Comment by credit_guy | Hacker News Reader

Comment by credit_guy | Hacker News Reader

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Sending light across interstellar distances is not a trivial affair. No light source is perfectly collimated; per wikipedia [1] "The divergence of high-quality laser beams is commonly less than 1 milliradian, and can be much less for large-diameter beams". Not clear how "much less" is (and I couldn't find other sources), but let's say it's 2-3 more orders of magnitude. That would make it about 1 arc second (~5 microradians). The closest star is at a distance of about one parsec, which means that two photons starting here and having an angle of 1 arc second between them end up at a distance of 1 AU (astronomical unit ~ 150 MM km) between them. Assuming you can make an antenna the size of Jupiter at the target, for every photon that passes through that antenna you need to send 1 million photons. Every order of magnitude lower in the size of the antenna requires you to send two orders of magnitude more photons. In other words, an antenna 1% the size of Jupiter requires you to send 10 billion photons for each photon received.

You can get around that by setting up a system of relays, but you will end up needing lots of relays, and that's a complex infrastructure you need to maintain.

In any case, even after we master inter-stellar travel, don't think we'll have channels of communication of terabytes/second, every byte will be expensive to send across.

Sending information in the quantities needed to 3d-print stuff at atomic level will most likely be prohibitively expensive; remember 1 gram of hydrogen contains 10^23 atoms, while one terabyte only contains 10^12 bytes. If a little latency is acceptable, it will always be many times cheaper to just send the stuff you need rather than the information to 3d-print it at atomic level.

[1] https://en.wikipedia.org/wiki/Collimated_light

> Sending light across interstellar distances is not a trivial affair.

I never said it was. But it's a hell of a lot easier (to say nothing of faster and more efficient) than sending matter.

Not necessarily. Remember, photons have mass. Sending a lot of them means you perform mass transfer. Since photons are not parallel, that transfer is not efficient. It's probably much more efficient to send a small package equipped with a large solar sail and push it from behind with a big laser until it reaches a relativistic speed, then decelerate it on the other side with another big laser.

Hm, that's a good point. Doing the math on that would make an interesting exercise.

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.

[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

> Remember, photons have mass.

Really? https://physics.stackexchange.com/questions/2229/if-photons-...

The concept that photons are massless entities is founded in obsolete information. It has long since been determined that photons do in fact carry mass.

When photons were initially conceptualized, the capacity to estimate their mass was too small to measure and thus prove to experimentalists. Theorists, therefore, focused on other concepts and problems.

Taking a look at the link you've provided actually reaffirms this fact. Mote that it is a question from a novice, and the very first answer refutes the idea that photons lack mass.

  In other words, a photon does have 
  relativistic mass proportional to its 
  momentum.

  And here we have it: photons have 'mass' 
  inversely proportional to their wavelength! 
  Then simply by Newton's theory of gravity, 
  they have gravitational influence.

Keep in mind that the top google results for stack overflow style Q & A forums aren't always going to line up in favor of your search queries.

You should read the top answer, since it's more likely to contain reliable facts, and not presume that the ideas expressed by the question are essentially correct.

Sorry I was trying to make the point that photons do not have mass in the same sense as other particles, they have no rest mass.

Uh, aren’t you just sending billions of photons then?

Sorry, I must be missing something, as this seams like you’re spending photons to move a package rather than just sending photons.

The problem is that all light sources are to some degree divergent. You can accelerate a package to a relativistic speed over a distance of the order of 100AU, which is thousands of times less than the distance to the closest star. Consequently, the light beam that you send spreads out thousands of times less over this distance. It’s still a lot, but you have a fighting chance.

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