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Comment: Not antigrav but still useful [Re: Negative ma...] (Score 1) 214

I might be made fun of for this but I'll ask anyway: If negative mass could be practically harnessef, would it allow for the antigravity/repulsorlift/mass effect technology of science fiction to be real?

Well, if you load your positive-mass vehicle up with an amount of negative mass, it will still fall downward, but it will have less overall mass and less weight. So it will only take a little amount of force to lift it or move it around.

The "if negative mass could be practically harnessed" is a big "if," though. Even aside from the fact that you have to figure out how to make negative mass.

Comment: Negative matter repels ordinary matter (Score 1) 214

Not so fast! Let me quote the GP:

Negative mass reacts oppositely to both gravity and intertia. Oddly, that means that negative mass still falls down in a gravitational field: The gravitational force is opposite, but negative mass responds negatively to force (a=F/m, where both F and m are negative). So negative mass particles repel each other gravitationally, but are attracted to positive mass objects.

Right

In other words, unlike normal matter, negative mass matter can never lump together under influence of gravitational force,

Right

but it will nevertheless attract normal matter.

You'd think, if it behaved like ordinary matter, that if it is attracted to positive matter, than it would conversely also attract positive matter. But no.

Negative matter particles attract each other, as you say, but repel normal matter. (They're attracted to it... but they repel it.)

The equations are: F = ma
and F = G mM/r^2

Comment: Pauli Exclusion [Re:Negative mass is weird] (Score 2) 214

by Geoffrey.landis (#47478321) Attached to: Cosmologists Show Negative Mass Could Exist In Our Universe

Ah, the

Pauli exclusion

principle. IANA physicist, but I've never been happy with this here thingy.

Fortunately, your happiness is not relevant to whether physics works.

...
Oh, BTW - this is just one of many examples where science does, in fact, depend on pure faith.

No, this is one of the many examples where science depends on pure observation. The Pauli exclusion principle was first arrived at from observations, and only somewhat later was the theoretical basis-- the spin-statistics theorem-- worked out.

Comment: Re:Negative mass is weird (Score 1) 214

by Geoffrey.landis (#47478215) Attached to: Cosmologists Show Negative Mass Could Exist In Our Universe

Out of interest, if there were pair creation events of involving particles of negative mass/gravity how would we detect them?

You're asking a lot, since we don't really know what the property of the particles are. A negative mass particle would curve in electric and magnetic fields (the usual way to determine what a particle is) just like a positive mass particle of the opposite charge. However, since negative mass particles also have negative kinetic energy, conservation of energy means that the remaining particles will have more energy coming out of the collision than they did going into it.

I'm not being critical, I'm curious - how would a particle accelerator, or a bubble chamber or whatever, look different with a negative mass particle?

Positive mass particles emit positive energy and slow down. Negative mass particles emit positive energy and speed up. If you see unknown particles exiting the scene at high velocity, and leaving behind more energy the faster they go, that would be a negative mass particle.

Comment: Re:Negative mass is weird (Score 4, Informative) 214

by Geoffrey.landis (#47477055) Attached to: Cosmologists Show Negative Mass Could Exist In Our Universe

Okay, as long as I've got you on the line... :)

What's supposed to happen when negative and positive mass collide?

If I throw a tennis ball at a wall, it bounces off (and the wall recoils imperceptibly). If I throw a negative tennis ball at a wall -- or throw it away, causing it to move toward the wall, whatever -- what happens when it hits? It seems like it would try to "recoil" in the same direction it was traveling, maybe even giving the wall a "tug" instead of a "push" when it hit. \

Well, I already said negative matter is weird.

Robert Forward proposed that when positive matter and negative matter touch, they cancel each other out, and vanish:
  (+) + (-) --> 0 (vacuum)
The mass cancels, and you're left with nothing there.

Unfortunately, we know that this can't happen, because if it did, then the opposite reaction could occur:
  0 --> (+) + (-)
--vacuum spontaneously generating pairs of positive and negative mass. If this could happen, it would happen, everywhere, all the time. But it doesn't. So there are rules (presumably conservation laws) forbidding this from occurring.

But it can't move forward, because presumably negative and positive matter can't simply interpenetrate -- or can they?

Of course they can interpenetrate. The reason that you can't walk through a brick wall is because of Pauli exclusion: the electrons in your body can't occupy the same place (the same quantum state) as the electrons in the wall. But, whatever negative matter is, it's not electrons (nor any of the other particles that make up "solid" matter). So, yes, it would pass right through ordinary matter.

Comment: Negative mass is weird (Score 5, Informative) 214

by Geoffrey.landis (#47475525) Attached to: Cosmologists Show Negative Mass Could Exist In Our Universe

What am I missing?

Nothing. Negative mass is weird.

What you're pointing out -- that a positive mass and a negative mass would chase each other-- was pointed out in 1957 in Bondi's paper about negative mass, "Negative Mass in General Relativity". Rev. Mod. Phys. 29 (3). Robert Forward, in 1990, then extended that analysis even further and pointed out that negative mass is even weirder than that.

A negative mass chasing a positive mass accelerates forever... but it doesn't violate conservation of energy, because the faster a negative mass moves, the more negative the kinetic energy, so the positive kinetic energy and the negative kinetic energy cancel out, leaving energy conserved.

There are weirder things than that, too.

If you think this is so weird that bulk negative mass can't exist... well, that's what Einstein thought (the "positive energy condition").

Comment: Dark energy is negative (Score 2) 214

by Geoffrey.landis (#47475381) Attached to: Cosmologists Show Negative Mass Could Exist In Our Universe

Is this similar to, unrelated to, part of, dissimilar, orthogonal, integral, or in any way linked to Dark Matter?

It's unrelated to dark matter (which has positive mass- that's how we know it's there), but dark energy is gravitationally negative (it causes expansion to accelerate: it's gravitationally repulsive)

Because I (and probably most of us) don't understand that either.

You're in good company! If you did understand it, you could publish, and you should be getting a phone call from Stockholm soon.

Comment: Negative mass- not antimatter, but odd (Score 5, Informative) 214

by Geoffrey.landis (#47474705) Attached to: Cosmologists Show Negative Mass Could Exist In Our Universe

Negative mass is very diferent from antimatter. Antimatter is opposite to normal matter in charge and quantum numbers (such as baryon number, etc.), but still has positive mass.

Negative mass reacts oppositely to both gravity and intertia. Oddly, that means that negative mass still falls down in a gravitational field: The gravitational force is opposite, but negative mass responds negatively to force (a=F/m, where both F and m are negative). So negative mass particles repel each other gravitationally, but are attracted to positive mass objects.

This has peculiar consequences. One consequence is that, for objects of negative mass, gravity and electrostatic charge switch. For normal mass objects, gravity is attractive, but like electrical charges repel. For negative matter, gravity is repulsive, but like electrical charges attract.

I wrote about this once, in the AIAA Journal of Propulsion and Power-- not a journal that physicists usually read, I'm afraid. If you have access to AIAA online, it's here: http://arc.aiaa.org/doi/pdf/10...

Comment: Re:"An anonymous reader" (Score 2) 112

by Geoffrey.landis (#47450861) Attached to: SpaceX Falcon 9 Rocket Blasts Off From Florida

It seems to me that NASA should simply contract those basic research payloads on top of SpaceX rockets, if SpaceX can get them into orbit for fewer dollars than NASA's own internal teams can. Why waste resources?

That's the way NASA currently does business: launch services are purchased.

SpaceX developed Falcon-9 on a NASA contract, specifically in order to be a vehicle that can be purchased for launch services. ("Commercial Orbital Transportation Services" was the name of the contract.)

Comment: Re:Could it be ... (Score 1) 95

While fast radio bursts last just a few thousandths of a second and have rarely been detected, the new result confirms previous estimates that these strange cosmic bursts occur roughly 10,000 times a day over the whole sky.

That's a lot of aliens.

Well, since there are 100 to 200 billion galaxies in the observable universe, not so many. One burst per galaxy every 50,000 years or so.

 

Or maybe we are inside of a slow thinking alien's head.

Comment: Beacon (Score 2) 95

It's worth pointing out that a good way to send a signal would be to have a bright but transient beacon, which doesn't itself transmit information (other than "here I am"), but serves to tell others where to point their high-gain radiotelescopes.

This could be what such a beacon would look like.

Not to mention the power output it would need to send a detectible signal from another galaxy.

From the summary: "bright flashes of radio waves that last only a few thousandths of a second.

A high power for a few milliseconds may not take an enormous amount of energy.

Comment: Re:Dimmable LEDs (Score 1) 278

by Geoffrey.landis (#47430539) Attached to: My most recent energy-saving bulbs last ...

Without dimmers, I've been hard pressed to see the difference between brands.

The differences are not easily visible from the outside. Some brands have gone through some pretty rigorous lifetime testing-- humidity, voltage variation, temperature-- while others are just "as long as it works long enough for the consumer to throw away the receipt" made, but you can't really tell by just looking at them.

Also, Lumens per watt can vary quite significantly from one to another.

FORTRAN is a good example of a language which is easier to parse using ad hoc techniques. -- D. Gries [What's good about it? Ed.]

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