# In principle, could any kind of wave (sound, water, electricity, gravity, matter, cmb, etc) be made to a “laser”?

This is of course not something that we can do at the moment, technologically speaking. But I was curious about the underlying principles of lasers and as they are, generally speaking, a form of concentrated,coherent, light radiation.

So this is for a fantasy project about another civilization that is very far advanced, with I guess what we might call "god-like powers".

Visible light is typically composed of a radiation that is a mixture of multiple wavelengths. In the universe we have other things that have a wave like presence/mode of propagation, such as:

1. sound
2. water
3. electricity
4. earthquakes
5. matter (as described e.g. here)
6. Cosmic Microwave Background (CMB) radiation
7. gravity (OK this may be more of a hypothesis at the moment)

The question is, would it in principle be possible, using the principles of lasers, to create concentrated/coherent waves of any categories of "things" listed above?

• 1: google.com/… Also, CMB 'laser' is like saying 'traffic noise laser': CMB is just there. Of course, a microwave laser is perfectly possible. – Sanchises Nov 23 '15 at 16:01
• Worth pointing out that the directional speaker isn't exactly a laser. It's a resonant driver that outputs a coherent wave, but it certainly isn't lasing. – Joe Bloggs Nov 23 '15 at 17:20
• Microwave laser: also known as maser, they predate lasers. Lasers were first described as "optical masers". – JDługosz Feb 17 '16 at 0:10

## No

Lasers (no longer an acronym for "Light Amplification by Stimulated Emission of Radiation"), require a couple of phenomenon to provide the beam and the other waves that you mentioned don't have a means of providing one of more of these 3 items.

### Design

First, you must have a "lasing medium", this is a material that can contain atoms with electrons in an "excited" state (meaning electrons are not in the "ground" / lowest energy state) AND remain optically transparent to the lasing frequency.

Second, you must have an energy source capable of stimulating those atoms by pumping energy into the lasing medium (aka gain media). Ideally, you want the electrons to be bumped up to the same "non-ground" level state (so they release the same frequency of photon when they drop into the ground state).

This can be done through chemical reactions (e.g. $DF$, $OI$, etc.), electrical discharge (e.g. $CO_2$), or by other non-coherent light sources.

Third, you must have a "resonator cavity" (usually a tube with a mirror on one side and a partially mirrored surface on the other).

### Operation

The order of operations is this:

1. The energy pump dumps energy into the gain media
2. The electrons in the gain media, jump into an excited state
3. At random one electron drops to the ground state, releasing a photon
4. The photon travels near another excited atom
5. This induces the second atom to release its energy in the form of a photon of the same frequency and direction
6. Photons continue their voyage inducing more and more atoms to give up their energy as photons of the same frequency
7. The photons reach the end of the resonator cavity and are either reflected or released.

As far as I know, no one has found a material to act as "gain media" for any of these other wave types.

### Details

Of the items in your list, #6 is a variation of em radiation. So it's possible that you could use the CMB as your energy pump. Just realize that there's not a lot of energy there so your laser will be very weak.

Items 1-4 on your list can participate in 1-3 different types of wave propagation

P (Primary) waves

These are compression-rarefaction (pressure) waves. A "gain media" for this type of wave would need to be able to release a pressure wave in phase with another pressure wave and in a single direction.

P waves can travel through solids, liquids, and gases - but not vacuum.

We know of no such material.

S (Secondary/shear) waves

These are transverse (side-side, like a wave on a rope) waves. A "gain media" for this type of wave would need to be able to release transverse motion in phase and in the same direction with another transverse wave.

S-waves can only travel through solids.

We know of no such material.

Surface (aka L) waves

a surface wave is a mechanical wave that propagates along the interface between differing media, usually as a gravity wave between two fluids with different densities.

In this case "gravity wave" does NOT mean a wave of gravity.

In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere and the ocean, which gives rise to wind waves.

Surface waves can only propagate along the boundary of two different materials (often two different phases of materials). Examples include ocean waves.

A gain media would need to operate on this same interface and provide a disturbance that only propagated along the material boundary.

We know of no such material.

For #5 & #7, we don't know how any of the components might work or have candidates for them. We know of nothing that would serve as gain media, energy pumping, or resonator cavity. We don't know how to reflect gravity, let along "pump up" gravity in an resonator.

• Technically the third point (resonantor) is not necessary while still common of course. It is possible to obtain lasing with only a single pass of EM radiation through the gain medium, without resonant cavity. (Still the answer remains right in most aspects.) – Ghanima Nov 23 '15 at 21:48
• Actually it is theoretically possible to obtain a laser ray without any of the listed prerequisites as long as you find some alternate "magical" way of producing a lot of coherent photons; that's just the technology behind production, not the inherent nature of the laser light. But yes, electromagnetic field needs no medium to exist; it's a property of space itself. Any waves that require medium interaction to exist can't be "lased". – SF. Nov 24 '15 at 10:23

An important thing about electromagnetic radiation is that is can pass through itself without causing interference. One 'wave' of light can be going up the body of the resonating cavity for the laser while another is going down, but the two won't meet. This means you can get their phase differences to match up just when you want them to in order to create a coherent (the waves are matching up and heading out of the laser) beam. Importantly, you can do this by just 'pouring' more light into the container.

Now consider a wavepool at a water park: You can't have both waves travelling through each other, they must interact, either constructively (you get a big splash as the two waves meet) or destructively (the waves just peter out, and it's rather disappointing). You can drive a single wave from either end of the pool and make it bigger, or you can throw more water into the pool to make a bigger wave, but you can't just keep adding water from one end and have the waves start to match up (be coherent) without the pool overflowing first.

Edit for the sake of clarity: The important point here is that physical waves can and must interact when passing through each other. EM radiation only causes increases in the likelihood of an interaction happening (photons don't hit photons). An antinode in a wavepool is always a huge wave, an antinode in a laser waveform is only a big thing if you stick something in it.

Further edit for the sake of clarity: For a solid material to have a signal-gain ratio of >1, you have to have a greater volume of material inside your laser than the volume of the laser. This isn't true of EM-radiation. That's the point I've been trying (and failing!) to illustrate.

If you have a physical phenomenon that can support waves travelling through each other, you can make a laser out of it. If it can't you just have to settle for resonant driving of the waveform. There's disagreement about which camp gravity falls into (if gravity is wavy at all).

What your super-beings could do is look at other methods of causing constructive resonance in various materials. Earthquakes would be a good example: If you can make three exactly calibrated earthquakes happen in precisely the right points in the earth, you could in theory have the shockwaves meet up at another point in such a way that they interfere constructively, and you get 3 times the earthquake at your target. Supergenius aliens shouldn't have any problem with the maths for that, but sadly it isn't a ground-laser.

• I've tried to make this simple by way of analogy. if anything's unclear let me know. – Joe Bloggs Nov 23 '15 at 17:06
• "An important thing about electromagnetic radiation is that is can pass through itself without causing interference." I am sory, I do not understand. There is standing wave, result of interference, in a laser. Light interference is generally well known. We do not see interference of normal light without special preparation because it is not coherent. Laser light is coherent and laser-light interference causes the characteristic look of laser-light spot. – BartekChom Nov 23 '15 at 18:15
• Interference patterns aren't a good example. Light does the same constructive and destructive interference as other waves. Waves in water moving in different directions will keep moving in their original directions after an interference event. – Jim2B Nov 23 '15 at 18:17
• Actually, waterpark waves travel through each other all the time. Its only once their energy gets high enough to break down the assumptions of linearity that they stop passing through eachother. We can see when that happens: its' called whitecaps. – Cort Ammon Nov 23 '15 at 21:10
• This is just... incorrect. Waves on a water's surface do pass through each other. Problems with large waves do not change that fact. You get problems with large waves with light anyway, due to the nonlinearity. – Dietrich Epp Nov 23 '15 at 22:34

Many waves about which you are talking are long (human scale as opposed to fractions of micrometers for visible light) and coherent (actually this means that they are normal strong series of waves, not chaotic perturbations) by nature and laser action is hard to apply to them, but sufficiently (fantastically) advanced super-civilisation could create something like laser (light amplification by stimulated emission of radiation, modification of maser (microwave amplification by stimulated emission of radiation)) i.e. generators creating coherent waves with the use of amplification in the medium, even at the atom level. However, it would be sometimes funny.

1. Sound is usually coherent. Incoherent vibration of matter is heat. In a crystal, quanta of sound are called phonons. Physics of solid matter is well developed and I would not be surprised if somebody had already created laser-like generators of phonons. EDIT: I have just found it. There are SASERs since 2010. "In this active medium, a stimulated emission of phonons leads to amplification of the sound waves, resulting in a sound beam coming out of the device."

2. Waves on water can be created as single waves (that contain very many coherent quanta - human length means very low energy of one quantum - the weakest possible wave (such that smaller ones are impossible due to quantum uncertainty) is probably smaller than one atom) or whole series. It is weird to treat something as trivial from the quantum theory point of view, but it is possible and maybe our super-civilisation could make the energy of very many very slightly excited molecules (it is not so easy to find molecules with energy levels near enough to each other) transform into energy of waves of water. EDIT: On the other hand, the way in which the wind is transferring its energy to waves amplifying them is somehow similar to laser action.

3. Electrical waves are more or less one-dimensional. They can also be created somehow similarly to laser action in electronic oscillators, but it is still not a full analogy. This would require something special.

4. Seismic waves could be amplified by causing explosions in the right places and moments, but to make it on a atomic level, would be again waird. They are mechanical vibrations like sound and also could be interpreted as coherent packs of phonons.

5. Coherent matter waves, like in Bose-Einstein condensate, are quite an achievement. However, the number of atoms (They can be treated as composed particles, as opposed to quasiparticles like phonons. Elementary particles building normal matter, electrons and quarks, as well as protons and neutrons, are always fermions, and there cannot be strong waves in the field connected with them - this would mean more than one particle/quantum in one state, and is impossible due to Pauli exclusion principle) is conserved, ant this is one big difference between matter waves and for example light: we can create photons, but we can at most transfer atoms to one state from the other ones. Another difference is that atoms are massive.

6. As sanchises has explained, "CMB 'laser' is like saying 'traffic noise laser': CMB is just there". "Microwave laser", maser, on the other hand, is older than laser.

7. Gravity waves such that we observe results of their emission (we see that binary pulsars change their orbital periods due to emission of this waves) are also coherent and have period of several hours, like the orbital period of the binary pulsars. Coherent gravitational waves with higher frequencies (from hertz to hundreds of terahertz (optical frequency) and further) would be something interesting and maybe could be somehow created with the use of stimulated emission, but currently we do not know any method to do it. Gravity radiation with energy of visible light would be very penetrating, but rather non-ionising. If the intensity were big enough, some gravitons would finally interact and heat the medium along the whole ray. Their interactions would be similar to the interactions of a photon of visible light, so could cause at most some chemical reactions and should not be very dangerous.

In modern physics, there are quasiparticles like phonons, composed particles like atoms and hypothetical fundamental particles like gravitons. To summarise: we can try to apply quantum field theory to this particles and justify the possibility that super-civilisation will build something that emits corresponding waves and is analogous to a laser. I tried to use my knowledge as graduate physics student and not to look at unorthodox ideas like a boring professional, but I am afraid that the result is not very clear. I hope that I did not do any serious mistakes and will be able to defend my ideas.

As a side note: Cosmic Background Radiation is light, but you wouldn't be able to turn it into a laser because there's no "container" for it. You could, however, "laserize" the radiation in that spectrum. I think you'd end up with one of these.

Electricity is also sort of already the same thing as well. There's a reason its called the "electromagnetic spectrum": electricity, magnetism, and light are all closely bound. Electric charge is the "force in the electric field," electricity is "electric charge in motion," magnetism is "the repulsiveness of the electric field," and light is "the waves in this field produced by the back-and-forth motion of electrons." That is, physics is weird and I am not a physicist to properly explain the differences in English, but MinutePhysics has a few videos on it. Which either makes it "not a laser" or "exactly the same thing as a laser" depending on how you look at it (and probably also the particle-wave-duality ((quantum)physics is weird)).

Gravity might be possible to laserize, it also might not. The problem is that we (as the collective knowledge of the human species) doesn't really know how gravity works yet (heck, we're still not entirely sure that it propagates at the speed of light, slower, or faster although experimental evidence suggests that it is exactly c). The main problem you'd have is moving a large enough mass around fast enough to make a difference (that or causing it to pop in and out of existence). It's kind of hard [citation needed] to pump more mass into something the same way you can pump energy in.

That said, a 'gravity laser' (a gaser? graser?) would probably just end up producing black holes (again, physics is weird). Which while certainly devastating (on a large scale), not exactly useful if it happens inside your reflection chamber.

• "Graser" is already in use for "gamma ray laser". Interestingly it is theoretically possible to emit a laser beam from a phased array (although we're very far away from actually producing one). This would be a "phased array laser" = "phaser?" – Jim2B Nov 23 '15 at 19:18
• Heh, so graser is already in use. I'm not surprised, that always happens when people try to come up with names for things (looking at you astrology). – Draco18s Nov 23 '15 at 19:20
• en.wikipedia.org/wiki/Gaser – Jim2B Nov 23 '15 at 20:18
• CAUTION DO NOT GAZE INTO GASER. – David Richerby Nov 23 '15 at 21:39

The question is, would it in principle be possible, using the principles of lasers, to create concentrated/coherent waves of any categories of "things" listed above?

Actually, yes. I think some people misunderstood and were trying to somehow convert any energy into a coherent light beam. That would be impressive, but is currently beyond our ken. But we already accomplish this feat on many different levels.

Back in the day, LASER was an acronym that stood for "Light Amplification by Stimulated Emission of Radiation". So naturally we old-timers don't simply apply that term to anything that presents a beam. (The term "X-Ray Laser" comes to mind - because it's actually a MASER, Microwave Amplification by Stimulated Emission of Radiation, but I guess the difference only means something to another physicist in today's world, so whatever...) That very thing is "yet another" application of creating a concentrated/coherent beam of waves - it just happens to be based on the principle of stimulated emission proposed by Albert Einstein. When atoms have been induced into an excited energy state, they can amplify radiation at a frequency particular to the element or molecule used as the medium (similar to what occurs in the lasing medium in a laser). Putting such an amplifying medium in a resonant cavity, feedback is created that can produce coherent radiation. How you release that radiation is relative to the wavelength/frequency of that radiation.

So you could effectively build a sound collimator device that produced a "beam" of concentrated sound (I think that would be devastating if deployed against a soft, fleshy target), or just about any commonly unfocused energy in existence. It's all in how you go about collecting, harmonizing, and focusing that energy into a coherent "stack" (or "beam") to be directed and released at your will.

tl/dr; Yes. It's do-able. Einstein said so, and I'm inclined to believe him.