Light & Gravity

Factual Questions
1

Here’s a conundrum for any physics mavens on the boards.

Let’s start from a good position - I know I’m wrong.

But here’s how the conundrum appears to me (not knowing jack shit about physics).

    • You cannot see blackholes, 'cause they have such strong gravity they suck in light.
    • Therefore light must be affected by gravity
    • In order to be affected by gravity, light must have mass, because only things with mass can be affected by gravity.
    • But things that have mass cannot travel at the speed of light, on account of various fancy laws that I’m not clear on…
    • So how can light travel at the speed of light ?

I’m sorry that this is such an idiotic question, but to a layman this is really weird…

Can anyone point out where my massive non sequitur is ?

Thanks

;j

2

No. The light is “infinitely redshifted” so that it has zero energy.

Incidentally, black holes can be seen due to the effect at the Schwarzchild surface whereby particle-antiparticle pairs are disassociated. (“Black holes ain’t so black” - Stephen Hawking.)

3

The gravity of the black hole warps space, and since light moves in space the “warping” affects it too.

4
  1. You can see bklack holes, though Hawking radiation is one product, the larger the black hole the less noticeable it becomes. The best way to see a black hole is from it’s accretion disc, which gives off huge amounts of energy due to the frictional forces of matter being sucked in.

  1. Yes light is affected by gravity, all mass objects bend light to some extent

  2. Light has relatvistic mass (an almost obselete defintion for mass, derived from it’s kinetic energy), but it’s better to think of light being affected by gravity as the curvature of spacetime by gravity

  3. Nothing can accelrate to the speed of light, but light is already at the speed of light and has zero rest mass anyway.

  4. Magic Pixies.

5

Bloody Pixies… I knew they were responsible.

But seriously - is this what you’re saying (more or less) “gravity curves the very fabric of space in which light travels, hence the curving of the light itself”… is this an accurate interpretation ?

By the way, thanks for the answers…

:@)

Tarantula

6

Yes, light can be thought of as always travelling in straight lines, so it’s curving is a result of the local altering of the topology.

7

I am pretty sure that I am correct in saying that light, i.e. photons, despite having no detectable mass, do produce gravity;
it is a small effect, but positive.
all the photons in the universe add to the mutual gravity and are part of the positive side of the equation, together with ordinary matter and dark matter, which act to slow down the expansion of the universe.
On the otherside of the scale is dark energy, whch acts to speed this expansion up, as far as I can understand it…
unless this is all a gross oversimplification?

SF worldbuilding at
http://www.orionsarm.com/main.html

8

The flaw in the OP’s argument is in point 3. Light does not have mass, but is affected by gravity. The other four points are essentially correct. Hawking radiation is negligible, and the accretion disk of a black hole isn’t really the hole itself, so it’s correct to say that we can’t see a black hole.

As for point 3, though, even Newtonian gravity allows massless objects to be affected by gravity. You take Newton’s two great equations, force due to gravity, and motion due to force. F = GMm/r[sup]2[/sup], and F = ma (G is Newton’s constant, M is the mass of (in this case) the black hole, m is the mass of the particle we’re dropping in, and a is the acceleration of the particle). You’ll notice that this means that a = GM/r[sup]2[/sup]: All particles falling in have the same acceleration, regardless of the particle’s mass. Now, true, you can’t really cancel out the m if m = 0, but it would make a mighty peculiar theory if a particle with zero mass behaved completely differently from a particle with really teeny tiny mass. So we conclude that, even in Newton’s theory, light should be affected by gravity.

It so happens that if you do the problem the right way, using General Relativity, you find that light is affected even more strongly by gravity. When Einstein and Eddington went to watch that solar eclipse, they weren’t interested in seeing whether the Sun’s gravity would bend starlight, they wanted to see how much it was bent.

As for photons producing (not just reacting to) gravity: A single photon does not have mass, but two or more photons do, so long as they’re not travelling in exactly the same direction. If we have a box full of electrons and positrons, and we let them all annihilate so that we have a box full of gamma rays instead, the total mass of the box is still the same. Photons do add to the “mass” of the Universe, although at present, that contribution is negligible (about 10[sup]-5[/sup] times the contribution from massive particles, as I recall). eburacum45, what you say is in fact an oversimplification, but it’s essentially correct.

9

What is the effect of gravity on light ? and other light [heavy] questions…

10

After reading the thread replies in this area, it appears that “Light” has tremendous inert energy. Whether the light comes from a Star, that may not exist anymore, or the Sun, which takes 8 or 9 minutes to get to Earth, or a match, lit on a dark night. In some way the “Light” travels a long way in a very short time (Relatively speaking). What propels it? Where does the propulsion energy come from? Is it pushed at all? Or does the Gravity of Universal Mass pull it home?

11

mac100, you may wish to familiarize yourself with Newton’s First Law of motion. An object in motion will remain in motion at a uniform velocity unless acted on by an outside force. Light doesn’t need any sort of energy to propel it, once it is moving in the first place.

The reason light is moving in the first place is that it is massless. A massless particle can only travel at one possible speed; c; which is approximately 300,000,000 m/s. A good conceptual way to think of this (which I must warn is far from strictly accurate) is through Newton’s equation F=ma. Light is massless, so if subject to any external force this equation gives “infinite” acceleration. However, in relativity, objects cannot go infinitely fast, but have a maximum velocity of c. Thus upon its creation a light particle instantly accelerates to c and gets “stuck” at that velocity.

12

Chronos - first off, thanks for the explanation, and again, I know you’re right; but secondly - how can one photon have no mass, but two of them have mass…
If the mass of one of them is nothing, as the old saying goes, “twice nothing’s nothing”…

I’m sorry that I can’t see the reason… please explain.

13

Slight nitpick: It is not the topology that is altered, but the geometry. What is the difference? Well, a topolgist cannot tell a coffee cup from a dougnut, but a geometer sure can. Roughly speaking, the one can only see how things are connected up (this is a vast oversimplification, but will do for the present question), while a geometer can make measurements and, in particular, find geodesics.