Does physics forbid such a device; a heat destroyer

Darwin123

Focusing the beam in this case may be difficult even in principle. The radiation is initially incoherent. The radiation is initially not uncollimated. There are several laws of physics that would prevent this chaotic radiation from being collimated over large distances.
Let us start with the laws thermodynamics. Initially, the radiation is incoherent and nearly isotropic. There are many degrees of freedom in this radiation. Therefore, the radiation has a large entropy content. When you collimate the beam, you are reducing the degrees of freedom. Therefore, in order to collimate the radiation completely you would have to destroy entropy. Therefore, there is no way to collimate a beam into a small area. The second law of thermodynamics places a limit on how tightly one can collimate a beam.
It turns out that even if you collimated the beam, you would need a large area to dissipate the energy. The thermal radiation would have to be collected over a large area before you collimated it. Otherwise, you would violate the second law of thermodynamics.
Now let us look at the problem from the point of imaging optics. The radiation from the hot object is incoherent. Therefore, the light coming from the hot object acts like an image. The imaging laws for lenses applies to all curved mirrors and curved lenses that are used to collimate the light. According to the laws of lenses, the image can't be shrunk to nothing. All you could do is make a smaller real image. Again, there is only a portion of the thermal radiation that can go into making a smaller image.
You could do this in terms of diffraction. The thermal radiation has a black body spectrum. Therefore, it has a spread of wavelengths. Therefore, diffraction effects prevent one from focusing or collimating the beam. There is an uncertainty relation here that may help.
A young man that I knew suggested making a death ray out of a flashlight and a large array of lenses. He suggested taking a large number of lenses to focus and collimate the beam from the flash light into a very small area. The beam would remain in a small area over a long distance. Within that area, material would be vaporized. One could then cut tanks and airplanes to pieces using this flashlight. I explained that it would not work due to the Second Law of Thermodynamics. The beam would spread out two quickly from the focal point.
He then asked me whether I ever tried it. The answer, of course, was yes. In the course of my work, I have often had a needed for a tightly focuses and tightly collimated beam of thermal radiation. I had tried many times to use lenses (refractive and reflective) to focus such radiation. It wasn't until I did some calculations using the Second Law that I gave up.
I have often wondered what would have happened if I was wrong. Suppose this young man had decided not to listen. Suppose three were a team of rebellious teenagers who didn't listen to mainstream scientists. They went ahead and made these death rays out of flashlight batteries.
Each one could split mountains in half on two Ever Ready batteries! Each one could destroy cities using one single candle!
They would also be able to cure the worlds energy woes. Using such an array of lenses, they could make perpetual motion machines that destroy entropy.
Maybe that would make a good science fiction story. Someone comes up with that hidden combination of lenses that break the Second Law of Thermodynamics.

Deeviant

The point is not to create a infinitely small beam of destruction, it is to simply avoid a omni-directional profile that average black-body radiation would generate and act as a "here I am" beacon in space.

Darwin123

Okay. I am just pointing out a limitation of this type of stealth technology.
The distances in space are huge. If the beam isn't completely collimated, the spread of the beam increases with distance. Any deviation from being fully collimation of the beam may be a serious limitation of the stealth aspects of this technology. There also may be a lower bound on the size of the instrument that creates this type of beam.
It is your story. You can work out the details to the appropriate degree of accuracy. The writers don't worry about this type of detail in "Star Trek."

Deeviant

Star Trek is "soft sci-fi", they can make up whatever they want, and they are not at all bound to our laws of physics. I am constraining myself to what is at least possible (hard sci-fi) because I want to get to the core of what sci-fi means to me: looking into our future and taking a best guess at what we will find there. I think the story part is good too, I am actually not writing that part, I outline the "universe" and another creates the story within the confines of the world. Obviously, I want to push the limits of known physics to give as many interesting tools for examining future possibilities, which I'm pretty sure ruffles the feathers of some that tend to stick to what is tried and true(for good reason, the type of physics I'm looking for isn't suitable for too much other than what I want to use it for: getting a hint at the future).

DaleSpam

If you want to do hard sci fi then you cannot break the second law of thermo.

Deeviant

I already agreed I am not going to try to use a black-body pumped laser, but merely radiate it out as directionally as possible. Where is the 2nd law being broken?

DaleSpam

I don't think it is in the current iteration, just the earlier versions we had discussed. Personally, I like the internal heat sink idea a lot. The temporary aspect could be a useful plot device.

rorix_bw

Assuming I have understood the relativistic tsolikov rocket equation, and that the equation is correct (Wiki says citation required!): http://en.wikipedia.org/wiki/Tsiolko...ocket_equation

18/22 14/18 of the ship's mass needs to be fuel, to get from "rest" to 0.9c. That is a 100% efficient drive, that ejects particles at 0.99c and all that fuel is used.

I understand this equation holds for all possible reaction drives because of conservation of momentum, and non-reaction drives are magic and thus Not Allowed in hard sci-fi. In order to significantly change direction of movement, or decelerate to its destination, the ship would be needing to refuel along the way, or have external booster stages like the space shuttle did.

In my book there is not much difference between "impossible because it breaks a law of physics" and "unfeasible to engineer, regardless of technology".

PS: You can work out the density of interstellar vaccum and thus the amount of mass available for fuel. Sci-fi author Larry Niven proposed a "ramship" thad had a large hydrogen collecting funnel on the front. It's just not enough. I think they would need to fly through a nebula or something. At 0.90c ... I suggest this could be explosive.

edit: The c and the units in the formula cancel, so just is needed. Someone please fix this if I messed it up.

mfb

In special relativity, you cannot add velocities linearly, but you can add the rapidity. Similar to the classical rocket equation, you have the acceleration at each step (where the rocket has ~0 velocity and classical formulas apply), but instead of the integral over the velocity changes you have to integrate over rapidity changes.
This is not a citation, but a good argument for the formula.

To get the required energy density, the ship needs antimatter/matter fuel or a hot black hole. In both cases, I do not see an option to get a cool exhaust.


If your reactor (or heat source in general) is hot enough, your required power low enough and so on, this is just an engineering problem. As an approximation for the radiated heat, use the solid angle relative to 2pi (a simple surface can emit in 50% of the whole sphere), multiply it with the area where the radiation leaves the ship, and use the formula for black bodies to evaluate the radiated power. The radiators have to be cooler than the reactor (heat source) to be useful.

rorix_bw

I don't think I wrote that 3rd paragraph (black body pumped lasers).

I don't think relavistic velocities are feasible in hard sci-fi. I suppose it depends how hard one wants to be. Even if you break no laws directly, you still end up with a system that cannot be built without violating other laws or some physical constant like requiring more atoms than exist in the universe.

Darwin123

Nothing. The problem is to radiate it out "as directionally as possible consistent with the second law of thermodynamics." Maybe it is not a real problem under the conditions of your story.
Go ahead. I see nothing fundamentally wrong with the concept.