In a 2002 paper, Laser Elevator: Momentum Transfer Using an Optical Resonator (available at your local school/library, possibly electronically — J. of Spacecraft and Rockets 2002), Thomas R. Meyer et. al. talk about a neat way to get a lot more speed out of light reflection than with a regular solar sail. The basic physics are pretty simple, and it’s a fun subject to think about.
When a photon hits a solar sail, it gives the sail momentum. If the photon has momentum P and bounces off a stationary sail, it looks like this:
Think of where the energy is in this system. Before it hits, the photon has energy E. After it bounces, the photon still has roughly energy E. But the sail’s moving, so where did it get its kinetic energy? (Remember, energy — unlike momentum — has no direction.)
The answer lies in the word “roughly”. The photon loses a tiny fraction of its energy to Doppler shifting when it’s reflected, but only a tiny fraction. It is this tiny fraction that goes into pushing the sail. This is a phenomenally small amount of energy — far less than a percent of what the photon has. That is, not much of the photon’s energy is being used for motion here.
This is why solar sails are so slow. It’s not that light doesn’t have that much energy, it’s that it has so little momentum. If you set a squirrel on a solar sail and shone a laser on the underside, do you know how much power would be required to lift the squirrel? About 1.21 gigawatts.
This is awful. If we were lifting the squirrel with a motor, railgun, or electric catapult, with 1.21 gigawatts we could send it screaming upward at ridiculous speeds.
This is where Meyer and friends come in. They’ve point out a novel way to extract momentum from the photon: bounce it back and forth between the sail and a large mirror (on a planet or moon, perhaps).
With each bounce, the photon loses a little more energy and adds another 2P to the sail’s momentum. The photon can keep this up for thousands of bounces — in their paper, Meyer et. al. found that with reasonable assumptions about available materials and a lot of precision, you could extract 1,000 times the momentum from a photon before diffraction and Dopper shifts killed you. This means you only need 1/1,000th the energy to levitate the squirrel — a mere megawatt.
This isn’t too practical for interstellar travel. It requires something to push off from, and probably couldn’t get you up to the necessary speeds. It may, they suggest, be useful for getting stuff to Pluto and back, since (somewhat like a space elevator) it lets you generate the power any old way you want (a ground nuclear station, solar, etc). But more importantly, it’s kind of neat — it helped me realize some things about photon momentum that I hadn’t quite gotten before. It’s like Feynman says, physics is like sex — it may give practical results, but that’s not why we do it.
Now we’ll let things get sillier. I spent a while trying to brainstorm how to use this with a solar sail (that is, using the sun). I imagined mirrors catching the sun’s light and letting it resonate with a sail.
But you really need lasers for this — regular light spreads out too fast. Maybe a set of lasing cavities orbiting the sun …
Supplemented by a Dyson sphere …
And since by this point we’ll probably have found aliens …
Why settle for interstellar communication when you can have interstellar war? And we could modulate the beam to carry a message — in this case, “FUCK YOU GUYS!”
Thank you Randall Munroe
"We can never obtain peace in the outer world until we make peace with ourselves"
Why can't the world help Tibet from the Chinese occupacy?
Ever since the occupation started in 1950 the people of Tibet have had to endure the worst kinds of torture and oppression.
Why is it that none of the worlds countries does a thing to help them? When Saddam Hussein invaded his neighbour countries all of the world stood up for them and helped them to regain their freedom. Maybe it's because Kuwait has lots of oil and Tibet is poor of natural resources. The occupation is on its 50th year now and I'm sad to say, it looks like it's going to be 50 more years.
Did you know that China cuts down large areas of Tibetan forests and ships it back to China?
Did you know that China melts down the religious treasures in Tibetan monasteries and ships it back to China?
Did you know that China imprisonates and tortures 14-15 years old nuns, because of their religion?
Did you know that China tortures nuns and monks whose only mission in life is to speak and act the message of love and peace?
Did you know that the Tibetan women are forced to sterilization and abortion?
Did you know that there now are 7,5 million Chinese in Tibet and only 6,1 million Tibetans?
Shakti was a group which played a novel acoustic fusion music which combined Indian music with elements of jazz; it was perhaps the earliest practitioner of the musical genre world fusion.
Its leading member was the English guitar player John McLaughlin, but it also featured the Indian violin player L. Shankar. It also included Zakir Hussain (on tabla), and R. Raghavan (on the Mridangam) and T. H. "Vikku" Vinayakram (on Ghatam).
In addition to fusing Western and Indian music, Shakti also represented a fusion of the Hindustani and Carnatic music traditions, since Hussain is from the North, but the other Indian members are from the South.
It came together in 1975, after the dissolution of the second incarnation of The Mahavishnu Orchestra, and toured fairly extensively during the period 1975-1977; it made only sporadic appearances (with personnel changes) thereafter.
Its eponymous first album, Shakti, has had a lasting following.
After 1977 the albums which L. Shankar recorded with Z. Hussain and T. H. "Vikku" Vinayakram stayed close to the music made popular by Shakti.
Some twenty years later McLaughlin and Hussain put together another band with the same concept, called Remember Shakti, including V. Selvaganesh (son of T. H. "Vikku" Vinayakram ), mandolin player U. Shrinivas and eventually Shankar Mahadevan.
There are a lot of videos on you tube, check them out.
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