Alcubierre Drive and dark energy

[Ags]

Apparently we can travel faster than the speed of light using the Alcubierre warp drive design. All this drive requires is small amount of "exotic" matter - no, not the Red Matter from the Star Trek movie, but instead negative mass matter. 

Negative mass matter would be repelled by gravity, so I doubt you would find any in a galaxy- instead such matter would be expelled. If there were clouds of negative mass matter in intergalactic space, could that explain dark energy observations? If positive mass would make the universe's expansion slow, presumably negative mass would make expansion accelerate?

[Ags]

OK, so this talk of negative mass matter sounds a bit nutty, but do remember NASA did try out an Alcubierre Drive experiment using quantum pressure to emulate negative mass:

https://en.wikipedia.org/wiki/Alcubierre_drive

https://en.wikipedia.org/wiki/White–Juday_warp-field_interferometer

Also, negative mass is a part of Physics' standard model:

https://en.wikipedia.org/wiki/Negative_mass

[Tiki]

Negative mass matter is not so nutty. Part of the gravitational field produced by a stream of moving particles acts in the opposite sense to the gravitational field produced by a static mass! (It is the part of the gravitational field that is produced by the momentum of the moving matter stream. cf. equations of gravitomagnetism).

[andrew s]

19 hours ago, Tiki said:

Negative mass matter is not so nutty.

Negative effective mass is very common in many body systems. For example an air bubble in water has a negative effective mass - it move in a direction opposite the pull of the local gravitational field. A single elementary particle with negative mass is yet to observed and is not part of the standard model of particle physics as far as I know.

Regards Andrew

[Ags]

Does this mean faster than light travel is possible... underwater?

Back to my original point, if we are prepared to accept that most matter is dark matter that does not interact with anything except gravity, why not also postulate "light matter" (geddit?) that is lighter than nothing and explains the dark energy observations?

[andrew s]

Light travels slower in water and the refractive index is greater than one.

You can postulate whatever you want to but it helps if it has some links to either current theory or better still observations.  Just throwing out ideas is easy. The hard bit is making specific predictions that can in principle differentiate one speculation from another. 

Regards Andrew 

[Tiki]

On ‎14‎/‎06‎/‎2017 at 15:27, andrew s said:

Negative effective mass is very common in many body systems. For example an air bubble in water has a negative effective mass - it move in a direction opposite the pull of the local gravitational field. A single elementary particle with negative mass is yet to observed and is not part of the standard model of particle physics as far as I know.

Regards Andrew

 

Perhaps it would be better to say 'Negative active gravitational mass is not so nutty.' After all, negative energies are allowable in quantum field theory and energy is a gravitational source. Pressure is also a source of gravity and tension is negative pressure, therefore if a system has sufficient negative pressure (ie. tension) then its active gravitational mass would be negative.

 

On ‎15‎/‎06‎/‎2017 at 11:32, Ags said:

Does this mean faster than light travel is possible... underwater?

 

Unlike the case for a vacuum, the speed of light in water can be exceeded in water.

[andrew s]

17 hours ago, Tiki said:

'Negative active gravitational mass is not so nutty.'

It is the case that some "fringe" theories propose this. (Fringe means not main stream physics but still respectable science.) The difficulty is our mathematical models admit may solutions not all of which are realised and it requires observations to constrain them. As I said above no negative rest mass elementary particles have been observed yet.

17 hours ago, Tiki said:

negative energies are allowable

Yes indeed but the zero of energy can be chosen to be whatever you want in both classical and quantum physics so I am not sure it supports negative gravitational mass directly. 

Regards Andrew

[Tiki]

2 hours ago, andrew s said:

 

Yes indeed but the zero of energy can be chosen to be whatever you want in both classical and quantum physics so I am not sure it supports negative gravitational mass directly. 

Regards Andrew

I  wasn't referring to the arbitrary choice for zero potential energy in a system as this would not contribute to the active gravitational mass. I was referring to short-lived negative energy virtual particles (important in Hawking radiation), negative energy densities in the Casimir effect etc.

 

[andrew s]

3 hours ago, Tiki said:

 I was referring to short-lived negative energy virtual particles (important in Hawking radiation), negative energy densities in the Casimir effect etc.

Sorry I misunderstood you. There are a lot of misconception's about virtual particles I would recommend this https://www.physicsforums.com/insights/physics-virtual-particles/ and the linked articles. This is a discussion in that series on misconceptionshttps://www.physicsforums.com/insights/vacuum-fluctuation-myth/ and might be a better starting place.

Regards Andrew