you get a force. Both approaches yield the same answers. It sure is nice when things work out that way! For a long time, this light pressure result was an interesting mathematical curiosity. After all, the pressure was always such a small number that it would never really matter, right? But, astrophysics is a realm of extremes. Three cases immediately come to my mind in astrophysics where the light pressure really matters. The first case is comet tails. Comets, are made of collections of dust and frozen gasses. The gasses sublimate and spew out from the nucleus along with dust grains. Light pressure from the Sun acts on some of these dust particles to push them away from the nucleus of the comet to form a tail. Gas molecules are caught in the solar wind and pushed away to form another tail. A second place where light pressure shows up in the Solar System is with rotating asteroids. As asteroids rotate, the side towards the Sun heats up, and the side away from the Sun cools off. This means that the side of asteroid where the Sun is setting is warmer than where the Sun is rising. Warmer means a tiny bit greater intensity of infrared light emited. This means that there is a slight push on the asteroid due to this assymetry, and this push can over millions of years alter the course of an asteroid. This effect on asteroids is called the Yarkovsky effect. A third place in astrophysics to see the effects of light pressure is with very bright objects. For example very high mass stars are very hot and very bright. If they are too bright, the light pressure starts to push away their outer layers. In effect, they tear themselves apart. But, it isn't just stars. The accretion disks of black holes or neutorn stars also get hot and bright. If they are too bright, then they too push material away. Sir Arthur Eddington first showed that there is a maximum brightness that something can be before it tears itself apart due to light pressure. So, we call this maximum brightness the Eddington limit. But, now that we know about light pressure, can we use it for anything? Let's look at these cases that I 've mentioned. One (the comet) involves sunlight pushing on very tiny things, dust particles. Another (the Eddington limit) involves exceedingly bright objects. The Yarkovsky effect is a tiny effect, but it adds up over sufficiently long time periods. Space scientists proposed several decades ago that perhaps we could use light pressure from the Sun to push spacecraft around the Solar System. Rockets have severe limitations. They push hard for a while, but only for a short while until they use up their propellant. Light pressure would not push hard, but it adds up. You'd have low acceleration, but if you keep it up, then that would still yield high speeds. So, if you had a big enough mirror to catch and reflect sunlight (remember, reflecting gives twice the effect of absorbing), then you could push a spacecraft around. The problem is making a spacecraft light enough. Remember, the dust particles in a comet's tail are tiny. But, if you have a big enough mirror, they you get a pretty decent force. As long as the spacecraft, and mirror, are light enough, then you could use this as a propulsion source. NASA is in fact working on this concept. Or at least they have been. There's no telling with all the budget cuts going on. But, comet tails are always pushed away from the Sun. Light pressure could push a spacecraft away from the Sun, but what if you wanted to come back? No problem! Remember the Yarkovsky effect pushes the asteroids in the direction of their cooler side (the warmer side has more push). If they rotate in a prograde manner (in the direction that they are moving around the Sun), then the back of the asteroid is warmer, so the asteroid speeds up and moves into an orbit farther from the Sun. But, if an asteroid rotates in a retrograde manner (the other way), then the front side is warmer, so the asteroid slows down and falls closer to the Sun. Solar sails on a spacecraft could work this way, too. Set one way, they could speed up the spacecraft and it would spiral outwards from the Sun. Set the other way, then the spacecraft would slow down and spiral inwards closer to the Sun. And best of all, there is no need for propellant. The problem, though, is that you need a huge mirror, and it has to be lightweight. This becomes an engineering problem: to build a mirror of several square miles area, but only a few tons of mass. Right now, we can't do that, but it is theoretically possible. If someone finally manages to make a working solar sail, then we could ride sunbeams anywhere we wanted in the Solar System, using just light pressure. -Astroprof 引用通告引用此项的网络日志
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