Gamma-ray laser moves a step closer to reality | Hacker News Reader

Gamma-ray laser moves a step closer to reality

phys.org

51 pointsdnetesn23 comments

If you're curious about the current state of the art see the high inte sity gamma source: http://faculty.virginia.edu/PSTP2013/Talks/HIGSCapabilities_...

This isnt a gamma laser because the gamma rays arent coherent, but they can be polarized more than 99%. The free electron laser makes optical laser light in a big cavity with dielectric mirrors, the next electron bunch in the storage ring collides with it and scatters it up to MeV level of energy. Flux and energy resolution can be exchanged by changing the diameter of the gamma collimator and the operating mode. Its generally something like 10^7 gammas/s for energies near 15 MeV with 3% resolution.

Newbie question: Aren't gamma rays just a particular frequency range of e.m.? If so, why would they be any less capable of coherence than other e.m. frequencies?

You can make them coherent but it's technically hard.

That is true that photons are photons, the trick is getting them to be coherent in the first place. Optical lasers use stimulated emission in some form. I presume that positronium from the article is able to somehow also undergo stimulated emission in the form of the particles anihilating and making 511 kev photons. The distinction between that and how HIGS works is that HIGS is effectively bouncing laser light off a fast moving mirror made of electrons, which actually destroys the coherence, even though the resulting gamma rays are still very similar.

To me this looks like someone has used a markov chain to massage together some buzzwords into something grammatically correct...

Can someone actually in the field comment on the legit-ness of this article?

The physics appears to be kosher, but I can't speak about the practicality of such as device. I suspect it's up there with graphene in that it'll generate a lot of papers, but no industrial applications.

It sounds like a powerful weapon to me.

Well, I can say for sure that it's gammatically correct.

Not that I'm opposed to speculative research but...

Does anyone know any actual uses for such a device? I know the article says medical and space propulsion. But it doesn't seem well suited to space propulsion (seems like a dangerous exhaust that doesn't do anything a microwave deam doesn't?).

Sorry if I'm being dull

I think they are referring to this passage in wikipedia? Kinda a stretch...

https://en.wikipedia.org/wiki/Nuclear_photonic_rocket#Power_...

> "A design proposed in the 1950s by Eugen Sänger used positron-electron annihilation to produce gamma rays. Sänger was unable to solve the problem of how to reflect, and collimate the gamma rays created by positron-electron annihilation; however, by shielding the reactions (or other annihilations) and absorbing their energy, a similar blackbody propulsion system could be created. An antimatter-matter powered photon rocket would (disregarding the shielding) obtain the maximum c specific impulse; for this reason, an antimatter-matter annihilation powered photon rocket could potentially be used for interstellar spaceflight."

This article is a bit out of date, and I'm not in the field, so I'm not sure what we've accomplished by other means :

https://doi.org/10.1063/1.3068817

Copying introduced some artifacts. I'm pretty sure it was OCR'd:

> "We believe it is worthwhile, because of the great potential for important ap-plication of radiation with enhanced co-herence in this spectral region. In every region of the spectrum in which coherent sources exist, they have great-ly multiplied the usefulness of radiation. Gamma rays have many impor-tant applications, because of their short wavelength, high penetrating power and ability to ionize. Controllable and directable sources of such radiation in which phase as well as intensity have significance, would greatly multiply its uses."

>" However, detailed speculation upon specific apdetailed speculation upon specific applications is clearly impossi-ble at this stage, because the form, per-formance, output characteristics, cost—even the existence—of a graser are obviously impossible to describe at this stage of our knowledge, beyond the few characteristics enumerated above."

>" Moreover, it is well known that the most significant applications of new de-vices are rarely foreseen. No one, for example, in 1958 could have predicted the use of lasers in computers, surgery, nuclear fusion, isotope separation, and soon."

> "The most exciting possibility for co-herent radiation in this short wave-length range is that of holography, which might permit direct observation of the structure of molecules, crystals, proteins, genes."

As an aside, SciHub is awesome. All of these publications should be open access. I wouldn't have been able to read many of the review papers in ML and biochem crucial for developing my own understanding if not for SciHub.

I think the idea is to achieve extreme energy densities and a high degree of control, with many practical benefits over a particle accelerator. This old article describes some of the stuff they were doing with a petawatt laser [0].

Essentially, they used it for microscale imaging and also generating extremely high energy electrons. There is also 'photofission' where gamma rays cause nuclear fission to occur [1]. There is some additional detail in this article [2]. They seem to be saying that obviously photon-electron interactions are incredibly useful in science, and a gamma ray laser would be just as useful but for photon-nuclear interactions.

Finally of course, a gamma ray laser would be a useful space weapon. For example, at [1] they could penetrate 6 inches of lead. It isn't clear how you could reliably stop a high energy gamma laser, as it would degrade even very dense shielding at the same time as generating showers of ultra high energy electrons, neutrons and protons.

[0] https://web.archive.org/web/20080614224504/http://newton.ex....

[1] https://en.wikipedia.org/wiki/Photofission

[2] https://spie.org/news/3681-nuclear-photonics-with-laser-base...

Surely to make a gamma laser, you at a minimum need a gamma mirror.

To my knowledge, such things don't exist (current gamma mirrors aren't 'shiny' - ie. they don't reflect the light, but instead diffuse it).

Coherence doesn't depend on the mirror, that's surely just about efficiency? I thought coherence came from quantum states of the lasing medium.

Coherent radiation in all directions usually isn't something that one imagines when hearing 'LASER'.

Put it in a giant tank of water with one exit hole. Highly inefficient...

It is not, but technically directionality is not required, only amplification by stimulated emission.

Coherence is also quantifiable - e.g. how many coherent photons travel into a certain direction within a time interval. A resonator increases that amount by many magnitudes.

Edit: I mean this in terms of a maximum norm. You get a smaller maximum amount of mutually coherent photons without an optical resonator.

I dunno, interstellar masers don't have mirrors. Kind of like a one shot pass through the lasing medium.

Free electron lasers generate X-rays using a single pass through the lasing medium, which is essential because you can't make good mirrors at those wavelengths either.

> Surely to make a gamma laser, you at a minimum need a gamma mirror

No. See free-electron lasers [1].

[1] https://en.m.wikipedia.org/wiki/Free-electron_laser

I really liked this talk on how free electron lasers generate such powerful pulses. https://news.ycombinator.com/item?id=16028723

I'd hypothesize that this would be useful for producing matter + anti-matter pairs

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