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Old Nov 11th 2012, 07:10 AM   #1
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General astrophysics question about light and interstellar travel

Hi, I have been pondering a scenario recently and I was wondering if anyone would think it to be something that would be feasible. My idea relates to light. Here goes. Is it possible to capture a light beam traveling at the speed of light, obviously, attach instructions to the light along with nano-carbon building blocks, and forward that light beam to another star where it would arrive in so many years and create a probe or whatever we choose? I was thinking that there are light beams that we capture now to find out what other far off worlds like like using the current telescopes we have. Why not use those beams that are traveling past our planet heading towards other worlds as a conveyor of instructions to build probes sort of like a fax machine uses light to transmit between machines to produce a faxed piece of paper. Granted that is a simplified way of putting it but it gets my idea across. If this is not the right for for this question, where or to who should I post it?

Dan
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Old Nov 12th 2012, 06:02 AM   #2
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I'm not quite following. On one hand you talk about using light to transmit information, which is commonly done and has been since radio was first invented. The example you gave is fax machines which operate over a communications network that transmits essentially at the speed of light, but other examples include radio, TV, etc. There was an attempt back in the 70's to communicate via broadcast (i.e., one way) using the Arecibo Radio Telescope in Puerto Rico as a giant antenna to transmit a radio signal with information encoded in it - whether that tansmission will ever be picked up by some intelligent extraterrestrial life is open to speculation. But the idea of transitting information using radio (which is a form of light) is quite well established - if you remember the book "Contact" by Carl Sagan (later turned into a movie starring Jodie Foster) that's exactly the premise - the aliens transmit information to earth via radio which the earthlings use to construct a machine.

But as for "attaching" an object to a photon - that doesn't make much sense to me. No object with mass can travel at the speed of light if that's what you mean. However, it is possible to use the momentum inherent in light to help push an object - there have been proposals to use giant "sails" in space that would capture light pressure from the sun to help accelerate space craft. It takes a huge sail to get a small bit of acceleration, and as you get further fron the sun its effectiveness is reduced, so no practical application exists yet.
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Old Nov 12th 2012, 07:54 PM   #3
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Light and interstellar travel

Chip, thanks for replying. I had forgotten about the similarity of what I was thinking about to the movie Contact. You say no object with mass can travel at the speed of light. According to Einstein, Energy is equal to mass traveling at the speed of light squared. What I propose is trying to figure out how to attach mass (nano-tubes) to photons that are already traveling at the speed of light to get the nano-tubes from here to there. Could we send 'smart' nano-tubes to a distant point that can build themselves into a probe that can send back information about where it has been sent? We use radio waves and light-waves to transmit data now. I am just thinking about this on a larger scale. I don't portend to know all of the physics behind how far a light wave can travel.
I have followed light-sail technology over the past several years and find that that does have some possibilities. I am trying to go faster than just pushing a screen using photons.
My thoughts here are something I hope can make someone else think outside the box in this arena and spur some new innovation in interstellar communication and travel. I know it may sound a bit far-fetched but I think these ideas are within our grasp. Thanks for listening. Dan
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Old Nov 13th 2012, 04:58 AM   #4
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The formula E=mc^2 has to do with the mass-energy equivalence, which says that an object's mass can be described by its energy content. If you convert mass to energy (such as happens in the sun, atom bombs. H Bombs, and nuclear power plants) the energy that is produced is immense, per E=mc^2. But that has nothing to do with transporting mass. No object with mass can travel at the speed of light, because it takes infinite energy to accelerate it to light speed. Reason is that the mass of an object increases with its velocity, according to the formula:



where p = momentum and gamma is a factor based on the Lorentx transformation:



Note that gamma approaches infinity as v approaches c. So as the object approaches the speed of light its mass gets larger, requiring an ever larger force to accelerate it a bit more, from F=dp/dt. It takes a huge amount of force to get close to c, and an infinite force to actually reach c. This is why particle accelerators are so huge - they must pump great amounts of energy into a particle such as a proton in order to accelerate it to near the speed of light before it smashes into the target. Clearly a single photon lacks the energy required to accelerate even the smallest nano device to near the speed of light.

Back of the envelope calculation: the energy in a photon can be found from E=hf, where h = Planck's constant and f = the frequency of the light. For a high-energy photon in the X-ray range with f=10^18 Hz, E is about 6 x 10^(-16) J. If you could capture all this energy and put it into a 1 microgram object, you could accelerate it to about 1 mm/s. It would be a very long journey to the nearest star system.

Last edited by ChipB; Nov 13th 2012 at 06:48 AM.
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