From: Jagger 4/02/99 11:28:49 Subject: Light and Gravity? post id: 234 ok, i'm not sure if this is going to go anywhere, but here goes. Does light have mass?, if yes, then light must have a gravitational force. So do you weigh less at mid-day so if you wanted to make an anti-gravity machine, could you put some light above, or compressed light or something like that. From: Cass 4/02/99 18:23:31 Subject: re: Light and Gravity? post id: 379 No, it doesn't. As far as I understand it, if photons had even the tiniest mass, because they are travelling so fast that mass would be increased so much that going out in the sun would be like someone dropping a planet on your head. From: Dr. Ed G (Avatar) 4/02/99 19:18:32 Subject: re: Light and Gravity? post id: 387 Does light have mass?, if yes, then light must have a gravitational force... Light has no mass, but it does have momentum. Now, I personally don't have a definitive answer (it is currently one of my quests, to find someone expert enough to convince me of the right answer), but I think I have a handle of where we're at in this forum. Firstly, by General Relativity (or "GR") things with mass warp the very fabric of space, or more accurately space-time. This warping effectively defines how both (other) matter and light moves through space as a result of gravity. This is not dis-similar to the way that the curvature of the Earth defines how we will move in 3 dimensions when we attempt to move in a straight line over its surface. Now, Chris argues that light cannot influence this warping as it has no mass. Therefore, light is influenced gravitationally by the warping of space, but does not have a gravitational influence of its own (unlike matter which does have mass and therefore gravity). In contrast, James argues that is not just mass that warps space-time but energy as defined by the equation (energy)2 = (mass)2c4 + (momentum)2c2 [which reduces to the famous E=mc2 when the velocity is small]. Therefore, light does influence the warping of space-time and does have a gravitational influence, although next to the energy contained in matter is phenomenally small in comparison. Like I said, I personally have not resolved this yet (although I've no doubt that someone else has), but at the moment I'm leaning towards James's position. I'll keep you posted. ... So do you weigh less at mid-day so if you wanted to make an anti-gravity machine, could you put some light above, or compressed light or something like that. If James is correct, and light indeed does have a gravitational influence, then the amount of light energy you would need in order to reduce your weight by 10% would be the equivalent of the energy released by 100 TRILLION HIROSHIMA BOMBS!!!!! I don't think I need to say more. :-) Soupie twist, Ed G. From: ray p 4/02/99 21:15:04 Subject: re: Light and Gravity? post id: 400 i agree with james and think i fact that the dark matter astrophysicists are looking for could be light matter but travelling light like every photon between here an andromeda + also the other energy types that exist in space with little significant measurable mass of their own but altogether measurable From: Tyrek 7/02/99 20:40:30 Subject: re: Light and Gravity? post id: 548 I'm not sure if light has mass, but it is certainly affected by gravity. "Black Holes" (if there actually are such things) are just that because theoretically their gravitation pull exceeds the spped of light - ie light cannot escape from it. You can add this in to the theories of faster than light travel and time travel and come up with all sorts of interesting theories (that is if you believe in black holes). From: Gigboy 10/02/99 12:07:41 Subject: re: Light and Gravity? post id: 723 Dear Ed G... I don't remember much from school but I remember the formula for momentum (P): P=mV where m is mass and v is velocity. If light has momentum then m cannot equal zero, as we know the speed of light. Or am I missing something (the more likely scenario as I'm not as well versed in physics as some).... Tony. xxx From: James Richmond (Avatar) 10/02/99 12:16:36 Subject: re: Light and Gravity? post id: 724 The formula p = mv only works for objects with mass which are travelling at much less than the speed of light. The relativistic formula relating energy to momentum is E2 = (pc)2 + (mc2)2, which has the advantage of working both for massive particles and for light. It also works at any speed. In the formula, E is the total energy of the object (including mass energy if the object has rest mass), p is the momentum, m is the rest mass and c is the speed of light. In the case of photons, which have m=0, the formula reduces to p=E/c. From: Steve 10/02/99 12:27:06 Subject: re: Light and Gravity? post id: 725 The thing to keep in mind is that light has two seemingly contradictory aspects. The first is as an electromagnetic wave (the one we're all most familiar with) and the second is as a particle. In its wave aspect, light has no mass. You can see, though, from James relativistic equation for momentum, that it does have a momentum and thus you can correlate what its mass 'would be' if it displayed one. These contradictory aspects of particles make more sense when viewed within the theoretical framework of field theory - which views a particle and photons (packets of light) as an extreme 'condensation' of the field. The reason that light is affected by gravitational forces (or, to put it in the context of field theory, by a gravitational field) is that the field curves the surrounding space (the greater the gravitational field, the greater the curvature) affecting everything - even light. This is why light 'bends' around massive objects. Not sure if this clarifies or confuses, but I hope it helps ;-) Steve This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.