A black hole has gravity of such magnitude, that light can not escape. So it sems that light do get affected by gravity. Now to the question: Is the gravity of the earth big enough to affect light, and if it is; how does it work. Does light bend around the earth, or even worse from a nothern habitat: does the sides of the earth (as viewed from space) attract more sunlight? (yes, that would make me a little bit jealous, since norway could do with a little bit more sun)
and; why is the speed og light constant? If you are in a rocket doing 900 k/ph, and flick on your flaslight, the speed of light is the same as if you were stralling in the park? Should the speed of light not increase with your speed? No it does not, but why?
Yes. It works because the Earth has a mass. Light doesn’t bend very much due to the Earth’s gravity, though. Even the Sun doesn’t bend it very much, but it was proven during a solar eclipse when some stars that should have been just hidden became visible.
Gravity (at least in the current understanding) is caused by the curvature of space-time, which in turn is caused by the presence of mass/energy. Though light is massless it still moves through space-time so it’s path is curved by the presence of matter. All matter bends light; it’s just a matter of degree.
Ditto EdwardLost, who makes an important distinction. The space around a massive object is warped. Light goes straight, but the space itself curves.
One way of doing astronomical studies of things floating around out there is to look beyond them and see how the light coming from objects past them is bent.
There is a universal constant called c, which has units of velocity. It represents several things, but one of them is the limit of how large relative velocities can be between things that have mass and can carry information. Light, and other forms of electromagnetic radiation, and also several other forms of radiation like gravity waves, move at this velocity. It is common but confusing to refer to it as “the speed of light” because this implies light itself is somehow key in all this.
You can study Einsteinian relativity and get more of an understanding why light goes past you at the same speed no matter how you are moving. There was an excellent paperback explaining this by George Mermin, and a more well known one by Lincoln Barnett that I thought actually confused things.
Presumably, gravity propagates at c. We have indirect evidence of this, but since we haven’t managed to detect gravitons yet, we cannot directly measure it. Nevertheless, we have no reason to believe differently.
Newton thought that gravity must act instantly because the position vector of a moving mass must point to the object’s present position not its retarded position. If this weren’t true then the solar system wouldn’t be stable. This forced Newton to postulate an “action at a distance” even though he knew it was ridiculous.
Once GR was developed it became clear that that the theory includes velocity dependent interactions that cause the position vector to be extrapolated forward so that to a first approximation it does point to its present position. Thus adding further to pretty much every scientists belief that gravity propagates at c.
Here’s a simple example. Suppose I have two parallel rods separated by some distance, with a massive bead on each of them. If I just let this apparatus sit for a while, eventually the two beads will settle down to be as close to each other as possible, due to their gravitational attraction to each other, thus:
–O--------
–O--------
But now, suppose I come to this apparatus after it’s settled down, and move one of the beads along its rod:
–O--------
--------O–
Now, this is going to start the other bead moving, to again match up with the bead I moved:
--------O–
--------O–
But when does it start moving? Instantaneously, or only after a delay? According to our best theories, it’s not instantaneous (and in fact, there is no such thing as “instantaneous”), but after a delay corresponding to the distance between the beads and the speed of light.
No one understands quantum mechanics and I would guess no one knows why this is so.
But let’s say c wasn’t independent from the speed of the observer. This would mean you could catch up to a light beam and you’d then see non-time varying electric and magnetic spatial waveforms.
But an EM wave consists of time varying electric and magnetic fields. The EM wave therefore couldn’t exist and we’d be in paradoxical conundrum.
Something like this was one of Einstein’s gedanken experiments that ultimately led him to the Theory of Relativity.
Ring, I don’t think I follow the logic of this. Wouldn’t this apply to any radiation at all? Sound, or the rings spreading from where a stone is thrown into the water?
As I remember Einstein’s thinking, based on a course I took now 30 years ago, it was a given that c is constant. Measurements of starlight, compared 6 months apart when the Earth’s orbit had it either moving towards the star or away (and at quite a high velocity), were one experimental demonstration. Given that and a few other simple things, we derived E = m c^2. As I remember the math was not hard, nothing beyond good high school algebra. The beauty of it seemed twofold: one, Einstein was able to recognize assumptions that everybody was used to making, assumptions so common that they were woven into the very language, and was able to think while not assuming these things. And, two, he was able to see the end point of the math, because of course there are many things in math that everybody can follow step by step and agree that it all must be so, while few people can picture the whole path and know to take this series of steps to reach an endpoint most of us can’t visualize from the starting point.
On the gravity waves moving at c (or no faster), there is a simple reduction to absurdity argument that they can’t go faster than c, if you accept Einsteinian special relativity (you don’t even have to accept his more complex general relativity to do this one). If gravity DID propagate faster than c, then scientists in certain perfectly legitimate (if hard to realize) reference frames could demonstrate that the effect occurred before the cause did. This would violate causality, the basic structure that says there can be causes and effects and that the causes must always occur before the effects.
The thing I’ve always loved about relativity explanations like this one are that I’m supposed to go “Yeah, that would be weird, so it must not be so.” and then in the next breath, they say stuff like “So anyway, there’s no such thing as simultanaity.”
Daniel, I’m a little confused about what you mean by “sides of the earth”. It’s a sphere- the whole surface is “a side”. In fact, at any given moment, the twilight sides of the earth go get more light than they (geometrically) should. It’s not gravity and relativity, though. It’s refraction. The atmosphere bends light like a prism and allows you to see the sunrise before the sun is “actually” risen.
You could be right Napier. I read this sometime in the dim past and never gave it much thought.
The only thing I could come up with is that unlike a sound wave’s pressure differentials, the magnetic field of the wave absolutely depends on the time varying electric field and vice versa. But I’m not really clear on this.
I did find this somewhat related statement, but again it really doesn’t say much.
Speaking of “sides of the Earth”… Everyone knows that the further away from the Earth you get, the more of the surface you can see, until when you’re very far away, you can see almost half of the surface. Few know, though, that once you get more than about a half a lightyear away, you can see more than half the surface, due to gravitational bending of the light (and this would be true even without atmospheric effects).
I meant absolute simultaneity. Well, substitute any crazy factoid from relativity then. I’m just saying that it’s funny to me to try to show logical paradoxes of proof of relativity.
Your reasoning is basically
“Isn’t it crazy to think that you could catch up to a light wave and see it standing still?”
“Yes”
“Therefore, if your twin flies around really fast, he’ll be younger than you.”
As I argued in another thread these problems arise solely from the difficulties of trying to translate the real language, math, into words. While it’s desirable to try to express these concepts in words, it’s often extremely difficult to do so. It also requires effort on the recipient’s end to understand that the words are not the physics.
The proper reasoning is something more like this.
Making the speed of light a constant in your axiom system leads to a number of equations from which physical effects can be derived and checked by experimentation.
One physical effect that can be derived from the equations that emerge, something that was not an assumption beforehand and is a surprise contained in the math, is that time passes at different rates for people in different reference frames. Experiment confirms this.
It is never the case that physicists say “the speed of light is a constant so lets brainstorm weird stuff”. This is what happens with non-physicists on the internet, however, which is one way we know they are wrong without even bothering to check out their results in detail.
The commentary on known effects here is not the same thing as the underlying math. The commentary is a convenience and an approximation only.
When I say, “But I’m not really clear on this,” what I mean is that in an EM wave the existence of the magnetic field explicitly depends on having a time varying electric field and vice versa.
But in a sound wave this isn’t really so (I think.) In other words the high pressure area isn’t created by the low pressure area except maybe semantically. And yet you certainly can’t have a sound wave without both. It doesn’t seem like the same phenomenon, but maybe it is. So I’m confused.
Maybe Chronos, Napier or Exapno could propound on the subject?
This is what puzzled Einstein also. For all kinds of waves, if you chased the wave (in a boat, on a train, whatever) the wave looked slower to you; if you traveled opposite the motion of the wave the wave seemed to move faster. For sound this was manifested by the pitch rising and falling. No mystery to this: the wave moves with constant speed within its medium (the air, the water, whatever) and you are moving with respect to the medium. But light is a wave with no observable medium! In Maxwell’s equations, the speed of light in a vacuum can be calculated from epsilon-naught and mu-naught, the permittivity and permiability of free space, constants of nature with no reference to motion. Severe mind-f*. Scientists of the time solved the conundrum by postulating an unseen electro-magnetic medium, the “aether” permeating all space. But Einstein took a different approach. He thought that some simple properties of nature everyone had always assumed to be true were, in fact, just wrong. The assumptions are (1) v1 = v0 + v, and (2) time is constant (the Galilean transforms) . In other words, if a car passes you at 20mph and you run begin running at 10mph, the car will move away from you at 10mph. Einstein showed that if you replace the Galilean transforms with the Lorenz equations (a more complex way of calculating velocities and time from within moving reference frames) then the light conundrum disappears: light can always appear be moving at its expected value no matter how the measurer is moving with respect to anything else. No aether needed. Experiments over and over again have shown that Einstein’s was not just a good idea, it was the law: Nature really does obey the Lorenz transforms, not the Galilean. The transforms incorporate a constant, “c”, with units of time. If you look a the equations, you can see that no calculation can yield a velocity greater than “c”. Nothing goes faster than “c” - not light, not radio, not rocket ships, not electons, not gravity - nothing. You simply cannot plug in any values that produce a velocity higher than “c”. Nature operates by these equations so nothing can ever go faster than “c”. Further work by Einstein showed that anything massless was actually constrained to always move at “c”. Light is massless and “c” is better known as “the speed of light”.