Particle Beams vs Lasers

[phongn]

How does the turret redirect the beam? (Lenses, I assume) How much loss is associated with the use of a turret?

No idea on either count.

[Admiral Valdemar]

So would a neutral particle beam suffer from the inverse square law, or would the focusing prevent that?

It'd suffer, but in space, not as much as a charged particle beam which would repel itself and bloom.

Charged for atmosphere, neutral for space. NPBs can't be redirected however.

[Arrow]

How about space to surface (going through atmosphere)? It would have to be a neutral beam then, wouldn't it, especially since you'd have to deal with a planet's magnetic field?

[1337n1nj4]

Depends on how close you get, I'd think. If you can get your ship to LEO, you could get around most of that.

[Admiral Valdemar]

It'd be better having a nuclear powered facility on Earth that is sunk into the ground as one big laser facility which fires vertically to a mirror sat constellation that can redirect the beam. It may lose some power, but the laser could be more powerful and not required to be in orbit which is expensive and risky.

[Symmetry]

While we're on the laser subject I've seen it claimed that any laser with an intensity of greater than 10Kw/CM^2 is useless in atmosphere because beyond that intensity the air is opaque to the weapon. Seemed kinda wonky so I thought I'd ask.

That seems about right. No gas is ever perfectly transparent to a laser, and so the laser's passage will heat the atmosphere as it goes. If you heat it enough, it will turn to plasma, which is far less transparent than normal atmospheric gasses.

[phongn]

IIRC, they're using a "precursor" laser on the ABL to 'burn' a path through the atmosphere first and then they fire the main burst to blast that pesky missile.

[buzz_knox]

How dense would a substance have to be for a gamma laser to interact fully, and what form would that interaction take? I had a thought that a FEL capable of emitting in the gamma and X-ray ranges would be an effective multipurpose weapon if the gamma were capable of penetrating the the target and causing a detonation/explosion in a vital area (engine room, magazine) when it struck a dense object (engine, reactor shielding). The X-ray would be available for less dense targets.

Forgive the potential breach of etiquette of quoting oneself, but does anyone know the answer to the above questions? I'm considering writing a fic of my own, and the answer to this would be helpful.

[Shroom Man 777]

Just say "incredibly dense, made from elements not found anywhere near earth" or some stuff.

[ClaysGhost]

How dense would a substance have to be for a gamma laser to interact fully, and what form would that interaction take?

It depends on the energy of the photons making up the laser beam. Photons carrying a few keV (wavelengths of about 1nm) will be stopped within a few mm or less by dense metals, and will deposit their energy on surfaces. Photons having energies of a few MeV (wavelengths of less than a picometre) get much further. An inch thick piece of steel will absorb about ~10-20% of the photons passing through it, at these energies (so about six inches thickness of steel would be necessary to absorb half the incoming photons). I don't know precisely how well dense metals absorb high-energy photons, but you could try looking up some "absorption coefficients" for particular materials and particular energies. I guess that the rate of absorption will increase rougly as e^(density2 / density1), so a material twice as dense as steel should absorb e^2 = ~7 times more photons given the same thickness at high energy (>1MeV) but it's probably more complicated than that.

The form of the interaction is complicated. Look up "pair production" and "compton scattering" if you're interested in the specifics. The practical effect is that below a few keV photons are absorbed relatively efficiently by electrons. At energies above a few MeV photons other less efficient mechanisms dominate and photons are not absorbed very well.

Is there any way to redirect a laser at all...

Optics, obviously, but wavelength matters. It gets harder to redirect photons as their energy increases. For example, the mirrors used in X-ray telescopes have to focus X-rays by using grazing incidence reflection; the mirror surface has to be nearly parallel to the direction of the incoming photons.

I don't understand how a laser beam can 'lose' energy after passing through a gravity-lens-thingy and coming out in a different direction. Surely it would blueshift as it neared the gravity source and redshift the same amount as it moved away, meaning no total loss of energy?

It would be so if the beam extended to infinity, both approaching and receding from the source of the gravitational field. Although this is fine for natural gravitational lenses because the fields are weak and the distances large, I don't see how this can be the case for a laser weapon on board a starship, because it's not generated at infinity and the field is strong. The amount of redshift or blueshift in frequency would depend on the details of the weapon, though.