Analog Science Fiction and Fact - June 2013

loved it when I read it the first time, and I
still love it. But now I’m a bit more critical.
For instance, take this “attach that one lead
to any portion of the mass that is to be affected”
business. Why? Why can’t the lead just
hang in the air? The air is part of the mass to
be affected, isn’t it? If the air doesn’t have the
inertia sucked out of it along with everything
else in the ship when the drive is turned on,
you’ll get one shredded ship in an instant.
A bit later, with the ship moving comfortably
close to the speed of light, but not yet
even outside the orbit of Mercury, Libby
wants to turn off his space drive. Lazarus is
concerned that the ship will slow down
again once it has its inertia back, but Libby
tells him that their current velocity is as real
as any other velocity. He points out that turning
off the drive will not instantly transport
them back to the spot where it was turned
on, so why should turning it off return the
ship to the velocity it had initially?
It’s a fair point. Made up physics is fine in
SF as long as you keep it consistent, and
Heinlein’s version of inertia removal in
Methuselah’s Children marks a sort of upper
limit for inertia-removal gizmos. That is, 1.)
inertia is removed instantly, and 2.) it is reduced
to zero, and 3.) there are no “penalties”
involving either energy or momentum
conservation.
But I’m not convinced this particular version
of the gizmo can ever actually be treated
in a consistent way. Consider point 2 and
look at the implications. Inertia is the tendency
of a mass to resist a change in its state
of motion. It doesn’t like to be sped up, but
once it has been, it doesn’t want to be
slowed down, and it doesn’t want to change
direction. Now suppose we have a spaceship
under thrust, and we drop the inertia to zero.
Regardless of the amount of push from the
propulsion system, our ship will suddenly accelerate
to—Well, to some very high speed.
The limiting speed will depend on the
propulsion method and the environment the
ship is in. In Heinlein’s tale, the push wasn’t
from the engines, but from the sun. If we are
pushing our ship with a big laser from the
Moon, it will be near lightspeed in very short
order, limited only by the build up of the resistant
opposing force of the tiny amount of
matter that exists even in “empty” space (es-
ON THE SUCKING OUT OF INERTIA
JUNE 2013
sentially, wind resistance). If the push is coming
from some kind of rocket, the limiting
speed will depend not on the total thrust,
but on the exhaust velocity of the jet (I
think—I’m willing to be convinced otherwise).
And what happens when the laser or the
rocket is turned off ? I don’t know. It isn’t defined.
Even if our ship was in perfectly empty
space, with the push gone, there is no reason
for the ship to keep going at all, since the inertia
is gone and that is the only thing that
keeps something moving when the force
goes to zero. But there’s also no reason for it
to stop, and no reason for it not to just go any
which way at any old speed.
So we’d be better off living with some restraints
on our inertia sucker. Like with point
one. Why should the inertia go away instantly?
You don’t cool a beer instantly by putting
it in the fridge, nor can you suck all the air
out of an airlock instantly. And let’s not have
all of it go away. As long as there is always
some inertia, we can continue to use Newton’s
laws and treat removal of inertia exactly
the same as a removal of mass.
Under these conditions, suppose we have
our spaceship out in deep space moving
along at some velocity. It doesn’t even need
to have the rockets firing. Now we turn on
our gizmo, reduce the inertia (or mass) of
our ship to a tenth of what it was, and assuming
momentum conservation is in effect,
what happens? Momentum is defined as the
mass times the velocity, so if we reduce the
mass to one tenth of its initial value, to conserve
momentum our ship is going to have to
move ten times faster. Keep sucking out inertia
this way and eventually we’re pushing
lightspeed, and to slow down, all we need to
do is put the inertia back in.
Okay, before you rush off to write a letter
to Trevor, I am aware that even though I imposed
momentum conservation, I ignored
energy conservation. As most of you know,
though momentum is directly related to the
velocity, kinetic energy is related to the
square of the velocity. So in general, most of
the time you are not going to be able to satisfy
the demands of both momentum and energy
conservation with this particular approach.
However, if you’re writing a
super-science SF story, what you can do is
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