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killium
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There is that little concept i think i don't get. If light has energy and energy is equal to mass (e=mc2), how can we affirm that light has no mass ?
Here's the problem energy does NOT equal mass. What you are quoting is the mass to energy equivilancy. It is rather subtle I guess but mass is not energy and energy is not mass. Mass can be converted to energy. Energy can be converted to mass. But they are definitely not the same.killium":3vh6em3n said:There is that little concept i think i don't get. If light has energy and energy is equal to mass (e=mc2), how can we affirm that light has no mass ?
killium":1igzf3ir said:how can we affirm that light has no mass ?
Huh?killium":ajnd4z1c said:i see. This would mean that the mecanism thru which matter is converted into energy and vice-versa is assymetric in regards to gravity...
Photons cannot move slower than light speed. That does not make any sense.kelvinzero":3u211z2k said:I dont know if this is valid, but I like to think of photons as always existing, even when moving below light speed.
No, this is where E=mc^2 comes in, the the mass is completely converted to energy there is no mass in the resulting photons.Another example,
You know how a positron and an electron can annihilate to form two photons? I reckon they have to have the same total mass before and afterwards.
That is actually sort of correct. If there was a magic bottle that could hold positrons and electrons and was also able to hold the resulting photons that result from anihilation, then that is just what would happen, you would have the mass from the particle pairs and after the anihilation you would have a bottle full of energy and the weight would decrease due to the disapearence of all of the particlesOtherwise you can sort of imagine an unobtanium bottle containing either matter/antimatter, or the equivalent energy in photons, and the weight of the bottle can be switched from one value to another depending what form the energy is inside it.. even though nothing goes in or out.
The mass of the black hole would increase due to the mass energy equivilancy E=mc^2Another example,
If you shine a light into a black hole, you would make that black hole more massive. How could it be otherwise? If energy could just vanish from the universe and not have any effect, such as increasing the mass of the black hole, then energy would not be conserved.
Thanks ramparts, you know this stuff alot better than me. When I first saw that you responded I thought, "oh, oh; musta screwed something up in the explanation".ramparts":3mmolcmj said:Exactly. Good explanation, origin. The key confusion here is that E=mc^2 and the mass-energy equivalence doesn't mean they're the same thing, just like dollars aren't the same as euros, even though they do similar things and you can convert one into the other.
me":3s130hx5 said:Light has mass, just not rest mass
Look.. things would proceed much more rapidly if people just assumed I was always right.ramparts":3s130hx5 said:Kelvin - No. Light does not have mass.
Most of the confusion in the rest of your message seems to be about a detail about the terminology of 'mass'. When I was taught relativity (admittedly only to a pretty basic graduate level) it was using the concept of total mass being equal to rest mass and relativistic mass. Apparently this terminology has gone out of fashion.origin":1qlaavlx said:Photons cannot move slower than light speed. That does not make any sense.kelvinzero":1qlaavlx said:I dont know if this is valid, but I like to think of photons as always existing, even when moving below light speed.
ramparts":3032uvuy said:That really wasn't clear from your posts, and if you want to be clear you should consider using terms in the way that they're actually used by scientists today. Light doesn't have mass in any meaningful sense. This relativistic mass concept in the way you're using it as far as I can tell hasn't been used since the first half of the 20th century. Mass in the way that everybody uses it - as a property of a particle's internal structure - doesn't change with a particle's speed, and it's something that for light is almost certainly zero. There's some decent info on this Wiki page (this section and the next, on modern views): http://en.wikipedia.org/wiki/Relativist ... c_concepts
Meanwhile you have yet to respond to any of origin's well-thought out objections to the other arguments in your last post.
origin":vnfcz189 said:Photons cannot move slower than light speed. That does not make any sense.kelvinzero":vnfcz189 said:I dont know if this is valid, but I like to think of photons as always existing, even when moving below light speed.
No, this is where E=mc^2 comes in, the the mass is completely converted to energy there is no mass in the resulting photons.Another example,
You know how a positron and an electron can annihilate to form two photons? I reckon they have to have the same total mass before and afterwards.
And what if the black holes are spewing out everything they take in,in another part of the universe? :?:Otherwise you can sort of imagine an unobtanium bottle containing either matter/antimatter, or the equivalent energy in photons, and the weight of the bottle can be switched from one value to another depending what form the energy is inside it.. even though nothing goes in or out.
That is actually sort of correct. If there was a magic bottle that could hold positrons and electrons and was also able to hold the resulting photons that result from anihilation, then that is just what would happen, you would have the mass from the particle pairs and after the anihilation you would have a bottle full of energy and the weight would decrease due to the disapearence of all of the particles
The mass of the black hole would increase due to the mass energy equivilancy E=mc^2Another example,
If you shine a light into a black hole, you would make that black hole more massive. How could it be otherwise? If energy could just vanish from the universe and not have any effect, such as increasing the mass of the black hole, then energy would not be conserved.
It needs some experimental verification. String theory is interesting but it has a ways to go to.Tritium":3dpmddvy said:What about "String Theory"?
Mmmm not exactlyAstro_Robert":1xvgbx6h said:momentum = mass * velocity. so zero mass times infinite velocity = finite momentum (heehee)
Photons were the dominant gravitational influence on the expansion of the universe for the first 50,000 years or so after the Big Bang, at which point the matter density overtook the photon density, and matter was then the dominant force on the expansion for another few billion years until the dark energy - or whatever is causing the acceleration - took over because the matter became too diffuse.Astro_Robert":1dea5j3d said:Ramparts,
I have a related question to your comment that light exerts gravitational influences.
I was once thinking about the expansion of the universe (I detest Dark Energy) and pondered if the cummulative photons emmited by the universe could be supplying the expansionist effect we perceive as dark energy. Ie, over time, most of the photons from the history of the universe are beyond the observable mass of the universe so their gravitational influence would be to 'pull' the universe towards the outer reaches and cause expansion we summon Dark Energy to solve.
At the time I decided that the distant photons would have such negligible gravitational influence due to the inverse r-squared nature of gravity that they could not have such an effect, even after accumulating for 13 Billion years. I am wondering if you know if any physicists have discussed this and if they concluded the same thing?