Re: Faster Than Light Travel - Reply

Whilst writing about light and stuff, Jon wrote:

> That would be impact of photons. See, light acts a particle (which
> by definition has mass) and an energy wave (which doesn't) depending
> upon how you look at it. 

Not strictly true. Light never has any mass, but it does have
momentum. The momentum is obviously not given by the Newtonian
formula of mass times velocity, but rather by Planck's Constant (h)
divided by wavelength.

Hence when light reflects of a mirror there is a transfer of momentum 
to that mirror of 2 x h / wavelength for each photon. This is the 
principle behind solar sails. The problem is that you need a lot of 
photons and a very, very light sail in order to get any appreciable 
acceleration. I once worked it out using very thin copper foil and 
the sail had to be bigger than a continent in area! And that was for 
a sail-ship operating near the Earth's orbit, further out you'd need 
an even bigger sail.

And strictly speaking light is always a wave and a particle at the 
same time, as indeed is everything else in the universe. How you look 
at it is simply a matter of picking which one is easiest to work with 
in approaching any specific scientific enquiry,

	Cheers,
		Steve

--
"My theories appal you, my heresies outrage you, 
 I never answer letters and you don't like my tie."  - The Doctor

Stephen Richard Pugh	 http://ds.dial.pipex.com/town/estate/ax16