# Fiction Writer Looking for Astronomy Help

Hi There,

I am working on my second novel, a kind of Buck Rogers-type adventure, and I want to make sure that I'm being at least reasonbly accurate in regard to what an "outer space" journey might be like. It's going to be light in tone and I'm not looking to get into lots of particle talk, but I take what I do seriously, and could use an astronomy advisor to help me out with some of the details.

If anybody is interested in participating, I'd love to hear from you, and you'll get a thanks from me in the book when it gets published.

If you want to see what I'm up to now, you can check out my web site and see that I'm for real.

Thanks,
Russ

www.findersk.com [Broken]

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mgb_phys
Homework Helper
If you post your questions on here - you will get lots of help.

That's great. I'll be back on the boards when I'm ready to go--which could be soon indeed.

Thanks,
Russ

Hi there Physics Guys,

So I'm taking you up on your offer and have my first round of questions for you that I would be most greatful if you could help me with:

* In regard to space travel, what is the top speed of the Space Shuttle?

* Has there been any formulas theorized as to how fast "warp" speed might be in relation to the speed of light? Is it the same? Less? More? And if so, by how much?

* How is the energy of the sun measured? And how can that be compared to energy produced in our everyday terms? For my purposes, what percentage of the sun's total energy/heat would be the approximate equivalent to an explosion strong enough to completely incinerate a major city--say, a 50 mile radius?

Thanks!
Russ

mgb_phys
Homework Helper
There are going to be a lot of - 'it depends what you mean' in the answers - that's science!

* In regard to space travel, what is the top speed of the Space Shuttle?
In orbit the shuttle is going around mach 25. In space there is nothing to stop you so if you run the engines you just keep accelarating while you have them running and then coast along at that top speed. The only maximum speed is the speed of light.
Each orbital height has a given speed so all it needs is fuel to reach that height. The shuttle doesn't have much fuel left once it is in orbit because it doesn't need to go any faster.

* Has there been any formulas theorized as to how fast "warp" speed might be in relation to the speed of light? Is it the same? Less? More? And if so, by how much?
Warp speed is just made up to make the story work - it can be any speed you like. I'm not enough of a geek to know what Warp speed was in the original star trek.
For the purpose of the story you either decide that travel is instant (as in Dune) or runs at some large multiple of the speed of light (Star trek) the problem with instant travel is you have to solve the plot problem of why you can't always pop back to HQ for orders or have instant backup.

* How is the energy of the sun measured? And how can that be compared to energy produced in our everyday terms? For my purposes, what percentage of the sun's total energy/heat would be the approximate equivalent to an explosion strong enough to completely incinerate a major city--say, a 50 mile radius?
You can measure the suns energy quite easily by just point a light meter at it and knowing the distance.
Total power is 4×10^26 W that is 4 with 26 noughts after it Joules / per second.
The atomic bombs in WWII were about 6 x 10^13 Joules so the sun is equivalent to about a million-million of them every second!
Note that large atomic bombs can equal the sun in power ( ie rate of energy released ) but only for extremely tiny fracions of a second, the sun keeps putting out this level of power for billions of years.

Compared to other astronomical events like supernova or gamma ray bursts this is nothing - they can out out the same total energy that the sun will give off over billions of years in seconds!

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Startrek's warp speed was totally made up. It is equal to the warp factor cubed times the speed of light. So a warp 5, would be 5^3 * c or 125 times the speed of light. It is not related to our current science whatsoever.

Janus
Staff Emeritus
Gold Member
A caveat to what mgb physics said about the Shuttle, you can keep accelerating only as long as you have the fuel to do so.

For a rocket there a formula you can use to determine how much fuel you need to make a given cahnge in velocity:

$$MR = e^{\frac{\Delta v}{v_e}}$$

MR is the mass ratio, or the ratio of the fully fueled rocket to the empty rocket.
$\Delta v$ is the change in velocity
$v_e$ is the exhaust velocity of your rocket.

The shuttle's main engines have an exhaust velocity of about 4500 m/s.

DaveC426913
Gold Member
Extraneous notes:

The space shuttle is a very poor choice for a journey beyond Earth orbit. Space travel by nature, puts every function at an extreme premium - weight fuel, capacity, performance, etc. That's why spaceships are all custom-made for their specific journeys.

The shuttle is meant to be general purpose and much of its design was put into reusability and return-to-orbit. This correspondingly compromises every other performance parameter.

If you were going to climb to Mt. Everest alone, you wouldn't pack all your survival equipment in general purpose cardboard boxes.

Wallace
* Has there been any formulas theorized as to how fast "warp" speed might be in relation to the speed of light? Is it the same? Less? More? And if so, by how much?
The cool thing about the 'science' version of warping (as opposed to the sci-fi ones) is that you do not need to move faster than the speed of light, indeed you do not need to be moving at all in order to get from one place to another in less time that light would! What I'm referring to is the 'Alcubierre drive'. This is a formal (i.e. fully scientific and mathemical and stuff) solution to the equations of General Relativity that allows this kind of warp motion. You could probably look up some details about this from the web somewhere, or ask here if you want more info about this particular type of warping.

Chris Hillman
Not a solution of the EFE!

[EDIT: The following comment occured in the context of a thread on science-fiction plot devices; Wallace has stated--- if I understand him correctly--- that he would have clarified his remarks in another context. Still, this being PF, I thought it important to enter a clarification.]

What I'm referring to is the 'Alcubierre drive'. This is a formal (i.e. fully scientific and mathemical and stuff) solution to the equations of General Relativity that allows this kind of warp motion.
No! The Alcubierre spacetime is a Lorentzian manifold, and it does have a very intriguing interpretation which corresponds quite well to many of the features of the fictional Star Trek warp bubbles, but it is not a solution of the EFE in any physically meaningful sense.

I have had to say this so often that I am rather tired of repeating myself, but to repeat myself: the EFE is $G^{ab} = 8 \pi \, T^{ab}$. Einstein's intention was that the problem of solving this be understood as finding a spacetime together with say the tensor describing an "empty space" solution of (the curved spacetime version of) the Maxwell field equations, such that when you compute the stress-tensor of the EM field (using the given EM field tensor and the given metric tensor) according to (the curved spacetime version of) Maxwell's theory, and plug the result into the RHS of the EFE, this matches the Einstein tensor computed from the given metric tensor. This would then be an exact electrovacuum solution of the EFE. Similarly one can define a notion of exact perfect fluid solutions, and then one can combine (by adding contributions to the RHS of the EFE) to define more general notions of an exact solution. We can say for short that we "reason from right to left" in solving the EFE, although this is potentially misleading because in all cases we are in effect finding simultaneous solutions of the EFE (for the purely gravitational part of the model) plus other relevant field equations (or the hydrodynamical equations). Indeed, electrovacuums are also known as "free space Einstein-Maxwell solutions".

Alcubierre OTH wrote down a metric ingeniously constructed using bump functions specifically in order to satisfy the requirements of the Star Trek "warp bubble". One can then "reason from left to right" in the EFE to infer what the putative stress-energy tensor would have to be, namely $\frac{1}{8 \pi} \, G^{ab}$. Unfortunately, the result turns out not to correspond with anything we can imagine making using physically realistic stress-energy tensors $T^{ab}$. This takes quite a bit of back and forth to explain, since effective field theories derived from considerations from QFTs briefly suggested otherwise, but the consensus has been for some time that warp bubbles appear to be unrealizable for a variety of reasons.

This review paper touches most of the technical points, although when Lobo says
All these solutions [Kerr vacuum, FRW dusts, etc.] have been obtained by first considering a plausible distribution of matter, and through the Einstein field equation, the spacetime metric of the geometry is determined. However, one may solve the Einstein field equation in the reverse direction, namely, one first considers an interesting and exotic spacetime metric, then finds the matter source responsible for the respective geometry.
one shouldn't read this as implying that it is always possible to find such a source! Indeed, it we could do that, metric theories of gravitation would be utterly vacuous! In fact, in gtr--- as you will soon discover if you try to solve the EFE "reasoning from right to left"--- the vast majority of Lorentzian manifolds are ruled out as candidates for spacetime models providing the geometrical arena for some physical scenario (e.g. "a spherically collapsing dust cloud interacts with a passing gravitational wave), simply because the Einstein tensor has the wrong form to be matched to any sum of contributions from physically reasonable stress-momentum-energy tensors.

IMO it only makes sense to "reason from left to right" when one is trying to figure out whether certain geometrical properties of putative spacetime models appear to be physically realizable eventwise (level of "jet spaces" in the sense of differential geometry) according to known physics, or not terribly wildly speculative physics. As I said, in the case of warp bubbles, the mainstream answer is currently "apparently not", even eventwise (at the level of jet spaces), and there are independent "local neighborhood" and "global" level arguments.

It is unfortunate that Lobo failed to stress these crucial points in his review. I can assure you that they are very widely appreciated by researchers in gravitation physics!

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Wallace
Thank Chris, I'm well aware of the workings of General Relativity. Of course the warp metric requires an exotic stress energy tensor, but we're talking sci-fi here so we can consign the required negative energy and other issues to problems for Engineers to work out. If this wasn't a problem we'd be warping to the stars already....

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mgb_phys