4 ms·

> Even on the crazy fringe frontiers of real science, I am not aware of any candidates for communication any better than EM. There is one, and it is essentiall

> Even on the crazy fringe frontiers of real science, I am not aware of any candidates for communication any better than EM. There is one, and it is essentially > people carrying things around on Segways Electromagnetic radiation must obey the inverse square law. It spreads spherically through space, so required power for a given SNR raises with distance squared. Moreover, there are severe interference issues. Once you have several systems communicating on the same bands you get more noise in the relevant frequency bands. Unlike waves which don't self-interact (i.e. obey the superposition principle, are linear), matter is coherent. If you shoot a hard drive at a distance galaxy (with greater than escape velocity), it may take significant time to get there, but you can travel arbitrarily far for essentially the same energy, as a first approximation. Of course, as a second approximation there is necessity of guidance/control (because of finite motor precision, target uncertainty, etc), but also the possibility of harvesting energy or photon momentum from stars en route. Aside from the expansion of the universe there's no theoretical limit as to why a sufficiently advanced probe couldn't propagate information arbitrarily far (if it "refuels" periodically, then its total energy usage is proportional to distance, i.e. linear, but launch cost stays bounded). In the worst case (perhaps for cosmically large distances) it'd need to colonize a planet en route to rebuild a pristine copy of itself. "Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway." –Andrew Tanenbaum, 1981 https://what-if.xkcd.com/31/ https://what-if.xkcd.com/31/ "The only way to actually reach the FedEx point is if transfer rates grow much faster than storage rates. In an intuitive sense, this seems unlikely, since storage and transfer are fundamentally linked—all that data is coming from somewhere and going somewhere (...)" https://en.wikipedia.org/wiki/Sneakernet https://en.wikipedia.org/wiki/Sneakernet

ajuc 8y ago
EM doesn't have to spread spherically. There is such a thing as directional antennas, spotlights, or even lasers. Yes they spread too, but throwing a hard drive at distances on the scale of light years isn't going to be precise either. Go far enough and you won't hit the galaxy you wanted to.
- int_19h 8y ago
  Directional antennas still obey the inverse square law, because the beam still diverges as it travels. So the amount of energy that you spend sending the signal gets spread over a larger and larger area, meaning that at any given point in that beam, the signal is weaker and weaker, until it gets lost in the noise. And even lasers diverge. An ideal laser has some minimal divergence that is inherent and cannot be overcome (if I understand correctly, it's because diffraction is non-zero even in vacuum). A true Bessel beam doesn't diverge - but it does not have a boundary, and hence requires infinite energy.
  - scythe 8y ago
    In order to achieve a Rayleigh length (effectively the "range") that is smaller than the distance from Earth to the nearest star, a (visible) laser should be approximately 40 kilometers wide. The effective range scales as the square of the beam waist, so you can communicate with the Andromeda galaxy using a laser just a little smaller than Neptune.
- darkmighty 8y ago
  > Go far enough and you won't hit the galaxy you wanted to This can be solved, in the limit of large distances (i.e. many galaxies away), by the aforementioned method of refueling/rebuilding/re-guiding along the way. But between each galaxy there's a vast, barren chasm to cross with little resources, I concur.

ajuc 8y ago

EM doesn't have to spread spherically. There is such a thing as directional antennas, spotlights, or even lasers. Yes they spread too, but throwing a hard drive at distances on the scale of light years isn't going to be precise either. Go far enough and you won't hit the galaxy you wanted to.

int_19h 8y ago
Directional antennas still obey the inverse square law, because the beam still diverges as it travels. So the amount of energy that you spend sending the signal gets spread over a larger and larger area, meaning that at any given point in that beam, the signal is weaker and weaker, until it gets lost in the noise. And even lasers diverge. An ideal laser has some minimal divergence that is inherent and cannot be overcome (if I understand correctly, it's because diffraction is non-zero even in vacuum). A true Bessel beam doesn't diverge - but it does not have a boundary, and hence requires infinite energy.
- scythe 8y ago
  In order to achieve a Rayleigh length (effectively the "range") that is smaller than the distance from Earth to the nearest star, a (visible) laser should be approximately 40 kilometers wide. The effective range scales as the square of the beam waist, so you can communicate with the Andromeda galaxy using a laser just a little smaller than Neptune.
darkmighty 8y ago
> Go far enough and you won't hit the galaxy you wanted to This can be solved, in the limit of large distances (i.e. many galaxies away), by the aforementioned method of refueling/rebuilding/re-guiding along the way. But between each galaxy there's a vast, barren chasm to cross with little resources, I concur.