xyzt Posted January 6, 2015 Posted January 6, 2015 Light (photons) fall as fast as lead balls in a gravitational field. This is not true, GR teaches us that photons move DIFFERENTLY from massive particles in a gravitational field. Photons follow null geodesics, massive particles follow non-null geodesics, the equations of motion are different.
Robittybob1 Posted January 6, 2015 Posted January 6, 2015 This is not true, GR teaches us that photons move DIFFERENTLY from massive particles in a gravitational field. Photons follow null geodesics, massive particles follow non-null geodesics, the equations of motion are different. This is the one thing that has baffled me over the last week, and you are the first person to answer that statement. When I was drawing my light clock in the free falling frame I thought the observer inside the frame would see the photon strike the light clock bouncing side to side, but from an outside observer the frame is accelerating in the gravitational field, so doesn't that imply the light inside the frame is falling at the same rate as everything else in that frame? Did you understand me there? What is the explanation for light not falling at the same rate as everything else, as Einstein said?
xyzt Posted January 6, 2015 Posted January 6, 2015 (edited) This is the one thing that has baffled me over the last week, and you are the first person to answer that statement. When I was drawing my light clock in the free falling frame I thought the observer inside the frame would see the photon strike the light clock bouncing side to side, but from an outside observer the frame is accelerating in the gravitational field, so doesn't that imply the light inside the frame is falling at the same rate as everything else in that frame? No, it doesn't, I already explained that to you in the previous post Did you understand me there? What is the explanation for light not falling at the same rate as everything else, as Einstein said? You start with the geodesic equations: [math]\frac{d^2x^a}{ds^2} =- \Gamma^{a}_{bc}\frac{dx^b}{ds}\frac{dx^c}{ds} [/math] For light, the left hand side of the equation is zero (light is not subjected to acceleration): [math]0 =- \Gamma^{a}_{bc}\frac{dx^b}{ds}\frac{dx^c}{ds} [/math] For massive particles, the left hand side of the equation is NOT zero. The right hand side is the same, for both photons and for massive particles. Edited January 6, 2015 by xyzt
Robittybob1 Posted January 6, 2015 Posted January 6, 2015 No, it doesn't, I already explained that to you in the previous post You start with the geodesic equations: [math]\frac{d^2x^a}{ds^2} =- \Gamma^{a}_{bc}\frac{dx^b}{ds}\frac{dx^c}{ds} [/math] For light, the left hand side of the equation is zero (light is not subjected to acceleration). For massive particles, the left hand side of the equation is NOT zero. The right hand side is the same, for both photons and for massive particles. Can you please explain it in terms of the light clock in the free falling frame, I can't yet relate to equations unless you can say them in words, sorry.
xyzt Posted January 6, 2015 Posted January 6, 2015 Can you please explain it in terms of the light clock in the free falling frame, I can't yet relate to equations unless you can say them in words, sorry. Tough. You need to take an introductory class. The language of physics is math.
imatfaal Posted January 6, 2015 Posted January 6, 2015 Tough. You need to take an introductory class. The language of physics is math. Harsh - but fair. Can you please explain it in terms of the light clock in the free falling frame, I can't yet relate to equations unless you can say them in words, sorry. We have been spoiled by pop-sci explanations of terrifically complex questions into thinking that these questions can have simple answers. I have just looked up Geodesic Equations to find out what courses at University teach them. For your guidance the School of Physics at Edinburgh University rates their General Relativity course as 11 on a scale of 1-12 with 12 being Doctoral Studies. So it is is MPhys sort of level.
Robittybob1 Posted January 6, 2015 Posted January 6, 2015 (edited) Harsh - but fair. We have been spoiled by pop-sci explanations of terrifically complex questions into thinking that these questions can have simple answers. I have just looked up Geodesic Equations to find out what courses at University teach them. For your guidance the School of Physics at Edinburgh University rates their General Relativity course as 11 on a scale of 1-12 with 12 being Doctoral Studies. So it is is MPhys sort of level. Wasn't it simpler than that? In the free falling frame all physics experiments would behave the same for the person within the free fall frame. So that seemed to me to suggest that the light photon would still hit the sensor. Does this mean it moves exactly like the rest of the frame. The lead ball would remain at the same height even staying level with the photon. I'm going to check out the physics - in the weekend. Edited January 6, 2015 by Robittybob1
xyzt Posted January 6, 2015 Posted January 6, 2015 Wasn't it simpler than that? In the free falling frame all physics experiments would behave the same for the person within the free fall frame. So that seemed to me to suggest that the light photon would still hit the sensor. Does this mean it moves exactly like the rest of the frame. The lead ball would remain at the same height even staying level with the photon. A free falling frame is inertial. By contrast, an observer hovering at a certain distance from the gravitating body must be accelerating (constantly), so he's not in an inertial frame. The two situations are not equivalent. I'm going to check out the physics - in the weekend. Excellent idea.
MigL Posted January 6, 2015 Posted January 6, 2015 You had better re-check that xyzt. In the frame of a falling observer, they are not accelerating. Otherwise you would have the curious case of seeing everything in a free-falling elevator become blurred, and horizontal light beams curving upwards.. Your equations and the resultant thinking are correct, but I don't think you're applying them correctly. Sorry, I don't do LaTex, but maybe Elfmotat can weigh in ?
elfmotat Posted January 6, 2015 Posted January 6, 2015 This is not true, GR teaches us that photons move DIFFERENTLY from massive particles in a gravitational field. Photons follow null geodesics, massive particles follow non-null geodesics, the equations of motion are different. This isn't true. The equation of motion is the geodesic equation for both massive and massless particles. Massless particles just have the additional constraint that [math]ds^2=0.[/math] In other words, they both follow geodesics, but massless particles only follow geodesics such that their velocity is locally constant. You had better re-check that xyzt. In the frame of a falling observer, they are not accelerating. Otherwise you would have the curious case of seeing everything in a free-falling elevator become blurred, and horizontal light beams curving upwards.. Your equations and the resultant thinking are correct, but I don't think you're applying them correctly. Sorry, I don't do LaTex, but maybe Elfmotat can weigh in ? I agree. A free-falling observer can be locally considered as an inertial frame. (I.e. ignoring all second-order effects due to curvature.) This is one of the core principles of GR: the equivalence principle. If you are in a free-falling elevator and shine a laser, the photons had better fall at the same rate that you do. Otherwise you'd be able to tell you were free-falling by measuring light-deflection. Equivalently, a uniformly accelerating elevator and an elevator sitting on the surface of a planet (with equivalent g-value) should also behave identically: light should be deflected by the same amount in both instances.
xyzt Posted January 6, 2015 Posted January 6, 2015 This isn't true. The equation of motion is the geodesic equation for both massive and massless particles. Massless particles just have the additional constraint that [math]ds^2=0.[/math] In other words, they both follow geodesics, but massless particles only follow geodesics such that their velocity is locally constant. Err, you claim that "That isn't true" and you follow by repeating my exact sentence. Pay more attention when you read, please. You had better re-check that xyzt. In the frame of a falling observer, they are not accelerating. Who said that they are accelerating? Are you reading what I wrote or are you making up things?
Robittybob1 Posted January 6, 2015 Posted January 6, 2015 Err, you claim that "That isn't true" and you follow by repeating my exact sentence. Pay more attention when you read, please. Who said that they are accelerating? Are you reading what I wrote or are you making up things? The light in the light clock will for an outside observer be falling and hence traveling further hence the time is dilated (the ticks of the falling clock will be getting longer by the tick.) Does that keep their "velocity locally constant" Further in a longer period of time sounds like the beginning of constant velocity. From the frame of the local observer they are not accelerating - I understand that.
MigL Posted January 7, 2015 Posted January 7, 2015 Robbitybob asked.. "so doesn't that imply that light inside the frame is falling at the same rate as everything else in that frame?" To which you replied... "No it doesn't. I already explained that to you in the previous post" Then you go on to present the geodesic equation and the distinction between massive and massless paths. So one of us needs to pay more attention to what they type or read, and stop making things up. I leave it up to readers to decide who that someone is.
xyzt Posted January 7, 2015 Posted January 7, 2015 (edited) Robbitybob asked.. "so doesn't that imply that light inside the frame is falling at the same rate as everything else in that frame?" To which you replied... "No it doesn't. I already explained that to you in the previous post" In the free-falling frame light moves at c. All other objects do not move wrt the frame (they all float within the frame since they are freefalling just the same). Do you have problems understanding this? Then you go on to present the geodesic equation and the distinction between massive and massless paths. Correct. Using math makes answers unambigous. So one of us needs to pay more attention to what they type or read, and stop making things up.I leave it up to readers to decide who that someone is. If you do not understand what I wrote, just ask. I will explain it to you. Does that keep their "velocity locally constant" Further in a longer period of time sounds like the beginning of constant velocity. From the frame of the local observer they are not accelerating - I understand that. This is incomprehensible, so it is not answerable. Edited January 7, 2015 by xyzt
MigL Posted January 7, 2015 Posted January 7, 2015 But that's not what Robbitybob asked. is it xyzt ? Do you have trouble understanding this ? Because ,if you want, I can explain this to you just as condescendingly.
xyzt Posted January 7, 2015 Posted January 7, 2015 But that's not what Robbitybob asked. is it xyzt ? You need to stop trolling at some point. Especially since you amply demonstrated that you clearly don't understand the basics. Sorry, I don't do LaTex, but maybe Elfmotat can weigh in ? In other words, you "don't do math". Well, the language of physics IS math, you should try learning how to express your thoughts in math form. Until then, any debate with you is leading nowhere.
Theoretical Posted January 7, 2015 Author Posted January 7, 2015 If I understand this correctly, gravity acting on a photon is not acceleration? What is it called? Since I don't know what else to call it, I'll refer to it as acceleration for now. Just substitute it for the correct word. Anyhow, I read that a photon experiences twice the acceleration(?) as objects such as atoms. Is that correct?
xyzt Posted January 7, 2015 Posted January 7, 2015 If I understand this correctly, gravity acting on a photon is not acceleration? What is it called? Photons do not accelerate. The local speed of light (in vacuum) is invariant. Since I don't know what else to call it, I'll refer to it as acceleration for now. Since there is NO acceleration you have no reason to call it...acceleration. Anyhow, I read that a photon experiences twice the acceleration(?) as objects such as atoms. Is that correct? Nope.
elfmotat Posted January 7, 2015 Posted January 7, 2015 Err, you claim that "That isn't true" and you follow by repeating my exact sentence. Pay more attention when you read, please. I quoted you directly. You said they have different equations of motions. They don't. The worldline of a photon satisfies the geodesic equation. "Constraints" are not usually considered to be "equations of motion." There's no need to get snippy about it, though I do find the irony of the sentiment amusing. That's just semantics anyway. The argument here stems from whether the question of "how does light fall" is being asked locally or globally. Massive particles and massless particles will locally "fall" identically to first order. Globally, second-order effects like curvature must be considered. That is, the answer depends on the distance scales you're working with and how accurate you want to be.
Theoretical Posted January 7, 2015 Author Posted January 7, 2015 Photons do not accelerate. The local speed of light (in vacuum) is invariant. Since there is NO acceleration you have no reason to call it...acceleration. If I understand this correctly, gravity acting on a photon is not acceleration? What is it called? Since I don't know what else to call it, I'll refer to it as acceleration for now. Just substitute it for the correct word. Anyhow, I read that a photon experiences twice the acceleration(?) as objects such as atoms. Is that correct? Nope. "In fact, if you work it through with Einstein's theory of gravity, you find that light bends by exactly twice as much as Newton predicted - and this agrees with experiment." http://physics.stackexchange.com/questions/122003/light-and-gravity-bending-of-light-around-a-massive-body "This means that a photon is effectively accelerated by twice as much as a slow-moving object." https://www.physicsforums.com/threads/bending-of-light-due-to-gravity.770226/ "Basically a horizontal photon will fall twice as fast in a gravitational field as a dropped object." http://www.researchgate.net/post/Did_Einstein_show_that_Galileos_Falling_Bodies_experiment_and_his_own_theories_of_Relativity_both_Special_and_General_have_deficiencies "Over a century later, in the early 20th century, Einstein developed his theory of general relativity. Einstein calculated that the deflection predicted by his theory would be twice the Newtonian value." http://www.einstein-online.info/spotlights/light_deflection "Curiously, however, Einsteins theory predicts that the path of light will be bent by twice as much as does Newtons theory, due to a kind of positive feedback. " http://www.physicsoftheuniverse.com/topics_relativity_general.html "Einstein's general relativity (the speed of falling light varies twice as fast as the speed of ordinary falling objects): " http://www.network54.com/Forum/304711/thread/1409005811/last-1409232873/HOW+DOES+THE+SPEED+OF+LIGHT+VARY+IN+GRAVITY,+EINSTEINIANS+- "Thus, the deflection predicted by GR is twice the deflection predicted by Newtonian gravity." http://www.reddit.com/r/askscience/comments/1nkc8m/why_does_einsteins_theory_predict_that_light/
xyzt Posted January 7, 2015 Posted January 7, 2015 (edited) I quoted you directly. You said they have different equations of motions. They don't. The worldline of a photon satisfies the geodesic equation. "Constraints" are not usually considered to be "equations of motion." There's no need to get snippy about it, though I do find the irony of the sentiment amusing. That's just semantics anyway. The argument here stems from whether the question of "how does light fall" is being asked locally or globally. Massive particles and massless particles will locally "fall" identically to first order. Globally, second-order effects like curvature must be considered. That is, the answer depends on the distance scales you're working with and how accurate you want to be. Let's try again, using math instead of psychobabble: "You start with the geodesic equations: [math]\frac{d^2x^a}{ds^2} =- \Gamma^{a}_{bc}\frac{dx^b}{ds}\frac{dx^c}{ds} [/math] For light, the left hand side of the equation is zero (light is not subjected to acceleration): [math]0 =- \Gamma^{a}_{bc}\frac{dx^b}{ds}\frac{dx^c}{ds} [/math] For massive particles, the left hand side of the equation is NOT zero. The right hand side is the same, for both photons and for massive particles." This is precisely what I wrote, in precise mathematical terms. This is basic textbook stuff. Do you have any issue understanding the above? "In fact, if you work it through with Einstein's theory of gravity, you find that light bends by exactly twice as much as Newton predicted - and this agrees with experiment." http://physics.stackexchange.com/questions/122003/light-and-gravity-bending-of-light-around-a-massive-body "Over a century later, in the early 20th century, Einstein developed his theory of general relativity. Einstein calculated that the deflection predicted by his theory would be twice the Newtonian value." http://www.einstein-...ight_deflection "Curiously, however, Einsteins theory predicts that the path of light will be bent by twice as much as does Newtons theory, due to a kind of positive feedback. " http://www.physicsof...ty_general.html The above is correct but this is not what you asked. What you asked is this nonsense: Anyhow, I read that a photon experiences twice the acceleration(?) as objects such as atoms. Is that correct? Do you understand the difference? Edited January 7, 2015 by xyzt
Theoretical Posted January 7, 2015 Author Posted January 7, 2015 (edited) The above is correct but this is not what you asked. What you asked is this nonsense: My post was very clear. I'm no longer interested in anything you have to say. Edited January 7, 2015 by Theoretical
xyzt Posted January 7, 2015 Posted January 7, 2015 My post was very clear. I'm no longer interested in anything you have to say. Yes, your post was very clear. Was also very wrong. Based on your follow-on, you still don't understand how wrong it was.
Theoretical Posted January 7, 2015 Author Posted January 7, 2015 "This means that a photon is effectively accelerated by twice as much as a slow-moving object." https://www.physicsforums.com/threads/bending-of-light-due-to-gravity.770226/
xyzt Posted January 7, 2015 Posted January 7, 2015 "Einstein's general relativity (the speed of falling light varies twice as fast as the speed of ordinary falling objects): " http://www.network54.com/Forum/304711/thread/1409005811/last-1409232873/HOW+DOES+THE+SPEED+OF+LIGHT+VARY+IN+GRAVITY,+EINSTEINIANS+- I find the above quote extremely amusing since it is a discussion between two very well known cranks (Pentcho Valev and John Duffield). One piece of advice: you will NEVER learn physics via (selective) quoting. "This means that a photon is effectively accelerated by twice as much as a slow-moving object." https://www.physicsforums.com/threads/bending-of-light-due-to-gravity.770226/ Mine quoting (crackpots) will never get you to learn physics. But I am repeating myself.
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