Webcomics! (Loads of images)

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Re: Webcomics! (Loads of images)

Postby Argali » Thu Apr 29, 2010 9:07 pm

So, what was all the hoo-hah about nothing being able to travel faster than light earlier in this thread.
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Re: Webcomics! (Loads of images)

Postby Arcand » Thu Apr 29, 2010 9:48 pm

Argali wrote:So, what was all the hoo-hah about nothing being able to travel faster than light earlier in this thread.


Depends on your definition of 'thing'. :)
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Re: Webcomics! (Loads of images)

Postby hoho » Fri Apr 30, 2010 5:45 am

Argali wrote:So, what was all the hoo-hah about nothing being able to travel faster than light earlier in this thread.
Radio waves are nothing more than different wave length of light so technically what they said is that light travels faster than light. Feel free to replace light with radio waves if you like :)
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Re: Webcomics! (Loads of images)

Postby theckhd » Fri Apr 30, 2010 6:06 am

Argali wrote:So, what was all the hoo-hah about nothing being able to travel faster than light earlier in this thread.

Because nearly every time a reporter gets their hands on a story like this, they try and play the "faster than light" angle, and do it poorly. There's no law that says "nothing travels faster than light," that's just the way most people word it (including me) when we're being careless. You can make a pulse of light travel faster than c very easily. In fact, you can even make the pulse leave a material before it enters. At the risk of revealing too much about my identity, that was the topic of one of my first publications.

The restriction of causality is that information can't travel faster than light. The pulse's peak may seem to move faster than light, but the peak contains no information whatsoever. In every case where the peak travels anomalously fast or slow, the information existed far in advance (i.e. in the leading edge of the pulse), and there's no causality violation.

I can think of two really simple analogies to illustrate this:

1) Imagine that I start playing a note on some sort of instrument. I start out very quietly (below your hearing threshold), and slowly increase the volume. You stand some distance away, and write down the time when you hear the first hint of sound.

Now we repeat the experiment, but you have a sensitive microphone and amplifier rather than your ear. Because of the microphone, you hear the sound earlier than you did in the first experiment. Did the sound travel any faster from my instrument to your ear? Of course not. The sound was already there, it was just below the limit of what you could detect. By adding some mechanism to adjust the volume, you were just able to amplify and identify it earlier.

In other words, the information (sound) was there already, long before the features we recognized and called the pulse.

2) Now imagine you have a laser pointer. You stand some distance D away from a wall, and aim the laser pointer 45 degrees to the right of the perpendicular between you and the wall. In ASCII:
Code: Select all
   WALL
---------------------
      |   /
      |  /
      | /
      |/
      o
     you

Then, you turn 90 degrees to the left in an amount of time T (pretend you adjust your speed sinusoidally so that the spot moves at a constant speed on the wall just for simplicity). The light spot moves from -D to D on the wall, or a total distance of 2D. That means the spot moved at speed 2D/T.

Now back away from the wall some, and repeat. D has increased, T has not. So the light moved faster, right? Well, no, the spot moved faster. But the transverse motion of the spot has nothing to do with the speed of light. The light traveled from your laser pointer to the spot on the wall at the speed of light; the light itself never moved "left to right," it always traveled radially away from you.

You can keep backing away from the wall until 2D/T > c, and you still haven't violated causality. The spot moves faster than the speed of light, but the spot carries no information, so that's ok. The "information" in this case is the photons of light emitted from the laser pointer, and that traveled from you to the wall at the speed of light, not faster than it.



....


Damnit, I've gone and derailed the webcomic thread again, haven't I?
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Re: Webcomics! (Loads of images)

Postby Arnock » Fri Apr 30, 2010 6:12 am

theckhd wrote:

....


Damnit, I've gone and derailed the webcomic thread again, haven't I?



here, I'll save it!


Image


oh wait...


and today's

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