261–270 of 274 posts
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Inadvertently starting an interstellar war, provided they survive the impact on their homeworld.
According to Wolfram Alpha, an iPhone 6 Plus which weighs in at 192 grams would have about 3.558×10^14 joules of kinetic energy at .2c, or about 85 kilotons of TNT. That's roughly 4 times the energy of the Fat Man bomb dropped on Nagasaki. Not the biggest boom in the world, but certainly nothing to scoff at.
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(I Agree.) As a rule of thumb, you can approximate sqrt(1 - v^2/c^2) = 1 - 1/2 (v/c)^2. So if v/c = 2, then the correction is approximately 1 - 1/2 .2^2 = 98% as you said. This formula is easier for back of the envelope calculations and the important point is that it's quadratic. (I always forget the 1/2 :(. ) The GP comment used the wrong correction of 1 - v/c, that is not quadratic so the correction is much bigger.
Cool trick. It starts to fall apart at higher speeds (at c, the approximation produces 1/2, when it should be 0), but it looks like up to 0.5 or 0.5 it's decent.
For speeds that are close to c, you have to use another "Taylor" series to get a good approximation. If v is close to c, then the approximation of the correction is sqrt(2(c-v)/c). So at 99%c you get 4.5%.
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You're right to be skeptical, because this is going to be very difficult and has many challenges that need to be overcome. But that doesn't mean we should sit here and say "nope, can't be done". People were skeptical when planes first flew, when it was attempted to put people on the moon, and so on. Jeez, people were skeptical when SpaceX said they could land a first stage on a barge at sea, and posted endlessly abou…
People were also skeptical of all the people before the Wright brothers who jumped off cliffs and killed themselves. Skepticism is the natural worldview of an engineer, because the universe is incredibly hostile to our ideas.
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Agreed on the extreme skepticism warranted. I am imagining these "solar sails" to be essentially spherical, both for structural reasons and so that their response to applied laser power does not depend on orientation. The electronics in precision guided mortars in the US army survive 50,000g. http://proceedings.ndia.org/C347/Davis.pdf (This is only for the fraction of a second when they are fired, and I don't know ho…
F=ma In this case m ~ 1 gram and a = 0.2c / 120 seconds =499654.097 m/s^2 F = 499654.097 m/s^2 * 1 gram = 500 newtons However this force is distributed over the whole sail which has an area is 16 m^2, so pressure applied to the sail is 500 N/16 m^2 = 31.25 pascals. This is a fairly low pressure. The pressure change going up or down a couple meters is about equal to this. Your typical cheap garbage bags can withstand…
Dude. 9 significant digits? You don't know m to better than 1 or 2 significant digits. Just call it 5e5 and be done with it.
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Spheres would cause a major issue - the laser would need to strike the centre of the circular visible surface. Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side. A flat mirror coupled with crazy fast gyros (also powered by the laser) might be stable enough that the mirror holds directed toward the earth.
Would a slightly conical mirror with its apex pointed towards the laser be self-stabilizing in tilt? Kind of like an aircraft with dihedral wings [1] is self-stabilizing in roll - imagine the laser illumination as an analog of the aero lift vector on the wings. If it tilts wrt the laser, the asymmetric illuminated area should counter the tilt. [1] https://en.wikipedia.org/wiki/Dihedral_(aeronautics)
The stabilization problem sounds really hard to me.
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Agreed on the extreme skepticism warranted. I am imagining these "solar sails" to be essentially spherical, both for structural reasons and so that their response to applied laser power does not depend on orientation. The electronics in precision guided mortars in the US army survive 50,000g. http://proceedings.ndia.org/C347/Davis.pdf (This is only for the fraction of a second when they are fired, and I don't know ho…
F=ma In this case m ~ 1 gram and a = 0.2c / 120 seconds =499654.097 m/s^2 F = 499654.097 m/s^2 * 1 gram = 500 newtons However this force is distributed over the whole sail which has an area is 16 m^2, so pressure applied to the sail is 500 N/16 m^2 = 31.25 pascals. This is a fairly low pressure. The pressure change going up or down a couple meters is about equal to this. Your typical cheap garbage bags can withstand…
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How could you store enough power in orbit to fire it?
Well, you could use your handy 200 GW solar array. With perfect conversion, and ~1300 W/m^2, you'd need to get your 12km x 12km panel array pointed consistently at the Sun.
Lithium ion battery energy density is 265 watthours per kg. So you will need 24.9 million kg of batteries. Or 24.9 thousand tons. It's a lot, Tesla factory makes more per year.
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A laser capable of propelling an interstellar probe is also a mighty space weapon. Some people proposed constructing that on the far side of the Moon so that it hass less of a chance of being used as against Earth targets (intentionally or by accident).
> A laser capable of propelling an interstellar probe is also a mighty space weapon. Yes, but maybe it could also give us the technology to quickly deflect perilous asteroids... ?
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Agreed on the extreme skepticism warranted. I am imagining these "solar sails" to be essentially spherical, both for structural reasons and so that their response to applied laser power does not depend on orientation. The electronics in precision guided mortars in the US army survive 50,000g. http://proceedings.ndia.org/C347/Davis.pdf (This is only for the fraction of a second when they are fired, and I don't know ho…
Spheres would cause a major issue - the laser would need to strike the centre of the circular visible surface. Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side. A flat mirror coupled with crazy fast gyros (also powered by the laser) might be stable enough that the mirror holds directed toward the earth.
> Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side.
No, because these lasers won't be focused to a point smaller than the size of the satellite. I'm not sure what the waist will be, but presumably it's much larger than the mirror.
The real reason these are sails is because they maximize the momentum transfer per unit mass. But unlike sails on boats, this force can't be transmitted to a displaced payload using tethers/struts, so the payload must be dispersed and integrated within the sail itself.
I don't know how to do general (hard) AI, but if I did, I can't think of a fundamental reason I couldn't shrink the AIs down to ~1g and make them able to survive 60,000G. So you could send a bunch of AIs on a 20 year trip to Alpha Centauri.
I would imagine Stephen Hawking has already put two and two together in this way; he often warns that humans should leave Earth to avoid extinction.
Even if we can't colonize other stars with people within this century, AIs could be thriving there within that timeframe. At least our "descendants" (the AIs) would be protected from extinction (by redundancy across stars).