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Comment by dmix | Hacker News Reader

How old would the radio signal be by the time it reaches us?

Edit: the answer was more obvious than I assumed :p, looks like our earliest AM/FM radio broadcast 110yrs on it's way leaving the milky way http://www.lightyear.fm/

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Surprisingly this is an ambiguous question to answer due to the nature of the expanding universe, but if the statement is that "the origin is 2.5 billion light years away" it was _probably_ transmitted 2.5 billion years ago.

http://curious.astro.cornell.edu/about-us/104-the-universe/c...

Interesting. Looks like it was around the time the earth's atmosphere was just beginning to populate with Oxygen. But not before there was life n earth https://en.wikipedia.org/wiki/Great_Oxygenation_Event

110yrs ago is only leaving the milky way

No, not even close. The milky way is 100,000 light years across.

Here's a graphic of the milky way with a dot that's 200 light years across (100 light years in each direction from earth.. how far our radio waves have gone).

http://www.planetary.org/blogs/emily-lakdawalla/2012/3390.ht...

While the edges of the Milky Way are not well defined, the solar system is about 25,000 light years from the center and 25,000 from the edge of the disk. The disk is about 1000 light years thick where the solar system resides[1]. Not sure where the Earth is relative to the Milky Way ecliptic, but odds are it is near it and that our radio broadcasts won't be leaving the Milky Way any time soon.

[1]http://www.universetoday.com/65601/where-is-earth-in-the-mil...

Waves broadcast at the beginning of 2013 would only reach 4LY out. For a radio signal to travel 2.5 billion light years it would need to have travelled for 2.5 billion years since radio waves travel at the speed of light.

Radio broadcasts are subject to the inverse square law. They will be buried in noise long before they reach any distant civilization.

Not to someone with a big enough receive dish antenna.

The free space path loss from Earth to Alpha Centauri (4.4 ly) is ~300 dB at 1 MHz (AM broadcast) and ~345 dB at 100 MHz (FM broadcast). Without directional antennas on both ends of the link (and remember, this is broadcast we're talking about), the situation is hopeless.

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