Inertialessness (the Lensman & Arilou sort), top speed, and spinning

[vicky_molokh]

Greetings, all!

Assuming a ship has its Inertial mass reduced to zero, but its Passive and Active Gravitational mass remaining at the usual level, such as with the inertialess drives of Lensmen, or of Star Control's Arilou, how does one figure what top speed the ship will achieve and how fast?

Likewise, assuming such inertialessness applies to a ship's rotation, how is the angular speed derived from its attempt to 'spin'?

Finally, a totally non-SF question: what is the best 'a spherical X in vacuum' way to attain an angular speed that is inversely proportional to an object's unmodified rest mass mass (whatever that mass may be, e.g. due to picking up stuff mid-turn), and does not depend on the duration of the application of given method (assuming that minimum application time is a measurable non-zero duration). (I'm assuming that the method is being reapplied constantly throughout the period of turning.)

Thanks in advance!

[Anthony]

Originally Posted by vicky_molokh

Greetings, all!

Assuming a ship has its Inertial mass reduced to zero, but its Passive and Active Gravitational mass remaining at the usual level, such as with the inertialess drives of Lensmen, or of Star Control's Arilou, how does one figure what top speed the ship will achieve and how fast?

Top speed will be c, and time required to achieve c will be zero (to within the limits of measurement).

[Bruno]

Originally Posted by Anthony

Top speed will be c, and time required to achieve c will be zero (to within the limits of measurement).

DIV BY ZERO ERROR REDO FROM START

[Anthony]

Objects with zero rest mass are incapable of moving at a speed other than c.

[malloyd]

Originally Posted by vicky_molokh

Assuming a ship has its Inertial mass reduced to zero, but its Passive and Active Gravitational mass remaining at the usual level, such as with the inertialess drives of Lensmen, or of Star Control's Arilou, how does one figure what top speed the ship will achieve and how fast?

Within the framework of those stories, top speed is set by friction with whatever the ship is in, so it's going to be strongly shape dependent. And on your conductivity I suppose, I expect the term from eddy currents from your motion in the local magnetic fields to be pretty significant for a spacecraft.

Within the framework of real physics, it's undefined. Actually I'm pretty sure that having no momentum and energy content also means you can build a thermodynamics argument it has no information content, and "something is here" is information, so going inertialess means you cease to exist.

Likewise, assuming such inertialessness applies to a ship's rotation, how is the angular speed derived from its attempt to 'spin'?

It's not clear to me that having zero (linear) inertia means you have no angular moment of inertia. For that matter, once you start redefining mass like that, it's not clear your linear inertia has to be the same in all directions.

Still, your angular velocity is going to be again set by friction.

Finally, a totally non-SF question: what is the best 'a spherical X in vacuum' way to attain an angular speed that is inversely proportional to an object's unmodified rest mass mass (whatever that mass may be, e.g. due to picking up stuff mid-turn), and does not depend on the duration of the application of given method (assuming that minimum application time is a measurable non-zero duration). (I'm assuming that the method is being reapplied constantly throughout the period of turning.)

Huh? If you're in vacuum, the only way to spin yourself up is to eject something with momentum (well OK, or pass through a properly asymmetric force field and use the thing generating the field as effective reaction mass, but that's not under your control and doesn't work so well if you're a uniform sphere anyway). For optimum spin-up you want to eject it from your equator defined by the axis you want to spin on and tangentially to your surface at the ejection point. Eject it as fast as you can for mass efficiency, as slowly as you can for energy efficiency. Note that you'd be better off if you can make yourself *non-spherical* and move your engine as far from your center of mass as you can.

[Bruno]

Originally Posted by malloyd

Within the framework of real physics, it's undefined. Actually I'm pretty sure that having no momentum and energy content also means you can build a thermodynamics argument it has no information content, and "something is here" is information, so going inertialess means you cease to exist.

!

I was joking (a little) about the DIV BY 0 ERROR but apparently I wasn't that far off.

[jeff_wilson]

Originally Posted by vicky_molokh

Assuming a ship has its Inertial mass reduced to zero, but its Passive and Active Gravitational mass remaining at the usual level, such as with the inertialess drives of Lensmen, or of Star Control's Arilou, how does one figure what top speed the ship will achieve and how fast?

Likewise, assuming such inertialessness applies to a ship's rotation, how is the angular speed derived from its attempt to 'spin'?

Lensman style inertialess (which ignores relativity) doesn't change the vessel's terminal velocity; it will always travel at whatever speed is necessary to balance the thrust of the engines against the drag of whatever medium or other objects it encounters. Other inertialess objects likewise move at whatever velocity balances the forces acting on it. As a consequence, the acceleration is always infinite.

This presumably works for rotation as well, which can make for some very wild rides if there is the least bit of misalignment of the thrust vector with the center of mass of the ship. Ships may use negative feedback control systems when in free space, but in battles they typically grapple one another and nearby planets with tractors and struggle for movement control superiority.

[vicky_molokh]

Originally Posted by jeff_wilson

Lensman style inertialess (which ignores relativity) doesn't change the vessel's terminal velocity; it will always travel at whatever speed is necessary to balance the thrust of the engines against the drag of whatever medium or other objects it encounters. Other inertialess objects likewise move at whatever velocity balances the forces acting on it. As a consequence, the acceleration is always infinite.

Okay, acceleration is infinite. Also, I realise that SC is made of pure handwavium. OTOH, Doc is one of the old-school hard-ish fellas, right? How did he arrive at the numbers? From school days, I'm vaguely (mis) remembering that drag is 'roughly proportional to the square, or, later, to cube of speed'*; I'm not even sure what sort of unit a dragness of a medium would be measured in.

* == Which means that most likely I'm misremembering.

[johndallman]

Originally Posted by vicky_molokh

OTOH, Doc is one of the old-school hard-ish fellas, right? How did he arrive at the numbers?

Well, no. "Doc" Smith was a practitioner of "Make it sound really cool"; his spiritual descendant is George Lucas.

He might have done arithmetic to pick a suitable speed for plot purposes, but since he never says how far apart anywhere is, I doubt it.

[whswhs]

Originally Posted by vicky_molokh

Assuming a ship has its Inertial mass reduced to zero, but its Passive and Active Gravitational mass remaining at the usual level, such as with the inertialess drives of Lensmen, or of Star Control's Arilou, how does one figure what top speed the ship will achieve and how fast?

Theoretically, it's the resistance of the interstellar medium. Smith treated this as equivalent to friction, which sort of implies that he's treating this as analogous to laminar flow of a fluid through a tube. I'm not sure that it works in "reality"; a more appropriate model might be that every time your ship impacts a molecule, the molecule has to be pushed out to the side to let the ship by, and since the molecule is not inertialess it cannot move faster than c, which means the ratio of the ship's speed to c is given by the angle of its sides. (The way Smith defined it, the ship would "stop" dead in space every time it hit a molecule, but that can't be reconciled with the description in the books, so presumably there's some minimal size below which gas and dust just gets shoved aside.)

Bill Stoddard