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Just wondered how they compare to sound waves, naturally they travel at the speed of light, but I was wondering if they diminish over time like sounds waves. Cheers.

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  • $\begingroup$ What a good question. So simple. But so thought-provoking. $\endgroup$ – John Duffield Apr 16 '18 at 15:42
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Classical waves from point sources disperse as $1/r^2$ . Sound waves and water waves are energy transfers in a medium and at some distance from the source there is not enough energy to move the medium and there is no wave.

Electromagnetic waves are built up by zillions of elementary particles, called photons, which even when the distance is so large that no longer classical light can be built, remain as individual photons unless they interact with matter.

In an effective quantization of gravity, the gravitons have the role in gravity corresponding to the role of the photons in electromagnetism, so even when the distance from the source is enormous so that no classical wave can be built up, they will exist as elementary particles until they interact with some other particle or radiation.

Extended sources can be considered to be built up by point sources, so at some distance will be acting like point sources ( as far away stars for light).

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  • $\begingroup$ So, they do diminish if there's matter in the way? $\endgroup$ – Sam Cottle Apr 14 '18 at 12:10
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    $\begingroup$ I do not agree that the classical light would disappear. Telescopes see light from really far away. One could argue that detection in CMOS sensors etc is a quantum process, but radio telescopes just work with ordinary antennas. $\endgroup$ – Pieter Apr 14 '18 at 12:16
  • $\begingroup$ @Pieter if the distance r is large enough so that a few photons at at a time exist per micron square, it will be like single photons from lasers , even though they were part of a light beam to start with, and ultimately antennas also work with quantum processes. You see stars in the galaxy but not the light reflected off their planets, the distances are too great $\endgroup$ – anna v Apr 14 '18 at 14:34
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    $\begingroup$ The material about gravitons is irrelevant. This is a question about classical general relativity. $\endgroup$ – user4552 Apr 14 '18 at 14:59
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    $\begingroup$ Sound waves and water waves are energy transfers in a medium and at some distance from the source there is not enough energy to move the medium and there is no wave. Maybe you mean that the amplitude approaches zero. This makes it sound as if the amplitude actually reaches zero at some finite distance. Electromagnetic waves are built up by zillions of elementary particles... You seem to be saying this in contradistinction to sound waves and water waves, but the analysis isn't logically different, and the quantum mechanical material is irrelevant. $\endgroup$ – user4552 Apr 14 '18 at 15:57
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If you stand in an open field and clap your hands, the sound spreads out as a spherical wave. As the wave expands at the speed of sound, the energy is diluted over a larger and larger surface area. Since the surface area of a sphere is proportional to the square of its radius, the intensity of the sound falls off like $1/r^2$.

Exactly the same logic applies to gravitational waves.

Sound waves can also interact with objects and be absorbed, transforming their energy into vibration of the objects, which then is transformed by internal friction into heat. Although this does in principle happen when gravitational waves interact with matter, the effect is extremely weak. Matter is nearly perfectly transparent to gravitational waves. This is why they're so hard to detect.

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