Moon Storms?, Where the dust blows form + to - Options

[edstrick]

Tommy Gold, like Fred Hoyle, was on of those tremendously gifted scientists who at times seemed constitutionally unable to a "Quantitative and Qualitative idea-matrix analysis" of the evidence for and against a theory or model. Rather, they seemed infinitely able to select and adjust the weighting on their evidence to optimally bias the interpretation of the evidence in favor of their highly, often over-quantitative conclusions.

But they were both so sharp, they were really hard to argue against. And Tommy Gold always (Fred not always, in later years) was worth listening to, no matter how cracked his idea might sound, cause there was always a sneaking suspicion he might well be right... AGAIN.

[Guest_BruceMoomaw_*]

The impression I got of Gold even when I was 15 -- based on his ever-changing Moon dust arguments -- was that he tended to regard scientific theory as Silly Putty. The low point of his career, however, surely came shortly before his death, when he announced that solar sails couldn't possibly work because any real physicist knew on thermodynamic grounds that photons couldn't exert pressure (and never mind that they'd been routinely used for spacecraft attitude control since 1964, and that the effect was the only thing that saved the Mariner 10 mission!)

[ljk4-1]

The Mysterious Smell of Moondust

NASA Science News for January 30, 2006

Long after the last Apollo astronaut left the moon, a mystery lingers: Why does moondust smell like gunpowder? In this installment of Apollo Chronicles, astronauts describe the surprising smell and taste of moondust, which they experienced first-hand inside their lunar landers. The dust gave one astronaut a case of hay fever. What does it all mean? To find out, read the FULL STORY:

http://science.nasa.gov/headlines/y2006/30....htm?list161084

Find out about the Science@NASA Podcast feed at

http://science.nasa.gov/podcast.htm .

[abalone]

The Mysterious Smell of Moondust

NASA Science News for January 30, 2006

Long after the last Apollo astronaut left the moon, a mystery lingers: Why does moondust smell like gunpowder?.

Aw that's easy, they never actually went to the moon and gunpwder was the cheapest black dust they could find to fill the movie studio.

No smoking on the set please!!

[ljk4-1]

Science/Astronomy:

* Solving Settlement Problems: Dealing with Moon Dust

http://www.space.com/adastra/adastra_moondust_060223.html

As scientists and engineers figure out how to return astronauts to the Moon, set
up habitats, and mine lunar soil to produce anything from building materials to
rocket fuels, they are scratching their heads over what to do about Moon dust.

[ectoterrestrial]

The impression I got of Gold even when I was 15 -- based on his ever-changing Moon dust arguments -- was that he tended to regard scientific theory as Silly Putty. The low point of his career, however, surely came shortly before his death, when he announced that solar sails couldn't possibly work because any real physicist knew on thermodynamic grounds that photons couldn't exert pressure (and never mind that they'd been routinely used for spacecraft attitude control since 1964, and that the effect was the only thing that saved the Mariner 10 mission!)

- I've chased you around this maypole before, elsewhere, and I won't start now. Here. I will say that I hope that after you've passed that you get more credit for your legacy than you give Tommy for his.

[Guest_Richard Trigaux_*]

The impression I got of Gold even when I was 15 -- based on his ever-changing Moon dust arguments -- was that he tended to regard scientific theory as Silly Putty. The low point of his career, however, surely came shortly before his death, when he announced that solar sails couldn't possibly work because any real physicist knew on thermodynamic grounds that photons couldn't exert pressure (and never mind that they'd been routinely used for spacecraft attitude control since 1964, and that the effect was the only thing that saved the Mariner 10 mission!)

How photon pressure happens?

It seems impossible that the light could exert any kind of mechanical effect.

But when a photon hits an atom on the surface of something, it is absorbed (it stops existing as a particule) but its energy is still there, and it is changed into mechanical energy. In some cases this mechanical energy is added to an electron's energy (the electron gets on an upper orbit, and is said "excited", it can even be expelled) but in most case it is transferred to the whole atom, which gets some impulse, into the direction the photon was going to.

And an atom moving inward a surface soon hits the other atoms. The gross result is that the movement is transfered from atom to atom as a wave, usually called a phonon (elementary quantum of sound) and many phonons moving into a body make heat, quite simply. But a part of the movement is transferred as an impulse to the whole set of atoms: a force which moves it. So the whole something recoils, pushed by the light.

[abalone]

But when a photon hits an atom on the surface of something, it is absorbed (it stops existing as a particule) but its energy is still there, and it is changed into mechanical energy. In some cases this mechanical energy is added to an electron's energy (the electron gets on an upper orbit, and is said "excited", it can even be expelled) but in most case it is transferred to the whole atom, which gets some impulse, into the direction the photon was going to.

Sound like a good story but how does one use this analogy to explain that a surface that emits a photon experiences a reactive pressure in the opposite direction, and a surface that reflects a photon experiences a pressure 2X that of a surface that absorbs it.

It is best to to regard a photon as having momentum which has to be consereved both during absorption, reflection and emission.

But when a photon hits an atom on the surface of something, it is absorbed (it stops existing as a particule) but its energy is still there, and it is changed into mechanical energy. The energy of the photon is much greater than this.

The photons energy is totally different to if moment. A photon does not exert pressure because it energy is changed into kinetic or mechanical energy

[Guest_BruceMoomaw_*]

The fact that photons have momentum is an inevitable consequence of E= MC squared. It is true that photons have no REST mass (which is one respect in which they differ from all particles of matter)-- but that's simply because, by their very nature, they are never at rest. They do possess MOVING mass -- or, as it's officially called, "relativistic mass" -- equal to the energy they contain. (In fact, ALL moving material objects also possess additional mass equal to the energy of their movement, which is why relativity predicts that the mass of a moving object actually increases as it moves at closer and closer to lightspeed.) There are some nice Google discussions of this at
http://www.physlink.com/Education/AskExperts/ae180.cfm
http://hyperphysics.phy-astr.gsu.edu/hbase...tiv/relmom.html
http://math.ucr.edu/home/baez/physics/Part...hoton_mass.html

[Guest_Richard Trigaux_*]

Sound like a good story but how does one use this analogy to explain that a surface that emits a photon experiences a reactive pressure in the opposite direction, and a surface that reflects a photon experiences a pressure 2X that of a surface that absorbs it.

It is best to to regard a photon as having momentum which has to be consereved both during absorption, reflection and emission.
The photons energy is totally different to if moment. A photon does not exert pressure because it energy is changed into kinetic or mechanical energy

This is not a theory, but what I learned at school, about how photons are absorbed. But I don't know all the possible processes.

In the case of an emission, I guess that there is some reverse process of that of the absorption (for instance in a heated surface, a phonon is absorbed, thus removing a momentum from the surface) but I am not sure of this.


In the case of a reflection, it is a very different process, as the photon is not absorbed. But, as an electromagnetic wave, it induces a current into the reflective surface. This current in turn results in another wave, the reflected wave, that the photon follows. But in order to give an impulse to the surface, the photon must lose energy, and thus lower its wavelength, a thing which is preciselly not observed into a reflection. Are you sure of what you say about a 2X force? If it is true, I don't know exactly what happens. Perhaps simply a Lorentz force between the incoming wave and the current induced into the surface.


What I am sure on the other hand is that a photon don't have a momentum, and you are both false, abalone and Bruce. Momentum is the product of mass per speed square (Ec=1/2 mv2), and it is an energy. Mass of a photon is null, and speed is the speed of light, equivalent of an infinite speed in classical mechanics. The current notions of mechanics don't apply into these conditions, and especially zero multiplied by infinite can give either zero, a discrete value, or an infinite value. It happens that the actual energy of a photon has no relation with its speed. You can no more take the energy of a photon, and put its speed in the formula to calculate its mass.

So radiation pressure result of subtle interactions of light with matter, not of a mechanical reflection of grains of light. If it were so, a beam of sun light (1kw/m2) would slam our faces, and sun light would project the planets into space.

[dilo]

Richard, you are right when you say "The current notions of mechanics don't apply into these conditions, and [..] the actual energy of a photon has no relation with its speed". In fact, energy of a photon is related to it's wavelenght. If you can accept that a zero-mass photon can transport energy, why shouldn't transport momentum too?
In fact, pressure of light is equal to it's energy density; this, considering the huge velocity of light, brings to a very diluited pressure... in fact, the light which impact the earth from sun transport 1370 W/m2 but, in a second, this light covers a distance c; this give a energy density of (1.37E3 J/s/m2)/(2.998E8 m/s)=4.57E-6 J/m3.
In other words, sunlight pressure is only 4.57E-6N/m2, about 22 billion times smaller than atmospheric pressure! (on the Earth surface, is 30% smaller due to atmosphere loss).
Not only this amount is impossible to feel, it is comparable or smaller than other effect perturbing planetary orbits (eg solar wind pressure or other planets attraction) so cannot really disturb orbits.
However, it push cometary tails and, perhaps, one day will help to make interstellar trips

[Guest_Richard Trigaux_*]

Richard, you are right when you say "The current notions of mechanics don't apply into these conditions, and [..] the actual energy of a photon has no relation with its speed". In fact, energy of a photon is related to it's wavelenght. If you can accept that a zero-mass photon can transport energy, why shouldn't transport momentum too?
In fact, pressure of light is equal to it's energy density; this, considering the huge velocity of light, brings to a very diluited pressure... in fact, the light which impact the earth from sun transport 1370 W/m2 but, in a second, this light covers a distance c; this give a energy density of (1.37E3 J/s/m2)/(2.998E8 m/s)=4.57E-6 J/m3.
In other words, sunlight pressure is only 4.57E-6N/m2, about 22 billion times smaller than atmospheric pressure! (on the Earth surface, is 30% smaller due to atmosphere loss).
Not only this amount is impossible to feel, it is comparable or smaller than other effect perturbing planetary orbits (eg solar wind pressure or other planets attraction) so cannot really disturb orbits.
However, it push cometary tails and, perhaps, one day will help to make interstellar trips

Errr... I don't want to be too affirmative, but a 1m2 sail receives 1300 watts of light energy. If only a small part of it was transferred into mechanical energy, this sail would be slammed away. We cannot reason as if photons were little bullets, with a mass and a finite speed. If it was so, of course, when bouncing off a reflective surface, they would transfer a part of their energy to the surface, from the collision. And with a very minute mass compared to the sail, they would have a very minute effect, what is observed. But photons have no mass, and they go at the velocity of light, so this reasoning cannot be made in this way. But the effects exist, no doubt. Perhaps the solution is a relativistic version of the mechanical analogy above. But I don't know it.