Can gravity exist in the absence of mass?

Question

People talk about powering spacecraft with "anti-gravity" drives. But if they are traveling in empty space, where mass is minimal, wouldn't the drive malfunction? My understanding is that gravity only exists in the presence of massive objects, like the Earth, stars and planets. The "universal gravitation field" is incredibly weak. You can overcome it by flipping a penny into the air. So how could "anti-gravity" work as a space drive? Makes no sense to me.

Answer 1

Questions like this about the feasibility of "anti-gravity devices" crop up every week or two.

You might want to read my fairly complete answer to a related question two weeks ago. The question was then posed as:

"Is it still impossible to build an anti-gravity car that floats around mid-air like real vehicles?" --- asked by ' MS '.

There I offer my argument why the search for a "practical" anti-gravity device is almost certainly doomed to fail. Putting it simply, gravity (in the sense of a local gravitational attraction or acceleration) exists because time runs differently in different places. In order to create an "anti-gravity device," one would have to have such control over spacetime, that one could LOCALLY and CONTROLLABLY REVERSE THE GRADIENTS IN THE RATES AT WHICH TIME RUNS, along one's intended path of travel! Do you think that will ever prove possible? I think not. (One might be able to BRIEFLY surf a passing gravitational wave, but most of them would be here and gone at the speed of light. Also, the "energy" in gravitational waves is not localizable, which presents yet another problem in "harnessing" them.)

Puttting it in more practical terms, you'd have to maneuver a more powerful source of gravity to be ahead of you in your desired flight path. But that (impossible though it would be in practice, anyway) still wouldn't be an "anti-gravity device" --- you would simply have overcome attraction to the "home body" by one to the "target body."

Live long and prosper.

Answer 2

Gravity applies to energy as well as mass; remember Einstein's equation. But an anti-gravity drive is not possible, as the sign of the gravitational force is always attractive. You don't need a huge mass to have measurable gravity -- do some research on Cavendish's experiment to measure the gravitational constant.

Answer 3

There is no such thing as antigravity. There is a theory that gravity waves can be deflected. If you could deflect away the gravity waves from one side, then you would be pulled towards the other side. Never been done yet.