For Questions That Don't Merit Their Own Thread

[Skyler4856]

This thread is an attempt to control repetitive, basic, or brief questions.

Rules:

Don't submit questions until a previous one has been answered

As always, be mindful of forum rules, and keep conversation cordial.

Ill start:

If an invisibility cloak must bend light, then shouldnt we utilize something similar to a cloth made of some kind of optical fiber to make an invisibility cloak?

[K^2]

In order to make an invisibility cloak/shield that works in air/vacuum, which have optical density near 1, you need a material with optical density less than 1. In other words, a material in which speed of light is greater than speed of light in vacuum. Fortunately, you only care about phase velocity for optical properties, not the group velocities. And so metamaterials with such property do exist. Unfortunately, nobody managed to make them into something that can actually be made into a solid object. So invisibility cloaks remain firmly in the realm of science fiction. There are plenty of people working on this, however, and some aerogel approaches are promising.

If you happen to have a medium with high index of refraction, like water, the problem is much easier. There are plenty of plastics with lower index of refraction, and transparent spheres that make their contents invisible when submerged under water do exist. Don't know why they aren't commercially available, to be honest. Would be a cool thing for hiding stuff in aquariums.

[Aethon]

If you're inside an invisibility cloak, how do you see outside?

[MBobrik]

If you're inside an invisibility cloak, how do you see outside?

You don't. Of course you could still leave a small opening to see, which would then be visible bud very hard to notice, or you could use other wavelengths like infrared to see through your own cloak.

[magnemoe]

In order to make an invisibility cloak/shield that works in air/vacuum, which have optical density near 1, you need a material with optical density less than 1. In other words, a material in which speed of light is greater than speed of light in vacuum. Fortunately, you only care about phase velocity for optical properties, not the group velocities. And so metamaterials with such property do exist. Unfortunately, nobody managed to make them into something that can actually be made into a solid object. So invisibility cloaks remain firmly in the realm of science fiction. There are plenty of people working on this, however, and some aerogel approaches are promising.

If you happen to have a medium with high index of refraction, like water, the problem is much easier. There are plenty of plastics with lower index of refraction, and transparent spheres that make their contents invisible when submerged under water do exist. Don't know why they aren't commercially available, to be honest. Would be a cool thing for hiding stuff in aquariums.

Yes, not sure if the fiber idea would work, you need to let all light pass from all directions and angles, i think it would be to much fiber to be practical.

[sgt_flyer]

Current research uses metamaterials - they can bend some EM radiations (like microwaves) or even some frequencies of light, but they need to miniaturise more and more the metamaterials patterns further they approach from the visible spectrum.

There's also using a pair of lenses to partially 'hide' an obect between the two (by using converging diverging light)

Although, if they are able to bend EM radiations with metamaterials, maybe they'll be able to create lightweight radiation shieldings for spacecrafts - no more need to block the radiations with heavy shielding (which would be unpractical for a spacecraft) , just bend the radiations path around the spacecraft to protect what's inside.

[shynung]

I've seen in one mod pack a monopropellant fueled engine that manages an I_SP of about 410 seconds, which happens to have an oversized nozzle bell. My question is, how far can one increase the I_SP of an engine through throat-and-nozzle geometry alone, for a given type of fuel?

[K^2]

I've seen in one mod pack a monopropellant fueled engine that manages an I_SP of about 410 seconds, which happens to have an oversized nozzle bell. My question is, how far can one increase the I_SP of an engine through throat-and-nozzle geometry alone, for a given type of fuel?

Assuming opration in vacuum, inviscid flow, and massless bell, you want it running to infinity to get 100% of thermal energy turned into thrust. In practice, gains start getting insignifficant pretty fast. But this "infinite bell" approach lets you compute the absolute maximum I_SP that a particular fuel mixture can have. Simply find the amount of heat produced, and assume that every molecule of exhaust has equal energy. Then compute total impulse per mass of material. If you do this for real fuels, you'll find that this maximum is somewhat optimistic. (I believe, it works out to well over 500s for LH2/LOX. Don't feel like doing the math right now.) The main reason is that exhaust does not have time to fully thermalize between the degrees of freedom, and that's because practical limitations requrie one to cut the nozzle short. Of course, there is some turbulence and drag as well that reduce efficiency.

Operation in atmosphere is a different story. Not only extending bell past certain cutoff doesn't give you extra thrust, but it actually reduces it. Which is why engines designed for operation in atmosphere are so much shorter.

[goldenpeach]

At which altitude a peregrine can fly?

Also, at what distance can they spot their prey?

[Hary R]

At which altitude a peregrine can fly?

Also, at what distance can they spot their prey?

1th question I don't know

2nd question this might gives you some element to find the answer

[gooddog15]

Whats the air speed of an unladen swallow?

[Hary R]

Whats the air speed of an unladen swallow?

which one? the European one or the African one...

[goldenpeach]

No one know...

All we know is that the speed is between 0 m/s and 299 792 458 m/s.

I can't more precise, sorry

[problemecium]

I was just looking for a thread like this! Props.

So anyway. Why is an alcubierre drive shaped like a ring? I looked it up and Google didn't point me anywhere that bothered to explain that part. I get that for some reason the ring shape results in an elliptical region of warped space, but WHYYY, and why can't we use a different configuration to make a different shape?

[K^2]

So anyway. Why is an alcubierre drive shaped like a ring? I looked it up and Google didn't point me anywhere that bothered to explain that part. I get that for some reason the ring shape results in an elliptical region of warped space, but WHYYY, and why can't we use a different configuration to make a different shape?

Alcubierre Drive corresponds to a specific geometry dictated by Alcubierre Metric. It is based around a spherical warp bubble. For this particular geometry, the bulk of energy density is located in a ring around the ship. This doesn't mean that the drive has to be ring-shaped. In fact, nobody's quite certain what you need to put that ring-shaped energy density there. But ring shaped drive seems intuitive because of that.

Can you have a different shape? In principle, yeah. There are infinitely many possible configurations. Alcubierre's was just the first one, and nowhere close to being most efficient. As you've pointed out, elliptical configurations are more common in modern research, since they require less energy. They still result in a ring of energy around the ship, though. But that doesn't mean that the only efficient configurations will result in a single ring.

[Mazon Del]

Actually, one of the big advancements that White did with his revising of the Alcubierre calculations shows that changing the shape can reduce (or increase if you did it wrong) the power required to operate.

[problemecium]

Sooo does anyone understand the Alcubierre Metric stuff enough to give a clear explanation of why a ring-shaped energy density is useful, as opposed to another shape such as a hollow sphere or a line?

[MBobrik]

Sooo does anyone understand the Alcubierre Metric stuff enough to give a clear explanation of why a ring-shaped energy density is useful, as opposed to another shape such as a hollow sphere or a line?

basic symmetry. you have one direction where you want to go, and two other axes where you generally don't want to go, that are equivalent, hence rotational axis symmetry. And you don't want to have strong gravity gradients in the middle, thus something similar to hollow cylinter. It is just that current geometries have length much shorter than diameter, and a flat hollow cylinder is usually called a ring.

[problemecium]

So we could easily design an Alcubierre Drive that is just a little block in the middle of the ship, as long as it generates a warp field big enough to enclose the whole structure?

[Skyler4856]

So we could easily design an Alcubierre Drive that is just a little block in the middle of the ship, as long as it generates a warp field big enough to enclose the whole structure?

As far as I'm aware yes

[K^2]

Sooo does anyone understand the Alcubierre Metric stuff enough to give a clear explanation of why a ring-shaped energy density is useful, as opposed to another shape such as a hollow sphere or a line?

Geometry gives you energy density. There's a formula. (See: Einstein Field Equations.) Alcubierre Metric has ring-shaped energy density distribution. It's not something you can change. If you take a different energy density distribution, you'll have a different metric with different geometry.

It can still be a warp metric. But it won't be Alcubierre Metric.

[shynung]

I've read from another thread that FTL travel enables time travel as well. Suppose one tried to use an FTL-capable spacecraft to travel back to the past; what kind of trajectory would he use? How much would the craft's maximum FTL speed affect the trajectory for the same destination?

[Iskierka]

That would only be for local FTL travel - all believable FTL techniques involve cheating by not locally travelling faster than light, but either generating a shortcut, or physically moving the space they exist in (which, not being inherently matter or energy, is not limited to c) at a speed greater than that of light.

That said, wormholes and alcubierre drives together could allow limited time travel. If you move a wormhole, it experiences time dilation, and thus ends up at a different point in time to its other end point. You couldn't use this to travel to before the wormhole, but in theory you could create it, move one end, then an alcubierre ship could travel through and then back through space to return to before its launch point. One issue with time dilated wormholes however is they would become unstable - when their field radiation propogates far enough to be able to travel through both, energy cycling through repeatedly would drain the wormhole's energy and cause it to collapse, so this would also be a very time-limited technique.

Main issue preventing time travel isn't directly physical inability to travel back, but simply that if you could, you would violate conservation of mass and energy - whatever was sent back would be removed from its current timeline and added where it already existed concurrently. The wormholes get a delay because the conservation would only cause the decay once energy can cycle, the space is physically connected so they are technically the same time, even if one has had more local time pass, thus energy is conserved until it starts infinitely cycling. The alcubierre drive further cheats by moving the existence of the additional energy while technically holding within causality. It's weird stuff, but fundamentally, outside a quirky and very limited cheat, time travel is impossible.

[K^2]

Warp to a remote location, accelerate to near light speed away from Earth (using reaction drive, not warp), warp back to Earth, decelerate. As you can see, it's practically impossible even if you have FTL.

P.S. Essentially, it's the same reason FTL + wormholes give you time travel. The extreme acceleration you need to pull this off causes extreme space-time curvature from perspective of the pilot. (Gravity indistinguishable from acceleration, etc.)

[gooddog15]

Could aliens understand what planet Pioneer 10/11 came from due to it's position?

Edited December 8, 2014 by gooddog15
Grammer errors