If gravity can accelerate a photon by bending its trajectory why doesn't it slow down a photon? This looks like magnetism effects on particles(changes direction but not magnitude of the velocity) ...
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maybe this related question answers yours?https://physics.stackexchange.com/questions/288170/since-quantum-mechanics-give-you-that-photons-have-relativistic-mass-m-frac?rq=1 – anna v Jan 21 '22 at 06:45
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@annav Why there is a problem with the 'river model'?It can explain gravitational time dilation for light, maybe frame-dragging... It considers photons as vibrations on a fluid space that sink in a mass volume causing gravity. How do we know that the photon near the event horizon cannot have unbelievebly small speed of receding from the BH due to 'river' going towards the black hole and slowly accelerates as the river is more slower as it is further from the BH? Simply, why the river model is less accepted?? – Krešimir Bradvica Jan 21 '22 at 13:02
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In the standard model of particle physics photons are zero mass point particles going with velocity c . The follow the geodesics of general relativity. A model of gravity as you describe, which I do not know and you give no link, even if succeeding in modeling macroscopic observations could not embed the standard model, imo, – anna v Jan 21 '22 at 13:56
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Because it's an experimentally verified fact that the speed of light is constant whether it is bending or not.
In ordinary speech, to accelerate means to increase your speed. In physics, acceleration is the rate of change of velocity. Now velocity is a normed vector and hence has both a magnitude and a direction. So in acceleration, both the magnitude and direction can change.
When light accelerates, its magnitude remains constant but its direction can change.
Mozibur Ullah
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Gravity does change the energy of photons, and it can both increase or decrease that energy depending whether the photon is going in or out of the gravitational well. That's called gravitational red- or blue shift. Of course photons still always travel at the speed of light.
rfl
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But bending of its trajectory is still change of velocity although only in direction.... – Krešimir Bradvica Jan 21 '22 at 13:04
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In everyday language "accelerate" implies making the photons go faster, but in physics parlance, it suffices to change the direction. In other words, if you change the direction but not the magnitude of a velocity, you still have an acceleration, but the object neither speeds up nor slows down.
That's what gravity does with photons - it can accelerate photons, but not speed it up nor slow it down.
Allure
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Does that mean gravity acts perpendicular to the velocity of the photons at all times? Or can it still not act perpendicular but GR somehow makes the speed constant – jensen paull Jan 21 '22 at 11:37
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1The OP has fallen victim to one of many semantic issues in physics: words that mean something in your kitchen mean something different in the physics classroom. On this particular problem I'll recommend never using the word decelerate. It has no real meaning in physics, so its use causes ambiguity. (Is an object decelerating in the negative direction speeding up?) My recommendation: reserve accelerate for any change in velocity, and speeding up and slowing down when you mean those things. – garyp Jan 21 '22 at 12:01
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@jensenpaull no, gravity is not necessarily perpendicular to the velocity of the photons, but if it isn't then there is a gravitational redshift or blueshift. See rfl's answer. There is no change in the speed of the photons, regardless. – Allure Jan 21 '22 at 12:58