There’s a little something that’s been bothering me since the first time I saw it. In the title sequence for Star Trek: Voyager, there is an impressive flyby through a nebula. As Voyager passes left to right, it leaves a dramatic trail of vortices in it’s wake. It looks very good, but would it happen?
I know what you’re thinking: It’s a computer generated animation of a fictional space ship flying through a non-existent nebula. Any normal person would realise this and move on to something important.
True.
Where were we? Oh, yes. So, what are the relevant processes involved as the Voyager passes through a nebula? I have no idea. I have no background in any of the pertinent fields of mathematics. I do have an Internet connection, however, so that automatically makes me an expert.
Rayleigh-Taylor Instability
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The Wikipedia fluid dynamics article has this picture of something called a Rayleigh-Taylor Instability. That looks pretty darn similar to the process in the video, doesn’t it. Does this mean I’ve figured it out already (assuming I want to read about Rayleigh-Taylor instabilities)? No, I don’t think so.
I don’t think fluid dynamics plays a big role here. This “RTI” thing is caused by something moving from one fluid to another of markedly different density. So my question now becomes, “is a nebula a fluid”? Well, strictly, yes. All gasses are fluids. Still, intuition tells me that it’s not going to react like one would expect when the density is this low. Anyway, the vacuum of space definitely does not qualify as another fluid of differing density.
At what point do discrete particles in a vacuum (well, near vacuum, then) stop acting independently and start behaving like a fluid? Another way to ask this would be, how many atoms of something do I have to add to a perfect vacuum of a given volume so that it stops being a ‘near vacuum’ and starts being ‘a low-density fluid’? I think we need to cross that line before we’re going to start seeing anything resembling those vortices.
Personally, I think what we would be more likely to see is something like a nearly conical wake behind Voyager. Nebulous particles hitting the ship would be given energy and move away in straight lines. Each particle, it seems to me, would behave more like a lone billiard ball bouncing off a cushion. You might see some strange interactions at the front of the ship as those particles “pile up”, I suppose.
So there you have it. One unanswered question that, through careful research, is now a whole bunch of unanswered questions. The type of unanswered questions that seem to bother only me. Welcome to my world.
Update (January 14, 2010): I posed this same question on Wikipedia’s Reference Desk. It generated more interest than I was expecting, and some well thought-out responses. I’ve pasted them below. The signatures you see are those of the Wikipedia users.
Wikipedia Reference Desk Replies
Well, there would be gas as well as dust – and gasses are “fluids”. However, through most of a typical nebula, the density of all of that stuff is spectacularly sparse. A nebula is hardly distinguishable from a hard vacuum. We really only see them because we’re looking through so many lightyears of material. At those kinds of densities, the spaceship maybe pushes a few hundred atoms out of the way each second…technically, I suppose that’s a swirl…but I doubt that many people would really classify them that way.
Having said that – there are places in nebulae where new stars are forming. In those places, gravity is bunching the material together – and the density goes up spectacularly. By the time the gas gets dense enough to ignite into a star, it has a density higher than iron. So somewhere between the center of a newly forming star and the near-vacuum of the rest of the nebula – there would have to be relatively small regions with a density similar to normal air pressure here on earth – or close to the density of liquid water. In those regions, our hypothetical spaceship would certainly leave a nice turbulent wake…but that would be a tiny, TINY percentage of the overall volume of the nebula – and they would be short-lived because once that amount of material gets together at that density, the gravity that’s compressing it would be around 1g so it’ll be falling inwards very fast indeed and very soon the star will ignite and our spaceship had better not be anywhere nearby!
SteveBaker (talk) 02:44, 14 January 2010 (UTC)
It must have taken Cpt Janeway forever to find a nebula with exactly the parameters Steve just described.
I suppose it might be meant as flying upwards out of a gas giant. (The next couple shots show Voyager flying near one of similar color) But I don’t imagine that it would be easy to find one made of such a brightly colored atmosphere that you could just fly through for a photo op.
What makes the shot really unrealistic is the impressive way Voyager “punches” out of the nebula/planet. For some reason there’s a reasonably well defined demarcation between “visible blue fluid” and “nothing”. (I think in real life you’d need fluid in the “clear” sections anyway, to get those sorts of fluid movements.) APL (talk) 03:09, 14 January 2010 (UTC)
The relevant factors are the density of the nebula and the speed of the spacecraft. If I’m interpreting this correctly, the centre of the Orion nebula (as good a nebula as any) has a density of about 3000 particles per cubic centimetre. That compares with about 3×1019 in air. You can get decent vortices at about 10 m/s in air, I’d guess, so it stands to reason that one would need to travel at about 1017 m/s to get them in a nebula. That is about 300 million times the speed of light (and the ship clearly isn’t at warp)… As Mythbusters would say: BUSTED! –Tango (talk) 03:31, 14 January 2010 (UTC)
I don’t think that’s right. You need to know effective viscosity, which does not scale between gas and plasma. –Dr Dima (talk) 03:37, 14 January 2010 (UTC)
Sure, there is a contribution from different viscosities, as there is from all kinds of other factors, but are they likely to account for more than 8 orders of magnitude? –Tango (talk) 03:43, 14 January 2010 (UTC)
Effective viscosity may vary by far more than 8 orders of magnitude under different conditions. More importantly, you just can not compare a hand moving in air with a starship moving in dusty plasma by simply scaling density and velocity. You need to at least evaluate the Reynolds numbers in both cases (that is, compare densities, velocities, sizes, and viscosities). And that only applies for unmagnetized plasma. However, plasma is likely to be magnetized, so things are far more complicated than that. –Dr Dima (talk) 04:01, 14 January 2010 (UTC)
Your question is very complicated indeed. That is because we don’t really know what exactly the conditions are in any particular nebula. Very low density does not imply lack of interaction; indeed, most particles in the nebula are probably charged (whether those are individual electrons, ions, atom/ion clusters, or dust grains) and therefore interact via long-range electromagnetic interactions, limited only by Debye screening radius which for a hot low-density plasma may be very large. And then there is also a magnetic field, which probably is carried along with the plasma flow. It is hard to predict how a macroscopic object would perturb such a medium. –Dr Dima (talk) 03:28, 14 January 2010 (UTC)
On a related subject, it has been hypothesized that Earth produces vortices as it passes through solar wind (there may occur a Kelvin-Helmholtz instability at the boundary between the Earth magnetosphere and the solar wind), but I do not know if this was experimentally confirmed. Starship may or may not possess a magnetosphere — I forgot to ask Scotty how the shields work 
— but if it does, it may produce a wake by this mechanism. –Dr Dima (talk) 03:45, 14 January 2010 (UTC)
You might be interested in The Physics of Star Trek. Excellent question, by the way.–Shantavira|feed me 08:25, 14 January 2010 (UTC)
Bow shocks are observed from stars moving through nebulae, but of course that’s a much larger scale than Voyager, and not quite the same as vortices. — Coneslayer (talk) 12:02, 14 January 2010 (UTC)
I don’t think the vortexes themselves are the crux of the unbelievability. Federation star ships seem to require a plethora of fields and beams unknown to modern (ie:real) science. So if you could find a place where Voyager might conceivably fly through such a wonderfully visible cloud of blue stuff, you could claim that any irregularities in the way it moved were caused by the (for example) navigational deflector or (for example) the bussard ramscoops. APL (talk) 18:00, 14 January 2010 (UTC)
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The video clip has some serious problems:
1) As I pointed out in my Wikipedia Reference Desk answer, there are places in a nebula where you might maybe make vortices. However, it’s not possible for there to be such an abrupt change between dense-looking gas and clear space – the pressure inside the dense stuff would pretty much instantly push it out into the hard vacuum of the clear stuff. I don’t buy that bit of the video.
2) The worst part of all is where Voyager is flying low over the planetary rings. We can see how close it is to the rings by the size of it’s reflection (which is also highly dubious) – yet the curvature of the rings shows the planet to be only about 100 times as big as the starship! Such a planet certainly wouldn’t be neatly spherical with a ring system. If you were flying only about one starship-length above the rings of (say) Saturn – then they would seem to be an enormous flat surface stretching off to the limits of your vision in all directions…even if you could see the edge of a ring – it would appear to be a straight line because the radius of a ring is spectacularly huge – many times the radius of Saturn – which in turn is many times the radius of the earth. There is simply no way to finagle a good explanation for that shot. Even in a “star trek universe” – that’s quite simply gravitationally impossible.
3) The reflection of Voyager in the ice particles of the ring is also impossible – the individual chunks of ice are rotated in random directions – even if one piece might reflect light from (say) the nose of Voyager towards the camera – the piece next to it might be reflecting the tail or one of the engines or something. Since the ice particles range down to the size of a grain of sand – the average effect of all of those random reflections would not be to form an image – not even a very fuzzy one such as shown in the video.
So this is a very pretty piece of footage – but it’s not remotely physically correct. The density of a nebula is the least of the problems!
My favourite goof is actually in the Star Trek: The Next Generation opening sequence. The Enterprise flies by what appears to be Saturn. The camera pans.
The stars seen through the rings move with the pan, but those outside do not. Not to mention you wouldn’t see stars at all with a camera set to an exposure suitable for such a close, and brightly reflective, planet as shown.
Another way to think of this: maybe the disturbance is caused by the navigational shields (or the full-on shields). Still doesn’t explain the vortices, but it does give another mode for the disruption of the nebula.
And in support of your theory of it not looking like RTI, I have some videos showing water droplets hitting a surface in very low pressures and they dont’ splash, so that supports that idea that the nebula in a vacuum is not the same as two different density fluids. New Scientist discusses it here: http://www.newscientist.com/article/mg18624935.200 I haven’t been able to find the videos online.
True. I guess it depends on if you took this as a question about how it would react in the Star Trek universe. That’s a valid approach for geek-chats such as this, without a doubt. My intention was the “any object moving through a real nebula”, though.
I’m actually shocked about the absence of a splash. I guess a splash is just the interface between those two dissimilar RTI fluids, then. Neat.