# Re: 1 g acceleration?

Hal Finney (hal@rain.org)
Mon, 6 Jul 1998 08:18:05 -0700

Damien Broderick, <damien@ariel.ucs.unimelb.edu.au>, writes:
> At 10:29 AM 7/5/98 -0700, Hal wrote:
>
> >By the principle of relativity (which says that you can't tell if you're
> >moving, i.e. all motion is relative)
>
> Of course you can tell you're moving if you're in an accelerating spaceship
> - the floor pushes on your feet.

Right, I was referring to velocity, not acceleration. The point is, the amount of power it takes for you to feel 1 g of acceleration is independent of your velocity, simply because there is no such thing as (absolute) velocity. We can conclude immediately that a fast-moving spaceship will not notice anything impeding its efforts to accelerate at 1 g, because otherwise this would imply that the laws of physics are somehow different for fast moving and slow moving objects, and the principle of relativity forbids this.

(The principle of relativity is different from the theory of relativity: the former merely says that you can't tell how fast you are going, while the latter develops all the ramifications of this.)

> >As the
> >stars approach the speed of light [...]
> >they undergo Lorentz contraction, flattening out in the
> >direction of motion.
>
> Well, they grow closer together. But they don't individually flatten, they
> undergo relativistic rotation. This curious truth has been known since
> 1959, and is due to James Terrell of Los Alamos; I met it in Milton
> Rothman's nice little book THE LAWS OF PHYSICS (Basic Books 1963).

This "rotation" is an optical illusion, not a real effect. It is true that you can see the back side of objects, but that is because of the way the light rays travel to your eyes. The objects are not really rotated, if you take what you see and calculate the light travel time back to when it was emitted. Likewise the "starbow" and aberration effects which make it look like the universe is compressed forward of you are illusions, as is the appearance that the universe is evolving quickly ahead of you and freezes into immobility as it passes.

Hal