grizzly

53652. Mon Feb 20, 2006 1:23 pm 


Kevino7 wrote:  light is an electromagnetic wave and carries energy (That would help explain the second question).
For question 1, i dunno when it moves it has a mass? 
Photons only have mass when they are moving. If a photon stopped moving then it no longer have any mass. 




Kevino7

53654. Mon Feb 20, 2006 1:24 pm 


ahh, E=mc2 




Celebaelin

53668. Mon Feb 20, 2006 1:51 pm 


grizzly wrote:  Kevino7 wrote:  light is an electromagnetic wave and carries energy (That would help explain the second question).
For question 1, i dunno when it moves it has a mass? 
Photons only have mass when they are moving. If a photon stopped moving then it no longer have any mass. 
Huh? If it has mass when moving how can it possess enough energy to move at the speed of light? 




dr.bob

53857. Tue Feb 21, 2006 6:06 am 


In relativity, there are two types of mass: Relativistic mass, which is how much a particles "weighs" (if you had some scales and a gravitational field) when moving, and Rest mass, which is how much a particle weighs when it is not moving.
The Rest mass never changes. It is invariant. However, when a particle travels faster, it increases in mass. This mass increase affects the Relativistic mass, and is given by the equation:
m(r) = m(0) / sqrt(1  v^2/c^2)
Where "m(r)" is the Relativistic mass, "m(0)" is the Rest mass, "v^2" is the velocity squared, and "c^2" is the speed of light squared.
Consider the case when the Rest mass is nonzero. As the velocity approaches the speed of light, "v^2 / c^2" approaches 1. Thus "1  v^2/c^2" (and, by extension, "sqrt(1  v^2/c^2)") becomes a smaller and smaller number.
If you divide a nonzero number (the Rest mass) by a very small number, you'll end up with a very large number. Therefore the Relativistic mass will increase, tending towards infinity, as the velocity approaches the speed of light.
However photons, and other particles that move at the speed of light, have no Rest mass. Thus m(0) = 0. This is a special case since, in the above equation, you're always going to be dividing zero by some positive number. The result of this is "undefined". It has been shown that such particles, with no Rest mass, have purely Relativistic mass equal to E/c^2 (a simple rewriting of E=mc^2).
So photons do have some kind of mass becuase they possess energy and energy has mass (E=mc^2). However they have no Rest mass, so they are able to travel at the speed of light. Having no Rest mass also implies that they can only travel at the speed of light. They really have no choice in the matter. So it's a bit confusing to say "If a photon stopped moving then it would no longer have any mass" since a photon can never stop moving.
Sorry if I've waffled on a bit here. 




Tas

53861. Tue Feb 21, 2006 6:21 am 


No worries....I likes waffles!
So, with what you are saying, is that light can be some of the missing mass? What I am getting at is that all those photons zipping about ought to have some mass.
It does seem to be that physicists have come up with another cop out....everything fits our equation, (i.e. nothing can travel at the SOL), except photons. Let's make a story up, as otherwise it spoils our work and means the equation is wrong!
LOL
:)
Tas 




dr.bob

53866. Tue Feb 21, 2006 6:35 am 


Tas wrote:  No worries....I likes waffles! 
Mmmmmmm, waffles
Tas wrote:  What I am getting at is that all those photons zipping about ought to have some mass. 
Of course, they do, although the mass of gazillions of photons is still pretty tiny compared to even a small amount of matter. The speed of light squared is a really big number. However I'm pretty sure that cosmologists are able to account for this mass since electromagnetic radiation is a well known phenomenon.
I have no souces to back this up, but in the main cosmologists are pretty clever people and I don't think they would have just forgotten about all the photons :) 




Tas

53869. Tue Feb 21, 2006 6:42 am 


So, all Dark Matter really boils down to is the amount of stuff that we do not know about, right?
So, we know have evidence to support the theory of super massive black holes, but cannot see them, and who knows just how much matter those buggers contain? We can't see them, so....that sounds like alot of matter that we theorise about. Has anyone done any serious calculations on how big these are?
:)
Tas 




Gray





eggshaped

53877. Tue Feb 21, 2006 7:25 am 


I imagine there's one or two contributors to this fascinating thread who may be interested in this. These two articles were written in the last week, one claiming that dark matter has been detected, the other that a new formula for gravity has been formulated which could negate the need for darkmatter.
I think it all adds up to "we're not really sure at the moment, but we'll sure as hell find out".
Quote:  Through the Cambridge team’s observations, they were able to create various maps of these galaxies and calculate the movement of their stars... With these maps, it was possible for them to outline the existence of dark matter in galaxies. 
http://thetartan.org/2006/2/20/scitech/5
http://news.bbc.co.uk/2/hi/science/nature/4679220.stm
Quote:  "There has always been a fair chance that astronomers might rewrite the law of gravity. We have created a new formula for gravity which we call 'the simple formula', 
http://www.universetoday.com/am/publish/improved_einstein_theory.html?1322006 




eggshaped

53879. Tue Feb 21, 2006 7:27 am 


dammit Gray.
:o) 




dr.bob

53904. Tue Feb 21, 2006 8:03 am 


eggshaped wrote:  I think it all adds up to "we're not really sure at the moment, but we'll sure as hell find out". 
I think that's the most succinct way of putting it.
Remember, scientists actually know nothing. They just have some models to explain how things work. Some of these models are more accurate than others. Some things (like the far off universe) are really difficult to measure 'cos WH Smiths don't sell rulers that big, so some models are always going to be a bit shaky. 




Celebaelin

53908. Tue Feb 21, 2006 8:11 am 


I'm having trouble with this.
If photons can only travel at the speed of light then their relatvistic mass, and thus their energy, must be fixed since m(0), v and c are all constant in a given medium. I didn't think that was what we observed but I'm willing to accept that making a detector out of old lollipop sticks, drawing pins, brown paper and PVA glue is probably not the best approach.
Quote:  It has been shown that such particles, with no Rest mass, have purely Relativistic mass equal to E/c^2 (a simple rewriting of E=mc^2). 
This is the clincher really isn't it. The "undefined" result means that this must have been approached in a different way. Can anyone tell me how this was shown? Is it pure maths or can it be put into, or at least approximated in, words?
Anybody willing to have a bash at the idiots guide to waveparticle duality? I’d put this in the frame of reference of Energy and Mass not being equivalent/in equilibrium through choice but I’ll waive that if necessary in the hopes of getting an answer.
I’ll not muddy the issue by giving my current, and doubtless wrong, understanding of this.
Meanwhile, back at the telescope…
Soooo, Veal, Lettuce and Tomato Sandwiches with Chile Sauce all round then! Roll on April.
Thanks for the information everyone, great thread. 




dr.bob

53919. Tue Feb 21, 2006 8:36 am 


Celebaelin wrote:  If photons can only travel at the speed of light then their relatvistic mass, and thus their energy, must be fixed since m(0), v and c are all constant in a given medium. 
'Fraid not. Since m(0) = 0, the equation for working out relativistic mass doesn't work becuase you're dividing zero by something and getting "undefined".
A photon's energy can vary. It's proportional to its frequency.
Celebaelin wrote:  The "undefined" result means that this must have been approached in a different way. Can anyone tell me how this was shown? 
I knew a while ago when I was doing special rel at university. I'd have to dig out my notes, though. I seem to recall it wasn't too complicated :) 




samivel

54058. Tue Feb 21, 2006 2:19 pm 


That's all right then :) 




Jenny

54229. Tue Feb 21, 2006 10:35 pm 


Interesting Nova programme on TV tonight, called The Ghost Particle, about neutrinos. They were once thought to have no mass, but apparently it has now been decided that they do. 



