If Light Has No Mass......

[peter]

Then how is it unable to escape from a black hole's gravitational pull? But forget that - Here's what I wanted to do; just tinker around with the idea of light a bit.

We know light energy can exist independantly of it's scource; if I point a good strong torch at the Sun and switch it on for say five minutes, then turn it off, someone on the suns surface [with an appropriately stong detector] will see the first of that light arriving eight and a quater ish minutes after I first turned on the torch [ie after I have turned it off again], it will continue to be detected for the five minutes and then stop thirteen minutes and a quater after .....etc - you get the idea. So the light travels through space independantly of its emitting scources behaviour.

Now in a room if I swithch on the light it is [almost] instantly flooded with light, and if I turn of the light, again it dissapears almost instantly. But if I had superfast eves I'd see the same thing as when the torch is turned off - a sphere of blackness spreading out from the bulb untill all the light had dissapeared. But where has it gone? In fact, why doesn't light 'fill up' the room like water in a tub [only everywhere] and accumulate, getting brighter and brighter all the while, untill your eyes can stand it no more? Where's it going? Well I guess the answer is it's being absorbed and converted into 'waste' hear energy by the very matter of the room I am sitting in. The walls are taking their share, and the various molecules of the air in the room are having their bit as well. So Ok - if this is the case it must mean that if I could introduce light into a perfectly reflective room containing a perfect vacuum, then the light energy would have nowhere to go exept as to remain as light bouncing around the room even after the emitting scource had been switched off. Now that would be worth seeing!

[Incidentally - the idea of containing light in a bottle is not as far fetched as it seems. The Future of the Mind book we are discussing 'in another place' gives detais of an experiment where just this is done by entrapping light in near absolute zero temps in viscous liquids where it is forced to 'bounce around' from molecule to molecule rathe than exit the container.]

[Hashi Lebwohl]

The massive gravity of a black hole creates a situation where the "path of least resistance" for the light to follow is not away from the black hole but back towards itself. Beyond the event horizon, the photons are being accelerated towards the black hole with such force that any energy they had is spent trying to escape; when they can't leave it creates the situation where there is an object which you could detect only because it blocks objects behind it.

The speed with which photons travel give them relativistic mass, the presumption being that they have 'zero' rest mass (or at least a rest mass so low that it cannot be detected, only calculated).

[I'm Murrin]

Altering the path of a particle requires energy transfer, so a photon contained in a reflecting box would lose energy over time to the walls.

[Vraith]

The massive gravity of a black hole creates a situation where the "path of least resistance" for the light to follow

Well mass curves spacetime...the light isn't being "pulled" on, it is following the shortest possible path through curved spacetime. [which may be what you meant by path of least resistance?]
It happens all over to greater/lesser extent. Black holes, at the horizon, curved it all into a circle/sphere. [inside the horizon things get weird...I don't know if anybody really knows how that works, except small bites of it...even the MATH just gives up and says WTF?] But gravitational lensing is the same thing, except the space is just a LITTLE bent.

M's got your light box answer, I think, peter, eventually...though having the right materials and watching it happen slowly instead of the normal instant would be cool.
[[the only way to do that, though, is to be in a completely self-contained suit inside the box...and if your suit was totally reflective, you wouldn't be able to see anything and if it WASN't totally reflective it would make the darkening happen faster.]]

On your superfast eyes and bulb...I don't think you'd see a sphere of darkness start expanding from the bulb when turned off...because some of that light in the room is left over, bouncing around in all directions. [that's how you can see the walls, and all the other stuff, instead of just the part of the bulb shooting photons straight into your eye].
I THINK you'd see sudden significant dimming...then a slower dimming to blackness. Stretching my visualization skills....I'm not sure, but that first sudden dim would PROBABLY, to your superfast eyes, suddenly appear as a dimmer line [or maybe very long/thin/pointy cone] between you and the bulb that then propagated in all directions to dim the whole room...then the room would slowly dim basically equally everywhere. Maybe.

[Hashi Lebwohl]

Yes, that is what I meant--gravity curves spacetime and objects will "fall" based on that curvature, even if that means turning around and heading for their source.

Things definitely get weird when the acceleration due to gravity stretches space until the time dilation, the good old 1/sqrt(1-v^2/c^2), becomes a complex number (v > c). Some conjectures are that objects begin moving spacewise rather than timewise (meaning they could possibly go backward or forward at a rate different from our own) while other conjectures are that the compression allows matter (it really isn't "matter" at that point but some "macro quantum" material) to move into one of those higher dimensions that the mathematics tells us are there. I like that hypothesis because it gives that material some place to go, which makes sense to me.

[High Lord Tolkien]

I'm fascinated now about what happens to the light when the bulb gets turned off. I don't get what happens to it.
Can someone dumb it down to a 10year old level for me?

[Hashi Lebwohl]

When the light bulb gets turned off yes, if we could slow it down we could watch the trail end of the light moving outwards and it would look like "darkness" spreading from the bulb, until the light hits the walls, gets absorbed, weakly re-emitted, then bounce again and re-light the dark areas until all the photons were of low enough energy that they don't get re-emitted from the walls. Drop a bounce ball on the ground and imagine it is a photon. Every time it bounces it loses energy until it sits on the ground.

[Vraith]

When the light bulb gets turned off yes, if we could slow it down we could watch the trail end of the light moving outwards and it would look like "darkness" spreading from the bulb, until the light hits the walls, gets absorbed, weakly re-emitted, then bounce again and re-light the dark areas until all the photons were of low enough energy that they don't get re-emitted from the walls. Drop a bounce ball on the ground and imagine it is a photon. Every time it bounces it loses energy until it sits on the ground.

I don't think so...I'm not sure my description is quite right. But I think it might be "less wrong."
Because: the ONLY direct sight of the light bulb is the photons that go directly from the bulb to your eye. Everything else you can see...INCLUDING the rest of the bulb itself...is reflecting/bouncing/room-filling light. [[you don't see, for instance, the "shell" of the bulb by direct light from the filament or whatever your source is...you see the shell by reflected light. And by many different reflections for almost everything.]]
So if you are looking directly at the bulb...you wouldn't see the last photon coming. You would just suddenly STOP SEEING the bulb directly. If no other light was hitting your retina in the same place, but from different angles, and your retina was ultra-high resolution, you'd just suddenly perceive a black spot where the emitter was...and likely an over-all dimming cuz total light to your eye just dropped. And shortly after [or longly...depending on exactly how much faster than light you are seeing] all the things you see by once-bounced photons would start dimming...but this would PROBABLY be distributed around the room.
SOME things visible way on the other side were coming back to your eye after a single reflection. At the same time, SOME things much closer...because of angles/shapes/reflections...are seen by you after MULTIPLE bounces.

Now...if that bulb was in the middle of NO room...or a very large space with little or no reflection back from boundaries/walls, your superfast eyes might see a sudden darkness expanding in all directions from the source.

Maybe.

[peter]

Do photons actually have a calculable mass?

[I have this 'bulb' shaped thing at home that has a sort of weather-vane affair inside it, with four sqare 'sails' balanced on a needle so they can rotate. Each sail has a black face and a white face ...... just checked, it's called a 'Crookes Radiometer'. I was told once that the motion of the sails produced by radient light was a demonstration of the momentum change of photons in bouncing of the white sides as opposed to being absorbed by the black sides. Sounds fishy to me.]

[Hashi Lebwohl]

I don't think so...I'm not sure my description is quite right. But I think it might be "less wrong."
Because: the ONLY direct sight of the light bulb is the photons that go directly from the bulb to your eye. Everything else you can see...INCLUDING the rest of the bulb itself...is reflecting/bouncing/room-filling light. [[you don't see, for instance, the "shell" of the bulb by direct light from the filament or whatever your source is...you see the shell by reflected light. And by many different reflections for almost everything.]]
So if you are looking directly at the bulb...you wouldn't see the last photon coming. You would just suddenly STOP SEEING the bulb directly. If no other light was hitting your retina in the same place, but from different angles, and your retina was ultra-high resolution, you'd just suddenly perceive a black spot where the emitter was...and likely an over-all dimming cuz total light to your eye just dropped. And shortly after [or longly...depending on exactly how much faster than light you are seeing] all the things you see by once-bounced photons would start dimming...but this would PROBABLY be distributed around the room.
SOME things visible way on the other side were coming back to your eye after a single reflection. At the same time, SOME things much closer...because of angles/shapes/reflections...are seen by you after MULTIPLE bounces.

Yes, your description is more accurate because we can see an object only after photons have reflected off it and made it to our retinas to be processed as information.

Do photons actually have a calculable mass?

Check out this little paper which I found attached to a note on the section "physical properties" of the photon page on wikipedia. So far, the best physicists can do is find upper limits on its rest mass.

[Vraith]

Do photons actually have a calculable mass?

[I have this 'bulb' shaped thing at home that has a sort of weather-vane affair inside it, with four sqare 'sails' balanced on a needle so they can rotate. Each sail has a black face and a white face ...... just checked, it's called a 'Crookes Radiometer'. I was told once that the motion of the sails produced by radient light was a demonstration of the momentum change of photons in bouncing of the white sides as opposed to being absorbed by the black sides. Sounds fishy to me.]

Photons do not have a "rest mass"...if you could make them sit still, no mass.
If they DID have rest mass then they wouldn't be able to travel at the speed of light.

The Crookes explanations I recall has nothing to do with photon mass at all. [some folk thought it did for a while...don't think anyone does anymore]. It is all due to heat differences between black and white side...and the effect this heat difference has on heating the air and the currents/pressure.
The Crookes does not work in a real vacuum...nor at higher air pressures. Only in a partial vacuum. [There are actually two different heat effects happening at the same time, they both push the same way in the end]

Photons do have relativistic mass [momentum] so there is an effect...like a solar sail. And it does transfer more momentum with reflective surfaces...so I suspect with a real vaccum in a Crookes it would rotate the other direction [though you'd probably need a much brighter light source to do it than the heating effect version...as a guess]

[peter]

Yes - just checked the beast out and if the vanes were a stalled car the black face would be pushing the car and the white pulling. this has to be consistant with the black faces absorbing radient heat, warming up over and above the white faces and thus causing the air molecules that strike the face to be reflected at higher velocity. To balance this additional velocity the vane moves in the opposit direction to the reflected air molecules. Were it a 'photon' relection effect, the additional momentum would need to be balanced by motion in the other direction [ie ith white pushing and black pulling]

[nb It's actually quite hard to explain which direction the thing rotates without messing it up - hence my car analogy, but I do realise there is no actual 'pulling' going on. ]