Avi Loeb

and it's simply because of the curvature of space-time that the earth is trying to go along a straight line but the space-time is curved and therefore it moves in a circle and if you were to remove the sun the source of gravity the earth would leave the solar system on a straight line in the same way that if you remove the bowling ball from the trampoline

the marble will continue to move on a straight line on the flat surface of the trampoline.

So that's the way Einstein thought about it.

Now, you can imagine a solution to Einstein's equations that involves curved spacetime.

And indeed, there is this solution where it's curved in a very unusual way such that it can propel an object at a constant speed.

So because light speed is the limit, so this is a way around the light speed limit.

As long as you can reach the configuration that this solution embodies, and it requires some form of energy that produces negative gravity, which we don't have, we never engineered it.

The universe accelerates, so there is some repulsive gravity acting on the expansion of the universe, but the substance that causes that expansion, dark energy,

you would need to somehow engineer it in a different way because it fills uniformly the universe, just as if it's the vacuum energy density.

But the question is, is there any anti-gravity source that we can shape and have quantum gravity engineers design an object like that?

The object itself is a solution to a steady state where this object is moving,

but we don't know how to produce it and what ingredients you need to put.

I mean, you can imagine a cake that is extremely tasty, but you just don't have the ingredients to make that cake or you don't have the oven to make that cake.

So that's the way you should think about it.

I can give you another example of a situation where you don't need rocket fuel to propel yourself.

Just imagine we had access to a negative mass.

All the masses that we know about are positive, and that's why gravity is attractive.

But in electromagnetism, we have positive and negative charges.

So imagine that just like electromagnetism, we would have negative masses, and they would repel an object close to them.

So now I take the negative mass, put next to it a positive mass of the same value, and the total mass of this system would be zero.