Discussion in 'Astronomy, Exobiology, & Cosmology' started by machiaventa, Jul 9, 2012.
Do you mean to say that, photons are affected by gravity due to its momentum ?
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In the electroweak symmetry breaking the Goldstone modes are eaten by the W bosons. And it isn't so much that the NG modes break the symmetry but rather they are a result of the broken symmetry. The electroweak Mexican hat potential illustrates this nicely. It's the Higgs scalar value which is involved in the symmetry and when broken the NG modes become apparent.
If particle photon is affected by gravity, then photon should accelerate or decelerate under the effect of gravity. But speed of photon is constant.
No. In the general relativistic picture, photons are affected by gravity because they follow the "shortest path" through the local curved spacetime.
In general relativity, local frames are inertial and photons travel at a constant speed. But I'm fairly sure that the global spacetime geometry can be curved, and in that case the speed of light will not be measured to be constant.
If spacetime is curved and particle photon is following this curved path; I think this is frame-dragging only because photon normally travels in a straight line.
Let us take the example with the Sun. A ray of light bends around the Sun. If this is due to the gravity of the Sun, then the light emitted by the Sun also should be affected by the gravity of the Sun. But is it so?
'c' (speed of light) is universal constant like 'G' and 'h'. So, 'c' is frame-independant; be it local frame or global frame or curved global spacetime geometry.
SR is based on constancy or frame-independance of 'c'. Is SR based on gravity?
Even if a photon particle is affected by gravity, why only tangential photon should be affected by gravity; why non-tangential photon particles are not affected by gravity?
The universal constant "c" and the speed of light you measure in a particular frame, especially and accelerated one, are not the same thing.
SR only applies in inertial frames. SR is a subset of GR - a "special" case.
What's a non-tangential photon particle?
As per SR, speed of light is constant with relative to an observer irrespective of the speed or acceleration of the observer. So, speed of light will also remain constant with relative to a measuring equipment.
SR happens when a mass is at a relativistic speed. At a relativistic speed, the mass may cause frame-dragging also.
SR is a special case of relative motion between an observer and light, where speed of light remains constant with relative to the observer. SR is not about gravity whereas GR is about gravity.
Non-tangential photon particle is a photon particle which is either emitted or absorbed by a star or a planet.
Tangential photon particle is the photon particle which travels tangentially or parallel to the tangent of a star or planet.
No. The speed of light is not constant relative to an accelerating observer. As I said, SR applies only in inertial reference frames.
SR applies at speed of 1 metre per second, and at 200,000,000 metres per second.
With respect, I don't think you havea clue about what frame dragging is, since understanding that concept requires an understand of general relativity. And it appears to me that you don't have a good understanding even of special relativity.
Basically correct. SR is GR in flat spacetime (i.e., roughly, without gravity).
That term does not appear in any of the scientific literature I am familiar with. Did you invent it yourself?
I coined these terms to distinguish between photons which are emitted/absorbed by astronomical mass and the photons which are not emitted/absorbed by the same astronomical mass but may bend around it.
Some of the photon particles emitted by the Sun will be absorbed by our Earth. With relative to the Earth, these photon particles can be considered as 'non-tangential photons'.
Some other photon particles emitted by the Sun but not absorbed by our Earth and travelling in the vicinity of our Earth can be considered as 'tangential photons' with relative to our Earth.
These tangential photons may bend around the Earth. If this bending of photons around our Earth is due to the gravity of our Earth, then the photons being absorbed by our Earth also should be affected by the gravity of our Earth. But is it so?
Yes, it is so. All photons are affected by gravity. While under the right conditions we have detected gravitational lensing, due to the sun's gravitational field, the earth itself does not affect a photon within the range of our ability to detect it.
You get stuck on the gravitational lensing and frame-dragging, while ignoring observations of a redshift associated with the affect of gravity, on large scales. A photon's velocity is not altered, but its wavelength can be. A strong gravitational field can red or blue shift a photon's wavelength, depending on if it is headed into or out of the gravitational field.
(GR explains this as being the result of changes in the curvature of space and/or, if you like.., changes in how time (or clocks) and measuring devices (or rulers) are affected by where they are in a gravitational field.)
So photon's of any vector, (tangential or non-tangential) are all affected by any gravitational field that they move through. A gravitational field can change a photon's wavelength and or its direction. Changes in wavelength, are easiest to detect over cosmological distances, where time and distance results in an accumulated detectable change.
If a gravitational field can change the direction of a photon, that means the gravitational field is causing angular acceleration to the photon particle.
If the gravitational field can cause angular acceleration to the photon particle, why gravitational field is not able to cause linear acceleration to the photon particle?
Gravitational force works on a mass for all the directions of motion of the mass. Whatever may be the initial motion(vector) of the mass, gravity being a force will apply some acceleration to the mass and its velocity-vector will change accordingly.
In the case of photon particle the velocity vector changes only in gravitational lensing, where its speed remains the same but only direction of motion of photon particle changes.
Red shift or blue shift of a photon particle can not be considered as an acceleration of photon as here velocity vector of photon remains unchanged.
If gravitational force can cause gravitational lensing of a photon particle, then gravitational force also should be able to cause linear acceleration or deceleration of a photon particle. But there is no evidence of linear acceleration/deceleration of photon particle due gravitational force.
So, perhaps gravity does not interact with photon particle. Something else may be interacting with photon particle to cause gravitational lensing.
There is also evidence that some massive particle do not interact with gravity. So, particle photon not interacting with gravity is not unlikely.
In classical everyday terms we think of gravity as a force. That is what it looks and feels like to us. That is also consistent with the perspective of Newtonian Dynamics. But this view point can only be said to accurately describe gravity in a weak field limit or locally...
General relativity as a description of gravity replaced Newtonian dynamics, retaining the Newtonian perspective as a locally defined limited case. GR does not describe gravity as a force. Instead it describes gravity as a consequence of a curvature of spacetime.
For lay discussions this is where most of the confussion begins. We tend to try to think of things in terms consistent with our everday experience.., where gravity feels like a force. However, our everyday experience is defined by that limited local weak field limit of GR, which is well described by the conceptual model of Newtonian dynamics.
Where it appears you are being confused is that what we observe as gravitational lensing, gravitational redshifts and other strong field examples of gravitation, cannot be accurately explained by Newtonian dynamics and gravity as a force. (An example is frame-dragging which cannot be explained by the Netonian view point.) It is well described by GR where gravity is described geometrically as a curvature of spacetime.
From that prespective, the perspective of general relativity gravitational lensing has nothing to do with a force of gravity changing the velocity vector of anything. Even though, in the weak field limit that is what it looks like to us. When objects are moving at classical velocities, everything looks and feels like gravity is a force. What is happening is that the photon is not moving at classical velocities. They travel a the speed of light and follow a straight path through curved spacetime, which from our local perspective does not appear to be straight. Instead it looks curved.
There is no easy or accurate classical everyday example of what the curvature of space looks like. The bowling ball on a rubber sheet, is often used but is a very crude an inaccurate example.
When talking about the effect that gravitation has on a photon, it is very important to remember that what we are talking about are not things that happen in that local weak field limit that can be described within the context of Newtonian dynamics where gravity feels like a force.
Photons always move in a straight line following the curvature of spacetime.
Curvature of spacetime around a rotating astronomical mass like the Sun, is frame-dragging only. Thats why gravitational lensing happens around the Sun.
Frame-dragging effect of our Earth is very weak(though it has significant gravity); thats why photon does not bend around our Earth.
Hansda, this is a basic relativity issue. You are definning the photon's path from an inertial or accelerating frame that is not the same as the frame of the photon. From the photon's frame of reference it does not beand around anything. When we think of, or imagine the path of the photon in the everyday classical terms of "our" frame of reference, it "looks" like it bends. But that is just how it looks from our outside point of view.
A very gross example might be, when you extend a pole into water, you know the pole is straight. but it "looks" like it bends when it enters the water.
The path of a photon only looks like it bends to us because we are observing its curved space path, from a flat space point of view.
And frame-dragging while it adds to the curvature of space around a gravitating object, does not play the primary or greatest role in the curvature of space that results in gravitational lensing. On a theoretical level, as I know of no observable non-rotating gravitational masses, even a non-rotating gravitation mass, would produce a gravitational lensing effect.
You really are stuck on the frame-dragging causes gravitational lensing bit. It is an important aspect of GR but it is not as significant as you think, where practical observations are concerned... The sun does not rotate fast enough for its frame-dragging effect to be the deciding factor in observed gravitational lensing... And cosmological gravitational lensing is observed to occur where there is no object to generate frame-dragging, hence the interjection of dark matter into the cosmological model.
As per GR, spacetime curves around a astronomical mass to generate gravity. So, the spacetime will curve around the Earth. Spacetime will curve around the Sun. Spacetime will curve around the Moon.
Consider an observer on the Earth. The observer on the Earth will see photon particle bending around the Sun but the observer on the Earth will not see any bending of photon around the Moon though spacetime curves around the Moon. So, just curvature of spacetime does not bend the photon ; something more is involved there.
This is totally different case of change of medium. Here no spacetime is involved.
Non-rotating gravitational mass can not cause frame-dragging. So, they can not cause gravitational lensing.
Our Earth causes frame-dragging to some extent, which is proven by GP-B experiment. So, the Sun also can cause frame-dragging.
Why wouldn't the Moon's effect be noticed? It'd be much much weaker but it would still be there.
Those are due to different things so you can have lensing within frame dragging. You can get lensing from the Schwarzchild metric. Lensing is little more than a more elaborate instance of light deflection.
You are using a common classical definition of curve or curvature. When within the context of GR the word curve or curvature of spacetime is used, it does not mean curve or cuvature as you would use those words, to describe a road you drive on.
And yes from our relatively fixed frame of reference, light does appear to bend around a star. That does not mean that from the photon's frame of reference it does anything but travel in a straight line.
As OnlyMe points out, it is possible to construct a space-time where a space-like section is flat, so light follows straight lines, but which overall has non-zero curvature. It's a somewhat counter intuitive concept and one which a lot of people find they have to churn through a few examples (when doing the mathematics of it) to develop a new kind of intuition for.
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