The Algebraist by Iain Banks

[psikeyhackr]

If the energy required to vaporize the wall requires a momentum much smaller than the momentum required to punch the wall outward

It is not energy of momentum that would vaporize the wall, it would be heat energy.

That vaporization would produce gaseous material at very high pressure which would expand outward damaging other things. I believe it was Jerry Pournelle who wrote stories involving lasers on the ground firing up the nozzle of a rocket to produce expanding gas to propel the rocket into space.

The momentum applied to the cannon would be extremely small.

The momentum of a photon is equal to Planck's constant divided by the wavelength of the light.

Planck's constant has a TEN to the MINUS 34 in it.

http://open.salon.com/blog/kwatts59/..._mass_of_light

Math, physics and engineering are three different things. There are situations in engineering where things are too small to pay attention to. Trying to channel the power to a laser that could produce a kilogram of measurable momentum would be WAY MORE of a problem than the recoil. I don't even want to do the calculation of the power. I presume this was an X-ray laser. Why mess with anything else? LOL

psik

[Fung Koo]

Photons do not have mass:

http://en.wikipedia.org/wiki/Photon#Physical_properties

Not that wikipedia is the be-all and end-all. But neither is nquixote's link to what is an argument for laser recoil in an RPG game.



Light does have momentum, but you're using the wrong equation. Keep reading on the above link for more.

[Chuffalump]

Originally Posted by nquixote

The action is pointless because:

A) It doesn't really have any "macro significance". Nothing much is at stake.

B) The characters are unsympathetic and shallow and it's impossible to care about any of them.

C) The action scenes seem like they were put in to be "set pieces" - e.g. there's really no reason for them to be randomly nuking a cruise ship, but the author just thought it would be cool to have a scene where they nuke a cruise ship.

D) The physics is TERRIBLE (no recoil from laser cannon that blow open thick metal walls, characters surviving being underneath a building-sized hovercraft, etc.), which makes suspension of disbelief difficult.

E) The scenes relied on unrealistic character stupidity (Horza, apparently a trained commando/spy, lets his two insanely dangerous enemies live, and takes both with him at the same time into incredibly precarious situations, using all his manpower to guard them!). Actually, unrealistic-stupidity-as-a-plot-device is something I notice a lot in British space operas, including Alastair Reynolds and Peter F. Hamilton...maybe they learned it from Banks?

All in all, there was nothing witty or surprising or original in the book. I briefly hoped that the whole story turned out to be some kind of mental exercise of the Culture Mind as it waited to be rescued - that would have been a cool twist - but it didn't materialize. I finished the last page and thought "Wow, I just read a book where nothing happened."

Oh well. Most of your points seem to be a case of horses for courses and all that sort of thing. I enjoyed it. You didn't. I find Banks' puts over a sense of scale in his books that is virtually unmatched by any other author that I've read.

However, I'll disagree with you on point D. Why should a laser cannon have recoil? Nothing physical is being moved so action/reaction is not applicable. As for surviving under a hovercraft, I have no idea if this is feasible but since it IS possible for a hovercraft (NOT a tiny one man jobbie either) to float over a minefield without setting the mines off (at the instant of contact), I wouldn't rule it out either.

Does anyone know if Jackie Chan really did get run over by the hovercraft in Rumble in the Bronx?

[Chuffalump]

Blimey. It took so long for me to write a reply that all my points have already been made.

Anyway, depending on which use of a laser to 'blow out a wall' is being discussed... wasn't there a great tank of water behind the wall which blew it out once it was weakened after being heated up by the laser?

[nquixote]

Originally Posted by psikeyhackr

It is not energy of momentum that would vaporize the wall, it would be heat energy.

Um...the kinetic energy of the coherent photons in the laser beam are converted to heat when they hit the wall. When the laser is fired, it's kinetic energy, with the velocity pointing in one single direction.

The momentum applied to the cannon would be extremely small

No. If you think the beam vaporizes the wall, then here is what you do:

1. Find the energy required to vaporize the wall (this will be BIG).

2. Divide that by the speed of light to get the momentum. That will be the "kick" the ship experiences when it fires the cannon.

Planck's constant has a TEN to the MINUS 34 in it.

But that's per photon, dude. There are a LOT of photons in a laser beam.

Originally Posted by Chuffalump

However, I'll disagree with you on point D. Why should a laser cannon have recoil? Nothing physical is being moved so action/reaction is not applicable.

Nope. Light is a physical thing. It has momentum even though it does not have mass. The recoil comes from conservation of momentum.

Now look, maybe in Iain M. Banks' world, laser cannons have some way of spitting a bunch of neutrinos out the back when they fire, so that they have no recoil. But Banks never said anything like that, so we'll have to assume that he just wanted to write about stuff blowing up without thinking about the physics...

[psikeyhackr]

Originally Posted by nquixote

1. Find the energy required to vaporize the wall (this will be BIG).

A wall made of what material? How reflective is it in the laser's wavelength? Is it conductive? How thick is it?

NO! You compute the total energy of the photons that would have to come out of a 900 Kg laser cannon to give it a recoil acceleration of 1 m/sec^2. That isn't too fast and there aren't so many variables.

BIG isn't a very precise term in physics.

I'm not even questioning the efficiency of the laser.

Whereas the He-Ne demonstrates an efficiency of ~ 0.01 - 0.1 %, the CO2 Laser operates with an efficiency of up to 30 %. Continuous power levels of a few kilowatts are possible yields from a moderately-sized laser. Thus this laser has applications in industry for cutting metals and welding.

http://www.phy.davidson.edu/StuHome/...-Final/co2.htm

psik

[Jeroen]

On Topic:

If you want to try Banks, I would recommend "The Player of Games" and "Use of Weapons", book 2 and 3 of the Culture novels respectively. The stories of the books stand on their own, but unfortunately, most of what you need to know about the Culture is explained in "Consider Phlebas".

Now Consider Phlebas is not a horrible book. It is actually quite a nice adventure tale with some pretty good ideas here and there. Yet it is also pointless action and nothing much is at stake. I would read it anyway. Approach it like a movie that you know hasn't much plot but has cool action. And afterwards you know what everybody is talking about in this thread.

[Chuffalump]

Originally Posted by Jeroen

On Topic:

If you want to try Banks, I would recommend "The Player of Games" and "Use of Weapons", book 2 and 3 of the Culture novels respectively. The stories of the books stand on their own, but unfortunately, most of what you need to know about the Culture is explained in "Consider Phlebas".

I think most of the books allow you to understand the Culture without having to read them in any particular order. I'd also recommend Excession and Matter although there are some sections of the first that seem irrelevant until the end of the book. I also enjoyed Against a Dark Background which is SF but not his Culture series. His non-SF is supposed to be good but I've only read The Wasp Factory which was a little weird/dark for my taste.

If you want a little taster of his work and style, go to the library and get his collection of short stories The State of the Art.

[Chuffalump]

Originally Posted by nquixote

Now look, maybe in Iain M. Banks' world, laser cannons have some way of spitting a bunch of neutrinos out the back when they fire, so that they have no recoil. But Banks never said anything like that, so we'll have to assume that he just wanted to write about stuff blowing up without thinking about the physics...

Ha ha. May be it's a double ended laser with the reverse beam reflected round to join the first beam to cancel recoil. Would you get light pressure on a perfect reflector? Maybe you could bend the light using the artificial gravity generators..

[Fung Koo]

First, the bloody lasers:

No -- there is almost no appreciable recoil. And what infinitesimally small amount of recoil there is is largely an indirect effect (a laser of sufficient strength projecting into a non-vacuum creates plasma right at the aperture when released, causing the energy to be converted to force, causing an equal/opposite reaction at the aperture point, exerting an external force outward from the aperture. Still not actual recoil though -- that's a shockwave travelling through a medium, and has more to do with sound than with light).
---

On to Banks.

I thought this thread was about the Algebraist, not the Culture novels?

The Algebraist is probably Banks' 2nd or 3rd best SF novel, IMO. (Excession and Player of Games are my other two faves.) Honourable mention goes to Against a Dark Background, but I thought The Algebraist the superior standalone.

The worldbuilding is fantastic. Banks depicts the logistic and political problems of living in a high-tech but relativistic universe with great clarity, while also keeping up with his sense of a universe absolutely jam packed with life.

The Dwellers were hilarious, and the the way Banks depicts the excesses of human and dweller existence have a ring of truth to them. (At least, I see truth in them, which is probably why I like Banks in general.)

The conclusion was imaginative, and also is scientifically plausible (which is more than can be said for the Culture novels).

I believe the OP said s/he wasn't finished yet, so I won't spoil it. But, the conclusion is worth it, to endure the apparent side pots that take over the story. It all ties together nicely in the end

[nquixote]

Fung Koo, you are very wrong. You are thinking in terms of classical mechanics (I think), but where light is concerned you need to think in terms of relativistic mechanics. There is so much wrong in your post that I can't go into it. Take a physics class, or read a book. Sheesh.

Psikeyhackr, to give a 900kg cannon a recoil acceleration of 1 m/s^2 requires 2.7e11 W of power.

[Fung Koo]

Originally Posted by nquixote

Fung Koo, you are very wrong. You are thinking in terms of classical mechanics (I think), but where light is concerned you need to think in terms of relativistic mechanics.

Thanks to my good buddy, Dr. of Quantum Optics, I've figured out where I went wrong. I screwed up on the mass-energy equivalence relating to momentum as a vector.

Any laser generates its beam, as I said before, by reflecting light back and forth between two reflecting surfaces (passing through the gain medium, which adds photons to the system). The back-wall mirror is as close to 100% reflective as possible. The front-wall (the output coupler) can vary in reflectivity, but for a visible spectrum laser is usually like 99.99% opaque (or, 0.01% transparent).

So the energy in the beam is only 0.01% of the total energy in the system. (The actual power of the output beam is a result of an optimum level of reflectivity within the optical cavity so that the light passes through the gain medium to collect the maximum amount of photons before the gain medium itself actually hinders the process.)

The recoil would be the same -- the output coupler's opacity, times the total amount of energy in the system. So, 0.01% of the total energy in the system will be experienced as "recoil," which would be equal and opposite to the amount of energy released from the laser.

Assuming that a weaponized laser could keep photons actively ricocheting inside the optical cavity/propagation chamber, maintaining a ready-state at a particular energy level by having a variable-opacity output coupler, the laser at rest would be experiencing equal "recoil" at both ends simultaneously until fired (no net force). With the opacity of the output coupler reduced to the desired firing intensity of the beam, the sudden imbalance in net force would only cause the recoil at the backend of the laser to become noticeable. And only by the percentage of difference in the opacity of the output coupler.

So it's a conceptually different sort of recoil than the recoil you find in a chemically-propelled weapon. Rather than the recoil being caused by force acting additively on the gun (the gun as a system moving from a potential energy system [inert] to a kinetic energy system), the recoil in a laser gun would actually be caused by a decrease of force being exerted on the front-wall of the laser. A sort of negative-recoil, if you will.

Regardless, the amount of recoil in a laser is negligible. And easily, easily counteracted if there was ever a laser powerful enough to get to levels where it was worthwhile to do something about it.

Take a physics class, or read a book. Sheesh.

Thank you, professor. I've taken several, and consider physics an interest -- though I confess, I led myself astray here. But remind me not to take a class from you, Professor Rude!

[nquixote]

Originally Posted by Fung Koo

I screwed up on the mass-energy equivalence relating to momentum as a vector.

You don't need mass-energy equivalence to find the momentum of light.

So the energy in the beam is only 0.01% of the total energy in the system.

That's not the issue. The issue is how much momentum the light that actually leaves the laser has.

So, 0.01% of the total energy in the system will be experienced as "recoil," which would be equal and opposite to the amount of energy released from the laser.

Since energy is a scalar quantity, to say "equal and opposite" here is meaningless.

The momentum of the light leaving the barrel of the laser cannon, in this case, can be found from the energy needed to vaporize a quantity of steel. That's a very easy calculation and has nothing to do with the inner workings of the laser.

Thank you, professor. I've taken several, and consider physics an interest -- though I confess, I led myself astray here. But remind me not to take a class from you, Professor Rude!

Well, sorry for being rude, but I get tired of people who don't what they're talking about telling me I don't know what I'm talking about...

[Ropie]

Perhaps Iain M Banks had a conversation like this with some scientist friends before he decided to write about a laser canon with no recoil.

[psikeyhackr]

Originally Posted by Ropie

Perhaps Iain M Banks had a conversation like this with some scientist friends before he decided to write about a laser canon with no recoil.

And maybe he could figure out the obvious.

There are a lot of people that try to make science seem more difficult than it really is. And also try to use psychological debating bullshit to win simple disagreements about physics.

Telling kids about entertaining sci-fi stories with ACCURATE SCIENCE would help them integrate real science into their thinking while their mental sponges are still working well. Harry Potter is a waste of their time.

http://www.gutenberg.org/files/24161...-h/24161-h.htm

http://economics.gmu.edu/wew/article...tingDumber.htm

psik.