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## Is There a Limit To a Laser's Energy?135

StartsWithABang (3485481) writes "For normal matter — things like protons, neutrons and electrons — there's a fundamental limit to the number of particles you can fit into a given region of space thanks to the Pauli exclusion principle. But photons aren't subject to that limit; in theory, you could cram an infinite number of them into the same exact state. In principle, then, couldn't you create a laser (or lasing cavity) with an infinite amount of energy inside? Perhaps, but there are some big challenges to be overcome!"
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## Is There a Limit To a Laser's Energy?

• #### Photons aren't the problem (Score:4, Funny)

by Anonymous Coward on Sunday May 04, 2014 @03:58AM (#46911753)

It's the number of sharks you can fit into a given region of space.

• #### Re: (Score:2)

It may be the number of D cells you can fit into the units on the sharks.

• #### E=MC^2 (Score:2, Interesting)

by Anonymous Coward

Eventually the laser energy will create a black hole, provided some other exotic effect doesn't occur first. Realisitcally though it's not possible to attain those kinds of photon densities (nothing can reflect anywhere close to well enough for starters).

• #### Kugelblitz (Score:4, Informative)

on Sunday May 04, 2014 @05:57AM (#46911953)

Eventually the laser energy will create a black
hole

There is a specific term in astrophysics for such a theoretical object:

http://en.m.wikipedia.org/wiki... [wikipedia.org]

• #### That's a limit on energy DENSITY (Score:4, Insightful)

on Sunday May 04, 2014 @12:14PM (#46913215) Journal

Eventually the laser energy will create a black hole, provided some other exotic effect doesn't occur first.

That's a limit on energy density, not total energy in the laser. In principle you could use a very WIDE laser opterating below the black-hole thrshold and focus the beam externally (which, if it's powerful enough, it might do eventually, by self-gravitation, after leaving the cavity, even if the cavity geometry made it emit a colimated, rather than a converging, beam.) Thus, making a kugelblitz with a (very wide) laser might be theoretically possible (if "some other exotic effect" didn't make the required laser cavity to wide to be physically realizable).

I'd imagine "Some other exotic effects" might include the electric field component of the coherent light becoming strong enough to polarize the vacuum and create particle-antiparticle pairs from multiple photons, dissipating their energy, somewhere WAY below the threshold of gravitic-collapse effects. So you'd need a REALLY WIDE laser and REALLY GOOD optics to make your external-to-the-laser black hole.

Of course the question, being phrased in terms of Bose-Einstein vs. Fermi-Dirac statistics and "infinite" energy was really about energy density in the cavity - just poorly phrased. So you answered the question that was REALLY being asked.

• #### Re:E=MC^2 (Score:5, Informative)

on Sunday May 04, 2014 @03:55PM (#46914679)

E = mc^2 specifically applies only to objects that have nonzero mass and are at rest with respect to the observer. Photons are massless and move at the speed of light.

The general equation is E = sqrt((mc^2)^2 + (pc)^2) for rest mass m and momentum p. If a particle has mass and is at rest, then p=0 so E=mc^2. If a particle is massless, then m=0 so E=pc.

(The "m" here refers to rest mass m0, not the "relativistic mass" m* which is defined as m* = m0 / sqrt(1-(vc)^2)). Relativistic mass is best thought of as a fake concept to hide the ugly sqrt denominator. People can imagine things getting heavier when they're moving, and can keep saying "Einstein discovered E=mc^2". But it still has division-by-zero problems with massless particles, and things don't really "get heavier" when they move, so if you try to avoid thinking in terms of m* you won't get as confused. Neither m nor m* makes E=mc^2 work with photons.

Imagine if a bundle of photons could gather and form a "black hole". The hole and its event horizon would be constrained to move at the speed of light, which you can't, since you have mass. so you might easily escape its event horizon- you wouldn't have time to fall in before the thing was gone. Real black holes have mass and don't move at the speed of light relative to anybody.

• #### Re: (Score:2)

(nothing can reflect anywhere close to well enough for starters).

... nothing that we have technological access to at the moment. You might manage to reflect light with a high-enough magnetic field, but getting it flat enough to form a lasing cavity isn't going to be easy.

• #### ExactoMundo (Score:2)

Napoleon, like anyone can even know that.
• #### There are some... er, limits: (Score:5, Informative)

on Sunday May 04, 2014 @04:32AM (#46911825)
• #### Re:There are some... er, limits: (Score:5, Interesting)

on Sunday May 04, 2014 @07:16AM (#46912089) Journal

But long before that happens the question is if the laser can remain a laser.

A laser needs some kind of nonlinearity in the medium. Any nonlinearity introduces a scale. So the real question is: At which power does of-resonant driving cause transitions (e.g. Landau-Zener) or of-resonant shifts (Stark shift) and can you actually theoretically contruct a medium which fulfills the criteria to serve as a lasing medium for an arbitrary large scale of power?

As a starting point for an examination of such questions i recomment the Quantum Optics Toolbox for Matlab by Sze Tan.

• #### Spoiler at the end. Answer is "No" (Score:5, Informative)

on Sunday May 04, 2014 @04:36AM (#46911831) Journal

"Update: After a conversation with Chad Orzel, it looks like although there's no limit to the photon energy you can produce, you will at some point--above about 1 MeV in photon energy--start spontaneously producing matter-antimatter pairs of particles whenever your photon interacts with a reflective surface. So at extremely high photon energies, your laser light begins to resemble a matter-antimatter thermal bath rather than merely coherent light."

So it would act like more Star Wars weapons?

• #### Re: (Score:2)

This is not only a problem for high photon energies on mirrors, but also for high energy densities in free space. If I remember correctly this starts at about 10^20 W/cm^2
• #### Re: (Score:2)

So it wouldn't be possible to construct a reflective surface that's not solid in the traditional sense but a reactive field of energy, which would guide the process at the point of interaction - to bypass the limit?

• #### Re: (Score:1)

Some sort of force field?

• #### Re:Spoiler at the end. Answer is "No" (Score:5, Informative)

on Sunday May 04, 2014 @07:25AM (#46912115)
I just searched for an answer to this question. Seems that pair generation by irradiation of matter (e.g. a mirror) is shown experimentally and can reach quite high intensities:
http://journals.aps.org/prl/ab... [aps.org]
Generation in vacuum though seems to be shown only in models until now:
http://iopscience.iop.org/0295... [iop.org]
http://journals.aps.org/pra/ab... [aps.org]
Seems that the reaction rate is much lower, so maybe this is not a limiting factor for building a laser.
Normally high intensities are achieved by building a pulsed laser. This produces a beam of laser pulses, which is then focussed into a tiny spot. Intensities in this spot can be alot higher than inside the laser cavity. You could achieve higher laser intensities just by building a larger laser (like http://en.wikipedia.org/wiki/N... [wikipedia.org] ).
Inside the laser cavity intensities are normally limited by the effects of nonlinear optics ( http://en.wikipedia.org/wiki/N... [wikipedia.org] ), which occur in all kinds of matter.
• #### Compton Scattering (Score:3)

This effect will not only kill at high energies but at high intensity too. With a high enough intensity you can have multi-photon interactions to achieve the same total energy. However pair production is not the only process you have to worry about compton scattering [wikipedia.org] will occur as well. This will impose an intensity limit well below pair production energies.

Essentially reflected photons will have less energy than incident photons and as the energy increases so too does this energy difference. It is cause
• #### Re: (Score:2)

Chad Orzel was my college physics professor. Cool dude.
• #### Re: (Score:1)

Correction, the answer is "yes" in terms of "is there a limit?".

• #### Not even for "normal" matter (Score:1)

For normal matter — things like protons, neutrons and electrons — there's a fundamental limit to the number of particles you can fit into a given region of space thanks to the Pauli exclusion principle.

Wrong, unless you assume space is discretized, which might happen around Planck's length, but has never been proven theoretically nor experimentally.

• #### History.... (Score:5, Informative)

on Sunday May 04, 2014 @05:07AM (#46911885)

Billions and billions of years ago, even before lord Xenu, there was a scientist who pulled this off.

Blext Telfrawd, an A type Hixoid, did get an infinite number of protons into a finite space. Then the containment field faltered, obliterating the iteration of his universe..

Most historians agree this was tragic for it ended his universe, and created one with Justin Bieber. Sentients who were able to achieve trans-dimensional universital access, send a message to you from the past: It's just too risky to repeat the so called "Bieber Event",

You've been warned.

• #### Putting the cart before the horse? (Score:4, Interesting)

on Sunday May 04, 2014 @05:13AM (#46911893) Homepage Journal

Okay. Interesting on a theoretical level.

The main problem with testing this is "how does one generate infinite or near-infinite energy" to power something like this?

Of course, if we've answered that, we're ALREADY in a place where we've either wiped ourselves out (accidentally or otherwise), or we've basically solved the greatest real-world problem in the history of humanity.

• #### Re: (Score:3)

I think the idea if that you could create a stable laser, that exists in some reflective box. And you would keep adding power to it forever.
Or at least until it exploded.

• #### Re: (Score:2)

The main problem with testing this is "how does one generate infinite or near-infinite energy" to power something like this?

Use your local power plant. It produces near-infinite energy as far as lasers are concerned.

• #### Re: (Score:2)

You're not acquainted with what "infinite" actually means are you?

• #### Re: (Score:2)

You're not acquainted with what "near-infinite" actually means are you? Talk to me again when you have a laser that uses 100-1000 megawatts continuously.

• #### Wrong interpretation of energy (Score:5, Informative)

on Sunday May 04, 2014 @05:56AM (#46911949) Homepage

The energy of a photon is characterized by its wavelength. In a laser, the wavelength is constant. You have a large amount of photons which are coherent but at an almost single wavelength. When the article is talking about 1 MeV, it falsely interprets this as if the laser is emitting a single photon at 1 MeV. That is not what happen. It emits many photons in coherence which the sum of energy of all the individual photons will reach 1 MeV or more. Each photon cannot create an electron-positron pair and all photons collectively cannot create an electron-positron pair.

A 1 MeV photon would be a gamma ray photon and it is not true at all, your laser doesn't change its wavelenght as more more "energy" is emitted. In fact, we should instead talk about the power of the laser rather than its energy. The power being the amount of energy emitted by unit of time.

• #### Re:Wrong interpretation of energy (Score:5, Informative)

on Sunday May 04, 2014 @06:47AM (#46912037)
Exactly.

This is the usual muddle up between energy and intensity.

There is no apparent upper limit to the energy of a photon. The galaxy Markarian 501 emits photons in the teraelectronvolt (TeV) range.

The question here is about intensity. The relativistic energy-moment dispersion, E^2=(mc^2)^2+(pc)^2, which applies to all on-shell particles, has a gap when m>0. This gap, which is about 1 MeV for electrons and positrons, can be overcome when the electric field (generated by a sufficient number of photons, irrespective of their energy) approaches the Schwinger limit of about 1.3 x 10^18 V/m. At this point, virtual electron-positron pairs can be created in abundance because the mass gap has been overcome, and electromagnetism then becomes non-linear. Pumping in more photons after this simply creates more virtual e-p pairs.

Hope that helps.

(IAAP working on this topic).
• #### Re: (Score:2)

(IAAP working on this topic).

I demand proof in the form of a youtube video of your laser!

• #### Re: (Score:2)

When the article is talking about 1 MeV, it falsely interprets this as if the laser is emitting a single photon at 1 MeV. That is not what happen

He is indeed talking about 1 MeV per photon. He's discussing the theoretical limits of photon power density in a hypothetical gamma-ray laser with an adjustable wavelength. An ordinary laser pointer stores more than 1 MeV of energy in its lasing cavity, although a physicist would not typically use eV to describe the combined energy of a light beam.

• #### Re: (Score:3)

He is indeed talking about 1 MeV per photon.

he is jumbling together a lot of nonsense, imnsho.

He starts with the idea of an ordinary laser. Those are not even in the X-ray range, nevermind the MeV gamma-ray range. Then he wants to 'compress' the lasing cavity to *ahem* reach black-hole level of energy densities. While you can transfer energy to the radiation field (thus shifting up photon energies from the visible/UV range) you'll need a HECK of a fast compression to reach the electron-positron generation threshold. So that's nonsense.

Second, lasing

• #### Re: (Score:2)

Then he wants to 'compress' the lasing cavity to *ahem* reach black-hole level of energy densities.

It seems pretty clear to me—I took that same first course—that a neutrino is just a white hole (moving at the speed of light) made up of photons which such a strong self-interaction they can't escape from themselves and thus refuse to interact with much of anything else.

This all seemed to fit with the gravitational contribution of the EM Stress Energy Tensor until I saw a post from Lubos on Stackex

• #### Even if there is no limit (Score:2)

Would it be effective against a Dikironium cloud creature?

• #### bomb-pumped X-Ray lasers (Score:2)

If you can channel the energy of a fusion explosion into many lasing-while-ionizing rods (think "Real Genius"s death ray laser, but MUCH larger) you could pack so many X-Ray photons into a burst that the impact (momentum transfer) alone destroys the target's armor, at least according to David Weber.

• #### Old news? (Score:5, Interesting)

on Sunday May 04, 2014 @07:42AM (#46912169)
There was an article from 2010 that talked about the theoretical limit to laser beam energy. From the article:

"At high laser intensities interaction of the created electron and positron with the laser field can lead to production of multiple new particles and thus to formation of an avalanche-like electromagnetic cascade"

Here's the link to the article in question: http://physicsbuzz.physicscent... [physicscentral.com]

That article was ultimately using this [nytimes.com] article as a source.
• #### Overcoming the "black hole" problem (Score:2)

It is well-known that once you exceed a certain energy density you create a black hole. This is why the Death Star superlaser consists of multiple small lasers that combine.

• #### File this under "smart people are stupid" (Score:1)

IANAP. Which means, i probably have a better understanding of the subject than they do. Theoretically.
• #### Eddington Limit (Score:3)

on Sunday May 04, 2014 @09:50AM (#46912597)

As a certain energy density, the radiation pressure from the photons will be stronger than the tensile strength of the optical cavity, and the laser will blow apart. In astronomy, a similar limit is called the Eddington limit, so this is really the Eddington limit for a laser.

The radiation pressure is (ignoring all factors of 2 or cos(incidence)) E / c. A tensile limit, T, of 500 mega pascals (reasonable for steel) thus would imply an energy intensity of c T, or 1.5 x 10^17 Watts/m^2. If the total cavity had an area of 1 m^2, then that's ~ 10^17 Watts.

Note that it is common in pulsed lasers to have a lot of energy in a very short pulse (so the actual power during the pulse is very high). If your pulses were a microsecond in length, then the Eddington limit per pulse would be about 10^11 Joules, equivalent to 24 tons of TNT.

• #### E=mc^2 (Score:4, Interesting)

on Sunday May 04, 2014 @11:23AM (#46912933)

At high enough energies particles are spontaneously created. They in turn will obey Pauli Exclusion (at least if they have spin I think). So enough photons and you make matter that will prevent you from making more particles ie pumping more energy into the space.

• #### "physics" is multidisciplinary (Score:3)

on Sunday May 04, 2014 @12:41PM (#46913359)

"Physics" is not just one thing anymore. The guy writing TFA, Ethan Siegel, is a bonified professional physicist. Reading the comments, you can see he just didn't know this one thing as well as he thought. How does that happen?

I don't know that there's any physicist going through training today or in the last 20 years who really understands "all" of physics.

Physics PhDs learn most of physics up to about 1910 (even that is a stretch, but at least the complete fields up to that point are introduced and sketched out), and the next 100 years are based on your specialty. The limits of energy density for photons are usually in this realm of "introduced only if directly important to your specialty."

It's up to the individual to fill in the gaps after formal classes, and it can be very hard to figure out what you don't know. It's particularly hard because of the oversimplified way physics is generally taught in undergrad, even to physics majors. Your old reference books may not actually be correct. I'm sure I've got a physics textbook around which claims almost exactly what Ethan said in his blog; the "why" of pair generation is just too distracting.

• #### On a tangent note... (Score:2)

On a related note, I asked this quessie at a laser pointer forum a while back. Would still be interested in hearing a real answer: http://laserpointerforums.com/... [laserpointerforums.com]
• #### 3 words: Bose-Einstein Condensate. (Score:2)

Bose-Einstein Condensate! In more detail, fermions cannot be crammed together but in certain conditions, Bosons can. Photons are a type of Boson but not the only one. The Pauli exclusion principle does not apply to Bosons! Looks like a non-specialist needs to read some books on this concept. I won't even go into deeper details without this point being crystal clear!
• #### Medium but ugly (Score:1)

man this medium page is but guly.

giant fonts, no use of the screen.

bleark.

• #### More importantly (Score:1)

How many balloons can you pop with that?
• #### Yes. (Score:2)

The article reads as somewhat naive to me.

Every material has an energy density limit, at which point it will breakdown or ionize.

And given that no material is completely transparent to any wavelength, once you produce enough energy density, every material will ionize and place an upper limit on laser propagation.

• #### Photon Torpedoes (Score:2)

Now you know how Photon Torpedoes work....

• #### Pauli exclusion limit (Score:2)

Is that when a sweaty guy in a Vneck, chest hair puffed out, gold chain on his neck, wont let you into "da club" ?