Perpetual motion machine.

[Kablooie]

It's the ultimate example used to illustrate an impossible technology.
The conservation of energy principle precludes it.

But wait a minute now.

Scientists have discovered dark energy in the universe which is repulsive instead of attractive like gravity.

In theory if that force were harnessed and set in cyclic opposition to gravity, a PM machine might actually be possible.

Of course no one knows enough about dark energy yet to realistically propose anything like this but it's interesting to speculate on possibilities.

(I'm reading The Physics Of The Impossible which got me thinking.)

[Confusious]

1. the only problem with that idea

Is the repulsion of dark matter only works on a galactic scale.

Or the theory of relativity is off a little.

[Kablooie]

2. What about a real big PM machine then?

Might run a huge pinball machine with galaxies as bumpers. Forever.
Make Nintendo jealous.

[CJCRANE]

3. "Scientists have discovered dark energy in the universe

which is repulsive instead of attractive like gravity."

The idea of a repulsive force holding everything in place was one of the ideas in Barrington J. Bayley's sci-fi novel The Zen Gun.

[DetlefK]

4. There was a similar thread some months ago: It doesn't work.

You can't harness Dark Energy.

It's not just the distance that's expanding. The ruler, by which you measure distance and with which you would somehow harness the kinetic energy, is expanding too.

For example:
If you expand a spring, you can harness to stored energy, because each atom is pulled away from it's equilibrium position and wants to go back.
But if you redefine the equilibrium distance between the atoms (-> expand space itself) the atoms would be in equilibrium again. And now you couldn't get some energy out of it.

[Kablooie]

5. Maybe, but the thing is we really don't know much about dark energy.

All we really know is that something is speeding up the expansion of the universe.
Everything else is merely conjecture with little experimental evidence to back it up.

My guess is that our current definitions of dark energy are too simplistic.
Like Einstein's theories which reconcieved the concept of gravity, I think dark energy will need a new paradigm to be understood.

[pokerfan]

6. We don't even know if dark energy is energy

Right now the words 'dark energy' and 'dark matter' are simply placeholder terms meaning we don't yet really know what's going on here. Amazing that we know so little about something that apparently constitutes the majority of the universe.



Whether or not it's an exploitable resource will have to wait for a more complete understanding of its nature but, unless it's infinite (as opposed to being really really big), I would expect some form of conservation principle would still apply. Still, it doesn't have to be infinite in order to be exploitable. Solar energy isn't perpetual or infinite but our sun is going to be around for billions of years.