A grade schooler asked me this and I didn't have a good answer. He
asked, "If a flashlight is moving the speed of light and then you turn
the flashlight on, what happens? Will the light from the flashlight be
going 2x the speed of light? Or will the light from the flashlight be
trapped?" It's been bugging me. Thanks.

Howdy sunny808-ga,

Since we want to make sure this particular grade schooler gets off on the
right foot, he needs to be informed that nothing with any mass, such as
our flashlight, can reach the speed of light.  This would take infinite
energy, and even if you could this, which you can't, then time would stop,
pretty well putting an end to our experiement.

So, we have to start with the premise that the flashlight is moving at a
very fast rate, say .9 the speed of light.  Remember that the speed of
light is a constant.

A constant is a constant, so when the flashlight is turned on, the light
from the flashlight travels at... the speed of light!

This would also be how you, holding the flashlight, would perceive it,
thanks to relativity.

For more, here are some NASA articles that speak to the subject.

"Ask an Astrophysicist"
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970102c.html

"Ask Us [-] General Physics: Speed of Light"
http://helios.gsfc.nasa.gov/qa_gp_sl.html

If you need any clarification, please feel free to ask.


Search strategy:

Google search on: flashlight "speed of light"
://www.google.com/search?q=flashlight+%22speed+of+light%22

Looking Forward, denco-ga - Google Answers Researcher

That was quick!

My pleasure, sunny808-ga, and thanks for the generous tip.

Looking Forward, denco-ga - Google Answers Researcher

But what about this?

http://www.newscientist.com/article/dn6092.html

Howdy mr_normsk_-ga,

Even if the theory of a "variable constant" that affects speed of light is
accepted, and as the article in question points out, "You would expect alpha
to stop [changing] five to six billion years ago," and thus the speed of light
would have become a "constant" constant at that point of time.

It doesn't really matter to the question, though, as even if we place it in
the very distant past, the light from the flashlight would travel at whatever
the speed of light was then, and not 1.9 the speed of light.

Looking Forward, denco-ga - Google Answers Researcher

In that case, very good answer!!

A small comment here.

One of the fundamental issues when you start learning relativity.
Mathematical transformations provided by Lorentz do the trick:
so, it is not classical 1+1=2, but more like 1+1=1.2 when things get relativistic.

However, imagine we rotate the flashlate and it is on. Then we can
observe the flashlight spot movement. Depending on how far it reaches
a "wall" it will move faster. So, after some critical distance the
speed of the spot movement will actually exceed the speed of light
itself. Velocity higher then the speed of light?

The answer is yes.

BUT

This is not a "movement". More strictly speaking - the two points that
are illuminated by flashlight spot are not casually connected. There
is no information transported by the higher speed then the speed of
light.

Hence, the higher velocities are possible, provided they are not
carrying any mass OR information.

The example is our universe that is thought to be expanding by the
speed higher then the speed of light during the inflation.

Keywords for more details: Cosomology BigBang Inflation