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Comment by spiralganglion | Hacker News Reader

That's something I want to know. With the amount of energy given to our broadcasts, they surely would decay long before traveling any noteworthy distance. I'm curious as to what our most powerful broadcasts have been, and how far they'd travel before becoming unnoticeable. Especially, when compared to the amount of EMR released by neutron stars, pulsars, background noise, etcetera.

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From the SETI FAQ: "Detection of broadband signals from Earth such as AM radio, FM radio, and television picture and sound would be extremely difficult even at a fraction of a light-year distant from the Sun. For example, a TV picture having 5 MHz of bandwidth and 5 MWatts of power could not be detected beyond the solar system even with a radio telescope with 100 times the sensitivity of the 305 meter diameter Arecibo telescope."

Now, the Arecibo message should carry over a couple of hundred light years easily, but it was transmitted for three minutes only, and if you miss it, you miss it.

Not if you have faster-than-light travel!

If they don't, it would seem pointless to contact aliens, by the time they send their ambassador on a multi-generational ship to Earth we humans may have already been extinct.

FTL is probably not ever possible no matter how technologically advanced a society may be as it would allow for the violation of causality.

You're assuming we already know everything there is to know about causality. It wasn't long ago that some people thought the sound barrier was insurmountable, or that trains would kill their occupants by going too fast.

FTL is equivalent to time travel into the past. If you have one you have the other.

People thought humans wouldn't break the sound barrier for engineering reasons, not because the laws of physics prevented it. And just because people in the past thought something was impossible doesn't mean it will one day be possible. 100 years ago, physicists thought perpetual motion machines were impossible.

"100 years ago, physicists thought perpetual motion machines were impossible."

Bad example, man, the "anything is possible" crowd hasn't necessarily conceded that point either....

Thanks for saying what I wanted to. Everybody who knows nothing about physics always brings up the "humans can't possibly travel faster than 25mph" thing when someone who does know talks about violating causality, but as far as I've seen it was only printed in a newspaper or two, not espoused by physicists. The speed of light limitation is a necessary part of one of the most precise and thoroughly tested theories of all time.

What about the bending of space for an instant travel time? That's more of an unknown in terms of possibility from what I have read.

If I understand GR correctly, FTL travel is (in principle) not necessary to reach an alien race in reasonable time, no matter the distance - from the point of view of an astronaut. Due to length dilation, from the point of view of an astronaut, the whole universe gets pretty small when he travels with almost-lightspeed, so that he can travel, say, one lightyear in five minutes (assuming he can survive the acceleration). The problem is that the wait on earth would be pretty long...

I'm not actually 100% sure my interpretation of GR is correct, if anyone of the people who know more about it could elaborate, I'd greatly appreciated it (for example, I think acceleration has effects on time, too, which could crush my "long distance travel in no time"-theory).

There is an analysis of the detectability of human radio signals at:

http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html

The main uncertainty is at estimating the level of radio technology that can be available to alien civilizations.

Not to mention whether they'd even bother with radio, if better technologies become available.

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