Gravity Propulsion

Question

I'm trying to figure out a concept for some sort of super-advanced space propulsion system that works by bending spacetime. As I understand it, planets' orbits are actually straight paths, but they move through a spacetime sufficiently curved that they travel in ellipses around their system's center of gravity. Taken to the extreme, stuff falls into a black hole because, if you get in close enough, spacetime is so warped that any direction you move in is toward the singularity.

Is it possible, within our current understanding of physics, to bend the spacetime around an object so that it moves relative to nearby objects? Like, something that translates the movement toward a center of gravity like a planet or star into directional movement?

Answer 1

The "within our current understanding of physics" makes this hard. Our current understanding is that the curvature of spacetime is intricately linked to the distribution of matter and energy in spacetime.

Now there is the well known Alcubierre drive which allows for ftl travel (in a global frame, nothing travels faster than light locally). However the Alcubierre drive requires exotic material (with negative mass) and extreme consumption of energy (the power of several billion stars) to work. Our current understanding does not permit for a functioning Alcubierre drive to be built.

There are similar, but low tech ways of bending spacetime. For example, if you fling a planet out into space, it can drag a spaceship behind it gravitationally. That's probably not what you were thinking, yet it does involve bending spacetime to translate motion towards a centre of gravity into directional movement.

Essentially the problem is this, to bend spacetime you need to move large amounts of mass around, and if you can move large masses around, why don't you just move the spaceship? There is a very exception to this. If the large mass is already moving, you can steal a little of its momentum, and gain a lot of speed. This is called a gravitational slingshot, and is very much "real physics".

Taken to extreme is the possibility of gaining velocity by dipping into the Ergosphere of a rotating black hole. This could allow for an object to gain a very large amount of energy from the black hole (up to 20% of the object's mass-energy). Again, this isn't a drive that you can take with you, but a way of boosting your speed. Flying your spacecraft around a black hole has a number of health and safety issues.

Answer 2

This is essentially science-fictional answer, but the science-based component isn't neglected. The reasoning is similar to an answer given to this question Internally consistent grav-plating

This answer assumes that gravitational propulsion works by artificially making spacetime curve and the spacecraft is accelerated towards this artificial gravitational field. Currently the only known way to influence the curvature of spacetime is to add mass. Assume that there exists a fundamental physical mechanism responsible for the curvature of spacetime and energy can be pumped into it. The amount of energy pumped will determine the amount of curvature. This in turn will determine the rate of acceleration of the spacecraft.

If the artificial gravitational field can be generated around the spacecraft in such a way that it "effectively" is falling in the direction of its destination, the ship will accelerate as if it was in free fall in a natural gravitational field.

Depending on the power output of the ship's reactors this acceleration can be set arbitrarily high. In free fall there are no crushing acceleration pressures to harm or injure the ship's complement.

A separate gravitational drive can set up around a major structural component of the spacecraft. This can be used to generate an artificial internal gravity field, so the space travellers can enjoy normal gravity during their journey.

Reality check:

There is no known physical mechanism that can achieve this effect. This is purely hypothetical. Remember this is essentially a science-fictional answer. Gravitation, in the general relativity model, is based on the concept that the curvature of spacetime is responsible for it. Therefore, this answer has postulated two things. That spacetime curvature can be achieved artificially without mass and that this curvature is directly dependent on the amount of energy pumped into the curvature-generating field.

Answer 3

No. You might use gravitomagnetism to make a launcher that is not part of the ship, but any mechanism will not be able to move itself. If you could project a fake mass ahead of the ship and fall towards it (as in a4android’s answer), you have the problem of the generator being on the ship. You can’t lift yourself by tugging on your bootstraps.

You have to contend with conservation of momentum. The gravity ring can launch a ship at 100 G acceleration without the passengers feeling a thing, but the launcher itself will recoil in doing so, and needs another propulsion mechanism to check the launcher’s motion and bring it back into place.

Likewise, the phantom mass projector would recoil when a ship was attracted to the phantom mass: the ship and the separate projector will move in opposite directions. Putting the projector in the ship will render it useless for this purpose.

Conservation of momentum is rather fundamental in the universe. Gravity does respect it.

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If you want to be more oblique, a dark matter rocket (as described in this answer) might involve gravity as the channel through which normal matter can be converted to dark matter.

Answer 4

Visualize the planets as balls rolling around on a taught sheet. It's easy to picture larger balls creating more pronounced curves. Now imagine you figure out how to control gravity to the point where you can space stretch like silly putty. You can grab a spot in space and stretch it way out. Now grab another from far away and stretch that one way out. Touch them together and allow a ship to pass between them. You move around by manipulating gravity, which really manipulates the bending of space. Travel is limited by how strongly you can stretch space, and which weird effects happen when you do it.

Answer 5

Yes, kind of, we haven't solved the math yet, we don't even know what a lot of it looks like, but we do understand that it's there. There are equations that the scientific community is working on that may eventually let us get a handle on direct manipulations of space-time, if we ever complete and solve them. If we can solve the math then in theory we can build artifacts that would allow us to bend space-time. That allows you to create and use artificial mass-energy and gives rise to many interesting applications; artificial black holes, "zero-point energy" (which actually amounts to the same thing), artificial mass (both real and virtual), artificial gravity, reactionless drive, inertial dampers, time-travel, etc... basically the full list of "McGuffins friends don't let friends have" and a few other toys that the writers never thought of.

But

We don't have the math, we're probably a paradigm shift away from completing the equations, and another from solving them and even then we'll still have to build a manipulation platform (that might just be the easy bit but I doubt it). We understand the relationships and phenomena we're trying to understand, describe, and recreate but not how they're created in nature.

Answer 6

I'm currently working on this exact machine right now and building the theory based strongly on relativity. By the way. Light getting stuck in a black star is because space is so dense around the star from being squeezed completely out of it's structure that light gets reflected back because a photons lack of mass. Like a bullet that's too slow and too light to penetrate a steel plate. And light coming in from an angle is deflected like the bullet passes through an angled plate, turning it in toward the star. To travel towards matter push space outward, in for the opposite. Beware that means you too, so hold on tight. The good thing is, the more the mass, the greater the push with equal power.