How long can electromagnetic waves last?

[musicgold]

Hello,

I am reading a book on the history of the universe. The book says that we can still detect the electromagnetic waves generated at the big bang in the form of white noise. I am not sure how it works, because the waves transmitted by AM or FM radio antennas die down after propagating some distance. How can waves generated 14 billion years ago continue to exist?

Can someone please explain this to me?

Thanks.

 

[Nugatory]

The waves transmitted by AM or FM radio get weaker with distance, but they never die out completely. It may seem that they've died out, but what's really happening is that they've just gotten too weak for your radio receiver to detect.

 

[sophiecentaur]

The waves transmitted by AM or FM radio get weaker with distance, but they never die out completely.

The Inverse Square Law has some unforeseen consequences which work very much in the Astronomer's favour (radio and optical). In simple terms, the ISL says that doubling the distance from an object will reduce the received signal power to 1/4 (proportional to the distance squared). Sounds bad at first but when you are dealing with large distances and already weak signals, you find that that inverse square thing means that you get the same proportional reduction going from a 'nearby' galaxy and a 'really distant' galaxy, as you get going from, say Mars to Pluto - or Low Earth Orbit to the Moon. Hubble makes full use of this because it doesn't have to deal with our mucky Atmosphere.
Compare that with sending radio signals through a coax cable - even the very lowest loss / expensive stuff. Every 10m of cable could lower the signal level to, perhaps 99% of its original level. After only ten lengths of 10m. the level has dropped to 90% (0.99¹⁰), and you can do the sums for a 5000km transatlantic cable. A radio link (on a straight path), on the other hand, would still be going strong.

 

[russ_watters]

The inverse square law doesn't apply to the CMB because it comes from everywhere, not a point source.

For those who remember, we've gone through this logic with discussions of Olbers' paradox.

 

[ZapperZ]

I'll "complicate" things even more here. If a photon were to "decay", it must have mass. If we apply one of the most recent upper limit for a photon mass (i.e. experiments that tried to find the mass of a photon, but couldn't, and thus, can put an upper limit on it based on the experimental capabilities) and then calculate its lifetime, it turns out that a photon's lifetime is longer than the age of our universe.

So if such a decay were possible, what are the limits on the lifetime of a photon? That is the question asked by a physicist in Germany, who has calculated the lower limit for the lifetime of the photon to be three years in the photon's frame of reference. This translates to about one billion billion (10¹⁸) years in our frame of reference.

So unless it is absorbed or interacted by something else, a photon will live forever.

Zz.