It's true that the beam will spread out as it travels. The diffraction effect doesn't even require an actual physical aperture like a hole. A collimated beam is the same as a beam that has been created by passing through an aperture, so diffraction effects apply.
Example of spread: a pencil-thick (say.5cm) laser fried from earth is a couple of tens of meters, IIRC, when it reaches the moon
Not sure where those numbers come from. When I studied laser physics the calculation using a 0.5cm beam produced a beam measuring many kilometres across at the moon. I think the beam was about 70m across at the moon when the initial diameter was 1m. This corresponds with standard diffraction principles in that the smaller the aperture, the greater the diffraction.
Thus, in order to communicate across interstellar distances they would have to start with a laser beam many kilometres in diameter if they wanted to minimise the effects of diffraction. With our current technology a laser this size just isn't going to happen, but that doesn't meant that it's impossible.
As for all the comments about how the lasers would have to be aimed specifically at the target, yadda, yadda, the article says that and says that the aliens would actually have to be trying to communicate with us, not necessarily simply firing out arbitrary bursts of information. ie. This search has a somewhat different focus to SETI, which is looking for communications, but also for arbitrary signals that could originate from an alien civilisation.
Using lasers would be preferable if you're just firing out in arbitrary directions hoping someone will pick it up, since laser beams have special properties that ordinary EM emitters do not possess, making them easier to distinguish from noise. But there are naturally produced laser beams coming at us from space, so the real deal is sorting out whether or not any laser sources are transmitting information.
It wouldn't bend around them, radio waves would pass through them. Radio waves tend to be too low of an energy to be picked up by atoms to induce electrion jumps, so they'll pass through objects with no problem (that's why you can get radio and attenna TV inside a house).. although with a planetary-sized body there will be some distortion. Light waves, on the other hand, are easily picked up by most metallic substances, and therefore will be absorbed.
First of all, "light" is made up of electromagnetic (EM) waves, just like radio waves, only of higher frequencies. Any metallic substance will reflect most of the energy, no matter what part of the frequency range you find it in and there are many substances that either absorb or scatter radio frequency EM radiation. We can pick up radio waves inside houses because most of the construction materials in the average house are transparent to radio frequencies and aren't dense enough to cause significant scattering (scattering does not require that an atomic transition exist at the given energy). But, assuming the earth has the metal core that geologists claim it has, radio waves won't pass through it anymore than visible frequencies will.
The sun is unlikely to have a metal core, though there is probably at least some metal in it. But, it is very dense, so there is a lot of EM scattering going on (I seem to remember that the energy released in the fusion at the core takes about 1 million years before it exits at the surface). The atoms do not have to have electron transitions at the energy corresponding to the frequency to scatter the waves, though scattering becomes less and less likely as the energy of the wave moves away from the transition energy. But with enough scattering centres the wave won't make it through.
Slashdot Mirror
It's true that the beam will spread out as it travels. The diffraction effect doesn't even require an actual physical aperture like a hole. A collimated beam is the same as a beam that has been created by passing through an aperture, so diffraction effects apply.
Example of spread: a pencil-thick (say .5cm) laser fried from earth is a couple of tens of meters, IIRC, when it reaches the moon
Not sure where those numbers come from. When I studied laser physics the calculation using a 0.5cm beam produced a beam measuring many kilometres across at the moon. I think the beam was about 70m across at the moon when the initial diameter was 1m. This corresponds with standard diffraction principles in that the smaller the aperture, the greater the diffraction.
Thus, in order to communicate across interstellar distances they would have to start with a laser beam many kilometres in diameter if they wanted to minimise the effects of diffraction. With our current technology a laser this size just isn't going to happen, but that doesn't meant that it's impossible.
As for all the comments about how the lasers would have to be aimed specifically at the target, yadda, yadda, the article says that and says that the aliens would actually have to be trying to communicate with us, not necessarily simply firing out arbitrary bursts of information. ie. This search has a somewhat different focus to SETI, which is looking for communications, but also for arbitrary signals that could originate from an alien civilisation.
Using lasers would be preferable if you're just firing out in arbitrary directions hoping someone will pick it up, since laser beams have special properties that ordinary EM emitters do not possess, making them easier to distinguish from noise. But there are naturally produced laser beams coming at us from space, so the real deal is sorting out whether or not any laser sources are transmitting information.
Cheers Craigus
It wouldn't bend around them, radio waves would pass through them. Radio waves tend to be too low of an energy to be picked up by atoms to induce electrion jumps, so they'll pass through objects with no problem (that's why you can get radio and attenna TV inside a house) .. although with a planetary-sized body there will be some distortion. Light waves, on the other hand, are easily picked up by most metallic substances, and therefore will be absorbed.
First of all, "light" is made up of electromagnetic (EM) waves, just like radio waves, only of higher frequencies. Any metallic substance will reflect most of the energy, no matter what part of the frequency range you find it in and there are many substances that either absorb or scatter radio frequency EM radiation. We can pick up radio waves inside houses because most of the construction materials in the average house are transparent to radio frequencies and aren't dense enough to cause significant scattering (scattering does not require that an atomic transition exist at the given energy). But, assuming the earth has the metal core that geologists claim it has, radio waves won't pass through it anymore than visible frequencies will.
The sun is unlikely to have a metal core, though there is probably at least some metal in it. But, it is very dense, so there is a lot of EM scattering going on (I seem to remember that the energy released in the fusion at the core takes about 1 million years before it exits at the surface). The atoms do not have to have electron transitions at the energy corresponding to the frequency to scatter the waves, though scattering becomes less and less likely as the energy of the wave moves away from the transition energy. But with enough scattering centres the wave won't make it through.