Variation of the Alcubierre warp drive

[grayson]

For those that do not know about the Alcubierre warp drive I have a Wikipedia link below to show you.

Wikipedia page for the warp drive

Now that we have that out of the way, we can move on to my variation. For the short story, it uses relativity of position to make an illusion of anti-gravity even though it is actually just gravity. Now for the long story. This warp drive is on a railroad, so it used massive spinning rings to warp spacetime around the drive and oppose the force at the front pushing it with gravity. So, I will show you a picture of the normal warp drive and how it works, and you can imagine this idea.

So, as you can see, the front of the drive warps gravity inwards. The outside warps the spaceship outwards. Normally, this outwards warping would take negative mass or negative energy density, but my idea uses a railroad that just warps the gravity like normal but to the other direction. There are many advantages and disadvantages of this idea. The advantage is that it would not take as much energy to warp it outwards. Another advantage is that it can be built easily but only theoretically. A disadvantage is it would take a railroad that would take years to build, and we would have to boringly travel the universe slower than light. Tell me your opinions

[Markus Hanke]

7 hours ago, grayson said:

relativity of position

I’m not aware of any such concept, at least not under this name. Can you explain what you mean by this?

7 hours ago, grayson said:

my idea uses a railroad that just warps the gravity like normal but to the other direction.

I’m afraid that’s not how spacetime curvature works. So long as you start with positive energy-momentum - irrespective of how this is distributed or oriented in space -, you’ll always end up with ordinary attractive gravity. This is not a question of position or orientation in space.

[joigus]

13 hours ago, grayson said:

For those that do not know about the Alcubierre warp drive 

It rings a bell, yes:

https://www.scienceforums.net/search/?q=Alcubierre&quick=1&type=forums_topic&nodes=29

6 hours ago, Markus Hanke said:

I’m not aware of any such concept, at least not under this name.

Neither am I. Perhaps translation invariance @grayson?

6 hours ago, Markus Hanke said:

I’m afraid that’s not how spacetime curvature works. So long as you start with positive energy-momentum - irrespective of how this is distributed or oriented in space -, you’ll always end up with ordinary attractive gravity. This is not a question of position or orientation in space.

Agreed.

[grayson]

10 hours ago, Markus Hanke said:

I’m not aware of any such concept, at least not under this name. Can you explain what you mean by this?

Sorry, I will explain it better. Basically, what I mean is that you can make the illusion of anti-gravity with normal gravity. It would work by surrounding the warp drive in massive objects that pull spacetime out away from the warp drive.

[grayson]

All I am saying is that where spacetime warps inwards, at the end of this inwards warp, it would oppose another gravity and appear to be an outwards warp. It is all relative 😉

[Markus Hanke]

11 hours ago, grayson said:

All I am saying is that where spacetime warps inwards, at the end of this inwards warp, it would oppose another gravity and appear to be an outwards warp. It is all relative 😉

No, not all quantities in nature are relative in the same way as eg speed is. A good counter-example is proper acceleration - it can be measured locally using an accelerometer, and all observers agree on this measurement. It is not relative to anything.

Gravity is similar - if you have an ordinary source of gravity, its effect is always attractive for all observers, never repulsive, irrespective of orientations, states of motion etc. True anti-gravity would require exotic matter to generate, which we have reason to believe does not exist. 

To put it more technical - “gravity” is a tensorial quantity, and tensors are covariant objects; all observers agree on them.

[grayson]

12 hours ago, Markus Hanke said:

No, not all quantities in nature are relative in the same way as eg speed is. A good counter-example is proper acceleration - it can be measured locally using an accelerometer, and all observers agree on this measurement. It is not relative to anything.

Gravity is similar - if you have an ordinary source of gravity, its effect is always attractive for all observers, never repulsive, irrespective of orientations, states of motion etc. True anti-gravity would require exotic matter to generate, which we have reason to believe does not exist. 

To put it more technical - “gravity” is a tensorial quantity, and tensors are covariant objects; all observers agree on them.

Yes, I know that the measurement will be the same but let me give a thought experiment. Imagine that you are sitting on earth. Suddenly, a massive celestial body, say Jupiter gets a bit too close to the planet. Obviously, you would die but saying none of that happens, the gravity of Jupiter would overwhelm you and you would feel as if you are being pulled away from earth. That is what I mean by relative.

[Markus Hanke]

10 hours ago, grayson said:

the gravity of Jupiter would overwhelm you and you would feel as if you are being pulled away from earth

This is just combining two sources of gravity to obtain a new spacetime geometry - gravity nonetheless remains attractive in nature, as it always does for ordinary sources. But for the Alcubierre drive you need actual anti-gravity, which is a completely different thing - it can be shown in a general way that the Alcubierre metric in its original form requires exotic matter; unfortunately you cannot “cheat your way there” just by cleverly arranging ordinary sources.

Also, even if you could construct an Alcubierre bubble, I think it would be completely unusable as a propulsion method, since it has some pretty nasty side effects and problems.

Just a word of warning - these diagrams depicting spacetime curvature are just visual aids to understanding the basic concept, they are not accurate depictions of actual geometry. Spacetime geometry is intrinsic to the manifold, so there is no actual “direction” to curvature, since spacetime is not embedded into any kind of higher-dimensional space. The crucial concept to understand here is the distinction extrinsic vs intrinsic geometry; this is very important if you want to understand GR.