Levitating With Light

The University of Pennsylvania has a team that did a little light research. Well, not light in the usual sense of that phrase. They used very strong light to levitate Mylar disks in a vacuum chamber.

Of course, it is no secret that light can exert pressure. That’s how solar sails work and some scientists have used it to work with aerosols and the like. But this appears to be the first time light lifted a large item against gravity. The team claims that their tests showed that a sunlight-powered flying vehicle might carry up to ten milligrams of payload. That doesn’t sound like much, but it’s impressive and the paper mentions that since the lift is not from aerodynamic forces, there might be applications in flying at very high altitudes.

The Mylar disks were 500 nanometers thick and had a 300 nanometer layer of carbon nanotubes beneath. The nanotubes absorb light, make the disks more rigid, and improve the Mylar’s surface-gas characteristics. The light source had a strong center beam and an even stronger ring around the center beam that causes the disk to remain over the center beam. The LED system used eight arrays, each consuming 100 watts of input power.

Preparing the disk might be difficult, but the LED power isn’t that hard. Even if you do like the researchers did and use water cooling.

30 thoughts on “Levitating With Light

          1. @fiddlingjunky: the blackbody radiation is in equilibrium with the walls (which have a certain temperature), and is therefore part of the thermodxnamical system. Have a look in a serious textbook about physics. BTW: I don’t know your concept of heat.

          2. @mm Your argument occupies a very narrow intellectual window between the colloquial understanding of the context and more generalized models of physics. I’ll break down the three regimes as I see them:

            Colloquial understanding: the context of this thread obviously points towards conduction. Now, conduction as a phenomenon is a can of worms, and one I’m not interested in pursuing.

            Where your assertion could be considered correct: the transfer of energy keeping the levitated object aloft can, indeed, be considered heat. All of the light emitted from the LEDs, including but not limited to IR emissions, would be included in this. Again, the non-limitation to IR goes against the colloquial grain of the thread, but technically correct.

            General physics: A classical definition of heat is the transfer of energy between two thermodynamic systems, not including any Newtonian transfers. I assume this is the definition you’re working from, though many physicists would quibble with this one. By this, any light can, indeed be considered heat, as one can consider any two systems (E.G. the sun vs. the universe) to be heat. However, light is not intrinsically heat, it is merely a mechanism that can be considered to transfer energy. For example, in this experiment, we wouldn’t consider any of the light that wasn’t absorbed or reflected by the levitated object to be heat, as it is not transferring energy between the systems of interest. Yet, it is still light.

            Go ahead and occupy that sliver of technicality if you wish, but I don’t personally consider it horribly useful technicality to hold to, and is less useful than either of the others surrounding it.

            Now, let’s get out of the weeds.

    1. They are scientists, they control for the parameters of the experiment, it’s the most basic part of their job, it makes zero sense presuming they wouldn’t account for this. They measured heat/IR, or used a IR filter, etc. They did everything that was required to make sure this wasn’t what was going on. It’s science 101.

          1. And sometimes they make mistakes despite genuine attempts to isolate or account for all the parameters.

            Then there is the lazy ones that are more Engineer in outlook – not bothered either way as long as they prove the effect they were wanting is sufficient to the task at hand, who cares about the minutia that makes up how it actually works as long as will do what they wanted…

          2. Sometimes they make mistakes, sure, if science was perfect, we’d all have flying cars by now.

            But it’s not the general rule. And there are lots of things in place to catch those mistakes (not working alone, peer review, etc).

          3. I agree with your con artist comment but if you’re suggesting that Pons and Fleischmann had ill intent then I believe you are mistaken. Their experiments were real but their scientific method was clouded by the results they wanted to see. This happens and that’s why it is prudent to question everything about an experiment.

          4. They became con artists when the issues with their experiments were pointed out, and they just kept acting like nothing was wrong anyway/walked outside of the normal process, to rake in all of the attention they would get if their experiment had no issues

        1. Presuming the most basic controls/precautions were not taken, is just unreasonable. It’s like going going to a random hot-dog stand, buying a hot-dog, and then suspecting that the vendor is secretely an extremely old HItler in disguise. It’s possible in the sense that anything is possible. But it’s not likely or reasonable to expect.

          1. Questioning methods is basic science. But so is reading, so just read the damn paper.

            From the paper:
            > We note that this lift force is not due to the temperature difference between the top and bottom…
            > We also note that the observed lift force cannot be caused by light (radiation) pressure because…
            >

  1. The wording doesn’t make it clear that this is not the same as solar sails in space. The lifting force is not generated by the light directly, but by differential heating affecting the recoil of gas molecules. It is not a hard vacuum in the chamber, but a low pressure environment (pressure between 10 and 30Pa).

    1. Exactly like the paradox in the famous Crookes Radiometer, where the dark side (absorbing the light) produces more thrust than the light side (reflecting the light, which should double the momentum transfer).

  2. If we can levitate with sound and can levitate with light, why not levitate with both light and sound? Maybe add some gravity in there too?

  3. I wonder how hard the vacuum was. Rather than trying to put something on top, where heat would rise. I would rather that the experiment was instead done vertically, operating on the z-axis, and they measured the pressure with a stain gauge.

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