Re:The Theory of Interstellar Trade
by credit_guy on Friday July 20, 2018 @03:48PM
(#17578321)
Ok, here's the math.
Solution 1: send information with a highly collimated laser. Assume 1 arc second divergence of the light beam and a distance to the nearest star of 1 parsec (the actual distance is 1.3). The diameter of the circle of light at the target is 1 AU. Assume there is a listening antenna at the target of the same size as the largest radio telescope today, which is 500m [1]. The ratio of the area of the telescope and the area of the light disc coming from the source laser is (500m/1AU)^2 = 10^(-17). You want to send about 1Tb/s of information. 1 bit/s requires about 4zW of power [2], so you need the antenna to receive about 10^12410^(-21) = 4e-9 W. You need to send about 10^17 more at the source, that means about 4e8W, or roughly 400MW. Let's make that 1GW, to account for various other inefficiencies. That's actually not so bad.
Solution 2. Send a package in a solar sail powered vehicle. I don't know how to do the math, but fortunately wikipedia has some examples [3]. A randevouz with Alpha Centauri for a vehicle with a final mass of 71 tons can be done in 41 years, and requires a power of 7200 GW here and 26000 GW in Alpha Centauri (for the braking stage). Let's say out of the 71 tons vehicle at the end of the trip, only 1 ton is useful cargo, and you fill that with SSD drives. Currently the largest SSD drive is about 100 TB, and it weights about 100 grams. That's 1TB/g, or 10^6 TB/ton. That's about 100 days worth of transmission at 1 Tb/s. So it wouldn't seem remotely comparable with Solution 1. However, by the time we reach the stars, I can easily see the information density going up by a factor of 10^20 or so.
So that's that. If you want to send information at limited bandwidth, you can send it using a big laser. If you can afford some latency (40 years instead of 4y), then you can send it by putting a lot of hard drives in a package.
Solution 1: send information with a highly collimated laser. Assume 1 arc second divergence of the light beam and a distance to the nearest star of 1 parsec (the actual distance is 1.3). The diameter of the circle of light at the target is 1 AU. Assume there is a listening antenna at the target of the same size as the largest radio telescope today, which is 500m [1]. The ratio of the area of the telescope and the area of the light disc coming from the source laser is (500m/1AU)^2 = 10^(-17). You want to send about 1Tb/s of information. 1 bit/s requires about 4zW of power [2], so you need the antenna to receive about 10^12410^(-21) = 4e-9 W. You need to send about 10^17 more at the source, that means about 4e8W, or roughly 400MW. Let's make that 1GW, to account for various other inefficiencies. That's actually not so bad.
Solution 2. Send a package in a solar sail powered vehicle. I don't know how to do the math, but fortunately wikipedia has some examples [3]. A randevouz with Alpha Centauri for a vehicle with a final mass of 71 tons can be done in 41 years, and requires a power of 7200 GW here and 26000 GW in Alpha Centauri (for the braking stage). Let's say out of the 71 tons vehicle at the end of the trip, only 1 ton is useful cargo, and you fill that with SSD drives. Currently the largest SSD drive is about 100 TB, and it weights about 100 grams. That's 1TB/g, or 10^6 TB/ton. That's about 100 days worth of transmission at 1 Tb/s. So it wouldn't seem remotely comparable with Solution 1. However, by the time we reach the stars, I can easily see the information density going up by a factor of 10^20 or so.
So that's that. If you want to send information at limited bandwidth, you can send it using a big laser. If you can afford some latency (40 years instead of 4y), then you can send it by putting a lot of hard drives in a package.
[1] https://en.wikipedia.org/wiki/Five_hundred_meter_Aperture_Sp... [2] https://en.wikipedia.org/wiki/DBm [3] https://en.wikipedia.org/wiki/Solar_sail#Swing-by_maneuvers