I'm currently writing something that explores the phenomenon of inside jokes, in which I use an astrophysicist joke that is meant to be undecipherable to the average reader. It was recommended to me by someone who, as it turns out, has as little understanding of physics as I do. After finishing a first draft, I've heard from an astrophysicist that the joke makes no sense. The reasons are so obvious that even I should have been able to realize how badly off this is. Here is the original version:

A young astronomer came rushing into the office of the head astronomer and announced: "Sir! I have just made an astonishing discovery! However, there is good news, and bad news! I have just discovered a new galaxy, that has just appeared, and is only 14 light years away from our own!", replied the young astronomer. "My God, that's fantastic, that's wonderful, amazing, etc. What bad news could there possibly be about that?" the old astronomer queried. The young astronomer replies, "Sir, it's BLUE!"

The joke is meant to be that the galaxy is blue-shifting, i.e. coming at us to destroy everything. But (as you obviously know) the distance/scale makes no sense; there are blue galaxies visible to us, which doesn't mean anything; and a galaxy actually coming towards our own will take billions of years to collide and means nothing to our solar system. I get it.

My questions are:

  1. Could it be that the original joke-teller may have meant a new star/solar system (and would it make a difference whether it was described as a "star" or "solar system")?

  2. I've understood that stars in other galaxies have been observed (or theorized?) to move at close to the speed of light. So would it be possible to have a super-fast star 14 light years away (or closer?), and that a collision would be imminent during a human lifetime?

  3. Most importantly, I've seen a lot of jokes based on the Doppler effect and the color blue, even though blue-shifting doesn't necessarily mean turning blue. There's the famous bumper sticker, "If this sticker looks blue, you are driving too fast." None of that makes sense, either, right? Why are these jokes still understandable? I'm thinking of this as a kind of meme--like the countless jokes that are based on a man's voice going falsetto when he loses his testicles, which would not actually happen.

  4. Just for my own curiosity and understanding, could really heavy-duty blue-shifting actually result in a star looking totally blue to the naked eye, or no? Everything I read uses the words "bluer" or "blue-shifting", but I can't seem to find descriptions of how celestial objects might actually look like.

My most important question is, of course, would the adjusted version of the joke make sense. It doesn't need to be funny, it can be tremendously un-funny, but can it be told in a way that has a semblance of internal logic, and is understandable to a physicist? Thank you!

  • $\begingroup$ The original joke is ok. Providing the galaxy is approaching fast enough it can be seen as blue. And it must be fast as it was unnoticed before. The only point is why it goes physically so fast (ie against the Hubble flow)..But is a joke. How fast it goes? You could apply Doppler shift. Or perhaps, in this case, the relativistic version of it. $\endgroup$
    – Alchimista
    Mar 20, 2018 at 8:53
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    $\begingroup$ Can't you just say you're a zebra? $\endgroup$ Mar 20, 2018 at 8:53
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    $\begingroup$ @Alchimista: Oh, really? I'm getting confused. Someone told me that the scenario is impossible even to imagine, because 14 light years would be deep inside our own galaxy. Also, would the blue color mean that the galaxy is moving close to the speed of light? Is that possible? That's necessary for the joke to work, i.e. that the collision will happen within the foreseeable future. $\endgroup$
    – Gordon
    Mar 20, 2018 at 9:33
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    $\begingroup$ I'm voting to close this question as off-topic because it is about analyzing a joke and not physics. $\endgroup$
    – Kyle Kanos
    Mar 20, 2018 at 11:10
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    $\begingroup$ You seem like someone who's best ignored, but (1) if you'd read my post before commenting, you'd know I didn't write the joke; (2) I'm not asking for "details" for "improvement", but wanted to know if the premises make sense, or could be made to, which has everything to do with physics; (3) if you'd followed the conversation, you'd know I don't care if anyone finds it "funny"; (4) I've received great answers, luckily before you showed up; and (5) your comments have way less to do with physics than what I wrote. Again, feel free to shut down the convo. You could start with yourself, though. $\endgroup$
    – Gordon
    Mar 20, 2018 at 16:01

2 Answers 2

  1. The joke makes more sense with a star, since a galaxy is too far away to be dangerous even if it is coming towards us with the speed of light. (For example, the Andromeda Galaxy is 2.5 million light years away from us.) If a galaxy was close enough to reach us within a human lifetime, then it would already be causing major troubles (since it would already be destroying the Milky Way), and the whole Doppler effect part of the punchline is completely irrelevant. The galaxy would be huge, very noticable and certain death no matter which way it is moving. So a star makes more sense. It does not matter whether it is a star or a solar system, since a collision with a star without a planetary system would still be catastrophic. There's an additional problem with using a star, though: they can be blue even without the Doppler effect, if they have a high temperature (O-type stars). For galaxies, these temperature effects average out and the joke works without further details, but for stars you'd actually have to explain that the spectral lines (e.g. of hydrogen) are blue-shifted, which probably doesn't make a good punchline.

  2. I am not saying that it is impossible, but it is highly unlikely. (Of course, the whole premise of this joke is highly unlikely.)

  3. Actually, blue-shifting does turn things blue. The sticker doesn't make sense because you would have to drive really fast in order to see a red sticker as a blue one, but that hyperbole is the heart of the joke, so it works. I would actually argue that your joke is missing this kind of hyperbole. For example, it would be funnier to have a punchline like "the reason nobody else found this galaxy/star is because it's so blue-shifted that it's in the UV". You know, take it to the extreme. (Having some kind of explanation why this astronomer is the first to find that object is also a good idea, and works well enough in this example.)

  4. The Sun has a spectrum that looks white to us. It can be described quite well as a blackbody spectrum for a temperature of about 6000 K, with the highest intensity at 500 nm (green light). If you blue-shift this spectrum, the highest intensity moves down to lower wavelengths, and while there will still be a "tail" in the higher wavelength ranges, it will be of relatively low intensity, while there will be high intensities in the blue wavelength range, so the effect will be similar to that of the blue sky.

For the reasons given in point 1, I think that the joke does not work well at all. It raises too many questions and the punchline is not effective. It is also impossible to translate the vague "it's blue" punchline into an approximation of a timescale - what's important is how blue-shifted is it? Even if you say UV, it's not obvious that this means that we are doomed within a decade or so.

  • $\begingroup$ Outstanding--and enlightening! My question about solar system vs. star had more to do with the wording of the joke: "I found a new solar system" at least sounds like a bigger discovery, even if it's not. For (1), would it be possible to name a type of star that an astrophysicist will know it shouldn't be blue? This would make it clear that the color is due to the Doppler effect. For (2), is there a way to imply that the star is coming fast enough and is close enough that the collision will happen in our lifetime? Or should it just be closer--4 light years, say, instead of 14? $\endgroup$
    – Gordon
    Mar 20, 2018 at 10:45
  • $\begingroup$ Actually, yeah. There are stars called Red Giants. They are big and have a low temperature, so that they look red. And I just did a quick and dirty calculation where I assumed that the intensity of a Red Giant is highest at 650 nm (in the red). If so, then it takes about 0.7 c (70% of the speed of light) for it to be so blue-shifted that this intensity maximum is in the UV. And this would mean that, if the star is supposed to kill us in 10 years, it would have to be... 14 light years away from here. So that actually works perfectly and might make a good joke! $\endgroup$
    – PoorYorick
    Mar 20, 2018 at 11:12
  • $\begingroup$ Haha, awesome! For a variety of reasons, I'm stuck with the general outline of this joke, so this fix is great. The only slightly awkward juxtaposition would be, "I've found a RED giant... It's BLUE." What about other types of stars, whose names don't involve a color? Assuming the distance mentioned in the joke can be less, and the time it takes for it to hit could be up to several decades for the joke to work. $\endgroup$
    – Gordon
    Mar 20, 2018 at 11:29
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    $\begingroup$ Well, not really red, more like orange… Also, why was this answer downvoted? $\endgroup$
    – pela
    Mar 20, 2018 at 14:00
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    $\begingroup$ @Gordon No, I didn't mean my comment, I meant why someone downvoted Fearabbit's answer, which I think is good. But yes, the joke could go like that. I posted an answer myself now which shows the actual color. $\endgroup$
    – pela
    Mar 20, 2018 at 14:50

To elaborate a bit on the physics of Fearabbit's fine answer, M stars have temperatures in the range 2400–3700 K and so have an intrinsic orange color (and are also very dim if it's main sequence M stars, so could in principle go unnoticed).

If an M star approaches us at 90% of the speed of light (i.e. $v=0.9c$), its spectrum will be shifted toward the blue by an amount $$ \begin{array}{rcl} \Delta\lambda & = & \left( \sqrt{ \frac{1+v/c}{1-v/c} } - 1 \right) \, \lambda\\ & = & -0.77 \, \lambda. \end{array} $$

At a distance of 14 lightyears, it would take the star 15½ years to hit Earth.

The figure below shows this shift for a 3000 K star, as well as the perceived color of the star. It goes from RGB = (255,215,137) at rest, to RGB = (191,208,255) at $v=0.9c$.


I will let the reader decide upon the quality of the joke. To be a killjoy, the measured blueshift only shows the component of the velocity along the line of sight to the star. The star can — and most likely will, in general — have a velocity component perpendicular hereto, and will thus most likely miss Earth.

Furthermore, stellar velocities in the Milky Way are more like a few 100 km/s. If $v = 300\,\mathrm{km}\,\mathrm{s}^{-1} = 0.001c$, the color is virtually indistinguishable from its rest color, with RGB = (255,215,138).

  • $\begingroup$ I'm impressed by all these thoughtful answers. To be clear, for my purposes, it's irrelevant whether the joke sucks or even if it doesn't entirely make sense, so long as an astrophysicist will "get it", whereas a lay person (obviously) won't. So this fictional M star, exceedingly unlikely to actually occur in the Milky Way, which is 14 light years from us and looks blue as opposed to orange, will at least be a cause for alarm. "This might be the end of the world!" Seems coherent enough for me as a joke, and if you all can follow that, I'm quite happy. $\endgroup$
    – Gordon
    Mar 20, 2018 at 15:35
  • $\begingroup$ @Gordon Well, at least I had fun doing the calculations :) $\endgroup$
    – pela
    Mar 20, 2018 at 16:44
  • $\begingroup$ And your purpose was clear enough, my astrophysicism just compelled me to describe the realism of the answer :) $\endgroup$
    – pela
    Mar 20, 2018 at 16:46
  • $\begingroup$ I'm curious about the -1… $\endgroup$
    – pela
    Apr 4, 2018 at 12:09

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