# The Naked Scientists Forum

### Author Topic: What is the speed of light leaving a travelling object?  (Read 4599 times)

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##### What is the speed of light leaving a travelling object?
« on: 07/12/2009 15:30:03 »

Hi Naked Scientists l best greetings can u help me with a question that's baffled me for ages,I did pass my  GSE Olevel in physics (about 1984).

My question is:

I'm  in a car travelling at 80kph I open the window and flick a pea at 20kph and I know the pea is now travelling at 100kph (wind resistance notwithstanding).

Now I'm in a speeding train which has a light on the front,and this is what gets me the speed of light is constant.

But why isn't the light travelling at the speed of light plus the train speed, and what is the light waveform coming from the train (lightwaves being constant).

I've pondered this for ages. If you do have the time to reply

yours greatfully
Beowulf2

What do you think?

#### JP

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##### What is the speed of light leaving a travelling object?
« Reply #1 on: 07/12/2009 17:15:16 »
You're in good company.  That kind of puzzle is exactly what led to Einstein coming up with special relativity.  The solution to your puzzle is that at high speeds, you can't just add velocities in the usual way.  If send a pea flying at speed vpea out the front of your car moving at speed vcar, the equation for its final velocity with respect to a stationary observer is

(vpea+vcar)/(1+vpeavcar/c2)

If both vpea and vcar are small, then (1+vpeavcar/c2) is basically 1, and you recover:
(vpea+vcar) for the addition of the velocities.  However, if you let vpea=c, then the entire expression simplifies and the final velocity with respect to the stationary observer is still c.

#### Farsight

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##### What is the speed of light leaving a travelling object?
« Reply #2 on: 07/12/2009 18:11:49 »
aade: it's really simple. The light travels at the speed it does because it's a wave, not a pea. Think of a sound wave. Its speed depends on the air it travels through, not on how fast the train's going.

The waveform of the light coming from the train is sinusoidal. Check out the doppler effect to find out how frequency is altered by relative motion.

#### JP

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##### What is the speed of light leaving a travelling object?
« Reply #3 on: 08/12/2009 04:45:46 »
There's a problem with that description, Farsight.  Sound moves in a medium, so if you're moving too, you can see waves traveling at different speeds with respect to you.  If you travel faster than the speed of sound, you could catch up to a sound wave.

Light travels in a vacuum and no matter how fast you're moving, it always seems to be traveling at the same speed.  You can't catch up to it as you can with sound.

#### yor_on

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##### What is the speed of light leaving a travelling object?
« Reply #4 on: 08/12/2009 22:17:25 »
JP is the one I would lean to in bad weather :)

Although there's an awful lot of stringently expressed mathematics in his answer he is correct.

Light will relative the observer always travel at 'c' relative what medium it 'bores through'.
Read this one and the next page http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html

You don't need to use the math to understand it.

http://www2.slac.stanford.edu/vvc/theory/relativity.html
http://www.squidoo.com/relativity_explanation

------------------------

Or if you like I wrote about that in my 'essay' here
http://www.thenakedscientists.com/forum/index.php?topic=25747.msg276327#msg276327

I come to how he defined this proposition at the end of that post after "And this one is very difficult to understand. How did he reach that conclusion?"
« Last Edit: 09/12/2009 00:17:02 by yor_on »

#### PhysBang

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##### What is the speed of light leaving a travelling object?
« Reply #5 on: 09/12/2009 15:00:00 »
In pair production we make an electron and a positron using a gamma photon:

<more crap removed>
Wow, it's like getting banned from that site with the actual scientists on it has driven you further towards incomprehensibility. What you have written there not only has little to do with pair production, it obviously has nothing to do with light travelling in a medium.

#### Mr. Scientist

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##### What is the speed of light leaving a travelling object?
« Reply #6 on: 09/12/2009 15:43:36 »
In pair production we make an electron and a positron using a gamma photon:

<more crap removed>
Wow, it's like getting banned from that site with the actual scientists on it has driven you further towards incomprehensibility. What you have written there not only has little to do with pair production, it obviously has nothing to do with light travelling in a medium.

Not would i say... rather is determination is a result of bad moderation in the past on previous forums.

#### The Naked Scientists Forum

##### What is the speed of light leaving a travelling object?
« Reply #6 on: 09/12/2009 15:43:36 »