Physics questions

coriej said:

pitfalls

1 - Speed of communications - it seems to me, given speed of light constraints, that a real time conversation between two people separated by very large differences (different planets)-
but is there a clever way around this?

I did find this: http://adsabs.harvard.edu/abs/1997JBIS...50..249T
"Real-time communication with distant objects in space might be possible through the use of the special properties of the quantum world."
but can't seem to access the entire article

2 - Explosions in space - limited fire, because of that pesky lack of oxygen - so likely a blowout, then a ship would crumple in due to the difference in pressure, right?

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I'll take a stab.
1. There are two ways to write around this issue. a) there is no FTL communication, so communicating takes ages. or b) the ansible! I believe Ursula LeGuin coined this hand-waving device that allows FTL communications for plot purposes, and it has been used by loads of authors. You can call it the ansible or give it your own name. It's well impossible so you may as well just do some hand-waving to get around it. But use whichever way works best for your story.

2. Smack me if I'm wrong, but I believe that some rocket fuels burn using a chemical other than oxygen as the oxidizer, so you could still get a decent blowout. If the ship is nuclear-powered, a crash could upset the control rods that keep the reaction in check thus leading to meltdown or EXPLOSION!

Re: the ship crumpling, I gotta say probably not. The pressure inside a ship is one atmosphere (or less!) The pressure outside a ship is essentially zero atmospheres. One atmosphere of pressure is 14 pounds per square inch. It's about the pressure of being 30 feet underwater. Not a very high-pressure situation.Overwrought climax scenes in sci-fi movies aside (I'm looking at you, Aliens 4!), a small hole in the wall of a spaceship could be easily plugged by placing one's hand over it without incurring serious injury. Beyond that, if an explosion causes the air onboard to escape, then there will be no pressure differential and thus no cause for the ship to crumple.

Submarines crumple from the pressures of the deep sea. A spaceship might blow OUTward into space, sort of like how they portray airplanes depressurizing.

Communications: contemporary radio communication is limited to speed of light -- and a fall off of energy as distances increase. It is impractical for any kind of conversation beyond the solar system. There is a hypothetical method of instant communication over vast distances, however, called "quantum entanglement." Simply, it has been demonstrated that if you have two sub-atomic particles (photons work) with the same spin, then separate them, changing the spin of one will instantly change the spin of the other. (Yeah, I know it's a great deal more involved than this, but hey, close enough, eh?) Anyway, it's conceivable to rig up a scenario where a bunch of linked pairs of sub-atomic particles scattered all over the region where you want to maintain communications. A story where this was used to good effect is "The Risen Empire" by Scott Westerfeld.

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Explosions in space: Assume a space ship (the NASA Space Shuttle works). It has a sealed hull containing compressed gasses (air at or near standard pressure). Touch off a stick of TNT inside it, and you definitely get a big whoomp of fire and shrapnel. Same thing if you shoot a bazooka rocket into the ship from outside. Whoomp! Most explosives don't depend on atmospheric oxygen to combust. Setting the wastebasket afire, does.

A firefight between two (very insane) astronauts with .45 cal pistols would likely hole the hull allowing the compressed gasses to escape into the vacuum of space. Meteroids can cause similar damage from outside the vessel. It would be unlikely for such events to be explosive; the requirements of containing atmosphere inside a vessel necessitate a pretty sturdy structure. Sure, everyone inside dies of asphyxiation within a short time, unless they can don pressure suits, but no bursting balloon.

The crumpling you mention happens to submarines that lose hull integrity: the pressure of the ocean is greater than that of the air inside. If a space ship loses hull integrity, it bursts apart because the air pressure inside is greater than the vacuum outside. However, such bursting is not explosive. Just a very rapid (and an almost immediately unfortunate experience for the occupants) loss of atmosphere.

The ansible is a sufficiently standard trope to be usable without requiring any justification.

If you know the science well enough to want to play with it, that's fine, but I don't see any reason to reinvent the wheel otherwise.

I would recommend not invoking quantum entanglement for the sake of FTL communication, because, according to the theory, entanglement doesn't actually transmit information, because it requires independent knowledge of the state of both particles. I'd stick with the hand-waving.

My galaxy has "communication spheres" stationed at various points, where ships can stop and perform long-range communications via recorded messages...That's as far as I go for an explanation.

It has a sealed hull containing compressed gasses (air at or near standard pressure). Touch off a stick of TNT inside it, and you definitely get a big whoomp of fire and shrapnel. Same thing if you shoot a bazooka rocket into the ship from outside. Whoomp! Most explosives don't depend on atmospheric oxygen to combust. Setting the wastebasket afire, does.

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Maintaining a fire would be difficult...once all the combustible air was evacuated. I rely on the explosive shrapnel dispersion effect a lot...

Another thing to keep in mind (since shrapnel was already mentioned): no friction in space.
That means parts flung away by an explosion will not lose speed with distance. Of course, getting hit by spaceship parts will get less likely the farther away you are from the explosion (proportional to the square of the distance) but if you get hit, it packs the same punch as if you were standig right next to the explosion.
Also, you will not get the same "sorting" you get in air, i.e. small debris losing velocity faster and travelling shorter distances, on the contrary, the lighter the particle the more speed it will have gained from the explosion.

So, if you have space packed with ships (like you do in star trek or star wars battles) there are no big burning fireballs, but you will have lots of secondary damage done by shrapnel and splinters.

The other handwaving device you can use (after entanglement) is the tachyon particle, which travels faster than light.

Tachyons aren't mathematically necessary to particle physics, but they don't actually violate Einstein's law against accelerating past the speed of light, since they come into existence already traveling faster than c. Of course, no one has ever found experimental confirmation of their existence.

Lhun said:

So, if you have space packed with ships (like you do in star trek or star wars battles) there are no big burning fireballs, but you will have lots of secondary damage done by shrapnel and splinters.

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Which is why people like to use things (at least in my universe) like double-hulls and emergency pressure bulkheads...nothing nastier than getting a hole in your ship's skin in the middle of nowhere.

coriej said:

My high school physics teacher I'm sure is spinning in his grave, but my brain is pulling blanks and I'm not quite sure how best to google this.

I'm almost done (finally) with the first draft of my YA science fiction - and I have a couple of scenes that I can write two different ways, depending on how the physics work.

Neither element is crucial to the story, I just would like to avoid some pitfalls

1 - Speed of communications - it seems to me, given speed of light constraints, that a real time conversation between two people separated by very large differences (different planets)-
but is there a clever way around this?

I did find this: http://adsabs.harvard.edu/abs/1997JBIS...50..249T
"Real-time communication with distant objects in space might be possible through the use of the special properties of the quantum world."
but can't seem to access the entire article

Click to expand...

Richard Morgan has "solved" this for himself in his Takeshi Kovacs stories, but it's very limited to his world, as the technology they use is derived from aliens. And in any case, the only way he explains it is by saying nobody really knows how it works.

Personally, I would use "wormhole" technology (or some other term to define the same concept) and still have a slight delay on transmissions, say one hour delay per million lightyears or something similar.

coriej said:

2 - Explosions in space - limited fire, because of that pesky lack of oxygen - so likely a blowout, then a ship would crumple in due to the difference in pressure, right?

Many, many thanks

c

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Air wants to escape from high pressure to low pressure in order to equalize. That's why divers always has to equalize their heads when going deeper.

A space-ship suffering depressurisation would most likely start ripping from the point of pressure-loss. I dunno though. This could just be so much horse manure. In my understanding of this though, the force of pressure on the limited area of the puncture would rip the section apart, unless the puncture is sufficiently large in the first place.

As for the explotions, you still get SOME flame if there were any oxygen in the bomb itself, or the target vessel. Depending, of course, on what kind of explosives is used.

Look at explosions under water and then look at flames in zero-G and then combine the two.

On a different note. Check out Richard Muller's "Physics for future presidents" class at Berkley University. You can see the whole class on podcasts, and it's BRILLIANT. Everything you've always wanted to know about physics, but without all the tedious maths.

EDIT: Oh yeah. Back to the question of depressurising space-vessels. You naturally wouldn't have this problem on a large scale because of emergency quick-seals locking of the damaged area.

Lhun said:

Also, you will not get the same "sorting" you get in air, i.e. small debris losing velocity faster and travelling shorter distances ...

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Is this right? I didn't know that. I understand inertia, but I would have thought that more energy is needed to build inertia on a heavier object, and thus it would still fly shorter distance than a lighter object (I'm not talking about dust and other particles). The light (a wheel-bolt) and the heavy object (a wheel) having recieved the same energy, the light one doesn't have the same potential for inertia, but the heavy one won't be affected as much because it's actually heavier. Oh my god, this is HARD to talk about for an uneducated person like myself...

Even though you didn't actually tell me, I feel I have to thank you for the information though. This just proves the need for Mythbusters

Little particles, having less mass, are easier to accelerate than are larger more massive particles.

Let's say you had a ship with sand as the protective insulation between an inner hull and an outer hull. (Yeah, it's stupid, but bear with me.) If a torpedo you shot at the ship managed to penetrate both hulls before exploding, the fine particles of sand would be nearly instantaneously accelerated to high speed (and, as Lhun suggests, would continue away from the explosion at that high speed for a very long time), but larger pieces would take more energy to get moving, so their velocity would be lower.

If said ship had an outer hull made of lead, say, and insulation made of fiberglass, and I was in the neighborhood when it exploded, I would worry more about the cloud of fine glass particles than I would about lumps of lead.

I was going to answer this, but my answers were all in here already

Derailment

Pthom said:

Little particles, having less mass, are easier to accelerate than are larger more massive particles.

Let's say you had a ship with sand as the protective insulation between an inner hull and an outer hull. (Yeah, it's stupid, but bear with me.) If a torpedo you shot at the ship managed to penetrate both hulls before exploding, the fine particles of sand would be nearly instantaneously accelerated to high speed (and, as Lhun suggests, would continue away from the explosion at that high speed for a very long time), but larger pieces would take more energy to get moving, so their velocity would be lower.

If said ship had an outer hull made of lead, say, and insulation made of fiberglass, and I was in the neighborhood when it exploded, I would worry more about the cloud of fine glass particles than I would about lumps of lead.

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yep. got that, just didn't know it was the other way around when affected by gravity and atmosphere. As you can tell, I haven't been around explosions alot. I also never had advanced physics at school.

James Blish used an idea for a FTL communications device known as the Dirac Transmitter. I don't have the energy to google, but as I remember, the premise was that if you alter certain quantum particles in a particular type of atom, you are instantaneously altering every similar atom in the universe.

Or something like that.

ProtoMatic said:

Derailment


yep. got that, just didn't know it was the other way around when affected by gravity and atmosphere. As you can tell, I haven't been around explosions alot. I also never had advanced physics at school.

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Gravity has nothing to do with this particular point it just makes things fall down.
There's two important differences in space compared to on a planet. No gravity, no friction.
No gravity just means that things don't fall to the ground. Since there's also no ground ins space this is hardly noticable at all. At least when comparing spaceships, when you're inside one you'll obviously notice you don't stick to the floor. Unless it's sticky.
No friction is much more important. No friction means unhindered newtonian movement, i.e. any object that acquires a certain vector (speed and direction) will keep that vector unless some force acts on it. A bullet will travel in a straight line until it hits something. A splinter from an explosion will. A spaceship will. Nothing ever slows down unless it is actively braking. Consequently of course, spaceships don't need to keep engines running to keep moving.

Lhun said:

There's two important differences in space compared to on a planet. No gravity, no friction.

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No, no. Gravity and friction both occur in space just the same as they do on the surface of the planet.

Gravity.
When you are on the surface of a planet, you are close (duh!) to a massive body. That means the force of gravity acting on you from from that massive body is strong. When you get out in space, you are farther away from any massive bodies. The force of gravity from any one of them is much weaker. Furthermore, there are likely to be several massive bodies in different direction, so their gravity pulls in different directions, which means one may partially or completely cancel out another. In space, the force of an explosion is likely to be much, much stronger than the force of gravity. Add to that the absence of a 'ground' for things to 'fall down' onto, and the effects of gravity become barely noticeable.

Friction.
Friction happens when two objects move in contact with one another. In an atmosphere, debris from an explosion rubs against air molecules, lots and lots of air molecules. With each contact, the debris gives up a tiny bit of its energy. In space, with no atmosphere, there is a lot less stuff for the debris to encounter. But when it does encounter a stray gas molecule or other particle, friction will happen and some energy will be lost. But since the force of the explosion gives the debris a lot of energy and there are significantly fewer such encounters to deplete it, again the effects are barely noticeable.

Lhun said:

any object that acquires a certain vector (speed and direction) will keep that vector unless some force acts on it. A bullet will travel in a straight line until it hits something. A splinter from an explosion will. A spaceship will. Nothing ever slows down unless it is actively braking. Consequently of course, spaceships don't need to keep engines running to keep moving.

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This part is correct. In the short term, at least. In the long term, the very tiny effects of the gravity of distant objects do add up. And space isn't a true vacuum; there are stray gas molecules and other small particles everywhere. So the effect of friction also adds up.

Smiling Ted said:

The other handwaving device you can use (after entanglement) is the tachyon particle, which travels faster than light.

Tachyons aren't mathematically necessary to particle physics, but they don't actually violate Einstein's law against accelerating past the speed of light, since they come into existence already traveling faster than c. Of course, no one has ever found experimental confirmation of their existence.

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The big problem with faster-than-light communication is that it can violate causality. Of course, many science fiction stories revolve around this very occurrence (even though some characters proclaim it a headache they'd rather be without) so there's nothing wrong with that! However, if you have superluminal travel or communication, then you may run into that issue.

It just depends where one is aiming to be on the science fiction spectrum. Hard science fiction would mean no superluminal travel or communication. Personally I like wormholes and tachyons. They're a bit softer, but not by much. Wormholes just require magnificent amounts of energy and possibly exotic materials to keep them stable. No big. Tachyons are ... well they're just cool.

I recently read House of Suns, by Alistair Reynolds. I haven't read any books by him before, but I was quite impressed by the way he dealt with the light-speed limit and the notions of causality.

FennelGiraffe said:

Gravity.
When you are on the surface of a planet, you are close (duh!) to a massive body. That means the force of gravity acting on you from from that massive body is strong. When you get out in space, you are farther away from any massive bodies. The force of gravity from any one of them is much weaker. Furthermore, there are likely to be several massive bodies in different direction, so their gravity pulls in different directions, which means one may partially or completely cancel out another.

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Space is big and lagrange points are small. It's extremely unlikely you'll ever be sitting in one, unless you specifically try. (Lagrange points are the points between two stellar bodies where their respective gravitic pulls are canceled out)
"Space" usually implies that you are far enough away from any major body to not experience a major pull. Yes you are correct that you experience the gravitic pull from all objects in the universe, wherever you are. But since that diminishes with the square of the distance you don't really notice it.

As a sidenote, since we already started nitpicking: When in a solar system you are always moving on an orbit. (or an a flight vector, or towards the sun. Both the latter won't last long) It may be in orbit around a near planet, but it's pretty much always in orbit around the sun. Suns are pretty massive.
You can ignore that too though, since it has hardly any effect on a more local scale.

In space, the force of an explosion is likely to be much, much stronger than the force of gravity. Add to that the absence of a 'ground' for things to 'fall down' onto, and the effects of gravity become barely noticeable.

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Actually no. In space there is no medium to transmit the force of an explosion so the only thing that transfers kinetic energy is the gas produced from the explosive. Which isn't much, so explosions will have next to no effect on something even a short distance away. (of course this depends on the size of the explosion)
This of course doesn't matter to a starship that has something exploding inside of it.
As another note, nuclear explosions in space will produce a massive radiation burst and pretty much nothing else. There'll be an expanding cloud of plasma from the materials of the bomb, but that's not much.

Friction.
Friction happens when two objects move in contact with one another. In an atmosphere, debris from an explosion rubs against air molecules, lots and lots of air molecules. With each contact, the debris gives up a tiny bit of its energy. In space, with no atmosphere, there is a lot less stuff for the debris to encounter. But when it does encounter a stray gas molecule or other particle, friction will happen and some energy will be lost. But since the force of the explosion gives the debris a lot of energy and there are significantly fewer such encounters to deplete it, again the effects are barely noticeable.

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Unless we're talking about a geological timescale, or something with a solar sail, we can safely ignore friction in space. Even in interplanetary space close to a sun, we need not even mention interstellar space. Encountering a hydrogen atom can be the biggest event of the week.

This part is correct. In the short term, at least. In the long term, the very tiny effects of the gravity of distant objects do add up. And space isn't a true vacuum; there are stray gas molecules and other small particles everywhere. So the effect of friction also adds up.

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The effect of friction adds up but is still totally neglible. And all gravity does is alter your vector ever so slightly. It is important in interplanetary space, when moving from one orbit to another, but you can mostly ignore it even then. At least, if you're writing a story, not a physics textbook.

For FTL travel/communications I really dislike wormholes for a variety of reasons - practicality (I think someone mentioned), the things are ridiculously hard to make, unless you have a good way of moving around solar masses and making black holes out of them. Also, they're (IMHO) totally cliche'. I recall an Isaac Asimov editorial in his SF mag about a saturday morning cartoon with characters going through wormholes to travel great distances through space. One might argue "poetic license" but he argued that it was teaching kids crap science.

I like the idea of quantum entanglement used for FTL communications, even if it's NOT the "spooky action at a distance" it's been advertised as. Here's something on it I mentioned before from Charles Sheffield at the "Quantum Transportation" link:
http://www.absolutewrite.com/forums/showthread.php?t=70038
I looked up recent news stories on quantum entanglement, and they (presuming the reporters can be trusted to to say what the involved scientists mean) indicate quantum entanglement does not provide a mechanism for FTL communication, which means Sheffield is wrong:

To this time, the most common reaction to the experiments demonstrating nonlocality has been to say, "All right. You can force action at a distance using `entangled' particle pairs; but you can't make use of this to send information." The new experiment shows that this is not the case. Quantum states were transported (teleported) and information was transferred.

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While the word quantum has become a mainstream cliche', entanglement (aka "The EPR Paradox") is perhaps the least-talked-about aspect of quantum mechanics.

Have instant communications with a paraphone, "the paradox telephone - scientists still don't know how it works, but engineers build it anyway."

Lhun said:

Space is big and lagrange points are small. It's extremely unlikely you'll ever be sitting in one, unless you specifically try. (Lagrange points are the points between two stellar bodies where their respective gravitic pulls are canceled out)

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You don't have to be in a Lagrange point to get partial cancellation.

Lhun said:

Actually no. In space there is no medium to transmit the force of an explosion so the only thing that transfers kinetic energy is the gas produced from the explosive. Which isn't much, so explosions will have next to no effect on something even a short distance away. (of course this depends on the size of the explosion)
This of course doesn't matter to a starship that has something exploding inside of it.

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OK, I condensed my statement a little too much there, but we're specifically discussing debris produced by an explosion. I should have phrased it something like "the kinetic energy imparted to the debris by the initial force of the explosion".

Lhun said:

The effect of friction adds up but is still totally neglible. And all gravity does is alter your vector ever so slightly. It is important in interplanetary space, when moving from one orbit to another, but you can mostly ignore it even then. At least, if you're writing a story, not a physics textbook.

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Oh, I agree completely, in this context, the effects of both gravity and friction are negligible. However, negligible is not the same as nonexistent. While I'm willing to give you credit for over-summarizing as well, what you actually said was extremely misleading. I've spent way too much time around those who truly believe there's no gravity in space to let such a statement stand.

Lhun said:

There's two important differences in space compared to on a planet. No gravity, no friction.

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In space there is no such thing as Negligible forces. This is simply because the distances involved require INSANE amounts of precision if you want to have anyway of navigating.

Think of it like this. Look at modern propulsion systems like Ion Drives and VASIMIR systems. In the case of the former it only produces an extremely small amount of thrust, yet is perfectly useable as a propulsion system for spacecraft in terms of navigating between planets. If forces measuring in millinewtons are useable as a means of driving spacecraft around, then I think you've got to be careful about how you choose to discount certain forces. Friction probably is negligible in most cases, but a solar sail would definitely be slowed down in interstellar space because it's surface area is so big and it's thrust so small(if not non-existant) that over time impacting small particles would slow the ship down(assuming you left the sail out of course).

Explosions in space would be very dangerous, in particular nuclear explosions. Because there is no medium to absorb any of the radiation or slow neutrons down, nuclear weapons would be extremely effective at killing the crew of spaceships. This in fact would make them a preferred weapon since anything within a 100km or so of a good sized nuke would be exposed to massive amounts of gamma rays and neutrons. In fact Nukes would actually facilitate the capture of ships, and given how valuable spacecraft likely are, who'd want to destroy one when you can just kill the occupants. However the raw destructive power of a nuke would be greatly lessened by the lack of an atmosphere. (Thus nukes would excell at killing crew, but not ships)

Personally I think space combat is pretty interesting if it's looked at from a realistic perspective. I put up a pretty good topic a few months back on the subject and it was the source of a lot of discussion on the topic.

Dommo said:

In space there is no such thing as Negligible forces. This is simply because the distances involved require INSANE amounts of precision if you want to have anyway of navigating.

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Tiny changes in vector can, if no further corrections are made, result in missing an interplanetary destination by vast distances. The problem is exascerbated when distances increase to the interstellar.

However, it strikes me that this is a problem only for robotic ships (ie: those that must be controlled remotely). With onboard navigation (big computers, telescopes, intuitive living pilots, etc) it seems plausable to me that to merely arrive in the general area of a destination is adequate. You know: "Oh look. Earth is over there. Change course to ... "

But planets are big targets. A spaceship, even one ten times the size of the Titanic, is tiny by comparison. Paint it flat black, and it can virtually disappear. Current stealth technology can confuse most EM detection systems. Compound that with very high velocities, and you, the bombadeer, have a miniscule opportunity to hit your enemy with anything like a projectile. So, yeah, weapons such as nuclear bombs that broadcast devastation over a wide area, make considerably more sense.

I am forever amazed that the "ray" guns (disruptors, phasers, et al) common in SF movies are pointed at the target as a sniper would aim his rifle. Seems to me that turning on the ray and waving it around would result in a higher kill rate.

Alternatively, fighting battles in port, as opposed to between destinations, would make a lot more sense: ships are moving at very slow speeds, compared to bashing about the galaxy at 0.5C, maneuvers are equally slow, and there is a LOT of real estate to attack and defend. (The death star in the Star Wars IV movie is an example... if only they hadn't made the fighters appear to be flying around in air!).

Now, of course, given a really good system of weaponry for space battles, our highly intelligent opponents will develop equally good defensive measures. (I personally like the flat black paint idea, with appropriate high tech adjustments, of course.)