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If I were in the vacuum of space, and I had a piece of paper in a perfectly stationary position in the middle of space...would it start moving if I turned-on a flashlight on it?

2006-10-13 18:33:36 · 13 answers · asked by entranced82 3 in Science & Mathematics Physics

13 answers

Light is electromagnetic energy. Energy can be captured and made to do work.

I've read about designs for craft powered by light-sails. A light sail would be hundreds of square miles of extremely thin material, tethered to a small craft in the center, something like a spider web with thin plastic stuck to it. The light pressure (not the solar wind) moves the craft away from the sun.

It's science fiction for now.... but who knows.

2006-10-13 18:38:48 · answer #1 · answered by KALEL 4 · 1 0

According to Wikipedia (see below), it isn't actually the force of light that causes the radiometer's vanes to turn. The article does mention, however, that light provides a very small force (so small that it can't turn the vanes of the radiometer, but it apparently answers your question). Interestingly, the explanation of what DOES turn the vanes appears to be still a bit uncertain.

Apparently, the answer to your question about the flashlight and paper in space is that the light would move the paper, but only imperceptibly.

Here's an excerpt from the Wikipedia article:

Over the years, there have been many attempts to explain how a Crookes radiometer works:

1. Crookes incorrectly suggested that the force was due to the pressure of light. This theory was originally supported by James Clerk Maxwell who had predicted this force. This explanation is still often seen in leaflets packaged with the device. The first experiment to disprove this theory was done by Arthur Schuster in 1876, who observed that there was a force on the glass bulb of the Crookes radiometer that was in the opposite direction to the rotation of the vanes. This showed that the force turning the vanes was generated inside the radiometer. If light pressure was the cause of the rotation, then the better the vacuum in the bulb, the less air resistance to movement, and the faster the vanes should spin. In 1901, with a better vacuum pump, Pyotr Lebedev showed that in fact, the radiometer only works when there is low pressure gas in the bulb, and the vanes stay motionless in a hard vacuum. Finally, if light pressure were the motive force, the radiometer would spin in the opposite direction as the photons on the shiny side being reflected would deposit more momentum than on the black side where the photons are absorbed. The actual pressure exerted by light is far too small to move these vanes but can be measured with devices such as the Nichols radiometer.

2. Another incorrect theory was that the heat on the dark side was causing the material to outgas, which pushed the radiometer around. This was effectively disproved by both Schuster's and Lebedev's experiments.

3. A partial explanation is that gas molecules hitting the warmer side of the vane will pick up some of the heat i.e. will bounce off the vane with increased speed. Giving the molecule this extra boost effectively means that a minute pressure is exerted on the vane. The imbalance of this effect between the warmer black side and the cooler silver side means the net pressure on the vane is equivalent to a push on the black side, and as a result the vanes spin round with the black side trailing. The problem with this idea is that the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the force on the vane should be exactly the same — the greater temperature causes a decrease in local density which results in the same force on both sides. Years after this explanation was dismissed, Albert Einstein showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there. The force predicted by Einstein would be enough to move the vanes, but not fast enough.

4. The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds, but first published by James Clerk Maxwell in the last paper before his death in 1879. Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behave like the pores in Reynolds's plate. On average, the gas molecules move from the cold side toward the hot side whenever the pressure ratio is less than the square root of the (absolute) temperature ratio. The pressure difference causes the vane to move cold (white) side forward.

Both Einstein's and Reynolds's forces appear to cause a Crookes radiometer to rotate, although it still isn't clear which one is stronger.

2006-10-13 19:32:05 · answer #2 · answered by actuator 5 · 0 0

I think the bulb in the flashlight would create heat which can produce forces, heat rises for example. In a vacuum there is no molecular activity however so I think the paper would not move in a vacuum in space. Have you ever seen those candle holders that have fan like wheels above them and the heat rising from the candle causes the wheel to spin and rotate a mobile or something. Well, I think a flashlight wouldn't be powerful enough but a high power halide lamp or something that gives off heat could do the same thing. Not in a vacuum though, a vacuum is the ab sense of anything right, so if you put a flashlight in it wouldn't/couldn't be considered a vacuum by definition. Your question is very thought provoking, I will be thinking about it for days. I think outside of a vacuum a strong light could be proved to give off heat which can be used as a force to be felt or move things.

2006-10-13 18:52:54 · answer #3 · answered by Sheila D 2 · 0 0

Yes, i've seen designs of spacecraft with sails on them. Since there is no resistence to movement in space, the extremely small amount of force from light would slowly accelerate the spacecraft. It would take millions of miles and years to accelerate the object at a significant rate, but it would work. If you think you could propel yourself by holding the paper and flashlight you would be wrong. The theory of Conservation of Momentum says that for any light and force emitted by the flashlight towards the paper, there will be an equal amount of force pushing the flashlight the other way, so no net movement.

2006-10-14 04:55:18 · answer #4 · answered by Nick G 2 · 0 0

The impact of photons can be translated into motion, even on Earth. There are little devices with glass surrounding a pin wheel like design and as long as it is in the light the device will rotate from the force of light, Check Edmund's Scientific catalog. On your specific example the type of paper might be important as space is a very hazardous environment for certain materials, cold and heat can be extremes. Also the angle of the paper to the sun is vital to being effected by photon propulsion if the force was to move the paper to an edge on attitude then very little force would be effecting it. This sun /photon force effect which is commonly called "solar wind", and devices called solar sails are designed to take advantage of this effect. I even designed a number of years ago a "Solar Chute" that could be launched via space probe and deploy a net to snag a meteorite, and then use a steerable solar chute to help guide it away from impact with the Earth. But the meteorite would need to be spotted well in advance, as the force exerted by Solar wind is very minor, even less than was calculated years ago.

Note also that as a "Force" photons can be directly transferred into multiple forms of "force", such as Solar cells which convert light into electrical force, which also is convertible into electromagnetic force and indeed can be reconverted yet again into another frequency, which is called light. Karma Physics.

2006-10-13 18:53:27 · answer #5 · answered by Mystery 3 · 0 0

Yes. A simple demonstration of this can be observed with a radiometer (a very inexpensive device that can be purchased at most planetariums and some museums.) It consists of a transparent glass enclosure (about the size of a light bulb) The air has been removed from the enclosure. Inside this vacuum is a spindle with four "leaves" attached at 90 degree angles. Each "leaf" is black on one side and white on the other. When the device is exposed to a light source, the force of the light causes the spindle with its "leaves" to rotate. The brighter the source of light, the faster the rotation.

2006-10-13 18:53:33 · answer #6 · answered by RG 4 · 1 0

Sure, it would. Light has momentum and energy just like a fllying tennis ball has and it will be partly transferred to the piece of paper when it gets absorbed or reflected on it. There have actually been some vague ideas of making light space ships flying far out in cosmos using the force of the light from the sun.

2006-10-13 19:09:27 · answer #7 · answered by First L 2 · 0 0

Yes and yes. But the acceleration would be *very* low. Probably only on the order of a few µm/s².

But the idea of using very large 'sails' (on the order of hundreds of km²) made from very thin plastic (aluminized mylar, or some such) to act as a 'Solar Sail' within the vicinity of a star has been seriously considered by NASA.


Doug

2006-10-13 18:39:28 · answer #8 · answered by doug_donaghue 7 · 2 0

You might find this article on light sails interesting. Apparently they are already in use for minor attitude corrections for satellites.

http://en.wikipedia.org/wiki/Solar_sail

The article on the Nichols Radiometer is also interesting.

http://en.wikipedia.org/wiki/Nichols_radiometer

2006-10-13 19:21:36 · answer #9 · answered by Helmut 7 · 0 0

Yes, and yes. And if you let the flashlight go, it would move away from you like a rocket... in very, very, ... very slow motion.

2006-10-13 19:04:52 · answer #10 · answered by Jason T 3 · 0 0

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