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Optical Levitation, Space Travel, Quantum Mechanics and Gravity 82

Posted by timothy
from the where-is-the-tractor-beam-I-crave dept.
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) <kenwd0elq@gmail.com> on Sunday June 01, 2014 @07:38PM (#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!

    • Don' forget Forward's Flight of the Dragonfly. Bonus: about half the novel is a technical addendum of the proposed starship design.

      Robert L Forward had the added bonus of being a bona fide physicist and engineer. He didn't goof around, he proposed a terawatt laser system to propel the ship ... with a return stage.

    • The Wikipedia article traces it back to Maxwell and Kepler. And proving has been done, since trajectory calculations have included it since the 60's.

      It just hasn't had enough potential to use in stead of other tech. It needs a killer app, one for which it is uniquely suited, to take hold. Name that and it is a done deal.

    • Propultion is not the problem, there's a glut of possible technologies that coud drive a ship to a nearby star. Food and water are the problem, the most sphisticated biodomes here on Earth only last about a year before they decay into poisinous organic goop. When we have the technology and political will power to fix the life support systems on spaceship Earth we will have the technology to feed interstellar colonists on their journey, only then then we can talk about getting off the solar merry-go-round. U
      • by kenwd0elq (985465)

        "Proven" is, perhaps, the wrong word. "Made to be practical", perhaps. A $5 radiometer from a craft store proves quite readily the idea that light has pressure. The trick will be, as with the nuclear fusion proposals that are perhaps twenty years in the future - and have been "20 years in the future" for thirty years now, to make it big enough and practical enough that we can extract usable amounts of energy from it.

      • by Smallpond (221300)

        Sending adult humans on a 70,000 year trip is pointless. If you want a human visit to the stars start thawing the fertilized eggs 20 years before arrival.

  • Hmmm. (Score:5, Informative)

    by jd (1658) <imipak@nOSPam.yahoo.com> on Sunday June 01, 2014 @07:45PM (#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 http://www.physicsforums.com/s... [physicsforums.com]

    • Re: (Score:2, Informative)

      by Anonymous Coward

      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.

      • by jd (1658)

        Space is filled with dust, so yes, as you travel away from the sun, as the solar sail cools (since less heat is reaching it, inverse square law) it does indeed get heavier. It also gets heavier as it accelerates, due to relativity. It would be interesting to determine what the precise function is. The density of space dust is given in Carl Sagan's book, Cosmos, that was a companion to the series.

    • Re:Hmmm. (Score:4, Informative)

      by pushing-robot (1037830) on Sunday June 01, 2014 @08:54PM (#47144303)

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

      • by jd (1658)

        The calculation was done by NASA and published in a peer-reviewed paper in New Scientist in 1988, I think. As best as I can recall, the solar sail was assumed to also have an initial mass of 5 Kg and to gain mass at a constant rate (since the remnants of the accretion disk should be thinner the further out you go, but you travel through more of it per unit time). I forget what the rate was. As I recall, the paper noted that there would be extreme difficulty in having a sail of such a size that was structura

        • by delt0r (999393)
          New Scientist is NOT peer reviewed. It is in fact full of quite a lot of crap.
      • mass of sail = mass of napkin on which the calculations were done

    • Re: (Score:2, Funny)

      by Nyder (754090)

      ...

      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...

      That seems pointless, because you are going in reverse, think you'd want to vaporize the stuff that would be in the way.

      • by jd (1658)

        If you're wanting to get out of Dodge, what could be better than removing Dodge?

    • Re: (Score:2, Informative)

      by Anonymous Coward

      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.

  • automatic transmission and transistor radio. Also X-ray, general relativity, computer and nuclear reactor. ACME, truck and road intersecting with rail tracks. Dynamite!, rocket engine and jet liner.

    • No, I'm sorry, contestants must phrase their responses as questions.

      Here are some more clues:

      Bucky-ball enema moon tube escalator, Jimmy-Carter-built Baba Yaga affordable housing on chicken legs and dephlogistonated Moebius Yorkshire Pudding.

      • by Thud457 (234763)

        Bucky-ball enema moon tube escalator, Jimmy-Carter-built Baba Yaga affordable housing on chicken legs and dephlogistonated Moebius Yorkshire Pudding.

        Great, you just spoiled the whole new season of Dr. Who.
        With the exception of flailing the sonic screwdriver around like a magic wand and yelling "RUN!". But those are a given with Moffat.

    • Is that to the tune of the The Big Bang Theory theme, or Billy Joel's We Didn't Start The Fire?

    • by Gibgezr (2025238)

      "Things I found in my father's barn."
      I'll take "Oddities of English Cooking" for 500, Alex.

  • the interesting thing is you can force photons out of the way... effectively creating your own pressure gradient which provides accelleration in a direction of your choosing.
  • That is great the levitation theory and radiation pressure using the concept of solar sails is being tested. But, even if this test prove to be a great success, There is no possibility that we will get to see Space Travel in our life times. Ironic Isnt it?
  • Usual /. (Score:5, Informative)

    by hansraj (458504) on Monday June 02, 2014 @05:01AM (#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.

  • So it's light smashing into matter and imparting its energy onto it so that it moves. What's quantum about that? That's only Newton level physics.
  • I imagine you would have more than 7 years bad luck for breaking this mirror!

Entropy isn't what it used to be.

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