[Spaceships] Your preferences regarding plausible/playable Reactionless Drives

[Flyndaran]

Originally Posted by vicky_molokh

...

There isn't such a thing as a quantum of speed, is there?

As a change in position over time, I imagine it might be a plank length over the time frame of the entire existence of the universe.
But what would you use such a term for?

[vicky_molokh]

Originally Posted by Flyndaran

As a change in position over time, I imagine it might be a plank length over the time frame of the entire existence of the universe.
But what would you use such a term for?

So be it. Though I suspect that in this case, the drive will have to have a sphere of influence comparable to the size of the universe. Which makes one wonder, would that classify it as a contactless reaction drive, or as something different?

[malloyd]

Originally Posted by vicky_molokh

There isn't such a thing as a quantum of speed, is there?

There isn't any quantum of distance or time either. The planck length and time are simply quantities with the right dimensions that fall out of manipulations of a handful of apparently fundamental constants - i.e. they are the natural measuring units in a system that sets all those fundamental constants to 1. They don't necessarily have any physical significance at all.

There *is* a fundamental unit of speed in the same sense as the planck length is a fundamental unit of length. It's c, the speed of light.

[martinl]

Originally Posted by malloyd

Ditching relativity, at least if you do it with a preferred frame, isn't likely to have any effect on the game. OK, technically magnetism goes away, light vanishes as electromagnetism becomes impossible, nuclear reactions stop working and the sun goes out for lack of an energy source, and atoms fall apart when the binding energy disappears

Bah. Won't get through the PC's DR.

[vicky_molokh]

What do you mean no quant of length? If there's such thing as a quant of angle, surely there should be one for distance/location?

[Anthony]

As a side note, something that behaves in a way similar to a reactionless drive doesn't really require any violations of GR, it just requires some forces that don't exist as far as we know. In particular:

If you had some sort of tractor/pressor beam, you can simply grab hold of the nearest large object and pull yourself around. This isn't fundamentally different from using a pole and a rope, other than the fact that you're using invisible beams rather than physical objects. The following constraints apply:

• Force can only travel through your beam at the speed of light; thus, if you're pulling on an extremely distant object, it will take quite a while to start or stop accelerating.
• Force can only be applied towards or away from an object. Thus, if you want to accelerate sideways, you have to basically push on one edge of you anchor, pull on the other. This will multiply acceleration by 2*sin(angular size/2); at long distances this can be approximated by multiplying by (size/distance).
• You're subject to power constraints -- the power requirement of your drive is at least (force applied) * (velocity relative to anchor). For objects in orbit around the earth, accelerating 'forward' requires about 70 MW per ton per G. 1 EP appears to be on the order of 100 kW/ton, so even there your acceleration would be 0.0014G, and it will go down as speed increases.

[Sunrunners_Fire]

Originally Posted by Anthony

As a side note, something that behaves in a way similar to a reactionless drive doesn't really require any violations of GR, it just requires some forces that don't exist as far as we know. In particular:

If you had some sort of tractor/pressor beam, you can simply grab hold of the nearest large object and pull yourself around. This isn't fundamentally different from using a pole and a rope, other than the fact that you're using invisible beams rather than physical objects. The following constraints apply:

• Force can only travel through your beam at the speed of light; thus, if you're pulling on an extremely distant object, it will take quite a while to start or stop accelerating.
• Force can only be applied towards or away from an object. Thus, if you want to accelerate sideways, you have to basically push on one edge of you anchor, pull on the other. This will multiply acceleration by 2*sin(angular size/2); at long distances this can be approximated by multiplying by (size/distance).
• You're subject to power constraints -- the power requirement of your drive is at least (force applied) * (velocity relative to anchor). For objects in orbit around the earth, accelerating 'forward' requires about 70 MW per ton per G. 1 EP appears to be on the order of 100 kW/ton, so even there your acceleration would be 0.0014G, and it will go down as speed increases.

A traction drive, basically. Change it so that it pushes/pulls off of space itself rather than requiring an object, and it should be able to very rapidly "inchworm" its way to whatever speeds.

[vicky_molokh]

Originally Posted by Sunrunners_Fire

A traction drive, basically. Change it so that it pushes/pulls off of space itself rather than requiring an object, and it should be able to very rapidly "inchworm" its way to whatever speeds.

That would make it true reactionless.

[teviet]

Originally Posted by Sunrunners_Fire

A traction drive, basically. Change it so that it pushes/pulls off of space itself rather than requiring an object, and it should be able to very rapidly "inchworm" its way to whatever speeds.

Then it would violate conservation of energy and momentum. "Space itself" is not a viable medium. You could accelerate arbitrarily with no energy input.

TeV

[Anthony]

Originally Posted by Sunrunners_Fire

A traction drive, basically. Change it so that it pushes/pulls off of space itself rather than requiring an object, and it should be able to very rapidly "inchworm" its way to whatever speeds.

The efficiency of a push/pull drive is dependent on the mass of the anchor relative to the mass of the object being accelerated. As vacuum has zero mass, efficiency is zero and acceleration is impossible.

You could, in principle, have something that attaches to the solar wind (a few hundred tons per cubic light-second) or dark matter (best as I can find, something like 10-20 tons per cubic light-second) and have absolutely enormous (multi-light-second) ethereal sails, but this tends to have its own issues. Also, the general energy content of a pressor beam is equivalent to an infinitely reflecting photon drive -- i.e. while a photon drive has a power requirement of F*c, a pressor beam of length L has an energy content of no less than F*L (tractor beams I believe require negative energy density, not sure how the math for attraction works).