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In the movie The Matrix, during "bullet time" sequences, bullets are shown trailing evenly-spaced refracting blobs:

which presumably represent shockwaves or vapour trails.

In reality, if it were possible to (almost) freeze time and move around the scene, what would a real bullet trail look like, if at all visible? Is it significantly different for different types of bullets (ignoring tracer bullets) or atmospheric conditions?

These video clips claim to show trails, but they are not at all obvious:

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2 Answers

up vote 8 down vote accepted

In reality, it is possible to "(almost) freeze time" and examine such phenomena, through the use of high speed photography. The fact that no clear photographs of "bullet trails" are readily found is a good indicator that such phenomena are not readily produced or observed. That said, there are two effects I can think of that could in principle lead to visible phenomena in your case of interest. They are both mentioned below, but the short answer is that neither will typically be "visible" in any reasonable sense of the word for a projectile the size of a bullet.

In the right regime ($M \approx 1$), the Prandtl-Glauert singularity can lead to condensation, producing a visible cone of condensed water vapor [gallery],[discussion]. The characteristic scale of this cloud is the same as that of the object. In this case, that means we are talking about a "cloud" of water vapor on the scale of a few $cm^3$. It is highly doubtful this effect would be readily visible for a projectile the size of a typical bullet, but nevertheless it could be present, given the appropriate conditions.

Supersonic jet condensation cloud, Credit: Ensign John Gay, USS Constellation, US Navy photo credit: Ensign John Gay, USS Constellation, US Navy

The second phenomena that could potentially be visually observed is distortion/refraction due to the changing density of air. I believe this is the effect present in the video clips you linked. This will visually act like a mirage (arising from the same physical cause, spatial variation of fluid density and therefore index of refraction), and it's visibility will depend significantly on illumination conditions and observational geometry. It doesn't "look like" anything, but rather distorts the path of light rays passing through it. It can be made much more readily apparent in controlled conditions as in this shadowgraph:

Shadowgraph of bullet, credit:Andrew Davidhazy

photo credit: Andrew Davidhazy

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Thanks, tmac. Based on your images and assuming supersonic bullets, am I correct to conclude that the main effects are (1) a small cloud just behind the bullet and (2) a blunt refracting cone shell just ahead of it? From equation BoatSpeed = WaveSpeed x tan(PI/2-Angle) at physicsforums.com/showthread.php?t=328445, is it right that the cone's half-angle is PI/2 - atan(Mach number) i.e. 39 degrees for 1380 ft/s bullets from Desert Eagles that Agents use? It's unclear from the shadowgraph how far the cone extends or how visible it is to the naked eye. Any idea? What about subsonic bullets? –  Gnubie Apr 12 '12 at 11:31
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Ugh, sorry. My answer was unclear, I'll try to improve it. Short version: 1) highly doubtful. First of all, the cone is about the scale of the plane, so in this situation (if it were even present) I'd expect it would be a cloud of vapor about the size of the bullet (not very visible). As for 2), it is about as visible as a temperature difference in air (not very). The reason I mentioned it is that under the right circumstances (mirage example) it can produce visible phenomena. "What does a mirage look like?" is not evident, what you can say is that it distorts the horizon seen through it. –  tmac Apr 12 '12 at 19:00
    
@Gnubie updated answer, hopefully that is better. –  tmac Apr 12 '12 at 21:41
    
Thanks for clarifying, tmac. That's what I expected. I wonder if you could comment on dmckee's answer below. In his images, I presume air rifle bullets are subsonic. Should any distortion be visible for bullets travelling below the speed of sound? –  Gnubie Apr 14 '12 at 22:25
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Surely the simplest thing to do is google for some images...

In particular several of the images on the bottom half of www.digitalpicturezone.com/digital-pictures/30-colorful-examples-of-high-speed-bullet-photography/ show a non-trivial stretch of space before the bullet's point of impact. You'll notice a marked lack of The Matrix-like effects.

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Could you please explain "a non-trivial stretch of space before the bullet's point of impact?" Perhaps my question was unclear: I wish to know what a bullet trail looks like as it is flying through air, not after it has hit a solid or liquid object. In your examples, most of the particles are debris from the target. –  Gnubie Apr 12 '12 at 11:07
    
@Gnubie A few of those picture show enough air to answer your question under the kinds of conditions that you find in a high speed photo lab (fairly dry, fairly warm). –  dmckee Apr 12 '12 at 13:16
    
Thanks, dmckee. It's interesting that your images (presumably) show subsonic bullets whereas tmac's are supersonic. I wonder what differences that makes. –  Gnubie Apr 14 '12 at 22:28
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