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#31
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Solar sailing DOESN"T break laws of physics'
On 6 Jul 2003 10:44:05 -0700, in a place far, far away,
(Alex Terrell) made the phosphor on my monitor glow in such a way as to indicate that: If photons have mass, and if they travel at c, how come they don't have infinite mass? Since they don't have infinite mass, they can have no mass, and therefore no momentum. You were doing fine until that last part. How do you conclude that no mass means no momentum? Please don't misintepret me. I do "believe" in solar sails. I'm just trying to figure out how it works, because, in the normal world: You mean the world in which light doesn't live? -- simberg.interglobal.org * 310 372-7963 (CA) 307 739-1296 (Jackson Hole) interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org "Extraordinary launch vehicles require extraordinary markets..." Swap the first . and @ and throw out the ".trash" to email me. Here's my email address for autospammers: |
#32
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Solar sailing DOESN"T break laws of physics'
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#33
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Solar sailing DOESN"T break laws of physics'
In article ,
Alex Terrell wrote: Momentum = mass * velocity If mass = 0 and velocity = 3E8, then momentum = 0 Please tell me what I'm missing? In a relativistic world, things moving at the speed of light follow different rules. Momentum is not m*v; rather, it is m*v/sqrt(1-v^2/c^2), which reduces to approximately m*v when vc. As is easy to see, for m=0 and v=c, momentum is 0/0, i.e. indeterminate. Which is not surprising, since photons of different energies carry different amounts of momentum. Photon momentum is, in fact, E/c, where E is the energy. -- MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer first ground-station pass 1651, all nominal! | |
#34
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Solar sailing DOESN"T break laws of physics'
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#35
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Solar sailing DOESN"T break laws of physics'
Alex Terrell wrote:
If photons have mass, and if they travel at c, how come they don't have infinite mass? Since they don't have infinite mass, they can have no mass, and therefore no momentum. You were doing fine until that last part. How do you conclude that no mass means no momentum? Please don't misintepret me. I do "believe" in solar sails. I'm just trying to figure out how it works, because, in the normal world: Momentum = mass * velocity If mass = 0 and velocity = 3E8, then momentum = 0 Please tell me what I'm missing? Photons have momentum inversely proportional to their wavelength, independent of their rest mass (which is zero). P = h / lambda This is also equal to their energy divided by the speed of light: P = E / c See for example pp 1109 in Halliday and Resnick 2nd ed. -george william herbert |
#36
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Solar sailing DOESN"T break laws of physics'
In article ,
Alex Terrell wrote: (Gregory L. Hansen) wrote in message ... If photons have mass, and if they travel at c, how come they don't have infinite mass? Since they don't have infinite mass, they can have no mass, and therefore no momentum. You were doing fine until that last part. How do you conclude that no mass means no momentum? Please don't misintepret me. I do "believe" in solar sails. I'm just trying to figure out how it works, because, in the normal world: Momentum = mass * velocity If mass = 0 and velocity = 3E8, then momentum = 0 Please tell me what I'm missing? Momentum isn't always mass * velocity. We find out what the real world is like by running experiments, and light has momentum but no detectable mass. It has energy, though, and mass itself is related to rest energy by m=E/c^2. -- "Is that plutonium on your gums?" "Shut up and kiss me!" -- Marge and Homer Simpson |
#38
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Solar sailing DOESN"T break laws of physics'
In article , (Alex Terrell) writes:
The effect of solar radiation pressure has been observed. This seems to be one of many experiments that appear to break a fundamental law of Physics (Carnot's rule). Nothing is broken here. (I read about another where light was made to trevel faster than light, but no information went faster than light). The interesting bit is always finding out how the laws of physics are conserved, and Geoffrey's done just that. May I ask a more basic question? If photons have mass, They don't and if they travel at c, how come they don't have infinite mass? Since they don't have infinite mass, they can have no mass, and therefore no momentum. Nope, wrong. Mass and momentum are separate things. Mati Meron | "When you argue with a fool, | chances are he is doing just the same" |
#39
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Solar sailing DOESN"T break laws of physics'
In article , (Alan Anderson) writes:
(Alex Terrell) wrote: If photons have mass, and if they travel at c, how come they don't have infinite mass? Since they don't have infinite mass, they can have no mass, and therefore no momentum. You were doing fine until that last part. How do you conclude that no mass means no momentum? Please don't misintepret me. I do "believe" in solar sails. I'm just trying to figure out how it works, because, in the normal world: Momentum = mass * velocity If mass = 0 and velocity = 3E8, then momentum = 0 Please tell me what I'm missing? You're missing the difference between "the normal world" and "Newtonian physics", that's all. The relativistic formula for momentum has an extra factor which increases as velocity approaches c, becoming infinite when v=c, making that formula inapplicable to photons. What is zero times infinity? Before you even start getting to relativistic physics, the point missed by many is that even in classical physics momentum *is not* defined as mv (yes, there is quite a lot of physics beyond high school physics). Momentum is defined as a gradient of the Lagrangian (yes, I know this doesn't mean much to whoever didn't study it but, as I said, there is lots of physics beyond high school physics). In the particular case of a classical massive particle, this *evaluates* to mv but that's a result, not a definition. For other entities you get different result. Thus, even within classical physics electromagnetic waves carry momentum even though they're massless. Mati Meron | "When you argue with a fool, | chances are he is doing just the same" |
#40
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Solar sailing DOESN"T break laws of physics'
"Steve Harris" wrote in message ...
If there's no change in photon energy to first order, then obviously that's a breaking of Carnot's law to first order, since Carnot requires an decrease in photon temperature (photon energy) for work to be extracted. True, I snipped context and all, but I love the way "to first order" sometimes takes on a meaning all of its own -- as if it meant something without even specifying "with respect to X". Carnot requires two thermal baths of different temperatures for kinetic energy to be gained. Which are present when a solar sail is working. But it's okay, because in any inertial frame where work is being done on the sail, you see two populations of photons (those coming and those leaving), and these two DO have two different temperatures. That's it. Ok. Start with a heat bath at temperature T. Extract a wee bit of energy from it to do some useful work. Wind up with a heat bath at temperature T - dt. Viola! Two different temperature heat baths! Ok by thermo! :-) No ... if that were the out, the second law would be empty. The two different temperature heat baths were there at the start of the problem, _before_ we introduced the sail. The are separated in momentum space rather than physical space. Of course the photons don't have to be charaterized by temperatures at all ... the sail could work with pure monochromatic radiation. Yet even if we had a complete Stefan-Boltzman distribution of frequencies characteristic of TK, but with the momentum vectors all pointing in one direction, that would _not_ correspond to envelopment in a heat bath at temperture T. If diferent regions of momentum space are populated by photons distributed differently in energy, whether or not according to some temperatures, that's enough to get work out of the system. And that's why a solar sail illuminated by the distant sun on one side and exposed to the icy cold inky blackness space on the other, can work. So saying, the great Shaman snapped closed his dearhide covered book of tales, and shooed the little braves off to bed to dream -- to dream of the illiminatable black crow winging forever in blackness on sooty wings while the coyote of the abyss howled his ceaseless song of unnerving beauty and infinite sadness and the rosy sun-squaw is tucked in her bed, waiting for a dawn that may never come. And darkness descended on the world. Amen. |
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