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Optical Levitation, Space Travel, Quantum Mechanics and Gravity 82

An anonymous reader writes "Light doesn't just make things brighter; it can also push things around. Normally this "radiation pressure" force is so small you don't notice it. But if you get a really big mirror then you could use it to power a space ship to the stars. This is the idea behind solar sails. The impact of light is more obvious on small things. Scientists are thinking about levitation of a mirror that would be large enough to see with the naked eye. If this turns out to work, the motion of the floating mirror could be used to probe the physics that connects quantum theory and general relativity."
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Optical Levitation, Space Travel, Quantum Mechanics and Gravity

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  • by kenwd0elq ( 985465 ) <> on Sunday June 01, 2014 @08:38PM (#47144017)

    There's nothing new about the idea of spacecraft being propelled by light pressure. There was an Arthur C. Clarke story published in "Boy's Life" in the early 60's about sunlight powered "sailing yachts" in a race from Earth orbit to the Moon. Or the Niven story "The Fourth Profession", in which an alien trading ship arrives at Earth, wanting humanity to build a launching laser to send the crew on the next leg of their journey.

    And it's been 30 years since Niven & Pournelle published "Mote In God's Eye" in which an interstellar probe riding a the combined beam of battery of laser cannons arrive in human space.

    So if actual human physicists are finally going to get around to proving the concept, so much the better!

  • Hmmm. (Score:5, Informative)

    by jd ( 1658 ) <> on Sunday June 01, 2014 @08:45PM (#47144051) Homepage Journal

    There are two sorts of solar sail, those that work off photons (and, no, you don't need a mirror, since you can't afford the extra mass) and those that work off ionized particles being emitted from the sun. Ionized particles have much more momentum and are generally considered superior.

    A solar sail that is 50 Km in diameter, attached to a 5 Kg probe, would accelerate that probe to 25% light speed by the time you reached the edge of the solar system.

    If you built a car whose headlights could accelerate the car in reverse with photonic pressure, the headlights would vaporize a considerable chunk of the planet in front of you. You can do the calculation yourself. The equations are at []

  • Re:Hmmm. (Score:2, Informative)

    by Anonymous Coward on Sunday June 01, 2014 @09:52PM (#47144289)

    It's just a detail in your post, but I didn't know solar sails got bigger and heavier as they cooled in such a way that their size in kelvin-meter and mass in kelvin-grams stayed constant. Fascinating.

  • Re:Hmmm. (Score:4, Informative)

    by pushing-robot ( 1037830 ) on Sunday June 01, 2014 @09:54PM (#47144303)

    What did you use for the mass of the 50km sail?

  • Re:Hmmm. (Score:2, Informative)

    by Anonymous Coward on Monday June 02, 2014 @12:17AM (#47144809)

    Actually you absolutely positively DO want a mirror for a photon drive, since it doubles the momentum imparted to you by each photon (sans losses), and only leaves you holding the bag on heat input from those losses, rather than the entire battery of laser cannons. And also somewhat importantly, you can dump most of it to bounce the laser beam back at you and decelerate as you approach your target.

  • Usual /. (Score:5, Informative)

    by hansraj ( 458504 ) on Monday June 02, 2014 @06:01AM (#47145519)

    The summary (and the headline) unnecessarily highlights space travel as a usage for radiation pressure and delegates the most interesting part as a footnote-ish last line. The /. crowd as usual starts shouting pros and cons of space travel, as if every comment on this page is not saying what has already been said a million time around here, and nobody to talk about the interesting part.

    I wish someone with the right background in physics posted something more interesting about the fact that a group of researchers have come up with prediction of how a non-quantized spacetime (gravity) would look in the presence of quantized matter/energy. Apparently this would look different than a quantized background with quantized foreground (IANAP, so I don't know what is this all about) in a measurable way. If they can levitate a tiny but macroscopic mirror using light and balance it then giving it a gentle push would create a pendulum with no friction slowing it down. By probing the frequency evolution one can potentially get closer to actually knowing whether a quantum theory of gravity is the right way to unify QM and GR.

    It's fascinating that such things are possible even in principle with existing technology. I wish someone would explain something more related to this.

No problem is so large it can't be fit in somewhere.