Dot Physics

The speed of exploding Galacticus

This is really random, but quick. I was channel surfing and saw the end of the movie: Fantastic 4 – Rise of the Silver Surfer. SPOILER: At the end someone really big explodes in space. There, that wasn’t too much of a spoiler, was it? (unless you read the title)

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I know I am a loser. I initially thought: wow, that seemed pretty fast for that scale. I wonder if that exploding stuff was going faster than the speed of light. OH! I know, I will use Tracker to measure it. It is video that is set for analysis since it has the Earth in there – and I am pretty sure I know how big that is. Here is the data from Tracker:

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Some remarks about this data:

  • I marked the right most edge of the explosion as it headed towards the Earth.
  • Unfortunately, this only gave me 2 real data points. That third point on the right I clicked on the edge of the frame even though the leading edge of the explosion had already moved out of the frame. But you know, I felt bad since otherwise there would only be two data points. So, I think that third point should really be a bit larger position (which would make a faster speed).
  • Since I am lazy and I want to be quick, I just took my Flip video camera and videoed the tv. I guess I could have gotten more data points if I looked a different source for the movie. Oh well.
  • I am not sure what vantage point this explosion was videoed from, but I am assuming the parallax and perspective errors are minimal.

Then how fast was the exploding stuff? Well, if the Earth is correctly scaled and I use all three data points then the explosion is moving at:

i-097e1ad560238f63873df4d36b8ecec9-2010-01-31_la_te_xi_t_1.jpg

If I just use the first two data points, I get a speed of 3.86 x 108 m/s. And? Well, the speed of light is around 3 x 108 m/s. And? Well, stuff can’t really go faster than the speed of light (relative to another observer). I am not sure I really want to go into all of this relativity stuff right now. Instead, I will assume that this gas is going just under the speed of light. If that is the case, then it should look different. Why? Doppler Effect. I can’t believe that I haven’t talked about this before (but I just searched my old posts and came up with nothing). Basically, the Doppler Effect says that light from stuff moving away from an observer appears shifted in color towards lower frequency (Red-Shift) and stuff moving towards us would be Blue-shifted.

This means that the exploding stuff should look more blue (or really, it would be such a short wavelength that you wouldn’t be able to see it) and the stuff moving away should probably in the radio wave spectrum.

Pre-emptive comment

“Hey dude. Don’t you know that is light that is moving away from the explosion?”

Reply: It’s not light that is moving out. If it were, then you would only see that light if it were moving right at you and then you would only see it when it gets to you.

Bonus comment I guess Star Wars made these circular explosions fashionable when they re-did the exploding Death Star in the second version of episode IV.

Comments

  1. #1 Eric
    January 31, 2010

    Hate to be picky, I am sure you get tired of that (see flying R2 post), but the villian’s name is Galactus. Otherwise, good stuff as usual.

  2. #2 Eff
    February 1, 2010

    Well, assuming the light is very intense, perhaps the explosion that you see is the reflection of light off the very thin dust of the interplanetary medium? But then it would have to be followed by a blinding flash several orders of magnitude brighter. Which would be bad for your screen.

  3. #3 guerra
    February 1, 2010

    How do you kill a thing that survived the Big Bang? Tell me that? BTW that movie sucked so much and made me die a little inside.

  4. #4 Rhett Allain
    February 1, 2010

    @Eric,

    Wow about the name. Can’t believe I missed that – I even used to read a lot of comic books. Oh well, I will just say I was trying not to spoil the movie with the name.

  5. #5 Eric Lund
    February 1, 2010

    Oh, and where’s the shock wave that an explosion like that should generate? How could an explosion that big happen so close to Earth and not have an effect?

    The explosion front won’t be perfectly spherical because of the Earth’s magnetic field. It looks like they got that much right, but the orientation is wrong: it should be spreading faster along the magnetic field than across it, because some of the energy of the explosion that moves across the field goes into displacing field lines. I don’t see a terminator in that shot of the Earth, so I would assume that the perspective is with the Sun behind the camera (of course, the light from that explosion could be suppressing the terminator). To get the observed shape of the explosion you would have to be looking down at one of the poles and the exploding object would have to be well off the magnetic equator (if it were near the equator both field-aligned fronts would be displaced toward the Earth).

  6. #6 guerra
    February 1, 2010

    I ask again. How can you kill something that survived the BIG BANG? Not to mention a Cosmic force? By the way, don’t worry about watching the movie it is horrible.

    for you non comic geeks here is the link to Galactus’ bio.
    http://en.wikipedia.org/wiki/Galactus

    Man I am such a geek!

  7. #7 dashpool
    February 2, 2010

    Apparent superluminal motion would also be possible from a near-lightspeed expanding disc/sphere:

    http://www.obscure.org/physics-faq/Relativity/SpeedOfLight/Superluminal/superluminal.html

    so even it the apparent speed is > c, that isn’t necessarily unphysical.

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