# Electro-Magnetic Alcubierre Drive

## Main Question or Discussion Point

An Alcubierre drive as I understand it is based on the idea of dangling a carrot in front of a horse with one hand and lighting its tail on fire with the other. Its desire for the carrot and its aversion to the fire drives the horse forward. The salient difference is that an Alcubierre drive would manipulate gravity to achieve the same effect. Expand the space behind the ship and contract the space in front of it.

So why can we not propel a spaceship forward by making it, say, positively charged in front and negatively charged in back, and then by dangling a negative charge in front of it and another negative charge behind it? The attraction in front and the repulsion in back would drive the whole thing foward. It would be an EM Alcubierre drive, would it not?

I don't doubt for a second that what I just suggested is impossible. I suppose I'm just asking "why is it impossible?"

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Staff Emeritus
2019 Award
Why don't you calculate the net force on the spaceship you get by doing that?

I would but I don't know how. I know f=ma but I don't know how to apply it here

russ_watters
Mentor
This happens to be a very easy problem. Try drawing a diagram of what you are describing and pick ANY values for the forces, then see what you come up with for the net force pushing/pulling the ship forward.

mfb
Mentor
And keep in mind that those charges in front of and behind the ship would be part of the ship as well... you do not even have to calculate numbers to see the problem.

(Also, electromagnetic charges do not contract or expand space like it would be required for the Alcubierre drive.)

Drakkith
Staff Emeritus
So why can we not propel a spaceship forward by making it, say, positively charged in front and negatively charged in back, and then by dangling a negative charge in front of it and another negative charge behind it? The attraction in front and the repulsion in back would drive the whole thing foward. It would be an EM Alcubierre drive, would it not?
Take the back charge as an example. It repels the negatively charged spacecraft so both it and the spacecraft experience a repulsive force. But you also have to account for the arm that is attaching the charge to the spacecraft. The repulsive force on the charge is transmitted along the arm to the spacecraft, so the spacecraft also feels the force that it itself exerts on the charge. In other words, the spacecraft pushes the charge, which then pulls the spacecraft, and the charge pushes the spacecraft which then pulls the charge. The result is zero net force and no acceleration. Same thing for the front.

Just to add my two cents , whenever you want to move somewhere your doing it against something or with respect to something else, like riding a bike you push against ground and move forward.hanging a bike in mid air would make you stationary because there would be nothing to push against.

in the original idea they want to push against gravity using it like some sort of a road. If you charge the ships surface then the only way it could move is if there was an electric field in which it could move , like if a nearby planet would be positively charged and then you would either charge the ship positively or negatively you would either repel from the planet or move towards it by attraction.
As long as I know in the average space there is no where near the field strengths required for this to work so you could charge the ship as long as you want nothing much would happen.

As drakkith pointed you would need the field and charges not only on the ship but on something else any physical object near by , otherwise you have nothing to push yourself against.

Bystander
Homework Helper
Gold Member
Try drawing a diagram of what you are describing
... or, if you don't want to draw a diagram spend a little time watching old animated cartoons, and ask yourself why the idea is funny enough to use in cartoons.

russ_watters
Mentor
... or, if you don't want to draw a diagram spend a little time watching old animated cartoons, and ask yourself why the idea is funny enough to use in cartoons.
Lol, yeah, good point!

Suppose the ship was only being pushed from behind by a seperate, non attached craft that is as large as the back of the ship but much thinner. (Imagine a cube shaped ship with a cube shape "magnet ship" behind it that is much thinner.) Could you use conventional rocket fuel to power the "magnet ship" forward, and therefore the larger ship in front of it? The "magnet ship" would be much lighter and require less fuel to propel the larger ship in front of it. But the magnetic repulsion could aid in propelling the larger ship forward, no? I'm no physicist but I got to thinking and maybe this is possible. Would the repulsion have to be greater than the thrust of the engines of the magnet ship?

Drakkith
Staff Emeritus
The "magnet ship" would be much lighter and require less fuel to propel the larger ship in front of it.
No. The fuel required would be equal to a single ship with a mass equal to the combined masses of the two ships.

No. The fuel required would be equal to a single ship with a mass equal to the combined masses of the two ships.
Dang, really? Why would the magnetic force pushing forward not apply?

Drakkith
Staff Emeritus
Dang, really? Why would the magnetic force pushing forward not apply?
It does apply. It's what's pushing the 2nd ship forward. But so what? The electromagnetic forces between atoms is what transfers the force from the engines to the rest of the first ship to begin with. Your idea is just another application of the EM force.

It does apply. It's what's pushing the 2nd ship forward. But so what? The electromagnetic forces between atoms is what transfers the force from the engines to the rest of the first ship to begin with. Your idea is just another application of the EM force.
I see. The em force is just transferring the energy of the rockets to the ship. Not providing any thrust itself... The mass of the larger ship is also pushing back against the smaller ship so that is why their total masses end up comines in this scenario. Is that right? I had the idea that because there is no friction the mass wouldn't matter and they would repel equally regardless of mass. Does that also mean that a larger ship requires more thrust than a smaller ship even in space to move it at the same speed?

Drakkith
Staff Emeritus
I see. The em force is just transferring the energy of the rockets to the ship. Not providing any thrust itself... The mass of the larger ship is also pushing back against the smaller ship so that is why their total masses end up comines in this scenario. Is that right?
Pretty much.

I had the idea that because there is no friction the mass wouldn't matter and they would repel equally regardless of mass.
The mass does indeed matter, even in a frictionless environment.

Does that also mean that a larger ship requires more thrust than a smaller ship even in space to move it at the same speed?
Kind of. A more massive ship requires more fuel to accelerate to the same speed as a less massive ship. Thrust doesn't really matter. I can use less thrust but thrust for a longer period of time to reach the same speed. This is just like how I can stomp on my gas pedal and reach 60 MPH very quickly, or I can lightly press on the gas pedal and take a much longer time to reach 60 MPH.

mfb
Mentor
Moving at a constant speed doesn't require thrust at all (if gravitational effects are negligible).
Accelerating more mass needs more fuel, sure. A rocket that can launch a small 100 kg satellite cannot carry a full space station to orbit.