# Light and Gravity

How does gravity effect light, since photons have no mass?

Short answer: yes. IANAP, can’t explain why…but thats the fact.

Gravity is the force felt due to the bending of space in the presence of mass.

In other words, for a body or particle or photon P in the presence of mass M (like the Earth or a Black Hole or even your left pinky finger), the change in motion towards the mass is caused by the bending of space.

So the photon’s moving along through space in a localstraight line happily, all the time. It just so happens that this straight line (as far as the photon sees it) is actually bent (because space itself is bent).

If you were to fire a ball bearing through space, it too would follow a ‘local straight line’ that would be bent because gravity bends space. The ball bearing doesn’t know the difference, and neither does the photon.

This has been an I Am Not A Physicist But Putz Around With It In My Workplace announcement.

From what I remember of my astronomy class this semester, photons actually do have mass. It’s just that they don’t have rest mass. I.e., they only have mass while they’re moving. I don’t pretend to know how this is possible.

…or is it that photons do have momentum, even though they don’t have mass?

There’s two answers to this, the Newtonian answer and the General Relativity answer. First of all, Newton. In Newtonian gravity, the force one object exerts on another is F = GMm/r[sup]2[/sup], where G is a universal constant, M is the mass of the object that’s attracting, m is the mass of the object being attracted, and r is the distance between them. Also according to Newton, force and acceleration are related by F = ma. What this means is that if you have an object near a large mass, (with no other forces on it) that it’ll have an acceleration of a = GM/r[sup]2[/sup]. Notice that this is true no matter what the mass of the object is. So if you have a single electron, it’d have that acceleration. A single particle with a trillionth the mass of an electron, same deal. No matter how small you made your particle, it’d behave the same way in a gravitational field. Now, this really only applies to objects with nonzero mass, but physics would look somewhat odd if there was a way to tell the difference between “really really tiny mass” and “exactly zero mass”, so it’s safe to assume that, in Newtonian gravity, photons would also be accelerated by gravity. So if a photon passes near, say, a star, its path will be slightly deflected.

Now on to Einstein. According to Einstein, gravity isn’t really a force at all. As long as there are no non-gravitational forces on an object, it’ll travel exactly on a geodesic, which is the “shortest” path between two points (I put “shortest” in quotes, because length is defined a little differently in relativity). Now, when there are no masses around, the geodesics are just straight lines, but the presence of mass curves space, in such a way that the geodesics appear curved. Light, then, like everything else travelling through space, follows these curved geodesics, which causes its path to be deflected.

So what’s all this about the deflection of light around the Sun being proof of General Relativity? Sincee both theories predict that, why does it matter? The key is that not only do both theories predict that light should be deflected, they also predict the amount. And it turns out that Einstein predicts that it should be deflected by exactly twice as much as Newton predicted. Since the actual measurements agree with Einstein’s predictions, and not with Newton’s, that’s taken as evidence that Einstein’s theory is correct (or at least, more correct than Newton’s).

I don’t suppose it’s an easy matter to explain to me why Einstein’s prediction was exactly twice as much as Newton’s without twenty pages of ugly tensor math… I’ve always wondered about that, but haven’t yet tortured myself with the math to prove it to myself.

Ok, if it is space that is bent, what is space? If there is a vacuum with just one object in it would light bend to the gravity of the object?

What is space? You might as well ask what’s the meaning of life. According to Einstein space means, well, space. What matter in our universe occupies. It isn’t really definable because, until another universe is discovered, there’s nothing to compare it to.

Does space have mass? How does gravity bend space?

Politics.

Afterdoing some more searching on the board I get the impression that light does have mass because it is moving. Is this correct?
Does the speed of a object deterine its weight?
For instance if a 5 lbs. ball was going around the speed of light (slightly less) would it still weight 5 lbs.

As I understand it, that 5-pound balls weighs a WHOLE lot more when you get it up to a significant fraction of c (lightspeed in a vacuum). In adding kinetic energy (a function of speed), you add mass. The thing acts like it’s much more massive, because it has much, much more energy, and for a relativistic physicist, energy = mass.

What is a photon? Can a photon only go the speed of light?

Just a pointer BZ, you might want to try using “mass” instead of “weight”. Weight is relative to the specific gravity, whereas mass is an intrinsic value.

ummm…I think…

Oh, and it’s a good idea to talk in terms of mass (the amount of matter in a thing), rather than weight (the force a gravitational field exerts on a thing).

Easier to talk about a 1.8 kilogram ball than a 5-pound one.

Weight changes based on where you are, but mass is consistent, aside from the effect I just described. And that effect is unrelated. There the variation in mass comes form the addition of kinetic energy, which equals mass.

In the presence of a gravitating mass an object that travels slowly wrt to c experiences a lot more time than space, and so the deflection is almost completely from the “curvature of time,” and this matches the curvature predicted by Newton.

A photon, travelling at c, experiences equal measures of space and time and therefore is equally deflected by both “time curvature” and “space curvature,” or twice what Newton’s theory predicts.

I won’t attempt the math here (because I don’t know it but I suspect you could start by playing around with E=MC[sup]2[/sup]).

Suffice it to say the faster you go the more mass you have. E=MC[sup]2[/sup] basically says that energy and mass are equivalent. Energy is mass, mass is energy. When I speed an object up I am in effect adding energy to the system and thus to its mass.

This effect happens when you drive in a car or walk or stand still (the earth itself is moving). However, the effect is very small at these speeds. The increase in mass doesn’t really take off till you start getting very close (a few percent) of the speed of light (if you graphed the mass increase the curve would be gradual and steepn sharply above 90% c). Of course, once you actually hit light speed your mass becomes infinite which is one reason why travelling that fast isn’t allowed for mere mortals (among other things you’d occupy the entire universe not to mention you’d need an infinite amount of energy to accelerate you to that speed).

A photon is a particle of light. Essentially. Of course, light is all-at-once a particle and a wave, so it can be confusing.

Generally, though, talking about photons is talking about discrete packages of light energy- not limited to the visual kinds of light either, but the whole spectrum.

In this thread the ways that photon travels at different speeds is explained. Short version: They don’t, but it looks like they do.

Interestingly enough anything going the speed of light cannot go SLOWER than the speed of light (in a vacuum). The light speed barrier works both ways so a photon always moves at light speed.