Well, the Alcubierre drive [1] uses negative energy, but at this point it is not clear whether this type of propulsion is just a mathematical gimmick. [1] https://en.wikipedia.org/wiki/Alcubierre_drive

Not only negative energy but insane amounts of it as well. Originally required converting more mass than the entire observable universe. That was lowered in later works. A recent work moreover made a drive using only positive energy.

Note that the newer, positive energy drive had the warp bubble itself moving slower than the speed of light. Still very interesting in principle.

We do not know if and I bet plenty of people do not believe such negative energy really exists.

Besides, all warp bubble metrics so far are inertial; meaning the ship is unable to accelerate, so they're a gimmick nowadays. I have hopes we find something workable though.

>We do not know if and I bet plenty of people do not believe such negative energy really exists.

Plenty of people believe that mental problems are all caused by disembodied souls unleashed by Xenu of the Galactic Confederation when he blew them up with A-bombs in Hawaii, back when Earth was called Teegeeack.

What "plenty of people" believe is not important.

Also, "plenty of people" used to believe the Earth was flat, or that the Sun orbited it.

Sure, but the fuel or batteries or whatever won't have to lift themselves out of the gravity well. The only fuel needed would be for inertial changes. It's a free G of acceleration!

"Just" need to plug in the anti-grav module and efficiency will soar.

How do you figure? With anti-gravity, you would need very little power for propulsion since gravity would not be a factor (or less, depending on how effective your anti-gravity tech is). The question is how much power the anti-gravity requires.

I suppose "we just need anti-gravity" is equivalent with "we just need to violate energy and momentum conservation".

It could also mean: We only have to feed the escape energy of the rocket itself into the rocket while it is on the ground, and without also carrying fuel upwards.

The “anti gravity” was just zero altitude momentum all along. Just use that big washing machine thing. Can’t stick humans in it though :-(

> The question is how much power the anti-gravity requires.

This is essentially a case of "draw the rest of the owl".

I would assume (with good reason) that the amount of energy needed to generate a counter to the gravitational field would be the same as the amount of energy required in thrust to do the same thing.

Still, having a drive that doesn't need to push stuff out of its back would be very interesting, IMHO - even if it doesn't work out better energy-wise.

Gravity is the shape of spacetime. Why would antigravity be a thing.

Can be a defect in spacetime. See: https://arxiv.org/abs/1803.09736

Alcubierre drive essentially changes the shape of spacetime, doesn't it?

Everything (that has mass or energy) changes the shape of spacetime.

Sure, my point is that the changes made with Alcubierre drive are controllable.

Not really. The Alcubierre "drive" is closer to a "shape", a static solution to Einstein's equations. There is no proposed device that actually achieves this shape. The only idea along those lines implies a kind of Alcubierre railroad - you set up a path of negative mass matter (which almost certainly doesn't exist) and then move this with regular means as the ship is approaching, thereby continuing the bubble for the ship.