Hacker News Reader

Comment by Steuard | Hacker News Reader

White claims to have calculated a way to substantially reduce the energy requirements to form a warp bubble, and without reading his papers I suppose I should provisionally believe him. But the required energy has to be negative (as he confirms here), and I have never, ever seen him or anyone else suggest how he plans to accomplish that. Negative vacuum energies like what he's relying on have never been observed in the universe (directly or indirectly), and would violate a number of "energy conditions" commonly believed to hold for realistic matter. It would be very exciting if those energy conditions really were violated, but it's an extraordinary claim that backed by zero evidence.

With that in mind, I continue to be puzzled by the amount of press that this story seems to receive.

View on HN

> and without reading his papers I suppose

http://members.shaw.ca/mike.anderton/WarpDrive.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2011001...

http://forum.nasaspaceflight.com/index.php?action=dlattach;t...

I believe negative energy has been observed in tiny amounts in the Casimir Effect, no?

That's a misconception I've come across a few times. It creates a negative energy density relative to the ambient energy density, not an absolute negative value. In other words if you consider ambient energy density to be zero the. The casimit effect gives rise to negative energy densities, but if course zero point vacuum energy means the ambient energy density is never zero. The casimir effect cannot lead to 'true' negative energy densities.

Dark energy has negative energy density... so I think there is hope that this is just a engineering problem ;)

I haven't studied this in detail, but my understanding is that dark energy (whether from a cosmological constant or other sources) does not violate the "null energy condition" (roughly speaking, the condition that a light ray would never "see" a negative energy density). I don't recall exactly which energy conditions the Alcubierre drive violates, but my impression was generally "all of them". (His paper explicitly talks about violating the weak, dominant, and strong energy conditions, but doesn't mention null one way or the other.) There are quantum effects that seem to violate some of these, but my impression has been that the known examples do not violate them when averaged along an allowed trajectory in the spacetime, which the Alcubierre solution would.

That's getting near the boundaries of my technical knowledge of the subject, though, so I won't argue further. It would be awesome if this could work out, but as I've said elsewhere there are multiple Nobel-worthy steps between what we know today and actually creating any sort of warp bubble in the lab. If I were White, I'd be publishing my solutions to those first!

Dark energy isn't an actual thing.

It's a term used to describe the "this doesn't make sense" part of the world. No one knows what it is, does, or if it exists.

Dark matter at least has some described properties (and no one knows if it exists either).

"With that in mind, I continue to be puzzled by the amount of press that this story seems to receive."

Let's make the supposition that a science idea has to meet some standard of plausibility in order to be publishable. The standard being applied to this is that it hasn't (yet) been categorically denied by any incontrovertible law of nature. Not much fails this test. If you postulate access to a source of negative energy or to the inside of a black hole, nothing is technically impossible.

As far as I know, negative energy is theoretically still sound; I may be quite mistaken though. What "energy conditions" would negative energy (or mass for that matter) be violating?

One reference is here: http://en.wikipedia.org/wiki/Energy_condition

These conditions are widely believed to apply to all realistic matter configurations (to the extent that the less-restrictive ones get used as postulates in some theorems in relativity), though there are some exceptions due to quantum effects (which I believe are usually short-lived). Just for example, a cosmological constant or other dark energy source does not violate the null energy condition.

It would be really cool to see any of the energy conditions violated for an extended time in a lab! I'm pretty sure, though, that it has not yet been done.

Weak energy condition[1]. In general the energy conditions are useful for analysis of the Einstein equations, but general relativity does not demand them. So violating the energy conditions is somewhat theoretically sound, but one can only say so much about general relativity with violated energy conditions. ( For example a lot of stuff we know about black holes only holds, if some energy conditions are satisfied.)

[1] https://en.wikipedia.org/wiki/Energy_condition

Negative refraction is just as strange a concept, and we seem to be slowly getting there technologically. So I'm actually hopeful this will happen someday.

Unfortunately negative refraction a secondary effect where you have negative permittivity and permeability at the same time. Negative permittivity is no big deal, metals act like an electron plasma gas around certain frequencies and get all weird, something like the color transmitted thru thin films of gold and how weird gold, silver, copper act in the THz range and all that. The "magic" is creatively negating both .. At The Same Time. Then the negative refraction / metamaterial effects take hold.

Kind of like you can compress ammonia (a bit) and nothing terribly interesting happens, and you can cool ammonia gas (a bit) and nothing terribly interesting happens, but do both at the same time and you've invented liquid ammonia and refrigeration and all kinds of nifty stuff.

On the other hand energy/mass seems rather fundamental. There are no multiple "easily" negatable components you can play with mixing together. There's no "add a to b to get the mass" going on. So you can't get sneaky with combinations and initial conditions to magically make negative mass/energy. It's really a different kind of problem.

The whole topic is pretty interesting to me because the first metamaterial stuff was constructed out of printed circuit boards with weird etchings right around the ham radio microwave 6cm and 9cm bands, which is right up my alley (doing weird ham radio things involving PCBs and such around those frequencies) Unfortunately I have been unable to figure out any practical engineering demo (like, an antenna or a waveguide or some such) that would be ham radio applicable in a real world demonstration. I guess I could make a really weird lens and place it in front of a 6cm horn waveguide antenna, but I can make a much simpler microwave lens antenna using about 1940s tech. Lens antennas using plates of styrofoam and aluminum foil work pretty well at 10 GHz ham radio band for example, but I digress.

Metamaterials are kind of a solution in search of a problem right now, like lasers in 1960s or home computers in the 80s or whatever. Most likely, much like the examples, it would be pretty hard to predict today just how important they might be in the future. Or maybe we're seeing the next "magnetic bubble memory" or magnetic amplifier technology or fluidic analog computing or whatever. Hard to predict.

Reply on news.ycombinator.com