[QUOTE=Chronos]
Wouldn’t work. The lifespan of particles like pions is just an average, not an absolute. So if some are decaying right at the alien relay satellite, some will also be decaying at lower heights. The net effect is that you’d see a glowing beam of light pointing up from the probe, like depictions of lasers in bad science fiction. You’d be better off using a laser beam, and just turning it off when atmospheric scattering was high.
[/QUOTE]
I disagree
Relativistic pion would decay in a upward directed shower. The main decay mode (98.8%) is into two gamma photons (~70 Mev on average, plus pion velocity), and the second largest (1.2%)is into an electron, positron and photon. All of these would initially be directed upward, at a close vector to the original pion, to conserve the momentum of the initial relativistic pion velocity. While some subsequent scattering, decays and interactions will occur for the electron and positron, they will be progressively more tenuous as the secondary products deviate from the beam, and undergo tertiary and higher reactions before some tiny mangled/obscured vestige arrives at any any human Earth-/space-based detector [presumable the alien orbiter isn’t parked next to on the ISS or any other Earth craft that could detect the narrow cone of primary decay products – though perching on the ISS would make for some interesting scenarios, wouldn’t it?]
We can only guess at the pion beam current of this hypothetical technology, but the 2 km CERN accelerator only manages miliamps of pions, and I think even that would be extravagant. Whatever the beam strength, any human detector would be on the wrong end of a long chain of fractions: > 1% of the pions decay in the atmosphere * 1.2% of pion decays produce charged particles * tiny fraction further scattered or otherwise affected by secondary interactions to produce, e.g. visible photons * the fraction of energy in whatever band we are detecting, vs. the overall spectrum of products the limited effective aperture of our detector (vs the surface of the potential sphere at that range). ** Basically, we could accidentally detect that tiny “side effect” fraction, the original transmission beam would’ve been millions (billions? quadrillions?) of times more powerful than needed by its intended reciever!* They would know the properties of the pion beam in advance, and the beam would be pointed. Let’s give the aliens some credit: if they communicate by pion, they must be able to use them efficiently.
Consider your own more conventional example: a laser beam is sharply defined, visible to the naked eye readily scattered, and monochromatic (easily identified), yet a laser beam in transit is for all purposes undetectable. Try taking a laser pointer out on a foggy day: the beam may be visible from on the order of a meter, but not tens of meters (due to rescattering and other effects) Except in special circumstances, like a kilowatt scale laser show on the bottom of a low cloud layer or highy reflective artificial “fog”, atmospheric scattering is a two-edged sword: it may scatter a fraction of the original beam for you to detect, but also scatters/obscures this desired scatter signal, rendering it useless or undetectable.
Consder, too, that it is highly unlikely that a suitable detector will be within 10 km of the beam (what a coincidence, that would be!) For naked eye detection – well, how many megawatts would it take to power a fluorescent bulb 100 miles long and even dimly visible at 10 miles or 100? I think it’s safe to say that the putative pion beam would be on the order of watts at best, and the side-scatter effects would be imperceptible over the entire beam surface, even from 1-10 meters
I’m not saying it’s impossible. One could for example, send scan the earth by satellite for the gamma range of [pion decay] positrons and [atmospheric] electrons, shifted by the initial relativistic velocity – but you’d have to be looking for them, and looking fairly hard, to have any reasonable hope of finding them. They’d look like background.