Amateur Astronomers Association of Princeton

by Ted Frimet

“you’re a good man, charlie brown”

Last month, I looked at the astrophotography image, produced by AAAP Club Director Rex Parker, of a blue comet. And was somewhat jealous of what he had accomplished. Well, maybe not jealous as much as knowing what can be done. Rex has talent, and years of experience to back it. This month, Rex reminds us that Proximus Centaurus came to roost, as a preamble to future science, being explored at a AAAP lecture. What a beautiful collaboration of color. This image showcases the star closest to our outside solar system ventures. In this red giants habitable zone, we note the recent discovery of Proxima B. This exoplanet is the target of “The Breakthrough Starshot Initiative”, discussed by this months, AAAP lecturer, Dr. Ed Turner.

The red dwarf Proxima Centauri and surrounding star field. Width of field ~28 arc-min, about the size of the full moon. Alpha Centauri is out of the field ~2 degrees away. Image by RAParker using Skynet/PROMPT5

Dr. Turner gave us his time and provided personal insight into the interstellar flight project. Combining advances in micro-electronics, nanotechnology, and photonics, our future generations stand to witness the return of information from Proxima B, precisely 54 years after flight departure. There are many technical hurdles to overcome. Amazingly, in a sense, there are already off the shelf solutions to attach to this scientific venture. Advances in non-related material science, for instance, will form the framework for a thin shell, reflective sail. This is not your parents solar-sail, mind you. Dr. Turner taught us that if we want to get to light speed, we need to leave the fuel behind. The sail material that will be attached to the postage stamp sized space craft, will be pummeled by an intense and coherent, ground based, laser light array. It will be responsible for accelerating the craft to 20 percent of the speed of light. “Meow”, says my photonic cat.

Meow to you, too. I asked why we have to have reflectivity, from the sail. Explained to me, satisfactorily by Dr Turner, and by fellow club Astronomer and Professional Planetarium Technician, William Murray, that there is a double kick. Momentum is imparted when the photon strikes, and on the reverse course (the reflection), there is another kick. This is akin, so I have been told, to a martial artists punch. It was a good analogy, however I remained stuck at the gate.

In my minds eye, I was seeing non-interaction of the photon with the sail, and the creation of virtual particles to do the job of excitation (momentum production). I started to see the quantum mechanics at work, and the emanation of light from the sail material. Untimely for me, I had already asked my question about using Snell’s Law and “reflected” on using refraction. I didn’t quite get it, yet, why we needed two kicks, and reflect light, when we could get one kick and let the light thru – or constrained within the sail – giving many kicks. Watch as one coherent wave goes in, yielding multiple wavelets, tugging at the outer sail in true Minkowski pressure! Not seeing it, yet? Think thru the double slit experiment, or water wave demonstrations. There it lay! My grandmother was a seamstress. Imagine her glee, whilst weaving thru the sail, incredibly small magnetic thread shielding. These intelligent nano-polymers threads will mark the way for a deterministic outcome.

Perhaps the physics of it, is that the cumulative small kicks equals the one big, reflective kick? I don’t think so. Richard Feynman did, years ago. So who am I to argue? In Feynman’s diagram, he would simply sum up all the smaller occurrences of refraction, with two arrows, and “close the triangle”, yielding the squared amplitude of the third side, as a definitive probability. But that was Feynman. And I am only poorly recalling one of his great lectures, again.

It would be akin to a laser light, refracting into an optical resonance chamber, where it would recursively push, like a subroutine caught in an endless loop. At the least, it would be a resource that would be used, repeatedly and not simply reflected back to the ground station. And then the voice of reason whispered into my ear – there will be heat!

Commercial break. I wanted to take that quantum step, and decided almost immediately that heat, is just another electromagnetic emanation. Is it destructive? Then upsample this heat of light into a frequency that is not. Can’t resample the frequency? Then alter the material. I have been mulling around with the idea of finding the correct frequencies to rotate a CO2 molecule. And in doing so, to upend it – so that it does not trap infrared energy. I would imagine that the sail will be fine tuned, so that an active material will permit heat to escape. Actually, this isn’t waste heat. I can’t be sure, if this is Minkowski or Abraham’s domain, however, the heat (or rather, this specific photonic pressure) may further drive momentum. Why not put it to good use? Ok. Maybe I just described a “good movie”, once again? Commercial time is over. In a true opto-mechanical fashion, where light resonates within a cavity, the light can in fact cool the surface, and not heat it, as opposed to what was whispered into my ear, earlier. Let’s continue.

I had to resort to an internet search, this morning. After speaking with Dr Turner, I was convinced that we should have available a tool to teach photonic pressure. After all, those fourth grade school girls, that had the heavens open for them, at Sacred Heart Astronomy Outreach March 2018, will have their next generation children be the recipients of Starshot image data. I wanted them to be agape at the continent wide images, when their adult children show them the Starshot results. I approached our guest lecturer, afterwards, and he suggested likewise. That we should develop and employ a science kit to demonstrate photonic pressure. This sounds like a good idea.

I thought, over the evening, and into the next day, that photonic pressure demonstration will go a long way, in seeding the mind of a child. I remember seeing, in grade school, a Crookes radiometer. However, I do not want to get caught up in the trap of thermodynamics driving this process. Even as amateurs, we have come a long way since Maxwell & Crookes. So, no Crookes. I want photonic pressure to drive this kit. Sensing how to manufacture this; the physical kit would fit in a shoe box, securing a low power laser and a bit of solar sail, within. While wearing the protective eye goggles, with a press of a button, the 4 centimeter square sail would deflect. Or would it? Could it?

The choice of laser might seem easy enough. Perhaps. Aside from making certain it is in fact, low power, the other over-riding principle would be matching frequencies. In terms of light propulsion, I do not have a firm grasp on momentum. Frequency times Planck constant, is all we really have to work with, here. Some have a slip of the tongue, and invite the talk of mass. However, we all know that light is massless (unless by creating a virtual particle, we have measurable mass, i.e. a virtual electron). I am inclined to believe that the frequency must match the material in question, for any deflection to be observed. A few solemn searches turned up what I expected. So for the pretentious Edison’s list of materials, we would have half as many lasers’ to choose from. And might I mention, that the realm of light, that cannot be seen (which is far more available in selection that the visible spectrum), might manage to do damage to our observers, while being, “unseen”?

I am at a crossroads, between material and matching laser light. I have no solution in sight. Maybe it is just best, to order some gold foil from eBay, shine a light on it, and hope for the best? I have a bottle of colloidal gold that can be applied to materials like tissue or plastic wrap. It is left over from the near infrared telescope that sits patiently in the attic, waiting for a reprise. (Oh, the projects you and I have set to the side!) I have no idea if that would work. And even if successful, I might just end up making a poor mans radiometer. Sigh.

I am pestered by the persistent thought of refraction, thru, or within a material, rather than the double kick reflection. And while searching for the truth of it, I found that in a liquid state, a photonics application can either, push or pull. That is, there can be either a push out, or a pull in, on a surface. Fortunately, the two theories that describe either activity, on its own merits, are known as Minkowski or Abrahams’. The theories work out best in empty space. Essentially, as long as there is a refractive index of less than 1, either theory may prove out. The citation below has the first quantitative proof to substantiate Abraham’s theory of photonic push.

Either way, I do not think we could do a kit in a vacuum. The box would collapse, and Schrodinger’s cat would not be meowing, anymore. I think I recall reading that the experimental containers were rather large. It would be desirable to keep it small, and simple, so that anyone can replicate it. However, if I fail to match material science (“a sail”) to a matching low power laser, it is still within the realm of possibilities to demonstrate either a push, or a pull, on a contained liquid to demonstrate photonic pressure. The study reference is from Li Zhang, et al. “Experimental evidence for Abraham pressure of light.” New Journal of Physics. DOI: 10.1088/1367-2630/17/5/053035

Read more about it at phys.org

By the by – this is some serious physics. Photonic pressure can be used to ignite a fusion reaction, called inertial confinement fusion. Momentum exchange is used in a laser cooling technique that stirs atoms into a quantum-mechanical ground state. I can finally relate to what was meant by laser cooling to produce a Bose-Einstein Condensate, in previous reads. At the end of the day, however, I only want to fly a kite. Call me Charlie Brown.

Following video: https://youtu.be/xv4vp9d1VP8