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Han Shot First (but in what reference frame?)

greedo_shoots_first.jpgI know I’m joining the debate on this one about 10 years too late, but the Han Shot First controversy has been a memeic stain on the Star Wars franchise since the 1997 Special Edition of Star Wars: Episode IV. Greedo, a bounty hunter working for Jabba the Hut, finds the fugitive Han Solo in the Mos Eisley Cantina. The androgynous green-one has every intention to bring Solo back to his boss in a body bag. Greedo holds Solo captive in a Cantina booth at blaster-point while he gloats over his lucky find. In the original release of Star Wars in 1977, after sufficiently distracting him, Solo carefully unlatches his own blaster and blows Greedo away through the table. Quite a glorious scene. I don’t think anyone faulted the character for doing what he did. No one interpreted it as cold blooded murder. Greedo was going to kill Han and the two obviously were not friends. A clear case of gritty, streewise self-defense.

When the movie was re-released in 1997, Lucas made a number of changes to the film “because he could.” He is the creator, after all. Most of them were benign, but a few were really irritating. However, the scene above in the Cantina was altered grievously. Greedo manages to shoot first and tag the back wall behind Han’s head (with no visible reaction by Han) even though Greedo is pointing the pistol at point blank range and is somehow anticipating Han’s furtive motions under the table. How can a bounty hunter stay alive if he frickin’ can’t hit a sitting target at point blank range? Han then gets his “retaliation shot” off (now filmed somehow OVER the table). Apparently, Lucas added the extra bits because he didn’t want to send the wrong message to kids about heros arbitrarily and secretly shooting their enemies (even if in a case of clear self-defense). I’m not sure what this says about Lucas’s opinion about my generation’s psychology, since we were apparently tainted by this gritty bit of reality. Perhaps the writing of this post is simply reinforcing Lucas’s point; an ironic testement to the damage done to my psyche by that original scene when I saw it as an unjaded 9-year-old.


In my mind, the real issue here isn’t whether Han Shot First, but rather Why Greedo Shot At All. Lucas’s crime was to include Greedo shooting and missing at point blank. It was a feeble attempt at some kind of heroic symmetry amongst theives. But for me, once Greedo is shooting too, I don’t really care who shot first.

Ok, but that’s not what this post is really about. That’s all background. For me, as a physicist, given that Greedo and Han both shot at all (we’ll just have to live with those versions of the movie forever), the real question physically is: “in what frame of reference did Han shoot first?” This is an issue easily resolved with Einstein’s special relativity.

What is a reference frame? Any physical system moving at constant velocity observing the sequence of events. For example, someone walking past the table watching the shootout is a reference frame. If they are accelerating, special relativity can certainly handle it, but things get complicated, so we’ll avoid that topic.

Strangely, according to special relativity, two events that are simultaneous in one frame can potentially be non-simultaneous in another. Also, non-causal (“spacelike”) events remain non-causal and causal (“timelike”) events remain causal. All observers must always agree that the catalog of events which occurred indeed occurred, but they won’t necessarily agree on the ordering.

To set up the calculation, let’s outline our assumptions. Everyone in all frames agrees the following events occured:
1) Han shot
2) Greedo shot
3) Han lives
4) Greedo dies

For fun, let’s say that:
1) Han and Greedo are about 1 meter from each other and Han is on the left in the Cantina reference frame;
2) In the frame of the Cantina (the same frame as Han and Greedo) in the 1997 Special Edition, Greedo shoots at time dt before Han (Lucas shortened this dt difference in in every release since 1997);
2a) Note, if Han is reacting to Greedo’s shot (that is, the events are “timelike” or “causal”) then Han is necessarily in Greedo’s future light cone. Therefore, Greedo shoots first in all frames of reference. Tough luck fanboys;
2b) But, if Han is NOT reacting to Greedo but spontaneously shooting by his own free will, Han can be outside of Greedo’s lightcone, so the events can be “spacelike” or “non-causal.” In this case, Han can still shoot first in some reference frames;
3) All action takes place in one spatial dimension (this just makes the calculation easier, but we could do it in 3D with minimal change in the result);
4) For simplicity, we will ignore time lags because of information propagation. It would be realistic to do so, but the calculation from special relativity won’t change because of it.

With these assumptions, when the events are spacelike (assumption 2b above), we can use the Lorentz transformations in combination with the invariant space-time interval to calcuate the frame velocities necessary to have Han shoot first.

I won’t bore you with the actual calculation (if enough people email me, I’ll post it) but, as a numerical example, if in the frame of the Cantina, Han fires 1 nanosecond after Greedo (i.e. dt above is 1 ns), he is outside Greedo’s future light cone so the shooting events are spacelike. This means that no information signal could have reached Han in 1 nanosecond from the moment Greedo shot. Han is shooting by his own free will. In this case, an observer moving at 0.3 times the speed of light to the left (Solo is on the left, Greedo on the right) would observe Solo and Greedo firing at the same time. If they moved any faster (up to the speed of light) to the left, they would claim Han Fired First. And they would be correct in their reference frame! This isn’t some optical illusion but rather “just the way it is” in that frame. Who is correct? Everyone. All observations in frames moving with constant velocity are equally valid. The idea of simultaneity is not an absolute concept.

I think Dr. John is right. Perhaps I do have too much time on my hands (but in what reference frame?).

The real message of this post? If someone claims event X occured before event Y, always ask: are the events timelike (able to effect each other by information moving at the speed of light or slower) or spacelike (unable to affect each other even with a light signal)? If they are spacelike, then the ordering can always be reversed by a change of reference frame and the question of “what happened first” becomes a moot, frame-dependent issue.

Apparently, today is Blog Action Day and everyone is supposed to post something on The Environment. I can say with all honesty that the issue of what frame of reference did Han Shoot First has no environmental impact whatsoever and contributes nothing to global warming.

Below: Han shoots first (original)

Below: Greedo shoots first (more recent version)

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