if photons in motion have mass and energy why don't they knock stuff over

Discussion in 'Digital Photography' started by Old Man, Apr 8, 2004.

  1. Old Man

    Old Man Guest

    Quantum or Classical, photon or field, p = E / c (sans h).
    Sans mass, waves and photons push, and they push twice
    as hard when reflected. Momentum is conserved. Observe
    Compton. [Old Man]
    Old Man, Apr 8, 2004
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  2. Do the math. Visible light has a wavelength of 5 * 10^(-9) m. Momentum
    of a photon is p = h/lambda. Thus p is approx. 1 * 10^(-25) kg m/s. Even
    for gamma rays, whose wavelengths are about 10^(-6) smaller, the
    momentum of a photon is only about 1 * 10^(-19) kg m/s. I don't think
    that could knock even an ant or a blade of grass over - do you?

    Bjoern Feuerbacher, Apr 8, 2004
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  3. Old Man

    4B Guest

    What about those small propels that "mysteriously" spin when they are put
    close to a source of light. Is not the momentum of the photons that make
    them spin?

    4B, Apr 8, 2004
  4. Old Man

    CeeBee Guest

    If morons like you have an Internet connection, why are they obsessed
    with trolling and crossposting?

    Go visit the dark side of the moon, and report back if light knocks you
    over there.
    CeeBee, Apr 8, 2004
  5. 1) Don't top-post, please.
    2) *Lots* of photons are acting there together.
    3) You need an apparat with very few friction forces for this to work.
    4) It is constructed especially to maximize the effect, by making one
    side black and the opposite one light-reflecting.

    Bjoern Feuerbacher, Apr 8, 2004
  6. Old Man

    Martin Brown Guest

    And the cheap ones sold commercially spin the wrong way since the vacuum
    in them is relatively poor. Warm gas molecules recoiling on the black
    side of the vane dominate the forces acting.

    When you pump them down to much lower pressures the recoil of photons
    off the mirror side eventually dominates. It is quite a fun demo.

    Martin Brown, Apr 8, 2004
  7. No. The vanes are black on one side, white on the other. This
    results in a temperature differential which causes a pressure

    Paul Cardinale
    Paul Cardinale, Apr 8, 2004
  8. How about making holes in a steel plate. A sufficiently powerful laser
    can put a hole in a 1/2 inch steel plate. If the light is coherent it
    can pack a punch.

    Bob Kolker
    Robert J. Kolker, Apr 8, 2004
  9. Old Man

    Don Stauffer Guest

    A number of deep space probes have had solar reflector attitude control
    panels. These use the photons to help stabilize or move the craft to a
    new attitude using the photon momentum. There have been many proposals
    over the years for solar sail propulsion for space probes.

    You don't ordinarily see the effect because the pressures are very small
    compared to pressures from even the most gentle breeze or air current.
    But they ARE there.
    Don Stauffer, Apr 8, 2004
  10. Dear 4B:

    I'll let you do your own search. Those devices are called "radiometers".
    If the envelope they are contained in contains any gas, then the black side
    moves away from the light source. If there is no gas, the white side moves
    away from the light source. I have never seen a radiometer with the white
    side moving away from the light, so I have never seen a completely
    evacuated one.

    I wonder if any of the space missions ever took a non-enclosed radiometer
    up with them?

    David A. Smith
    N:dlzc D:aol T:com \(dlzc\), Apr 8, 2004
  11. Old Man

    puppet_sock Guest

    The usual setup for such a critter is a small "turbine" with,
    most often, four blades. Each blade is black on one side and
    silvery on the other. The "glib" explanation is that the silvery
    side reflects light, and so gets double the push from each photon
    that hits it.

    The problem with the glib explanation is, the typical device of
    this type that you can buy in a novelty store or such, turns the
    wrong way. They spin such that there would appear to be more
    push on the dark sides of the turbine than the light. Then you
    find out that the bulb this thing is built in does not contain
    a vacuum, but only a very thin gas.

    See, what's really happening is, at least in the "novelty store"
    version of this thing, the black sides are warmer. And warmer
    means the gas next to them is warmer. And that means the gas
    molecules move faster. So it's a little gas turbine, powered by
    the energy of the light involved.

    Let's think about the approximate force that would be involved
    from light pressure. Say you've got bright sunlight, which is
    roughly 1 killowatt/m^2. (Depends on lattitude, clouds, etc.,
    but we are just doing an estimate here.) Suppose the fins were
    2 mm across, giving an area of 4E-6 m^2. So you've got a power
    of 4E-3 Watts. The force is power/c or a bit more than
    1E-11 Newtons. What does the turbine in one of these gizmos mass?
    Say it's .01 grams, or 100,000 of them to the kilo. Probably
    masses more than that, but let's play. So you've got F/m to
    give you the approx acceleration. You'd need to work out the
    torque and know the shape of the silly thing to get it correct.
    This will probably get us within a factor of 3 or so. So F/m
    gives 1E-11/1E-5 or 1E-6m/s^2. So to get the thing going to
    1 cm/s would take about 1E4 seconds, or about 3 hours. And that
    is neglecting any friction. And my experience with these gadgets
    is that they speed up very quickly in bright sun, and slow down
    again fairly quickly when you put them in less light.

    So it's probably not light momentum turning the fins.
    puppet_sock, Apr 8, 2004
  12. Old Man

    puppet_sock Guest

    Um? A number of craft have had solar power panels. Which craft
    have had solar reflector attitude control?
    Proposals yes. Examples built, none.
    puppet_sock, Apr 8, 2004
  13. Old Man

    John Navas Guest

    [POSTED TO rec.photo.digital - REPLY ON USENET PLEASE]

    Mariner IV (the first Mars probe, which I worked on back in 1964) had solar
    pressure vanes, mounted on the ends of each of the four solar panels, that
    adjusted automatically when the attitude jets fired. After they found the
    right balance, the need for attitude gas was so greatly reduced that the
    spacecraft used only a fraction of the attitude gas that would have otherwise
    been required. <http://nssdc.gsfc.nasa.gov/nmc/tmp/1964-077A.html>

    The earlier Mariner II (the first Venus probe) had a solar sail, but it was
    less effective than the four solar pressure vanes on Mariner IV.
    John Navas, Apr 8, 2004
  14. The solar panels on Mariner 10 were used for attitude control
    via radiation pressure, by tilting them appropriately between
    Mercury encounters. This conserved attitude-control gas and
    helped give the third flyby. See

    This was possible because the solar panels had to be large enough to
    deliver power not only at launch but during a Venus swingby, so
    there was more tolerance for off-normal pointing. In fact, the
    status report at
    indicates that the controllers had to tilt the panels on some direction
    anyway for thermal control after the first Mercury encounter.

    Bill Keel
    William C. Keel, Apr 8, 2004
  15. Lets take a mol of these photons then.
    You can indeed lift glass spheres with a laser.
    It needn't be especially powerfull actually.
    A few mW are sufficient.

    Rene Tschaggelar, Apr 8, 2004
  16. Old Man

    Rudy Garcia Guest


    And if the radiometer (the small propeller thingie) has a vacuum inside
    the glass bulb, the propeller spins in one direction, but, if the there
    is air inside, it spins in the other direction.
    Rudy Garcia, Apr 9, 2004
  17. Old Man

    Old Man Guest

    In sci.physics, regardless of the OP's expectations, the point
    is always physics, and there's plenty of that in this thread.
    Jonathan is requested to peddle his cracked pottery elsewhere.

    [Old Man]

    [snip diarrhea]
    Old Man, Apr 9, 2004
  18. Old Man

    Old Man Guest

    At the time of writing, Poincaré probably knew little about
    relativistic invariance. He is forgiven. Jonathan isn't. Even
    Newton knew that acceleration and rotation rate weren't
    relative. Both are absolute and self-referential. Overcast
    skys that hide the "fixed stars" aren't an obstacle to their
    measurement. Local, self-referential, measurements are
    guarantied to reveal Mother Nature. Global concoctions
    are the fairy tales of metaphysics. The local speed of light
    is absolute. E(dot)B and E^2 - B^2 are local relativistic
    invarients. For electromagnetic plane waves, they are both
    absolutely zero, no matter the observer's relative velocity.
    [Old Man]
    Old Man, Apr 9, 2004
  19. Old Man

    Don Stauffer Guest

    What is the one leaving solar system? Voyager, I believe it is. The
    attitude control paddles are on the ends of the solar array panels.
    These paddles are not the entire attitude control. If I remember right,
    there are CMGs for primary, gas jets to desaturate wheel, and the solar
    paddles. The paddles take some of the load off the CMGs, so they do not
    approach saturation as often. There is a limited supply of gas, and
    using it too often to unload CMGs would exhaust gas sooner.

    Now, my memory is not as good as it should be, so maybe it was not
    Voyager, but one of the later probes that went out to Jupiter and Saturn
    (Mariner?). I know it was a deep space probe series, but it has been a
    lot of years since I worked with it, so I am having a hard time
    remembering its name. Anyway, it is one with four solar arrays, with a
    paddle at end of each array. Clearly visible in pictures.
    Don Stauffer, Apr 9, 2004
  20. Well, that's a common misconception: actually, photons are knocking things over
    all the time.
    Richard Schumacher, Apr 9, 2004
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