[luke warm]

elementary question for some of you, but does light itself (say the light of a star) have any mass whatsoever?

[barmar]

Light is made up of photons, which are strongly believed by physicists to have zero rest mass. However, they have energy, and the equation e=mc^2 can be flipped around to m=e/c^2; this form of mass is called "relativistic mass".

If you look up "photon" and "mass" at Wikipedia you can get lots of details about these concepts.

[luke warm]

ok, thanks... i was reading an article on black holes and wondered what property of light allowed it not to escape the gravitational pull (same for x-rays)... i know that's very elemental for some of you guys, but i've already admitted my relative ignorance re: some scientific matters

[gwnn]

Light has momentum, which is quite interesting. It's the same momentum that you'd get by multiplying the mass from barmar's post by c. So it can rotate very small stuff (NOT solar mills - I just found out about this) http://en.wikipedia....hols_radiometer .

[blackshoe]

Light's momentum is the principle behind the solar sail.

[Al_U_Card]

luke warm, on Jun 7 2008, 01:03 PM, said:

ok, thanks... i was reading an article on black holes and wondered what property of light allowed it not to escape the gravitational pull (same for x-rays)... i know that's very elemental for some of you guys, but i've already admitted my relative ignorance re: some scientific matters

The curvature of space-time causes the light to follow a "bent" path. Once the singularity (black hole) is massive enough, the "bend" will not allow anything that crosses its "surface" to move away from the source. The light (or anything else) "falls" into the singularity.....to an end that even physicists tend not to ponder...

[Al_U_Card]

btw that "event horizon" around the black hole is still not strong enough to impede quantum tunneling so that, eventually, the entire mass of the black hole will just "evaporate"....

[Al_U_Card]

Jlall, on Jun 7 2008, 03:18 PM, said:

You guys are smart

I would have thought that dealt a "lemon", you might try to make some lemonade with a "squeeze"....lol

[helene_t]

Al_U_Card, on Jun 7 2008, 10:11 PM, said:

The curvature of space-time causes the light to follow a "bent" path.  Once the singularity (black hole) is massive enough, the "bend" will not allow anything that crosses its "surface" to move away from the source.

I never understood the explanation that black holes have too strong gravity fields to allow things to escape. After all, when a comet approaches the sun it will bounce back with the same (give or take mass loss due to evaporation) energy as it came with, and according to classical mechanics it doesn't matter how heavy the central body is. Sound as if I would understand it if I could understand general relativity, lol.

[Al_U_Card]

I always envisioned it as a sort of funnel that instead of narrowing to an exit, it re-opens up so that anything descending its surface towards the "exit" gets past a point of no return and can keep going "down" but can't "back-up" to get "out" of the gravitational well.

This is how I also see the quantum tunneling phenomenon but with those little virtual buggers seeing the "up" as "down" so that they CAN get out...

[matmat]

snipped out for future edit . . .

[luke warm]

Al_U_Card, on Jun 7 2008, 04:11 PM, said:

luke warm, on Jun 7 2008, 01:03 PM, said:

ok, thanks... i was reading an article on black holes and wondered what property of light allowed it not to escape the gravitational pull (same for x-rays)... i know that's very elemental for some of you guys, but i've already admitted my relative ignorance re: some scientific matters

The curvature of space-time causes the light to follow a "bent" path. Once the singularity (black hole) is massive enough, the "bend" will not allow anything that crosses its "surface" to move away from the source. The light (or anything else) "falls" into the singularity.....to an end that even physicists tend not to ponder...

this is a little confusing to me... barmar said there is what's termed 'relativistic mass' present in light, but no actual (resting) mass... does this bent path you speak of mean there is "friction" of a sort in light's path? even if that were the case, if light has no mass there would seem to me to be no net effect... i'm getting a headache

[Al_U_Card]

No friction, per se, but extra length so extra time that it takes to make the "passage" thru the bent spacetime continuum.

Does that help, or should I get you some Advil?

[gwnn]

The point is that there are two kinds of concepts of mass. There's the mass of inertia (how hard it is to accelerate the particle further) and the gravity mass (the one you need to put in F=G*m*M/(r^2) to get Newton's gravity force). Now, many many scientists worked out that these two are equivalent. Then Einstein postulated that the mass of inertia actually grows as the particle accelerates. So, in an effort to get rid of the potential difficulties this variable mass could cause, he just said gravity is not a real force, it's just caused by the curvature of space-time. So it's not determined by the product of the two masses and all, it's just some sort of geometrical weirdness. I never studied GR however so I am going to shut up for now.

[helene_t]

luke warm, on Jun 8 2008, 01:18 AM, said:

does this bent path you speak of mean there is "friction" of a sort in light's path? even if that were the case, if light has no mass there would seem to me to be no net effect... i'm getting a headache

The relativistic mass of light is very real - it can be seen when the sight-line to a star is very close to that of the Sun and it looks as if the star has moved away from its normal position - what happens is an optimal illusion caused by the light from the start being bent by the gravity field of the Sun. Another example is when sunlight pushes small objects such as spaceships and comets' tails away, something that can be explained by the light's momentum being transfered to the object. A third example is gamma decay where the emitting atoms suffer a mass loss which can only be accounted for if the gamma ray took some of the mass with it.

So there is nothing mystical about the mass of light, and it is not something that requires extreme circumstances such as black holes to take effect.

Everything gets trapped in black holes - again, there is nothing special about light. Apparently it is better explained in terms of the geometry of space rather than in terms of forces - a "straight" line within the event horizon of a black hole is a circle confined to the black hole.

[luke warm]

ok, i'm muddling along i think... i guess, given the very small child understanding i have of this, the thought of light mass (of any kind, relativistic or otherwise) makes me wonder why C should be a constant

[NickRW]

Well, for a start off, don't worry about the "small child understanding" thing. One of my sons has a good mate who is a physics graduate and he confesses to the fact that some of these things "do your head in". More accurately put, they are not intuitive.

Also, the speed of light is only a constant for the purposes of formulae like e=mc^2. If I remember correctly the "c" they use is the speed of light in a vacuum. It does vary slightly in different media, just like sound.

Nick

[luke warm]

the in a vacuum thing makes sense, if it's a constant... but does that mean space itself is not *really* a vacuum? see, we know (or at least i think i know) that an object in motion (say an asteroid) will stay in motion unless something acts on it... it just seems to me that all things with mass must eventually lose momentum, vacuum or not... very perplexing

[NickRW]

Space isn't quite a perfect vacuum and the moon, for example, is suffering a small amount of friction as it goes round the earth - so, unless it's orbit is on an escape trajectory, which I don't think it is, then one day it will eventually crash down into the earth. But I don't think you need to worry about it happening to you or your grandchildren.

Nick