### Why do some electromagnetic waves continue travelling while others disappear?

• Cosmic background radiation emitted when the Universe was very young still exists. But my wifi signal seems to disappear a short distance from my apartment. Why?

100mW WiFi radio transmitter vs most of the universe's known mass (CMB)?

Imagine to be at the center of a sphere made of routers or WiFi emitting devices. This should answer your question. Yet Connor Garcia put up a nice answer.

@fraxinus It's not really a fair fight since the CMB has around $1.4 \times 10^{89}$ photons, which is two orders of magnitude greater than all the photons emitted from any other source (star, supernova, black hole jet, etc...) since the great scattering . See https://physics.stackexchange.com/questions/276890/how-to-calculate-how-many-photons-are-in-the-universe

@ConnorGarcia this number is over the observable universe or what?

@fraxinus Yep, over the observable universe

@ConnorGarcia So a googol is still more than *everything* ;-).

• Electromagnetic radiation will continue to travel until it is absorbed. Some of your wifi signal is escaping to space where it may continue traveling for a very long time.

However, the strength of your wifi signal will degrade with distance according to the inverse square law.

So if you double the distance between your device and the wifi transmitter, your signal will be reduced by 3/4. When you move far enough away, eventually your signal will degrade enough that your device will no longer be able to decode the signal. So the radiation doesn't "disappear" but instead, your device will just be able to pick up less of the energy. Of course, if you separate the device and transmitter by a concrete wall, some of the signal will actually be "disappearing" as it is absorbed by the wall.

The CMB (Cosmic Microwave Background) on the other hand, was not emitted from a point source, and is not subject to the inverse square law. Instead, as wikipedia points out, the CMB

is very nearly uniform in all directions, but the tiny residual
variations show a very specific pattern, the same as that expected of
a fairly uniformly distributed hot gas that has expanded to the
current size of the universe.

So one can imagine that wherever we are in the universe, we can point an antenna in any direction and get about the same amount of signal (excepting Doppler shift) from the CMB. Of course, if we point our antenna towards existing matter, we will expect some or all of the CMB to have been absorbed in that direction. In many cases, astronomers can tell a lot about the composition of gas in absorption based on the narrow frequencies of the CMB that was absorbed. Back in the early 2000s, I spent quite a bit of time looking for cosmic formaldehyde in absorption lines of the CMB near the center of our galaxy, as an indicator for early star formation.

Turns out, the birth of a universe is a rather powerful signal source, and also that the entire observed universe is a rather large emitter surface.

Another factor is the size of the antennas being used to pick up the signal. You tablet, cell phone, or other device used to pick up your WiFi signal is pretty tiny. The antennae used to detect the CMB are quite a bit bigger: ttps://en.wikipedia.org/wiki/Discovery_of_cosmic_microwave_background_radiation If you used a similar-sized antenna for your WiFi, you could detect it quite a bit further away - assuming nothing absorbed it, of course.

@jamesqf Yes, with an antennae that size on both ends, you'd have signal for at least hundreds of kilometers (or miles, it really doesn't matter on this scale).

Also: CMB is more or less noise, while Wi-Fi is a discrete signal. If you trim your receiver to just verify that there is *a wifi signal* your "range" would also see significant improvements.

@jamesqf Now to get some radio telescope time to verify this hypothesis... /s

@user25375: The problem is that you'd have to use a space-based telescope, otherwise those WiFi signals would be blocked by the horizon. However, we can go the other way: New Horizons transmits at 12 watts, the Voyagers at 22.4 watts, yet with a large enough antenna that signal is detectable from well beyond the orbit of Pluto.

@jamesqf oh but you're ignoring the practical frivolities involved in wasting two radio telescopes to get an Internet connection.