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What the question really amount to asking is, if (as LIGO said) the gravitational waves emitted from the black hole collision were emitted as 'pure energy' this surely means that they were emitted as photons. If this is the right interpretation does it therefore mean that when the waves interacted with the lasers at LIGO, were they self-interacting photons that were interacting with the lasers not 'ripples in spacetime'?

Qmechanic
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Sam Cottle
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    Sam, photons aren't pure energy. They are massless particles. So are gravitons, which are the hypothetical particles that make up gravitational waves. – John Rennie Jun 17 '17 at 16:49
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    More precisely a gravitational wave is a coherent state of many gravitons. – John Rennie Jun 17 '17 at 16:50
  • Well as far as I'm aware all photons have an energy, so the way I interpreted energy was as being comprised of photons. – Sam Cottle Jun 17 '17 at 16:50
  • What the question really amounts to is asking whether or not the emission of 3 solar masses by the black hole was in the form of photons or not, and if so, whether those photons can self-interact causing the reading at LIGO. – Sam Cottle Jun 17 '17 at 16:54
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    All spiders have eight legs, but not all eight-legged animals are spiders. Similarly, the fact that photons carry energy does not mean that all processes that carry energy do it through photons. – Emilio Pisanty Jun 17 '17 at 16:54
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    Hi Sam. No, the 3 solar masses worth of energy wasn't emitted as photons. It was emitted as gravitational radiation, which may or may not be made of of gravitons depending on whether gravitons really exist. – John Rennie Jun 17 '17 at 16:56

3 Answers3

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General relativity is a purely classical theory and it describes the emission of gravitational waves without involving gravitons, photons or indeed any other elementary particle. The energy in a gravitational wave is basically the self energy of the spacetime curvature. To use a rather crude analogy, you can imagine spacetime as an elastic material and the self energy is the energy required to bend it. A gravitational wave carries energy in an analogous way to a wave in an elastic material carries energy.

When you ask about what particles involved you are asking how we describe the gravitational wave, and the energy it carries, using quantum mechanics. Given that we have no theory of quantum gravity there isn't a firm answer to this. However if we attempt to use quantum field theory then we find gravitational waves are coherent states of gravitons just as light waves are coherent states of photons. Note that a gravitational wave isn't a hail of bullet like gravitons just as a light wave isn't a hail of bullet like photons. The relationship between the wave and particle is more subtle than that.

So there is no reason to invoke photons in considering interactions of gravitational waves. The interaction with the detectors at LIGO is essentially perfectly described using the purely classical approach of GR. If you wished to use a quantum description then you'd need to consider scattering of gravitons, though it's far from clear if this would be a useful approach.

John Rennie
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This is flat-out incorrect:

the gravitational waves emitted from the black hole collision were emitted as 'pure energy' this surely means that they were emitted as photons.

Photons carry energy, yes, but that does not mean that all processes that carry energy do so through photons. In this specific case, the energy was emitted in the form of gravitational waves ("ripples in spacetime") which also carry energy, and which are entirely unrelated to photons.

To be clear, interpretations of the detection in terms of "self-interacting photons that were interacting with the lasers" are very much in direct contradiction with known physics.

Emilio Pisanty
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  • Consider the hypothesis that, via Stueckelberg QED, the photon has mass and can interact with other photons kinetically, albeit with a very low probability of doing so. Wouldn't it be possible then? – Sam Cottle Jun 17 '17 at 17:06
  • @SamCottle Anything is possible if you're OK with non-standard physics. If you're willing to go that route, why not say that gravitational waves are just beams of magic? You seem to be hell-bent on denying a core part of our understanding of the universe, without anything but a not-even-superficial understanding of the phenomena your new theory is meant to explain. I can tell you from experience that this has so far had a 100% failure rate. You're welcome to try - but this site is not the venue for it. – Emilio Pisanty Jun 17 '17 at 17:09
  • (And, to address that specific concern, no, for the same reason. Photons, whatever their properties, carry energy, but that does not mean that all energy-carrying processes do so through photons. I will not address any further examples, though.) – Emilio Pisanty Jun 17 '17 at 17:11
  • you should read this..https://arxiv.org/abs/hep-th/0304245 – Sam Cottle Jun 17 '17 at 17:14
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    @SamCottle You seem to be disinterested in the answer, which makes me wonder why you posted the question in the first place. Either way, I have nothing else to add here. Have a good day! – Emilio Pisanty Jun 17 '17 at 17:17
  • That's because I already know the answer :) – Sam Cottle Jun 17 '17 at 17:18
  • @SamCottle what does it mean for photons to interact kinetically, and what would a non-zero photon mass have to do with that? [also, a Stückelberg QED isn't compatible with the electro-weak interactions, so it is already pretty much incompatible with the data] – AccidentalFourierTransform Jun 17 '17 at 18:41
  • It would only mean that there's a small probability that the two masses (of photons) could collide with one another, like classical particles, and then scatter. Also, I've found nothing to suggest that Stueckelberg QED is incompatible with the electroweak interaction, can you link me a paper that shows that it is? There may be parity violation but there's parity violation in beta decay so...I don't know. – Sam Cottle Jun 17 '17 at 19:54
  • I'd give this a read as well...https://arxiv.org/abs/hep-th/0104242 – Sam Cottle Jun 17 '17 at 20:35
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As the comments and the two answers so far have mentioned, photons are the carriers of Electromagnetic Waves and Gravitons (supposedly) are the carriers of Gravitational Waves — please see this link for the carrier particles for each of the fundamental forces. This answer would, however, attempt to address how the "interference pattern" generated at LIGO could only be caused by the gravitational waves passing through it.

  • LIGO is engineered to be "blind" to EM and sound waves. See source.

  • All of LIGO's optical components reside in an ultra high vacuum chamber. A laser beam is first passed through a beam splitter, such that it splits the laser into two and sends each beam down along the two precisely identical L-shaped tubes. These laser beams are then incident upon the mirrors, from where they're reflected back to be detected by the detector at the base. This arrangement itself ensures that there will be no interference pattern where the laser beams recombine.

  • However, when the interference pattern is still detected, it means that there was the displacement of mirrors used in the setup, and not by the interference of any other light with the laser.

  • This displacement can arguably be caused by seismic activities too, but then it won't necessarily be detected at the other LIGO site, at least not with the same frequency and the exact time difference that it would take for a signal commuting at the speed of light (speed of gravitational waves) between the two sites.

  • In other words, it was the "spacetime" between the mirrors itself that experienced the distortion (because of shrinking and expansion of the distance between the mirrors and the detector as the gravitational waves passed through) and thereby generated the interference pattern thus observed.

  • The intensity of the recombined laser at the detector gives us the frequency of the gravitational waves and this was found to be exactly the same at both the LIGO sites in all the three events detected so far.

Dhruv Saxena
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  • What about gamma rays? – Sam Cottle Jun 17 '17 at 20:59
  • https://phys.org/news/2016-02-gamma-ray-accompany-ligo-gravity.html – Sam Cottle Jun 17 '17 at 21:04
  • Gamma rays are extremely high frequency and therefore very low range. LIGO laser beams have the wavelength of about $808nm$ (near infra-red), whereas the wavelength of gamma rays is much shorter, about $1pm$. I don't think that the interference caused between these two would be something that the LIGO detectors are designed for (particularly when they start off with no interference in the first place). Additionally, the link above itself notes, the GRB was detected $0.4seconds$ after the gravitational waves, not with it. – Dhruv Saxena Jun 17 '17 at 21:34
  • Yeah, but it's a funny coincidence isn't it? – Sam Cottle Jun 17 '17 at 22:21
  • Well, it's an unconfirmed discovery yet. Even if the GRB originated from the same source though, it wouldn't be very surprising. – Dhruv Saxena Jun 17 '17 at 23:18