51
$\begingroup$

The sources I've read compare gravitational waves to electromagnetic waves. I'm curious to what extent this is. In theory, could gravity be harnessed in similar ways to how we've used electromagnetic radiation such as in lasers?

If so: what differences would this have to a regular laser?

If not: What differentiates gravitational waves from electromagnetic waves to make this theoretically impossible?

$\endgroup$
  • 31
    $\begingroup$ Michael Bay, are you reading this? $\endgroup$ – corsiKa Mar 13 '15 at 20:50
  • $\begingroup$ In theory as opposed to in practice? What do you mean? $\endgroup$ – l--''''''---------'''''''''''' Mar 15 '15 at 22:20
  • 4
    $\begingroup$ I mean that I'm aware that we currently have no way of measuring any of this in practice and I would prefer to avoid people pointing that fact out when trying to answer the question. $\endgroup$ – Misc.nerdiness Mar 15 '15 at 22:23
  • $\begingroup$ In a recent blog post Stephen Wolfram is considering a graviton laser as a "far-out idea", but cool enough to be used in the Sci-Fi Movie "Arrival": blog.stephenwolfram.com/2016/11/… $\endgroup$ – asmaier Nov 13 '16 at 18:19
  • $\begingroup$ Very high frequency gravitational wave background in the universe arxiv.org/abs/gr-qc/0406089 discusses graser beams in interstellar plasma on p.3. "Transformations between gravitational and electromagnetic waves [take] place in the presence of ionized media, magnetic fields, or curvature... Under special matching conditions (”wave synchronization” or ”equality of phase velocities”) EM waves generate the GW beam which is amplified by the coherent quadruple oscillations of atoms (as well as of electrons or molecules) of the media..." $\endgroup$ – Mike Stay Sep 14 '17 at 23:20
29
$\begingroup$

Laser light generation is intimately related to processes that generate single photons. To date, gravitational waves have not been detected, and there are no known processes that produce single gravitons (not to mention there is no direct evidence that the gravitational field is quantized at all -- just logical arguments based on the structure of general relativity and quantum mechanics extrapolated to the relevant regime).

Since there aren't any processes known to produce single gravitons, there is no known means by which one could produce a gravitational wave laser.

EDIT: I agree with anna v's answer and John Rennie's comment, and I hadn't thought about free electron lasers when I wrote this. It would take relativistic planets or something like that, but it wouldn't be impossible.

$\endgroup$
  • $\begingroup$ If someone were to create gravitons, what kind of GASER could we make? $\endgroup$ – corsiKa Mar 13 '15 at 20:51
  • 3
    $\begingroup$ @corsiKa: Ender's Game sequels refer to it as a "Glaser", which I think sounds cooler, but makes less sense as an acronym. $\endgroup$ – Mooing Duck Mar 13 '15 at 21:25
  • 10
    $\begingroup$ I'm not sure this true because lasers don't have to rely on a population inversion of an excited state. Couldn't we build the equivalent of a free electron laser? A free mass quadrupole laser perhaps? $\endgroup$ – John Rennie Mar 14 '15 at 7:01
  • $\begingroup$ @MooingDuck: The term graser (with R, not L) is quite common in science fiction, and seems better for this purpose. But it's already taken for "gamma ray amplification..." $\endgroup$ – Ben Voigt Mar 16 '15 at 1:48
  • 1
    $\begingroup$ Since LIGO has clearly detected gravitational waves, do you have any updates on your answer? $\endgroup$ – Xiaodong Qi Oct 10 '16 at 21:08
14
$\begingroup$

I thought the answers that there can be no stimulated emission from excited gravitational states are correct, as there are no discrete bound states with the gravitational potential,but while exploring the suggestion by John whether one could make a free gravity laser in analogy to a free electron laser I found the following :

QUANTUM STATES OF NEUTRONS IN THE EARTH’S GRAVITATIONAL FIELD: STATE OF THE ART, APPLICATIONS, PERSPECTIVES

If one goes through the article, it is not a simple matter. A bound state of a neutron with the whole earth is not something one can build an inverted population with to create a laser. So the no should be qualified: beings who could experiment with ensembles of earths might be able to create inverted populations.

Now going to the gravitational analogue of a free electron laser I think we would have the same problem. The gravitational constant is very small, and it is necessary to use a neutral candidate so that the effect would not be masked by the electromagnetic changes, that one cannot see how an undulator with gravitational forces only could be attempted.

undulator

Schematic representation of an undulator, at the core of a free-electron laser.

The schema, because of the changes in acceleration might radiate coherent gravitational waves even as it is working with the charged electrons but due to the smallness of the gravitational constant these would be very weak , and , in my opinion , undetectable.

Of course all of the above with the assumption that gravity is quantized in the same way as the other three forces.

$\endgroup$
  • 1
    $\begingroup$ Why would you even need to assume anything about quantization of gravity? The free electron laser can be described satisfactorily even with classical theory of electromagnetism (unless the wavelengths are too small). $\endgroup$ – Ruslan Mar 14 '15 at 15:28
  • 1
    $\begingroup$ @Ruslan well gravitons enter the picture in the case of stimulated emission and gravitons come with quantized gravity. I would not know what would be "too small" on a gravitational set up. Maybe you are right for the case of the free gravitational laser . $\endgroup$ – anna v Mar 14 '15 at 16:01
  • 1
    $\begingroup$ "Beings who could experiment with ensembles of earths might be able to create inverted populations" /shiver $\endgroup$ – Cort Ammon Mar 16 '15 at 3:11
9
$\begingroup$

The key to making lasers work is the concept of "stimulated emission". When you have a population inversion - a larger number of atoms / molecules in an excited state than in the corresponding ground state - you can tap into their energy by stimulation emission. When such an atom/molecule is excited with a photon with energy corresponding to the transition, it emits a photon that is perfectly in phase with the incident photon.

In this way you get two photons that are in phase, and traveling in the same direction. This mechanism results in a beautiful amplification, and a coherent beam of light.

There is no analog (that I can think of) for stimulated emission of gravitational waves - and so a GASER (gravity amplification by stimulated emission of radiation) is fundamentally impossible AFAIK. Fundamentally, a massive system (potential emitter of gravitational waves) cannot be in an "excited" state from which emission is possible by excitation with another gravitational wave.

$\endgroup$
  • $\begingroup$ I have the same criticism of this that I have of Jerry's answer. $\endgroup$ – John Rennie Mar 14 '15 at 7:02
  • $\begingroup$ There is an analog of stimulated emission, it is superradiance as I've discussed in my answer below. The problem is making a device that uses the stimulated emission $\endgroup$ – cesaruliana Mar 15 '15 at 19:59
7
$\begingroup$

The physical principle behind the laser is stimulated emission. This is the easy part. There is a close analog of stimulated emission in general relativity, a phenomenon known as Superradiance. In short terms if you have a rotating black hole and emit gravitational waves in it with the right parameters you can get more gravitational waves out. The black hole loses angular momentum to feed the energy necessary to produce the amplification of gravitational waves. This is just the wave counterpart of a Penrose process. In a very heuristical point of view superradiance in a black hole is to stimulated emission as Hawking radiation is to spontaneous emission. Besides black holes superradiance has been theorized to occur also for rapidly rotating neutron stars.

So it is theoretically known that there are gravitational systems which do exhibit amplification of gravitational waves, without relying on population inversion or such things.

But a laser is not only amplification of waves, it is a device that operates with this mechanism in a stable way to produce coherent light. And this is where theoretically the "gravitational laser" dies. In a conventional laser one needs a cavity that a) provides a feedback mechanism so the device is stable (i.e. does not stop working because the population goes to ground state) and b) selects a given wavelength to produce the coherent light. And, as is well known, since gravity couples to everything there is no such thing as a gravitational cavity, or a mirror for gravitational waves.

In summary, it should be possible to produce gravitational waves of desired frequency, and is is possible to amplify the waves, the problem is that you cannot build a stable device. This covers the usual architecture of lasers, although it cannot rule out the free electron laser design, since it dispenses with a cavity. The issue would be to design the equivalent of a quadrupole wiggler, of which I have no idea how would work, but it doesn't seem to be theoretically ruled out

$\endgroup$
  • $\begingroup$ Two questions: 1. You write that "there is no such thing as a a mirror for gravitational waves." But a smooth gravitational potential deflects gravitational waves, and the deflection angle may easily be 180 degrees (e.g. if you project a light ray near a black hole with the right impact paramameter). 2. There are devices which select a wavelength, such as the normal modes of membranes (such as black holes). $\endgroup$ – bkocsis Nov 15 '16 at 21:51
  • 1
    $\begingroup$ I see your point, and it is indeed worth pondering on. In general my point in this answer was that a laser is not a physical phenomenon but a device, and although one can reproduce the main physical phenomenon there is a problem in having a stable configuration to sustain the lasing action. I would guess that although what you suggest could reproduce the desired effects it would not be efficient, therefore not sustaining the coherent wave production. But this is reductio ad ignorantia, just because I can't think of a way doesn't mean is not possible. $\endgroup$ – cesaruliana Nov 16 '16 at 20:43
  • $\begingroup$ How about reflection via the proposed Heisenberg-Coloumb effect? google.co.il/url?sa=t&source=web&rct=j&url=https://… $\endgroup$ – A. Ok Jan 15 '18 at 14:09
  • $\begingroup$ I apologize for giving a incomplete answer, that's a big paper and unfortunately I have no time now for a deep analysis. The way I see it the experimental setup is a bit strange. They are using High-Frequency waves so that the ion lattice can be taken to be in free-fall. Thus the wavelenght must be smaller than the interatomic distance, otherwise the lattice would vibrate as the wave passes along. It seems to me that in such a high frequency is exactly the regime in which classical consideration suggest that the film would be transparent to waves. I'm not saying the it is wrong, just strange $\endgroup$ – cesaruliana Jan 15 '18 at 23:23
  • $\begingroup$ I'd like to add that not every laser requires a mirror. I know of at least the home-brew, table top nitrogen lasers (see [en.wikipedia.org/wiki/Nitrogen_laser#Optics]). Crude, but still a laser, albeit pulsed. $\endgroup$ – Wraith of Seth Mar 29 at 0:28
4
$\begingroup$

The only thing I can imagine can work in theory, is by exploiting the mixing of gravitons and photons in an external electromagnetic field, see here. You then produce ordinary laser light and this gets partially converted to coherent gravitational radiation. This is analogous to the Primakoff effect where photons could be converted to hypothetical axions in an external field. So-called "light shining through walls experiments" are being performed, where experimentalists let laserlight move through a strong magnetic field, shine that onto a wall, and then attempt to detect axions on the other side of the wall by applying a strong magnetic field which would convert the axions back to photons. One then exploits the fact that these photons would be in precise coherence with the laserlight on the other side of the wall to set strong limits on the properties of axions.

$\endgroup$
0
$\begingroup$

If not: What differentiates gravitational waves from electromagnetic waves to make this theoretically impossible?

Electromagnetic waves couple to charge whereas gravitation waves couple to mass. Thus the process for generating gravitational wave would have to be very different from the standard Stimulated Emission/Population Inversion process used in a conventional laser.

$\endgroup$
0
$\begingroup$

As far as my understanding of physics go,the waves of gravity are like electromagnetic waves. The place where this idea falls apart is how these waves are made.

EM waves, especially those generated in lasers, are made by electrons absorbing/losing energy. There is no analog for gravity waves. Since lasers require EM waves being emitted (mostly) in sync with existing waves, and you cannot do that with gravity, you cannot have gravity lasers.

$\endgroup$
-1
$\begingroup$

There's no known way to make a gravity wave mirror. The ability to do that would suggest something like Cavorite, aka unobtainium

$\endgroup$
  • $\begingroup$ A smooth gravitational potential deflects gravitational waves, and the deflection angle may easily be 180 degrees (e.g. if you project a light ray near a black hole with the right impact paramameter). $\endgroup$ – bkocsis Nov 15 '16 at 21:54
  • $\begingroup$ Note that gravity waves and gravitational waves are different. $\endgroup$ – Qmechanic Nov 22 '17 at 20:46

protected by Qmechanic Mar 14 '15 at 12:36

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.