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Comment: Invention that makes beams do what they do anyway (Score 1) 115

by Geoffrey.landis (#47524975) Attached to: 'Optical Fiber' Made Out of Thin Air

So the channel itself... has the diffraction, scattering, and beam spread of an unchanneled beam.

The beams making the channel are channeled by themselves, they create filaments that self-focus the beam. Self-focusing beams in air have been pretty well established at this point and will go quite far if you have enough power because of the attenuation involved.

So, what you just said is that the beams self-channel anyway.

So, if beams self-channel, this innovation does nothing, right? It's a complicated system of multiple beams to make the beam channel, which is to say, self-focus. But you just told me "self-focusing beams in air have been pretty well established at this point."

Comment: Little, as far as I can tell [But what does it do? (Score 1) 115

by Geoffrey.landis (#47518689) Attached to: 'Optical Fiber' Made Out of Thin Air

air is not transparent

To the extent that air is not transparent, this doesn't work.

and does cause beam scattering.

This does not address beam scattering. If the air is scattering the laser beam, it still scatters the beam.

by creating a refractive channel like this they absolutely will reduce beam dispersion.

It would reduce beam spread... except that the beams that create the channel are not themselves channeled.

obviously it doesn't eliminate beam spread

on this we agree

but even a fiber channel perfectly designed for a single mode will have some diffusion so whats your point?

My point is that from a surface-level analysis, it doesn't do anything useful.

they may be able to increase snr by 10^4 over current technologies at 100 m. that's a serious improvement that shouldn't simply be dismissed so thoughtlessly.

Let me repeat. The beams that create the channel are not themselves channeled. So the channel itself... has the diffraction, scattering, and beam spread of an unchanneled beam. The net result can't be better than an unchanneled beam, because it is made out of an unchanneled beam.

Comment: But what does it do? (Score 1) 115

by Geoffrey.landis (#47517539) Attached to: 'Optical Fiber' Made Out of Thin Air

I'm puzzled as to what this does or what it's good for, exactly.

... they have turned thin air into an "optical fiber" that can transmit and amplify light signals without the need for any cables.

1. Air already transmits light signals. It's transparent.
2. They haven't mentioned anything about amplifying light signals. This would be hard.

So, they are creating a "pipe" that can transmit light... but it doesn't stop beam spread (since the beams that make up the "pipe" still have diffraction-limited beam spread), and it can't bend light around corners. So, they now have a pipe that will funnel a laser beam along the path made by other laser beams, which take it exactly the same path that the beam would go without the pipe...

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.

The secret of success is sincerity. Once you can fake that, you've got it made. -- Jean Giraudoux

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