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Thread: How does light gain momentum?

  1. #1 How does light gain momentum? 
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    Take the sun for example... is the light propelled away from the sun? by what?

    Does light ever slow down? or is the nature of light to keep racing as fast as it can in every direction?


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    Quote Originally Posted by question for you View Post
    Take the sun for example... is the light propelled away from the sun? by what?
    No. Never.
    Quote Originally Posted by question for you View Post
    Does light ever slow down?
    There is a small effect on the speed of light due to the gravitational potential.


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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by question for you View Post
    Take the sun for example... is the light propelled away from the sun? by what?
    No. Never.
    Quote Originally Posted by question for you View Post
    Does light ever slow down?
    There is a small effect on the speed of light due to the gravitational potential.
    Thank you pbm.

    So if it isn't propelled... then... is it attracted to something? My head feels funny.
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    Forum Professor pyoko's Avatar
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    Light follows gravitational lines and the shortest path between two points. If the gravity there, or rather space-time curvature is bent, it will follow that. Space-time bends around a planer, a star, or a black hole. This is why we see gravitational lensing. Light is not really "pulled" or pushed by anything in the usual physical sense. It has zero rest mass but has momentum. If a massive particle were to travel at the speed of light, it would need to be infinitely massive.

    From Spacetime - the relativistic view of time and space :

    Gravitation bends light rays.


    Since light has no mass, it is not subject to Newton's law of gravity, and hence, in Newtonian physics gravity has no effect on light. If space is curved, however, it follows that a ray of light seemingly moving in a straight line really travels in a curved line following the curvature of space. This is comparable, in some way, to the itinerary of a plane. Because the earth is a sphere, the shortest path between two points on earth is described by a geodesic, a curved line. While moving along the geodesic it would appear to the passengers of the plane that they are moving in a straight line, although they are not. Similarly, the light of distant stars travels through the curved geometry of space before it reaches Earth. This proposition is supported by observation.
    When the light of a star passes close to the Sun, it is deflected by the Sun's gravitational field, which causes it to appear slightly displaced. The star appears to be farther from the Sun than it should be. The displacement has been measured by photographing the apparent position of stars during a solar eclipse and comparing these positions with those observed in the night some time later. Apparent shifts of less than 2 seconds per arc have been measured this way, in close agreement with the predictions of General Relativity. Likewise, the mentioned deviation in the orbit of Mercury when the planet reaches its perihelion (=closest position to the Sun), which is in contradiction with the laws of Newton, can be explained with Einstein's model of curved space.
    It is by will alone I set my mind in motion.
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    Quote Originally Posted by pyoko View Post
    Light follows gravitational lines and the shortest path between two points. If the gravity there, or rather space-time curvature is bent, it will follow that. Space-time bends around a planer, a star, or a black hole. This is why we see gravitational lensing. Light is not really "pulled" or pushed by anything in the usual physical sense. It has zero rest mass but has momentum. If a massive particle were to travel at the speed of light, it would need to be infinitely massive.
    Hmm ok. I'm not sure what you mean by 'follows gravitational lines', what are they and why does light follow them?

    light is bent by gravity or space time curvature... so gravity acts on lights direction. But gravity is not the force which gives light its momentum...

    so is it unknown how light moves? but it nearly always travels at speed of light? except when gravity slows it down a bit? so when the light leaves the sun, does the suns gravity slow it down? so that it isnt actually going at the 'speed of light' and once the light is out of the sun's gravitational feild, does it automatically acellerate to maximum light speed?
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    Quote Originally Posted by pyoko View Post
    Light follows gravitational lines and the shortest path between two points. If the gravity there, or rather space-time curvature is bent, it will follow that. Space-time bends around a planer, a star, or a black hole. This is why we see gravitational lensing. Light is not really "pulled" or pushed by anything in the usual physical sense. It has zero rest mass but has momentum. If a massive particle were to travel at the speed of light, it would need to be infinitely massive.

    From Spacetime - the relativistic view of time and space :

    Gravitation bends light rays.


    Since light has no mass, it is not subject to Newton's law of gravity, and hence, in Newtonian physics gravity has no effect on light. If space is curved, however, it follows that a ray of light seemingly moving in a straight line really travels in a curved line following the curvature of space. This is comparable, in some way, to the itinerary of a plane. Because the earth is a sphere, the shortest path between two points on earth is described by a geodesic, a curved line. While moving along the geodesic it would appear to the passengers of the plane that they are moving in a straight line, although they are not. Similarly, the light of distant stars travels through the curved geometry of space before it reaches Earth. This proposition is supported by observation.
    When the light of a star passes close to the Sun, it is deflected by the Sun's gravitational field, which causes it to appear slightly displaced. The star appears to be farther from the Sun than it should be. The displacement has been measured by photographing the apparent position of stars during a solar eclipse and comparing these positions with those observed in the night some time later. Apparent shifts of less than 2 seconds per arc have been measured this way, in close agreement with the predictions of General Relativity. Likewise, the mentioned deviation in the orbit of Mercury when the planet reaches its perihelion (=closest position to the Sun), which is in contradiction with the laws of Newton, can be explained with Einstein's model of curved space.
    Its amazing but when i first quoted this post, the bit about gravitation bending light wasn't showing!

    While reading this i imagined that the sun I see in the sky... really has a different position to how I see it, due to the curvature of space time, or gravitational lines. So its like when you look into water and see a fish, the fish is really in a different position than it apears to the eye. But with the sun, the amount of variation between what i see and where the sun is, depending on what point of the earth'r surface i'm at, im geussing sun set and rise is when the sun will be most 'illusional, and mid day is when the light will be travelling at its straightest to my eye on the surface.

    Does it sound like I'm on the right track with that idea?
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    Quote Originally Posted by question for you View Post
    Take the sun for example... is the light propelled away from the sun? by what?

    Does light ever slow down? or is the nature of light to keep racing as fast as it can in every direction?
    Light (and radio and other electromagnetic waves) always travels at the speed of light. It needs no momntim other than what it has when the photon is formed.That's the way our universe is.
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    Quote Originally Posted by question for you View Post
    So if it isn't propelled... then... is it attracted to something? My head feels funny.
    No. Its a result of time and space changing where the photon is moving.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by question for you View Post
    So if it isn't propelled... then... is it attracted to something? My head feels funny.
    No. Its a result of time and space changing where the photon is moving.
    Can you give me any more details pmb? It's mind boggling for me. The light always moves away from the sources at light speed, but where does it go? what becomes of it? it always travels in a straight line apart from any wave like travel and a bit of space curvature. it's affected by gravity ( I think) so it is absorbed and held in the earth?

    Is light even a thing?
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  11. #10  
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    Unless the light encounters an electrically charged particle, i.e. and electron or an ion, it will travel in a straight line forever. The CMBR is the light which was released 377,000 years after the Big Bang.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    Quote Originally Posted by AlexG View Post
    Unless the light encounters an electrically charged particle, i.e. and electron or an ion, it will travel in a straight line forever. The CMBR is the light which was released 377,000 years after the Big Bang.
    Thats great, what does it mean?

    are electrons and ions the only electricly charged particals? why does this stop light?
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    Quote Originally Posted by AlexG View Post
    Unless the light encounters an electrically charged particle, i.e. and electron or an ion, it will travel in a straight line forever. The CMBR is the light which was released 377,000 years after the Big Bang.
    does that mean light needs an electron or ion to reflect?
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  14. #13  
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    Yes. Photons are absorbed by electrons which raises the electron's energy level. When the electron returns to it's lower energy level, it radiates a photon.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    Forum Professor pyoko's Avatar
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    Effects of gravity on light: photons are deviated instead of following a straight
    course like Newton would have predicted. Both "trajectories" are strictly speaking "straight lines", even if they "look" curved. They are both the shortest path.

    It is by will alone I set my mind in motion.
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    Quote Originally Posted by AlexG View Post
    Yes. Photons are absorbed by electrons which raises the electron's energy level. When the electron returns to it's lower energy level, it radiates a photon.
    How long does the electron hold on to the photon for? this is very new to me.

    In terms of light reflecting from a random surface... the photon hits the electron or ion, then the electron or ion pretty quickly eject it?
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    Quote Originally Posted by question for you View Post
    Quote Originally Posted by pmb View Post
    Quote Originally Posted by question for you View Post
    So if it isn't propelled... then... is it attracted to something? My head feels funny.
    No. Its a result of time and space changing where the photon is moving.
    Can you give me any more details pmb? It's mind boggling for me.
    Quote Originally Posted by question for you View Post

    Sure. As a photon moves through a gravitational field its spatial position changes by the amount dL the time it takes to move that distance is dt. The velocity is given by v = dL/dt. If the gravitational field is uniform then this quantity can be shown to have the value (1 + gz2)c, i.e.

    v = (1 + gz2)c

    Therefore as z changes as the photon climbs higher in the gravitational field then so to does the speed of light.

    I created a web page to show that v = (1 + gz2)c. It’s at

    http://home.comcast.net/~peter.m.brown/gr/c_in_gfield.htm
    For a photon moving in a gravitational field of a spherically shaped body scroll down to where it says Speed of Light in a Schwarzschild Gravitational Field.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by question for you View Post
    Quote Originally Posted by pmb View Post
    Quote Originally Posted by question for you View Post
    So if it isn't propelled... then... is it attracted to something? My head feels funny.
    No. Its a result of time and space changing where the photon is moving.
    Can you give me any more details pmb? It's mind boggling for me.
    Quote Originally Posted by question for you View Post
    Sure. As a photon moves through a gravitational field its spatial position changes by the amount dL the time it takes to move that distance is dt. The velocity is given by v = dL/dt. If the gravitational field is uniform then this quantity can be shown to have the value (1 + gz2)c, i.e.

    v = (1 + gz2)c

    Therefore as z changes as the photon climbs higher in the gravitational field then so to does the speed of light.

    I created a web page to show that v = (1 + gz2)c. Itís at

    http://home.comcast.net/~peter.m.brown/gr/c_in_gfield.htm
    For a photon moving in a gravitational field of a spherically shaped body scroll down to where it says Speed of Light in a Schwarzschild Gravitational Field.
    Thats great stuff.

    The thing i don't like about maths is that theres always a load of characters and symbols and never a key to explain what they mean.

    What does gz mean? gravity? and with dl/dt what does the / mean? divide by?

    As for your webpage the amount of questions i need answering befor i can makes sense of that is unreal! looks like some very impressive stuff
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  19. #18  
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    Quote Originally Posted by question for you View Post

    Thats great stuff.

    The thing i don't like about maths is that theres always a load of characters and symbols and never a key to explain what they mean.
    That’s not really a problem for physicists since anybody who knows calculus knows what these symbols mean. The problem with forums is we don’t know what the mathematical abilities are of the other person.

    Quote Originally Posted by question for you View Post
    What does gz mean? gravity?
    g is the acceleration due to gravity at the origin of the coordinate system. Z is the z-coordinate of the photon in the xyz coordinate system.

    Quote Originally Posted by question for you View Post
    and with dl/dt what does the / mean? divide by?
    dL/dt means the derivative of L with respect to t. See Derivative -- from Wolfram MathWorld

    Quote Originally Posted by question for you View Post
    As for your webpage the amount of questions i need answering befor i can makes sense of that is unreal! looks like some very impressive stuff
    The mathematics of General Relativity can be daunting but that part is rather simple. One only needs to know what a metric is and know calculus.

    I'm sorry that I can't help you better. I don't know who to explain something something calculus based to someone who doesn't know calculus.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by question for you View Post

    Thats great stuff.

    The thing i don't like about maths is that theres always a load of characters and symbols and never a key to explain what they mean.
    Thatís not really a problem for physicists since anybody who knows calculus knows what these symbols mean. The problem with forums is we donít know what the mathematical abilities are of the other person.

    Quote Originally Posted by question for you View Post
    What does gz mean? gravity?
    g is the acceleration due to gravity at the origin of the coordinate system. Z is the z-coordinate of the photon in the xyz coordinate system.

    Quote Originally Posted by question for you View Post
    and with dl/dt what does the / mean? divide by?
    dL/dt means the derivative of L with respect to t. See Derivative -- from Wolfram MathWorld

    Quote Originally Posted by question for you View Post
    As for your webpage the amount of questions i need answering befor i can makes sense of that is unreal! looks like some very impressive stuff
    The mathematics of General Relativity can be daunting but that part is rather simple. One only needs to know what a metric is and know calculus.

    I'm sorry that I can't help you better. I don't know who to explain something something calculus based to someone who doesn't know calculus.

    Ok, so 'calculus' is the name for all the symbols and letters used in maths? and I thought it meant pebbles!

    My mathematical ability: Well im uneducated except from high school maths... I was always good at picking up on mathematics at school, I got a brain that can work out equations, I'm pretty good at mental arithmatic. But i'm not educated to the point where I can read symbols and codes. Any idea where the best place for a beginer to start learning about the laguage of maths? is there a mathimatical dictionairy?

    So is gz is the acceleration and z is the axis or coordinate... what does one do with the gz in the equation? multiply one by the other? add them? divide them? keep them?

    L is time, t is distance? ... my question was: what is the reason the write it like this dl/dt... whats is the / symbol? we were taught at school, that is the division symbol, is that correct? I can't understand much of the wolfram stuff as I don't know about calculus.

    I was managing to vaguely grasp the original stuff you posted, just wasn't sure about dl/dt... I even managed to geuss g has something to do with gravity which im pleased with.

    You better stop explaining the maths of relativity to me, your welcome to explain calculus though!

    A metric? im going to geuss that means a unit measurment within a metric distance, or a distance, divided by 100. whats the difference between metric and decimal?
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    Decimal means ten or a division of ten where as metric is 100... right?
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