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What happens to an electro-magnetic wave "in the long run"?

It quickly gets weaker with distance but is it ever completely "destroyed"? Or, if you had equipment that was sensitive enough, could you pick up all electro-magnetic waves that have ever been generated? Is there a theoretical maximum for how sensitive a detector of electro-magnetic waves can be?

Depending of the shape of space, will such a wave, with time, "bounce back", be destroyed or something else when it reaches the end/edge of the Universe?

Sorry for the multiple questions but they are all "intertwingled".

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Electromagnetic waves are made of photons. These will continue to travel along geodesics unless they scatter or are absorbed. Currently the main model of cosmology has an infinite Universe so if this is correct a photon could keep travelling indefinitely if it is not absorbed.

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    $\begingroup$ Photons are the quanta of the electromagnetic field. The field isn't "made up" of them. $\endgroup$ – Acccumulation Apr 17 at 1:40
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    $\begingroup$ @Acccumulation There is no scientific proof the light waves come before photons. There is no way to even physically describe a light wave without incorporating billions of coherent Photons. When a light wave spreads out across the universe in the way the OP is asking, it makes more sense to consider single photons continuing on their separate paths. Otherwise what do you have left for your wave? What is it if it’s not photons? $\endgroup$ – Bill Alsept Apr 17 at 2:09
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Classically, there would be no hard limit. In a perfectly silent, classical universe, you could imagine a device to detect the wave. But practically, the strength would eventually decay below the ability of your device to discriminate it from other sources of noise (including those from within the device).

In QM, any detector has a lower and lower probability of interacting with any photons from the source as the distance increases. A very skilled detector might pick up a photon now and then, but it would have to discriminate those from detections of noise and other sources.

There is no "edge of the universe" for such a wave to bounce back from. In most cosmological models, the energy of the wave continues into ever larger volumes of space, reducing the strength and ability to be detected. In some possible models, space has a large-scale curvature. It would be possible in those universes for the waves to eventually be concentrated into a smaller region, increasing the strength. This would be similar to creating a splash on a perfect water world. Waves would propagate from the splash and move outward, decreasing in strength. But if they were not attenuated, they could continue to the other side of the planet, where they would be concentrated once again at the exact opposite side.

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The Cosmic Microwave Background (aka CMB) is light radiated from hot glowing matter as it cooled from the Big Bang. It took a few hundred thousand years for the universe to cool enough that electrons could stick to protons, forming neutral H atoms. Before this, the universe was full of free charges, which absorbed light. After this, the universe was transparent. So the glow that left at this time is the earliest light in the universe. It is about the same age as the universe, 13.7 billion years.

The universe has expanded by a factor of 1100 over the last 13.7 billion years. This means light that has been travelling all this time now has a wavelength 1100 times longer. It is no longer visible light. It is microwaves.

We measure this light today.

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