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Thread: Is light considered matter?

  1. #1 Is light considered matter? 
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    Is light considered matter?

    The reason I ask is because I often hear scientists explain how the gravitational pull of a black hole is so great, not even light can escape.
    That would mean that light is matter because only matter is affected by gravity. Right?
    But if, on the other hand light itself does not consist of matter, then how could light be affected by gravity?
    I understand there are certain materials that can bend light. But gravity would only pull those materials and not the light itself right? The way I see it, light is just a beam of energy. Can energy be affected by gravity?
    Please help me understand this.


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  3. #2  
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    Quote Originally Posted by Thrillseeker View Post
    Is light considered matter?

    The reason I ask is because I often hear scientists explain how the gravitational pull of a black hole is so great, not even light can escape.
    That would mean that light is matter because only matter is affected by gravity. Right?
    But if, on the other hand light itself does not consist of matter, then how could light be affected by gravity?
    I understand there are certain materials that can bend light. But gravity would only pull those materials and not the light itself right? The way I see it, light is just a beam of energy. Can energy be affected by gravity?
    Please help me understand this.
    Well the opening para of this Wiki article suggests definitions of matter can either exclude particles with zero rest mass, such as light, or include them. So your question is rather a good one.

    However, as far as gravitation goes, my understanding is that any MASS is subject to gravitation. It does not matter (excuse the pun) whether the mass is rest mass or not. As light has mass by virtue of its energy, it is subject to gravitation, even though it has no rest mass.

    More generally, all energy is associated with mass. This is why in nuclear fission the mass of the products is less than the mass of the starting element - the missing mass is represented by the energy dissipated into the surroundings in the reaction. And the same goes for any chemical reaction too. A charged battery (electrochemical cell) has more mass than a discharged one, though the change in this case is far too tiny to be measured.


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    In General Relativity gravity is the curvature of space-time which can be caused by mass but also by momentum or energy and the rate at which they are changing (I've been told this is called the "Stress-Energy Tensor"). So whether light is classed as matter or not doesn't really matter (the puns are really easy in this one), as light has energy it can cause space-time curvature and affects (and can be affected by) gravity.
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    As light has mass by virtue of its energy, it is subject to gravitation,
    So light = energy = mass = subject to gravitation.
    Thanks for that info. I'm still trying to comprehend the idea of light being pulled by a gravitational force. It's not easy forming this picture in my mind.


    So technically, by this theory, could it mean that the stars we see that are billions of lightyears away, are influenced by gravitational fields from black holes, stars, planets and maybe even dark matter?
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    as light has energy it can cause space-time curvature and affects (and can be affected by) gravity.
    So light is not only affected by gravity but it can actually affect gravity itself?
    Now I'm completely lost
    I don't even understand what light has to do with time itself.

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  7. #6  
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    Quote Originally Posted by Thrillseeker View Post
    As light has mass by virtue of its energy, it is subject to gravitation,
    So light = energy = mass = subject to gravitation.
    Thanks for that info. I'm still trying to comprehend the idea of light being pulled by a gravitational force. It's not easy forming this picture in my mind.


    So technically, by this theory, could it mean that the stars we see that are billions of lightyears away, are influenced by gravitational fields from black holes, stars, planets and maybe even dark matter?
    Exactly. There is a phenomenon called "gravitational lensing" by which certain distant objects appear to be in a different place from they really are, due to the bending of the light from them as it passes close to known massive objects, en route to us. But I'm not an astronomer so I can't give you examples.
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  8. #7  
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    Quote Originally Posted by Thrillseeker View Post
    as light has energy it can cause space-time curvature and affects (and can be affected by) gravity.
    So light is not only affected by gravity but it can actually affect gravity itself?
    Now I'm completely lost
    I don't even understand what light has to do with time itself.

    Anything that has mass, momentum or energy can curve space-time and as such "affects" gravity. I'm not sure what you mean by "I don't even understand what light has to do with time itself." where does time come into the questions you have asked?
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    [/QUOTE]I'm not sure what you mean by "I don't even understand what light has to do with time itself." where does time come into the questions you have asked?[/QUOTE]

    You were talking about "space-time".
    Is there a difference between space-time and time as we know it?
    The way I see it, time is the chronological passing of events.
    So when you said "as light has energy it can cause space-time curvature" i was wondering how light can affect time.
    Maybe I just misunderstood?
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    There is a phenomenon called "gravitational lensing" by which certain distant objects appear to be in a different place from they really are, due to the bending of the light from them as it passes close to known massive objects


    So astronomers can never really make accurate calculations on the trajectory of planets, asteroids, galaxies and stars because what we see up there could mostly be illusion.
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    Ah, OK I see where the confusion comes from. Space-time is not the same as time. Basically time and space are not considered separately but are combined into a single continuum which is not space and not time but incorporates both. A basic introduction to the concept is here:

    Spacetime - Wikipedia, the free encyclopedia
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  12. #11  
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    Quote Originally Posted by Thrillseeker View Post
    There is a phenomenon called "gravitational lensing" by which certain distant objects appear to be in a different place from they really are, due to the bending of the light from them as it passes close to known massive objects


    So astronomers can never really make accurate calculations on the trajectory of planets, asteroids, galaxies and stars because what we see up there could mostly be illusion.
    Not really, most of what we observe isn't lensed as the density of gravitational objects in space is very low and it needs a very large amount of mass (like a galaxy or a black hole) to produce an observable effect.
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    Quote Originally Posted by exchemist View Post

    As
    light has mass by virtue of its energy,
    What if a photon is slowed to zero velocity.

    Does this really mean anything as it will always be moving with respect to all the other reference frames ?

    But at this point , with regard to the particular reference frame does it cease to have any interaction with "gravity"

    Maybe this is a meaningless question?
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    Quote Originally Posted by Thrillseeker View Post
    So light = energy = mass = subject to gravitation.
    That is generally correct. Although, I would not say "energy = mass" but they are equivalent in some ways. And mass can be converted to energy and vice versa. For example if a particle and an antiparticle meeet, they destroy one another (no mass left) but prouce two photons whose energy is exactly equivalent to the combined mass (e=mc2).

    I'm still trying to comprehend the idea of light being pulled by a gravitational force.
    As others have said, we don't consider gravity a "force" in GR (it is an effect of the geometry of space and time) however, even Newton predicted that gravity (a force) would affect the path of light. The amount predicted by Newtonian gravity is half the amount predicted by GR. This was one of the first calculations by Einstein and one of the first tests of the theory.
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    Quote Originally Posted by geordief View Post
    What if a photon is slowed to zero velocity.
    It can't be, photons always travel at c.

    Quote Originally Posted by geordief View Post
    Does this really mean anything as it will always be moving with respect to all the other reference frames ?
    Irrelevent, they move at c in all reference frames.

    Quote Originally Posted by geordief View Post
    But at this point , with regard to the particular reference frame does it cease to have any interaction with "gravity"
    No because photons always move at c.

    Quote Originally Posted by geordief View Post
    Maybe this is a meaningless question?
    Yep
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    Quote Originally Posted by Thrillseeker View Post
    So astronomers can never really make accurate calculations on the trajectory of planets, asteroids, galaxies and stars because what we see up there could mostly be illusion.[/COLOR]
    Absolutely not. The various effects are well understood and taken into account in analysing what we see.
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    light
    has mass by virtue of its energy

    Now I'm confused because I just read this on Wikipedia:
    "The photon is currently understood to be strictly massless. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum,
    c."
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    In General Relativity gravity is the curvature of space-time which can be caused by mass but also by momentum or energy and the rate at which they are changing
    So could momentum and energy and the rate at which they change be what accounts for Dark Matter?
    Because I hear there's not enough matter in the universe to account for all the gravity.

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    Is light considered matter?
    No. Light is considered radiation.

    The reason I ask is because I often hear scientists explain how the gravitational pull of a black hole is so great, not even light can escape.
    Correct.

    That would mean that light is matter because only matter is affected by gravity. Right?
    No. All forms of energy are affected by gravity, and are themselves a source of gravity. This includes mass, but also massless things such as electromagnetic radiation.

    So light is not only affected by gravity but it can actually affect gravity itself?
    Yes, absolutely.

    "The photon is currently understood to be strictly massless. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum, c."
    That is correct. Only particles without rest mass can move at the speed of light - in fact, the can do nothing else. The only two ( known and confirmed ) particles without rest mass are the photon and the gluon.
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    Quote Originally Posted by Thrillseeker View Post

    So could momentum and energy and the rate at which they change be what accounts for Dark Matter?
    Because I hear there's not enough matter in the universe to account for all the gravity.
    No, dark matter would be made of particles which interact only very weakly ( or not at all ) in the electromagnetic spectrum; their presence can be deduced only from their gravitational effects.
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    No, my understanding is that dark matter is hypothesized to account for the "extra" gravity after all the mass, energy, momentum etc. is taken into account.
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    Thank you very much for this interesting information Markus and PhDemon.
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    Quote Originally Posted by Thrillseeker View Post
    Thank you very much for this interesting information Markus and PhDemon.
    No problem
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    Quote Originally Posted by PhDemon View Post
    No, my understanding is that dark matter is hypothesized to account for the "extra" gravity after all the mass, energy, momentum etc. is taken into account.
    Maybe our universe is part of a multiverse with a gravitational field that influences our own universe? Could that explain dark matter?
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    Quote Originally Posted by Thrillseeker View Post
    Quote Originally Posted by PhDemon View Post
    No, my understanding is that dark matter is hypothesized to account for the "extra" gravity after all the mass, energy, momentum etc. is taken into account.
    Maybe our universe is part of a multiverse with a gravitational field that influences our own universe?
    How could something outside our universe influence things inside it?
    Quote Originally Posted by Thrillseeker View Post
    Could that explain dark matter?
    I would say no, but I'm no expert in this area, maybe Markus will give you a better answer.
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    Quote Originally Posted by PhDemon View Post
    Quote Originally Posted by geordief View Post
    What if a photon is slowed to zero velocity.
    It can't be, photons always travel at c.

    Quote Originally Posted by geordief View Post
    Does this really mean anything as it will always be moving with respect to all the other reference frames ?
    Irrelevent, they move at c in all reference frames.

    Quote Originally Posted by geordief View Post
    But at this point , with regard to the particular reference frame does it cease to have any interaction with "gravity"
    No because photons always move at c.

    Quote Originally Posted by geordief View Post
    Maybe this is a meaningless question?
    Yep
    Actually I was confusing atoms with photons

    I was remembering the time the people at IBM were able to write "IBM" with individual atoms using what they called "tweezers" and "molasses"

    Even so , how come you cannot slow down photons since "c" only applies in a vacuum? Is it theoretically impossible to slow photons to a stop in the same way as with atoms?
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    Photons always travel at c, the apparent slowing of photons in dense media is due to them being absorbed (and ceasing to exist) and re-emitted (moving at c again) by atoms in the medium. The delay between absorption and re-emission leads to the average speed through the medium being less than c despite the fact that photons always travel at c.
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    How could something outside our universe influence things inside it?


    I just thought that if galaxies are affected by a gravitational force in our universe, maybe our universe is affected by some larger gravitational force outside our universe in the same way. Does that make sense?
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    Not to me, I can't see how gravity or anything else outside our universe can affect things inside our universe.
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    Quote Originally Posted by Thrillseeker View Post
    I just thought that if galaxies are affected by a gravitational force in our universe, maybe our universe is affected by some larger gravitational force outside our universe in the same way.
    I believe there are some hypotheses similar to this based around string theory (i.e. that gravity could leak between "branes"). Not something I know much about.

    One problem is that dark matter is concentrated around galaxies in our universe; why would matter in that other universe "coincide" with the position of matter in ours? Another problem is that the distribution of dark matter is very different from normal matter; because dark matter doesn't interact electromagnetically it doesn't form structures in the same way as normal matter. Therefore the matter in our galaxy has, largely, collapsed to a disk but the dark matter is distributed as a large sphere.
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    Quote Originally Posted by PhDemon View Post
    Photons always travel at c, the apparent slowing of photons in dense media is due to them being absorbed (and ceasing to exist) and re-emitted (moving at c again) by atoms in the medium. The delay between absorption and re-emission leads to the average speed through the medium being less than c despite the fact that photons always travel at c.
    thanks that is a handy piece of information for me.
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    Quote Originally Posted by Thrillseeker View Post
    Maybe our universe is part of a multiverse with a gravitational field that influences our own universe? Could that explain dark matter?
    That may be the case with dark energy which is not required for a particular class of theory (below) but not as far as I am aware with dark matter which appears to be a resident within our classical universe (unless of course more speculative theories such as MOND rise to the challenge).

    You might have been reading a bit on M Theory and/or Steinhardts and Turoks hypothesis regarding a cyclic universe as a particular version of M Theory in a pop science book or article. This version of M theory theory (an extension of String Theory) offers a unification of spacetime and gravitation in a multi-dimensional multi-brane universe where our classical universe represents a brane of 3D and 1T in this context. In this theory these branes periodically collide and in doing so re-create a big bang each time. Gravity is hypothesised to not be restricted to a particular brane hence its effects are posed to leak between branes and result in some of its characterictic properties (it's weak long range nature etc.) In the cyclic universe model, dark energy is posed to be the force between the branes. :-))

    EDIT: You beat me to it Strange. *puts gun back in holster* :-))
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    Thank you very much for that info "Strange" and "Implicate Order" (cool nicknames by the way)
    That was indeed very interesting.

    would matter in that other universe "coincide" with the position of matter in ours?

    Maybe our universe is colliding with another universe? Just as galaxies collide with eachother.
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    As far as my understanding goes it is not the same as colliding galaxies. In order to collide the two objects must occupy the same space at the same time. Another universe will have a different space-time manifold that is not connected to the one in this universe (otherwise it would be part of our universe) so there can be no direct interaction of this type.

    This is why I said: "How could something outside our universe influence things inside it?" earlier.
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    In order to collide the two objects must occupy the same space at the same time. Another universe will have a different space-time manifold that is not connected to the one in this universe (otherwise it would be part of our universe) so there can be no direct interaction of this type.


    Thanks. I understand.
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    Quote Originally Posted by PhDemon View Post
    As far as my understanding goes it is not the same as colliding galaxies. In order to collide the two objects must occupy the same space at the same time. Another universe will have a different space-time manifold that is not connected to the one in this universe (otherwise it would be part of our universe) so there can be no direct interaction of this type.

    This is why I said: "How could something outside our universe influence things inside it?" earlier.
    This is one of the counter-intuitive notions that the 'cyclic model' addresses. The momentum energy of collision is transferred to each brane and provides the requisite energy for the next expansionary cycle.

    What 'the Cyclic universe' model of Steinhardt and Turok proposes is that at the point of collision of two inflated and nearly isotropic parrallel branes, a big bang event is created and the energy associated with the momentum of this collision event is added to each brane and provides a sufficient amount to recommence the entire expansionary cycle of standard cosmology. The collision itself causes sigificant undulations (source of gravitational waves) in the brane at the point of contact. This energy dissipates through the standard conventional cosmological process towards ultimate heat death where at this point this energy is uniformly spread throughout the brane and provides a future layer of vacuum energy following the next collision event. The momentum energy of the collision event also serves over time to once again flatten each brane as they trend towards equilibrium until they once again become virtually parrallel. As we approach this equilibrium point (towards heat death) the theory proposes that once again the dark energy 'between' branes overcomes each branes distributed expansionary momentum resulting in a future collision event for the next cycle. Note that the energy associated with the momentum of the prior event is represented in the next cycle as vacuum energy (isotropic state of energy from prior collisions) and hence we get a layer...upon layer.....upon layer effect of vacuum energy as each cycle occurs giving a cause for vacuum energy......then the next cycle occurs where more energy is added to the mix .....and so on.

    I think a geologist (sedimentologist) or palaeontologist would like this theory. :-))
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    Quote Originally Posted by PhDemon View Post
    Photons always travel at c, the apparent slowing of photons in dense media is due to them being absorbed (and ceasing to exist) and re-emitted (moving at c again) by atoms in the medium. The delay between absorption and re-emission leads to the average speed through the medium being less than c despite the fact that photons always travel at c.
    I apologise in advance for a confused (hopefully not too confusing) observation but I find that strange behaviour (I appreciate that probably everyone else here is well used to it ).

    So does this mean that light never undergoes acceleration and its speed is either zero or "c" with nothing in between no matter how closely you look wrt spacetime (is the step change a quantum phenomenon?)

    Does it make sense to say that "speed" is not an accurate description of "c" insofar as it pertains to light itself? Does the "speed" only apply to the observer's measurements while really "c" as it applies to light itself is more like an intrinsic "property" of light itself. (if that makes sense)

    Again ,if light cannot be accelerated except for in a total instantaneous way what about this bending that occurs around black holes etc ? Is that not classed as "acceleration" (I am used to thinking of circular motion as an example of acceleration)

    Please again feel free to disregard these observations if they are just down to wooly uneducated "thinking" (which mightn't surprise me ).
    Last edited by geordief; January 9th, 2014 at 06:35 AM.
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    Quote Originally Posted by geordief View Post
    Quote Originally Posted by PhDemon View Post
    Photons always travel at c, the apparent slowing of photons in dense media is due to them being absorbed (and ceasing to exist) and re-emitted (moving at c again) by atoms in the medium. The delay between absorption and re-emission leads to the average speed through the medium being less than c despite the fact that photons always travel at c.
    I apologise in advance for a confused (hopefully not too confusing) observation but I find that strange behaviour (I appreciate that probably everyone else here is well used to it ).

    So does this mean that light never undergoes acceleration and its speed is either zero or "c" with nothing in between no matter how closely you look wrt spacetime (is the step change a quantum phenomenon?)

    Does it make sense to say that "speed" is not an accurate description of "c" insofar as it pertains to light itself? Does the "speed" only apply to the observer's measurements while really "c" as it applies to is more light itself is more like an intrinsic "property" of light itself. (if that makes sense)

    Again ,if light cannot be accelerated except for in a total instantaneous way what about this bending that occurs around black holes etc ? Is that not classed as "acceleration" (I am used to thinking of circular motion as an example of acceleration)

    Please again feel free to disregard these observations if they are just down to wooly uneducated "thinking" (which mightn't surprise me ).
    It seems to me the idea that you don't accelerate light from rest when it is created is easier to grasp if you think of it as a wave. You do not accelerate a wave when you make one: it goes at a set speed, determined by the properties of the medium through which the wave propagates. You create a wave by disturbing the medium (e.g. drop a pebble into a pool), whereupon the wave instantaneously begins to propagate at its pre-set speed.

    A light frequency photon is typically propagated by an electron in an atom changing state, creating an oscillating dipole. This change of state is not instantaneous (though it is extremely fast), so it takes a finite time to send out the complete photon if you like.

    I'll have to leave your other points to those whose grasp of relativity is better than mine.
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    It is my understanding that photons always travel at c (they cannot travel at a lower speed or aspeed of zero, when absorbed they cease to exist so do not have a speed).

    "c" is an intrinsic part of the structure of our universe (for example c2 is the constant of proportionality between mass and energy, and it crops up in other areas of physics that on the surface have little to do with light, speed of light is a bit of a misnomer, it just arises that massless particles (like photons) have to travel at c because of the way "c" is a fundamental part of how the universe works.

    As for the bending around black holes I don't think this is acceleration as the photon is moving along a geodesic (a "straight line" in curved spacetime).

    Someone with a better handle on the fundamental physics (I'm a chemist by training) may give you a better answer but this is my understanding of it.
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  40. #39  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by geordief View Post
    So does this mean that light never undergoes acceleration
    Correct.

    and its speed is either zero or "c" with nothing in between
    It is always c - photons are "created" that way.

    Does the "speed" only apply to the observer's measurements while really "c" as it applies to is more light itself is more like an intrinsic "property" of light itself.
    It is an intrinsic property of vacuum.

    Again ,if light cannot be accelerated except for in a total instantaneous way what about this bending that occurs around black holes etc ?
    The bending is due to the curvature of space-time.

    Is that not classed as "acceleration"
    It is classed as coordinate acceleration, as opposed to proper acceleration. In Newtonian physics these are the same, but in relativistic physics ( in curved space-time ) they are not. The difference is that coordinate acceleration is what a far-away reference observer sees ( a bent ray of light ), whereas proper acceleration is what an accelerometer co-moving with the light ray would physically measure ( if such a thing was possible ). Coordinate acceleration doesn't vanish in this case, and depends explicitly on the chosen observer, which is why the ray is bent globally, but proper acceleration vanishes everywhere locally, since an accelerometer in free fall will measure exactly zero. This difference between local and global measurements is a crucial thing to understand when it comes to GR.

    Please again feel free to disregard these observations if they are just down to wooly uneducated "thinking" (which mightn't surprise me ).
    It isn't. Quite the contrary, really - it is a very good question, and understanding the answer is crucial to the understanding of GR.
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by geordief View Post
    Again ,if light cannot be accelerated except for in a total instantaneous way what about this bending that occurs around black holes etc ? Is that not classed as "acceleration" (I am used to thinking of circular motion as an example of acceleration)
    The definition of acceleration in GR is rather different from the classical version (which is just change of velocity; i.e. speed and/or direction).

    For example, classically, you think of an object in free fall as accelerating but in GR that is not acceleration. Sitting in your chair at your desk so you feel the "force" of gravity is acceleration. Weird, huh?
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Quote Originally Posted by Strange View Post
    The definition of acceleration in GR is rather different from the classical version (which is just change of velocity; i.e. speed and/or direction).
    The correct and complete definition of an object's 4-acceleration in the context of GR would be the covariant derivative of 4-velocity with respect to proper time, which must vanish for free-fall objects :



    Locally, space-time can always be considered Minkowskian, which means the connection coefficients vanish, giving us just the usual



    Globally, however, since space-time is curved, this is no longer true, and we get the geodesic equation instead - the solutions to which are not "straight" from the point of view of a far-away reference observer ( but they are as straight as is possible in a curved space-time ).
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