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They are not a form of electromagnetic radiation, are they? So, why do gravitational waves move at the speed of light?

SG8
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2 Answers2

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Here I summarize the most important results:

  • Gravitational waves are predicted by Einstein's theory of general relativity.

  • According to general relativity, they are disturbances in the curvature of spacetime (so they are not a form of electromagnetic radiation).

  • LIGO and Virgo observations have experimentally confirmed the existence of gravitational waves.

  • According to the Einstein's general relativity, gravitational waves have two degrees of polarization and, as a result, they travel at the speed of light. This is because, according to the field theory, the wave equation of massless spin-2 particles (gravitons) must have two degrees of polarization and this is the case for the gravitational wave equation in general relativity. So, the gravitational wave (which consists of massless gravitons) travels at the speed of light the same as the electromagnetic wave (which consists of massless photons). (See more details in this link.)

  • A number of different analysis show that the speed of gravitational waves can be measured using the data of LIGO and it has been confirmed that gravitational waves propagate at the speed of light (See more information in my another answer here).

I recommend to see these interesting links:

  1. Gravitational waves (WikiPedia)

  2. Did LIGO measurements prove that the speed of gravity equals the speed of light?

  3. Why does gravity travel at the speed of light?

  4. How fast does gravity propagate?

  5. Particles Associated With Gravitational Waves

  6. Large gravitational waves

SG8
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    Why does having two degrees of polarization imply that they travel at speed of light? – Rafael Rodríguez Velasco May 26 '21 at 08:20
  • @SG8 So the reason for travelling at the speed of light is not Spin 2 but because it has no mass, correct? How about photons? They have Spin 1, but no mass as well, and travel also at the speed of light. – Charles Tucker 3 May 26 '21 at 10:57
  • @CharlesTucker3 - I did not say such things. The answer and the links are clear enough. Massless particles travels at the speed of light. A spin-2 particle with only two degrees of freedom (polarization) is necessarily massless. – SG8 May 26 '21 at 11:02
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    @SG8 Thanks for clarification! The first point (massless -> speed of light) is as expected and the second point (Spin 2 and two degrees of polarization -> massless) wasn't clear to me! – Charles Tucker 3 May 26 '21 at 11:14
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    @CharlesTucker3 - A spin-1 particle with two degrees of polarization is massless, so it travels at the speed of light. Otherwise, it has one extra degrees of freedom like vector massive bosons and in such cases they cannot reach the speed of light. The same as true for spin-2 particles. If they had more that two degrees of freedom (they could have at most 5 degrees of freedom), they will travel at the speed slower than light. – SG8 May 26 '21 at 11:19
  • @CharlesTucker3 - Parts of this link may be useful to you: https://physics.stackexchange.com/questions/630301/what-is-the-analog-to-the-electric-field-for-those-particles-different-to-photo/631977#631977 I think there are more nice discussions about these in PSE. Also, see those links in the answer, please. Cheers – SG8 May 26 '21 at 11:19
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The physical background here is not that one type of wave has the same speed as the other by some sort of coincidence, but rather that both have the speed limit associated with spacetime itself.

The speeds of either wave are of course predicted by the equations that describe the different physical phenomena (Maxwell equations and Einstein equation(s)), but it is no coincidence that both have the same speed $c$. It is because this speed is part of the nature of spacetime. The Maxwell equations are pretty much the simplest set of equations you can find for a wave-like field that respects the principle of relativity in a spacetime with a speed limit. The Einstein equation is pretty much the simplest one that can express a link between matter and geometric properties of spacetime.

When we say that spacetime itself has a natural speed limit, we do not need to refer to light or gravitation or any other type of disturbance. Rather, the assertion influences the types of mathematical quantity that accurately reflect physical phenomena. In this case the types are called tensors and spinors, and they have certain symmetry-like properties which enable them to express correctly the way temporal and spatial separations relate to one another when any given phenomena are considered from the perspective of different frames of reference. In such a framework it is found that no signals of any kind (where by 'signals' we mean influences that can allow one event to influence another) travel faster than the speed limit, and the simplest kinds of physical phenomena happen at the speed limit.

Andrew Steane
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