Hatrack River Writers Workshop: Smart physics people needed (aka-HELP!)

This is topic Smart physics people needed (aka-HELP!) in forum Open Discussions About Writing at Hatrack River Writers Workshop.

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Posted by Robyn_Hood (Member # 2083) on :

For those of you better versed in physics than I, I need your help.

I'm creating a scenario for a short story I'm working on; it is as follows:

There is a habitable planet that is in orbit around twin stars. The twins rotate parallel to each other.

In an adjoining solar system, the sun goes nova and begins to collapse, eventually forming a neutron star.

The nova and eventual increase in gravity draw the twin system into rotation around the neutron star.

So as it lies when my story begins, the habitable planet is suspended between the gravity of the neutron star and the twins. The twins continue to rotate around each other presenting a path between them every few weeks (or something). The planet continues to rotate, but the increase in temperature from the neutron star is causing a planet-wide drought.

i.e.

ns-----hp-----(ts)(ts)

Orbit is around the neutron star.

I know this is Spec Fic, but is this scenario the least bit plausible?

[This message has been edited by Robyn_Hood (edited August 06, 2004).]

Posted by ambongan (Member # 2122) on :

I know enough about astronomy to get myself into trouble...

But then, most people do. So I say, use your senerio. Few people will know if it is accurate or not.

But what I'd do first is to go to your local collage and ask the physics department.

Posted by Doc Brown (Member # 1118) on :

I hate to say it, but no. This cannot happen.

One problem is that the formation of a neutron star does not create gravity from nowhere. The neutron star is made out of the same stuff as the former star, minus anything thrown off, so it won't suddenly start attracting a neighbor that it didn't previously attract.

You might create a habitable planet trapped in the Lagrange points between a normal star and a neutron star, but if you make the star binary then the Lagrange points are unstable and the planet is in peril.

The alternative is to have a binary star system in which one of the stars is a neutron, and place your habitable planet in a distanct orbit around the neutron star. Unfortunately this would give you some really crazy climate (and perhaps radiation) problems to solve.

Is your goal to have a planet near a neutron star? Or a binary star? Or what?

Posted by EricJamesStone (Member # 1681) on :

Doc Brown is right, your scenario is scientifically impossible.

Why don't you tell us what you absolutely need for the story to work (i.e., It must be a binary star system because the plot hinges on something happening at second sunrise), and then we'll try to come up with possible astronomical arrangements.

Posted by Robyn_Hood (Member # 2083) on :

Basically what I need for the story:

1 - Inhabitted planet that somehow becomes inhabitable through a loss of resources (i.e. new global drought) causing the inhabitants to have to leave.

2 - There is only one way out of their system and that is by following the "sweet-spot" between two gravity entities (i.e. my idea for the twin stars).

My original idea was to have the nova result in a black hole but when I checked into things I thought a neutron star or pulsar might be better. If I went with the black hole, that would be reason enough to leave the planet and a drought would be unnecessary. Who wants to go down a black hole anyways ?

[This message has been edited by Robyn_Hood (edited August 06, 2004).]

Posted by Survivor (Member # 213) on :

Ditto Doc and EJS. And I don't even know what you meant by "The twins rotate parallel to each other."

Lots of things could make a planet untenable. But I think that you're correct to assume that it would have to be a stellar event if simply leaving the system would be easier than fixing the problem. As to what that event should be, there are plenty of candidates.

But the second point...I don't get it. This business of there being only one way out of the system...is that really an essential part of your plot?

Posted by EricJamesStone (Member # 1681) on :

OK, I've been trying to come up with a real-physics reason why you'd want to go between the two stars of the binary in order to escape the system, rather than around them, and I haven't had any luck.

While there would be a region between the two stars where their gravitational pulls balanced each other (the "sweet spot" you mentioned), I can't see any real-physics way that this would be an advantage.

So, if traveling between the two suns to escape is essential to your story, you may want to make up some sort of hyperspace jump point theory (i.e., the gravitational stresses in that sweet spot will allow FTL travel to another system.) Not the hardest of science, of course, but it fits well enough within the accepted conventions of science fiction.

Posted by Doc Brown (Member # 1118) on :

Keep in mind that binary stars are usually pretty darned far apart. If you are imagining the setup on the horizon in the Star Wars movies, with two suns that looked like ours, you are not picturing a real binary. Typical binary stars are farther apart than our sun and Pluto, and one is usually much smaller than the other.

Not that many readers will care. The fans who like Star Wars stories will have no trouble believing your Tatooine-like binary star. They would probably buy your gravity-making neutron star, too.

Posted by babylonfreek (Member # 2097) on :

With all due respect to Star Wars fans, of which I am a big one, you can get away with lasers you can see in a vacuum, ships that bank in space, and too-close binary stars. In movies.

It is a lot harder to get away with it in writing. Trust that you readers are going to be quite intelligent and science-savvy. Stick to as much good science as you can muster in the long run. It'll make the science fans come back. And those who don't know much in science? They won't be turned off by good science. Win-win situation IMHO.

That aside: the problem with catastrophic scenarios (scenari for you latinophiles) on a stellar magnitude is that something big enough to kill a star or throw a planet off-course is likely powerful enough to wipe puny, fragile life right along with it. A star system doesn't outlive its primary, and neither do the neighbors.

But don't let facts get in the way of a good story

Posted by Robyn_Hood (Member # 2083) on :

Yes it is important that there only one way out, or at least one path to their objective.

I was inspired by something from the first or second season of Andromeda. Becca is doing multiple slip-streams to get to this one point (can't remember the objective). Anyhow at one point they only have one slip-stream point that they can go through in order to continue in the direction they want. Only thing, it's on the other side of two stars or something. In order to get to it, they have to travel through the sweet-spot to avoid being pulled into either one.

My story needs to involve people who are forced to travel to get to a new home in order to survive. They need to be forced through one small point in order to get where they are going.

I can't explain much more than that.

I'll probably just write the story and figure something out as I go along. Physics has never been a strength for me (that's why I dropped it in high school).

Thanks for the help, I'll do some more research and see where it goes.

Posted by EricJamesStone (Member # 1681) on :

I have to disagree with Doc Brown on the distance between binary stars. The majority of them may be far apart, but not all.

Some binary stars are so close they are almost touching. Here's an article about a binary system where the two stars are about 80,000 km apart -- much closer than the Earth and moon are to each other. The stars orbit their common center of gravity once every five minutes.

http://physicsweb.org/article/news/6/3/16/1

So you can realistically have two stars in a binary system that aren't as close as that.

Posted by Survivor (Member # 213) on :

I still don't understand why you need a bottleneck. But if you do, then using a wormhole or some other kind of jump point seems your best bet. Is it really necessary that this be the point between two stars? Because that's been done to death and isn't very plausible to begin with.

Having the star wear out is simple, all G-type stars eventually wear out and start to turn into red giants (our own will do so in a few billion years).

Posted by punahougirl84 (Member # 1731) on :

And maybe I'm being ignorant, but wouldn't your ship be torn apart by the gravitational forces you plan to encounter? Ok, maybe you have planned for that too... just something that came to me

Posted by Survivor (Member # 213) on :

Short answer, not by a normal star.

Posted by Doc Brown (Member # 1118) on :

Of course Eric is right, there are some binaries that are close to each other. They are pretty rare. I believe the reason is not so much that that close binary systems don't form very often, it's that they don't last very long.

Flying a spaceship between those points would be challenging, but not as challenging as flying an Apollo capsule back to Earth from the moon and hitting the atmosphere just right. It both cases the positions of the gravitating bodies are predictable, so the trajectory is predictable and a computer can calculate the thrust vectors.

It's not really plausible to create a seat-of-the-pants navigation challenge using bodies in Newtonian freefall because their motion is easilly predictable using a computer. Instead, I suggest you give your characters something unpredictable, like a living enemy. If some person or thing is shooting at them, pursuing them, intercepting them, heading them off at the pass, or whatever, then your characters will need to react and maneuver on the spot.

Posted by Robyn_Hood (Member # 2083) on :

Thanks again for all the feedback; it has given quite a bit of food for thought.

I've decided to get rid of the "planet in peril" and just have these people on a journey to a new planet. They get to a point in their journey where they encounter a binary system. If they go around they will run out of fuel and/or food before getting to their destination. This is what forces them to choose the "bottleneck".

I have the rest of the story mapped out but needed a plausible motivation to begin the story.

Thanks again.

Posted by Survivor (Member # 213) on :

I still don't understand why they would have to go between the binary stars, nor what would make this particularly difficult.

Posted by Robyn_Hood (Member # 2083) on :

In time, all will be made clear as mud.

Posted by punahougirl84 (Member # 1731) on :

I was doing some research, and ended up reading some interesting info on neutron stars and binary systems here:

http://chandra.harvard.edu/field_guide.html

I actually started out with the star map:

http://chandra.harvard.edu/photo/map/

which is what led me to some interesting articles - I printed several of the field guides.

Anyway, thought if you didn't know about it, you might find it helpful. I didn't realize the similarities between black holes and neutron stars, and this site has given me some options I hadn't thought of, and some info that is really helping me out!

Lee

Posted by rickfisher (Member # 1214) on :

The thing is, they can't "run into" a binary system. They'll know long before they get there that the system is there, and a tiny, tiny deviation in direction will be enough to put them well out of the way of either star by the time they get there. If somehow you devise a reason where they have to go through the "bottleneck" (or even simply choose to), the problem would not be gravitational. If the two stars were close enough for the gravity to be any sort of problem at all, the radiation from the stars would be much more of a problem. But also, the closer they are, the easier it is to go around them. Of course, even this represents a very two-dimensional way of thinking. If the two stars are next to each other, just go over top of one. If they're above and below, just go to the side.

In short, I don't see how you can possibly make this work for your purposes. If you manage it, you have my sincerest admiration.

Posted by EricJamesStone (Member # 1681) on :

Rickfisher makes some excellent points, and a rather convincing case that from a conventional physics model, there just isn't any realistic need to go between the two stars in order to reach a point beyond them.

So, you have two choices:

1. Switch to an unconventional physics model. There's a hyperspace jump point (or whatever you want to call it) between the two stars.

2. Change the mission. The reason they have to go between the two stars is because there's a damaged ship or scientific research station there, or there's an alien plasma-based lifeform there, etc.

The problem of radiation can be dealt with easily enough just by claiming you have adequate shielding. (Of course, it adds to the risk if the shielding is only barely adequate, and won't be enough for prolongued exposure.)

Posted by Robyn_Hood (Member # 2083) on :

These are all great suggestions. I already planned to give them sufficient shielding to pass between the stars and have come up with a scenario to force them into the situation I want. I think I may have even found an appropriate twin star system to fulfill my needs (thanks punahougirl84 for the chandra links, they were extremely helpful, I'll have to bookmark them for future reference

Posted by Jules (Member # 1658) on :

As a potential answer for the 'why would you want to?', I suspect that routes between the stars would give you a larger gravity-assist acceleration than equivalent routes around them (although I haven't tried working this out accurately, it just intuitively seems likely). Of course, this is really only useful if you don't have any of the traditional SF FTL drives (I can see that there may be some FTL drives where a gravity assist would be helpful).

Posted by rickfisher (Member # 1214) on :

quote:
I suspect that routes between the stars would give you a larger gravity-assist acceleration than equivalent routes around them....

They wouldn't. Generally, if you go by a massive object, you speed up as you approach and slow down as you go past. The effects cancel out (except that you'll change direction). (Going between a pair of stars might allow you to speed up and then slow down without changing direction.) If the massive object is moving (with a component of its velocity perpendicular to your direction of motion), it is possible to transfer some of its motion to you by way of gravitational fields, which is how we got some of our space probes to the outer solar system and beyond. On the other hand, if it's the wrong kind of motion, you can transfer some of your velocity to it. Going between the stars would mean that the perpendicular component of the gravitational attraction would be reduced, so that the gravitational assist would be less than going around the system. (My daughter corrected my physics errors in this post.)

[This message has been edited by rickfisher (edited August 11, 2004).]

Posted by Survivor (Member # 213) on :

There is also the fact that at a higher velocity, a reaction mass based drive becomes more efficient.

So if you take a tight hyperbolic of a massive gravity well, fuel used to accellerate/decelerate when you are very close to the planet will have a substantially greater effect on your total energy.

I wasn't aware that we have deployed any spacecraft physically large enough to collect the rotational energy of any of our planets from their gravitational fields.

In any case, this would be an argument against using the route between the stars, because the gravity well will be very shallow at that point, besides which you could only get a good boost in the plane between the stars rather than being able to apply a thrust advantage in whatever direction you wanted.

Posted by Robyn_Hood (Member # 2083) on :

Would the mass of the object travelling between the stars make a difference to how the gravity would affect it?

I have a few different space crafts that need to take this route, each one is slightly different in size.

Theoretically, would a larger ship travel the gravity bridge faster than a smaller ship? I know this is space, but because we're dealing with gravity, are aerodynamics an issue?

My gut reaction says, "yes", but as I've stated before, my physics skills are minimal.

Posted by rickfisher (Member # 1214) on :

The mass would not make any difference to the speed (just like a 1 pound ball and a 10 pound ball fall at the same rate). And aerodynamics would not matter, either, unless you're going very fast through a very dense (relatively speaking) gas cloud or something.

quote:
I wasn't aware that we have deployed any spacecraft physically large enough to collect the rotational energy of any of our planets from their gravitational fields.

I didn't say rotational energy, I said motion. I meant orbital motion. Sorry if that wasn't clear.

Posted by Survivor (Member # 213) on :

Hmmm...now I'm just confused.

Posted by rickfisher (Member # 1214) on :

I'm trying to understand your confusion. The only thing I could come up with is, you said:

quote:
to collect the rotational energy . . . from their gravitational fields. [emphasis added]

Whereas, I said "by way of" their gravitational fields. In other words, the gravitational fields were the means of transmission, not the source of energy. If that isn't what confused you, you'll have to be more specific.

Posted by EricJamesStone (Member # 1681) on :

The best explanation I've seen of how the gravitational slingshot effect works is here: http://denbeste.nu/cd_log_entries/2003/09/Orbitalslingshot.shtml

Posted by Survivor (Member # 213) on :

Oh, right.

Sorry, I got stuck in a false dilemma there because of the use of the term "moving".

Posted by Alias (Member # 1645) on :

Robyn,

What I'd like to know is how you expose all of those details through your narrator to your audience.

Posted by Robyn_Hood (Member # 2083) on :

This is becoming more complex than I originally intended. All of this is just so I can create a half-way plausible story. It isn't going to be a long story (3K max.) so I don't need a lot of hard science for it, but I do want it to be believable.

Posted by punahougirl84 (Member # 1731) on :

Robyn - I hate to say it, but that is exactly what happens - very typical! Your trying to figure out just enough science to make an idea work, and then the science keeps letting you know all the issues with your idea! So you try to rework your idea, but, but, but!

Meanwhile your story gets longer, and longer, but you are devoted to the thought of not going on and on with exposition because you are not supposed to and will bore anyone who is not just looking at the science - you are trying to write a story about people who just happen to be in this interesting situation... only they CAN'T be for whatever reasons...

Actually, the best thing you could end up with is all these really smart people figuring out the issues for you, so you end up with some plausible science at the end of the day!

My stories start simple, and wind up complex... that is how many really great stories get written though, so I'm getting over it. I've been trying to keep my stories under 7,500 words based on mag requirements and what they will take from newbies. My current story is failing - already past 9k, with more to go.

I might have something to e-mail that will help - if I find it I will send it.

Posted by Robyn_Hood (Member # 2083) on :

Thanks. I'm enjoying some of the science discussion here just for the sake of science. I think I know how I'm going to do this, I've just been too busy to get started this week. Oh well, I can't procrastinate forever (well, technically I could but then I wouldn't make the deadline

)

Posted by Robyn_Hood (Member # 2083) on :

As promised, the story is finished and ready to read.

Check out the first 13 under the First Ever Hatrack Re-Write Challenge.

Let me know and I'll send a copy your way

p.s. Thanks for all the help.