# If gravitational waves exist are they technically just another form of light/electromagnetic wave?

I would imagine a gravitational wave would have very similar characteristics to electromagnetic wave, what kind of differences are there?

• tapir.caltech.edu/~teviet/Waves/differences.html – Hasan Jan 9 '15 at 0:12
• A little bit of research would shows you that they are very different entities. Also, a source emitting gravitational waves does not have to have electric charge, which would seem to disprove your hypothesis. Stick a bunch of neutrinos together in a certain asymmetric configuration and you can create gravitational waves. – HDE 226868 Jan 9 '15 at 0:24

If gravitational waves exist are they technically just another form of light/electromagnetic wave?

No.

Electromagnetic waves are (classically) disturbances in the electromagnetic field that propagate with speed $c$.

Gravitational waves are disturbances in the geometry of spacetime that propagate with speed $c$.

I would imagine a gravitational wave would have very similar characteristics to electromagnetic wave

Electromagnetism is (classically) linear. However, the gravitational field equations are non-linear (which are approximately linear in the 'small-signal' approximation).

While there is dipole electromagnetic radiation, the lowest order gravitational radiation is quadrupole. This is related to the fact that the gravitational field is a rank 2 tensor field as opposed to a vector field.

So, other than the fact that they are waves and propagate at $c$, they aren't similar at all. In a comment, Hassan has provided a relevant link for further reading.

gravitational waves and electromagnetic waves might be the same thing if mass and charge were the same thing (as far as i understand reality they are not the same thing).

imagine that we are standing some distance from each other and facing each other and you're holding a negative charge and i am holding a positive charge. they attract each other but we are restricting motion so that they cannot move toward each other along the line connecting them. but they can move in the left/right dimension and the up/down dimension.

so i move my charge up and your charge follows it up. i move my charge to my right and your charge follows it (to your left). then i wave my charge back and forth (actually left-and-right) a couple times per second and your charge follows that waving motion. that is literally an EM wave and i am a transmitting antenna and you are a receiving antenna. if i moved my charge back-and-forth a million times per second, you could tune that in with an AM radio. if i move it back-and-forth 100 million times per second, you could tune it in with an FM radio. if i move it 500 trillion times per second, you would see it as a blur of orange color.

that's what EM waves are. if i perturb the position of my charge, the position of your charge is perturbed. if a third-party observer who is equidistant from both you and me was watching, he/she would see the perturbation of your charge occur at a time that is $\frac{L}{c}$ later where $L$ is the distance between us.

now imagine you and i are big as gods and we're holding planets (that are attracted to each other due to gravity) and we do the same thing: i perturb the position of my planet and your planet responds with a similar perturbation. again, for the third-party observer who is equidistant from both you and me, the observer would see the perturbation of your mass occur at a time that is $\frac{L}{c}$ later than my perturbation where $L$ is the distance between us.

you get EM waves by waving a charge around (and the other folks holding a charge have their charges perturbed) and you get gravitational waves by waving large masses (a planet or a star) around and the other masses (that are attracted to it) are perturbed in their positions. $c$ is not just the "speed of light" or the speed of EM, but is the speed of any "instantaneous" interaction.

that speed is really only an expression of the units we use to express it. as long as $c$ is real, positive, and finite, it doesn't matter what value it is. if all dimensionless fundamental constants remained the same, everything else in physical reality would scale so that, for us mortals, $c$ would continue to be measured as around 299792458 meters per second (assuming the meter definition preceding 1960, now it is defined so that $c$ is that value).

• One small addition for the sake of clarity: While AM (amplitude modulation) and FM (frequency modulation) radio are in the frequency range that you specified, technically you'd have to modulate the frequency (i.e. the speed with which you're waving) or the amplitude (the distance you wave to the left/right) to be able to tune into it with a radio. – chopper Nov 25 '16 at 9:43
• no @chopper, that is incorrect (and being an electrical engineer, i feel that i have sufficient chops and credentials to say so). in both cases you can "tune it in" and in both cases you would hear silence (as opposed to the band noise you would hear in an empty channel) because both signals are unmodulated. i.e., you can tune in unmodulated AM and FM signals (and you will hear a nice silence rather than the hiss between stations and the reason you may not hear hiss with a modern receiver is because of muting/gate function in the receiver, not because the hiss isn't there). – robert bristow-johnson Nov 25 '16 at 19:36

## protected by Qmechanic♦Jun 26 '16 at 11:01

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