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### Author Topic: Can we prove light is affected by gravity?  (Read 9685 times)

#### yor_on

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##### Re: Can we prove light is affected by gravity?
« Reply #25 on: 28/01/2014 02:24:47 »
OK then here is the argument. Gravitational field strength is proportional to both time dilation and length contraction. The stronger the field then the more intense both these effects are. It follows that if we say the gravitational field is cancelled at the centre of a mass then so must the effects of time dilation and length contraction be cancelled. No field (cancelled out) no effect on spacetime. It cannot be argued any other way. If a force is cancelled out then it CANNOT produce an effect.

A gravitational time dilation and Lorenz contraction is related to mass, and, according to Einstein, to a uniform constant acceleration. Then you have 'energy', but I would prefer to leave that one aside for this as it hurts my head. But it also involves two frames of reference. Defining the 'empty center' as one frame, and the shell as another there will be a time dilation involved, as the two 'imaginary clocks' we compare between can't give us the same 'time'. Doesn't matter from where you measure, although if you measure from a center, it's no longer empty, sort of, so then we would have to add whatever mass that is to our consideration.

#### yor_on

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##### Re: Can we prove light is affected by gravity?
« Reply #26 on: 28/01/2014 02:41:42 »
You might be thinking of it as the 'center' being the more correct clock though? Well, I don't know, from a 'global perspective' as in 'containing a universe' you might want to argue that a flat space, without gravity is some lowest 'state' of 'time'. From a local definition the 'right clock' always must be your 'wrist watch' though, and that one never change relative yourself (ideally defined). So from a local point of view all other clocks you measure your own against must go 'wrong', but as your local clock never change relative yourself, no matter where you are, you can define that local clock as being 'global' too, although now from a ideal local definition.

That one doesn't care for mass, or speed/velocity. It's what makes us age and die, no matter what we do, or where.

#### yor_on

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##### Re: Can we prove light is affected by gravity?
« Reply #27 on: 28/01/2014 02:57:07 »
You can use a local definition, to define a 'point' though, scaling it down. But that is not equivalent to compressing matter into a black hole. When you scale you magnify.

#### JP

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##### Re: Can we prove light is affected by gravity?
« Reply #28 on: 28/01/2014 03:06:50 »
OK then here is the argument. Gravitational field strength is proportional to both time dilation and length contraction. The stronger the field then the more intense both these effects are. It follows that if we say the gravitational field is cancelled at the centre of a mass then so must the effects of time dilation and length contraction be cancelled. No field (cancelled out) no effect on spacetime. It cannot be argued any other way. If a force is cancelled out then it CANNOT produce an effect.

A gravitational time dilation and Lorenz contraction is related to mass, and, according to Einstein, to a uniform constant acceleration. Then you have 'energy', but I would prefer to leave that one aside for this as it hurts my head. But it also involves two frames of reference. Defining the 'empty center' as one frame, and the shell as another there will be a time dilation involved, as the two 'imaginary clocks' we compare between can't give us the same 'time'. Doesn't matter from where you measure, although if you measure from a center, it's no longer empty, sort of, so then we would have to add whatever mass that is to our consideration.

Gravitational field is not proportional to time dilation or length contraction, though.  Time dilation and length contraction always involves comparing the point of view of two observers, so it involves comparing their two reference frames, including their relative motions and the difference in the gravitational field between them.

So two observers at rest WRT each other inside the spherical shell both measure the same, zero net gravitational force and they agree on measurements of clocks and meter sticks.  If one observer is inside and one is distant and both are at rest WRT the spherical shell, they they DO observe length contraction/time dilation, since any communication they have with each other has to pass through the gravitational field outside the shell.  They observe this despite the fact that each locally measures no gravitational force.

#### jeffreyH

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##### Re: Can we prove light is affected by gravity?
« Reply #29 on: 28/01/2014 10:12:37 »
OK then here is the argument. Gravitational field strength is proportional to both time dilation and length contraction. The stronger the field then the more intense both these effects are. It follows that if we say the gravitational field is cancelled at the centre of a mass then so must the effects of time dilation and length contraction be cancelled. No field (cancelled out) no effect on spacetime. It cannot be argued any other way. If a force is cancelled out then it CANNOT produce an effect.

A gravitational time dilation and Lorenz contraction is related to mass, and, according to Einstein, to a uniform constant acceleration. Then you have 'energy', but I would prefer to leave that one aside for this as it hurts my head. But it also involves two frames of reference. Defining the 'empty center' as one frame, and the shell as another there will be a time dilation involved, as the two 'imaginary clocks' we compare between can't give us the same 'time'. Doesn't matter from where you measure, although if you measure from a center, it's no longer empty, sort of, so then we would have to add whatever mass that is to our consideration.

Gravitational field is not proportional to time dilation or length contraction, though.  Time dilation and length contraction always involves comparing the point of view of two observers, so it involves comparing their two reference frames, including their relative motions and the difference in the gravitational field between them.

So two observers at rest WRT each other inside the spherical shell both measure the same, zero net gravitational force and they agree on measurements of clocks and meter sticks.  If one observer is inside and one is distant and both are at rest WRT the spherical shell, they they DO observe length contraction/time dilation, since any communication they have with each other has to pass through the gravitational field outside the shell.  They observe this despite the fact that each locally measures no gravitational force.

How can length contraction and time dilation not have a proportionality WRT gravity. They are caused by either the gravitational field or the relativistic momentum of a mass. We rely too much on observer perspectives. The universe doesn't need observers for the laws of physics to apply. The other problem is the complexity of interactions. I know the Higgs field should have an interaction with gravity but this can't be possible if the graviton is massless and travels at c. If the graviton has mass it no longer travels at c and gravity falls apart. Also if it has mass it isn't going to pass through matter. This tends to suggest another as yet unknown particle that provides an indirect mechanism. I can't find a way for this to happen. So to answer a previous question, yes, gravity SUCKS!

#### JP

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##### Re: Can we prove light is affected by gravity?
« Reply #30 on: 28/01/2014 13:46:18 »
How can length contraction and time dilation not have a proportionality WRT gravity. They are caused by either the gravitational field or the relativistic momentum of a mass.

Just because something is caused by something else does not mean it has to be proportional, which is why your argument doesn't hold.  The other problem is that you're assuming length contraction and time dilation are something that happens locally, and they're not.  An observer at a point in space observes the laws of physics to hold in his reference frame.  This means he measures length and time normally.  Length contraction/time dilation only happen when comparing his observations to that of another observer in a different reference frame.

Quote
We rely too much on observer perspectives. The universe doesn't need observers for the laws of physics to apply.
The question of observers is a complex and philosophical one, since we can't do science without observing.  Putting that aside, as I pointed out above, length contraction and time dilation is an effect of comparing processes--they don't even have to be measurements in a standard sense--in two different reference frames.  This is why it's not proportional to local gravitational effects, but depends on the way the structure of space-time varies between the two observers (and their relative motions).

Quote
The other problem is the complexity of interactions. I know the Higgs field should have an interaction with gravity but this can't be possible if the graviton is massless and travels at c. If the graviton has mass it no longer travels at c and gravity falls apart. Also if it has mass it isn't going to pass through matter. This tends to suggest another as yet unknown particle that provides an indirect mechanism. I can't find a way for this to happen. So to answer a previous question, yes, gravity SUCKS!
There are several problems with these statements: First, the Higgs field predicts inertial mass, i.e. the resistance of an object to a push or pull embodied in the mass in Newton's famous F=ma.  Although we assume inertial mass is equivalent to gravitational mass, we don't have a fundamental reason why this should be so and the Higgs field doesn't tell us anything about gravitational mass.  There's a missing piece to tie quantum field theory into general relativity and explain this.  Next, the graviton is a hypothesized particle in some models of quantum gravity, but getting a consistent theory in which is appears is not easy, and it may not even exist.  Third, having mass does not necessarily have to do with how easily something can pass through matter.  The most important factor that determines interaction with matter is charge or interaction with charge.  Photons have no mass, but they can be easily absorbed by a lot of matter.  Neutrinos have mass (admittedly small mass), but interact weakly with charged particles and zip right through most matter.  Fourth, maybe there is an unknown particle and maybe there isn't.  It's all well and good to say that a theory is incomplete (we know gravity/QM are incomplete descriptions), but this incompleteness just means we have more to discover.  It doesn't predict an unknown particle.  Maybe it predicts many particles.   Maybe it predicts one.  What it does require is coming up with testable theories that are consistent with prior observations (and also therefore with current QM and theories of gravity).

#### lightarrow

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##### Re: Can we prove light is affected by gravity?
« Reply #31 on: 28/01/2014 15:24:11 »
OK then here is the argument. Gravitational field strength is proportional to both time dilation and length contraction. The stronger the field then the more intense both these effects are. It follows that if we say the gravitational field is cancelled at the centre of a mass then so must the effects of time dilation and length contraction be cancelled. No field (cancelled out) no effect on spacetime. It cannot be argued any other way. If a force is cancelled out then it CANNOT produce an effect.
And where is the problem? Outside you will have warped spacetime, inside no.

#### jeffreyH

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##### Re: Can we prove light is affected by gravity?
« Reply #32 on: 28/01/2014 15:25:49 »
My point is that there is a varying gradient through spacetime of both length contraction and time dilation. This would only be apparent if viewed from infinity where gravity has no influence. Since we can't do that experimentally we have to devise some means of theoretically assuming we are working from infinity. After all g is taken as negative and referenced from infinity. If all we do is chop the universe up into an infinite set of frames of reference that can only be viewed locally or transformed one at a time we are missing the bigger picture. To solve the Einstein field equations we have to think outside the box. The tools we are using haven't worked for a solution to gravitation for the last 100 years.

#### yor_on

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##### Re: Can we prove light is affected by gravity?
« Reply #33 on: 28/01/2014 16:29:33 »
I totally agree JP :)

Relativity is a science of philosophy to me, and logic. That's also what makes it fun. And "How can length contraction and time dilation not have a proportionality WRT gravity." is indeed a very nice question to ask oneself. Think of a guy in a rocket, traveling at a constant uniform gravity of one gravity. Ignoring spin we now can stipulate that he should find a equal 'field' of gravity inside that rocket, as what he found resting at Earth.

How is that possible?

He expends energy to get to this acceleration and gravity, Earth doesn't, to give you that same G. If we translate his rocket and fuel into energy then? And then do the same for Earth? Not enough, although Earth translated into energy becomes a magnitude of energy I can't even imagine. But there must be a point while this little rocket accelerates at one uniform G, where it, in that instant, surpass all the energy of a Earth, just to gain one small displacement more per second.

It's crazy, at least very hard to understand, how it can work this way.

#### yor_on

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##### Re: Can we prove light is affected by gravity?
« Reply #34 on: 28/01/2014 16:52:27 »
And if you use 'light clocks' you will see that time dilations and Lorentz contractions aren't solely connected to inertia, or gravity. It's a result of the fact that 'c' is a constant invariant factor, belonging to all 'inertial frames of reference', aka uniform motions. It's always 'c', doesn't matter how fast your uniform speed (geodesic/velocity) is relative some other reference frame. So describing it as a proportionality to gravity solely becomes a tough proposition.
=

What I personally think is that there must be a connection to 'c' for gravity too though. And 'c' is a description of displacements over time, but, as locally defined. If you want to be strict we define a proper mass from uniform motion too btw. Accelerations plays havoc with your scale, as you weight yourself, so we need to include proper mass there too. And then we have transformations and 'energy' which are the ones making it make sense, although not locally measurable, except as gravity/inertia under accelerations.

So not only 'c', but 'local'. The last, to me, means that we need to wonder what 'motion' is, and how we define a clock, and so also a 'distance'. All of those are local definitions made from a uniform motion.

And then there is relativistic mass, which is the sort of mass you can't measure locally in a uniform motion, or acceleration, but becomes very tangible in any collision, expressed in kinetic energy. And that is a added problem as it neither is measurable from the vacuum, nor locally in your uniformly moving, or accelerating, rocket. What you can measure locally is a blue/redshift, and a gravity/inertia under a acceleration but it's not proportional to your relativistic mass, as you easily can see thinking of accelerating at one uniform gravity, considering the kinetic energy expressed in a collision, at different times. And if in a uniform motion this too will be absent, locally.

Einstein gave us one description, QM gives us one more, with Higgs defining inertia under accelerations. None of them seems wrong, but none of them seems to tell the whole story either. Not to forget, we have classical physics too, as Newtons and Maxwell's that works really nice under normal circumstances. Einstein said himself that it was Maxwell that gave him the idea of 'c', if I remember right.
« Last Edit: 28/01/2014 17:28:15 by yor_on »

#### jeffreyH

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##### Re: Can we prove light is affected by gravity?
« Reply #35 on: 28/01/2014 17:42:10 »
And if you use 'light clocks' you will see that time dilations and Lorentz contractions aren't solely connected to inertia, or gravity. It's a result of the fact that 'c' is a constant invariant factor, belonging to all 'inertial frames of reference', aka uniform motions. It's always 'c', doesn't matter how fast your uniform speed (geodesic/velocity) is relative some other reference frame. So describing it as a proportionality to gravity solely becomes a tough proposition.
=

What I personally think is that there must be a connection to 'c' for gravity too though. And 'c' is a description of displacements over time, but, as locally defined. If you want to be strict we define a proper mass from uniform motion too btw. Accelerations plays havoc with your scale, as you weight yourself, so we need to include proper mass there too. And then we have transformations and 'energy' which are the ones making it make sense, although not locally measurable, except as gravity/inertia under accelerations.

So not only 'c', but 'local'. The last, to me, means that we need to wonder what 'motion' is, and how we define a clock, and so also a 'distance'. All of those are local definitions made from a uniform motion.

And then there is relativistic mass, which is the sort of mass you can't measure locally in a uniform motion, or acceleration, but becomes very tangible in any collision, expressed in kinetic energy. And that is a added problem as it neither is measurable from the vacuum, nor locally in your uniformly moving, or accelerating, rocket. What you can measure locally is a blue/redshift, and a gravity/inertia under a acceleration but it's not proportional to your relativistic mass, as you easily can see thinking of accelerating at one uniform gravity, considering the kinetic energy expressed in a collision, at different times. And if in a uniform motion this too will be absent, locally.

Einstein gave us one description, QM gives us one more, with Higgs defining inertia under accelerations. None of them seems wrong, but none of them seems to tell the whole story either. Not to forget, we have classical physics too, as Newtons and Maxwell's that works really nice under normal circumstances. Einstein said himself that it was Maxwell that gave him the idea of 'c', if I remember right.

There are a lot of complexities and we haven't mentioned pressure, density and vibrational energy.

#### yor_on

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##### Re: Can we prove light is affected by gravity?
« Reply #36 on: 28/01/2014 19:16:44 »
Well, it's one of the hardest ideas to encompass in relativity. I like measurements, and using those you have no extra 'energy' stored locally, in a added uniform motion, also called relative motion. But we have measurably different velocities inside this universe.

Neither will you measure the vacuum outside to contain more energy due to that added motion.

But Einstein describes it in his stress energy tensor. In that gravity becomes a result of the geometry of SpaceTime, as I get it :) defined by mass energy and the geodesics something will take, 'time' is another parameter necessary, naturally, for it to exist. In that geodesics will describe the geometry. "Mass tells space-time how to curve, and space-time tells mass how to move.". How that is translated into a uniformly accelerating rocket, and its relativistic mass, aka kinetic energy, as measured from a collision is a trickier one, that I really would like to understand from measurements, aka experiments. I definitely have to agree with Pete in that 'relativistic mass' must exist though. I think you also might translate it into 'potential energy', well possibly? But I like measurements/experiments.
=

what you can define in any motion though is a changed relation between frames of reference. If you measure light from a sun, and then start to move towards it, you will find light to blue shift. But is that the same as a perfect vacuum? And how can I prove a perfect vacuum to change, using no suns, measurable radiation? Another sore point of such an idea is the question, if I now, by defining the vacuum as able to unmeasurably 'change' due to my motion, now won't have to present it (the vacuum) as a 'absolute frame of reference'?

There are no absolute frames of reference in relativity, only locally definable constants, those proven to exist by anyone doing a (equivalent) local experiment, giving us our definition of 'repeatable experiments'.
« Last Edit: 28/01/2014 19:53:52 by yor_on »

#### jeffreyH

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##### Re: Can we prove light is affected by gravity?
« Reply #37 on: 28/01/2014 20:12:11 »
Well, it's one of the hardest ideas to encompass in relativity. I like measurements, and using those you have no extra 'energy' stored locally, in a added uniform motion, also called relative motion. But we have measurably different velocities inside this universe.

Neither will you measure the vacuum outside to contain more energy due to that added motion.

But Einstein describes it in his stress energy tensor. In that gravity becomes a result of the geometry of SpaceTime, as I get it :) defined by mass energy and the geodesics something will take, 'time' is another parameter necessary, naturally, for it to exist. In that geodesics will describe the geometry. "Mass tells space-time how to curve, and space-time tells mass how to move.". How that is translated into a uniformly accelerating rocket, and its relativistic mass, aka kinetic energy, as measured from a collision is a trickier one, that I really would like to understand from measurements, aka experiments. I definitely have to agree with Pete in that 'relativistic mass' must exist though. I think you also might translate it into 'potential energy', well possibly? But I like measurements/experiments.
=

what you can define in any motion though is a changed relation between frames of reference. If you measure light from a sun, and then start to move towards it, you will find light to blue shift. But is that the same as a perfect vacuum? And how can I prove a perfect vacuum to change, using no suns, measurable radiation? Another sore point of such an idea is the question, if I now, by defining the vacuum as able to unmeasurably 'change' due to my motion, now won't have to present it (the vacuum) as a 'absolute frame of reference'?

There are no absolute frames of reference in relativity, only locally definable constants, those proven to exist by anyone doing a (equivalent) local experiment, giving us our definition of 'repeatable experiments'.

Seeing that you mentioned light. It is noticeable that if you look at a light bulb from across a room it is dimmer than looking at it from an inch away. Think of this. If photons are moving in a straight line then the same number of photons should reach your eye whether you are across the room or an inch away, UNLESS some of them miss at the greater distance. That is they describe a transverse trajectory that increases with distance. So not all the photons reach your eye as some pass by at the outer edges. May not be correct but would answer some questions if it was.

#### JP

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##### Re: Can we prove light is affected by gravity?
« Reply #38 on: 28/01/2014 20:50:56 »
My point is that there is a varying gradient through spacetime of both length contraction and time dilation. This would only be apparent if viewed from infinity where gravity has no influence. Since we can't do that experimentally we have to devise some means of theoretically assuming we are working from infinity. After all g is taken as negative and referenced from infinity. If all we do is chop the universe up into an infinite set of frames of reference that can only be viewed locally or transformed one at a time we are missing the bigger picture. To solve the Einstein field equations we have to think outside the box. The tools we are using haven't worked for a solution to gravitation for the last 100 years.

Sure they have.  We can use them to make lots of calculations, including correcting for gravitational time dilation in GPS.  Obviously general relativity is incomplete since it doesn't work with quantum theory, but that doesn't mean it's wrong.  It's a model that works where its supposed to work and will have limitations like any model.  Saying we need to think outside the box is true (if thinking in the box worked, we'd already have a working theory of quantum gravity), but it doesn't tell us anything.  If you want to say that the idea of an observer in GR is flawed, you need to come up with a replacement theory that makes new predictions and explains why GR works so very well for observers, because it does!

#### jeffreyH

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##### Re: Can we prove light is affected by gravity?
« Reply #39 on: 29/01/2014 09:56:43 »
My point is that there is a varying gradient through spacetime of both length contraction and time dilation. This would only be apparent if viewed from infinity where gravity has no influence. Since we can't do that experimentally we have to devise some means of theoretically assuming we are working from infinity. After all g is taken as negative and referenced from infinity. If all we do is chop the universe up into an infinite set of frames of reference that can only be viewed locally or transformed one at a time we are missing the bigger picture. To solve the Einstein field equations we have to think outside the box. The tools we are using haven't worked for a solution to gravitation for the last 100 years.

Sure they have.  We can use them to make lots of calculations, including correcting for gravitational time dilation in GPS.  Obviously general relativity is incomplete since it doesn't work with quantum theory, but that doesn't mean it's wrong.  It's a model that works where its supposed to work and will have limitations like any model.  Saying we need to think outside the box is true (if thinking in the box worked, we'd already have a working theory of quantum gravity), but it doesn't tell us anything.  If you want to say that the idea of an observer in GR is flawed, you need to come up with a replacement theory that makes new predictions and explains why GR works so very well for observers, because it does!

I completely agree with everything you have just said. I know relativity works and has revolutionized physics, as did the work on the standard model of particle physics. The idea of an observer is not flawed, arguing that would be nonsensical. Gravity involves a complex interrelationship of properties. This complexity doesn't lend itself to being dissected. By viewing it locally we remove variation. If we use remote observers there are always observers either more or less affected than them. We are stuck in the middle. As an example of this thinking, if you are relating all your measurements of a wave function from a position halfway up the rising edge of a peak you don't see it in the same way as removing yourself and viewing from a distance. Your calculations are much less complex when you can see the whole thing.
« Last Edit: 29/01/2014 09:59:15 by jeffreyH »

#### JP

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##### Re: Can we prove light is affected by gravity?
« Reply #40 on: 29/01/2014 16:11:25 »
As an example of this thinking, if you are relating all your measurements of a wave function from a position halfway up the rising edge of a peak you don't see it in the same way as removing yourself and viewing from a distance. Your calculations are much less complex when you can see the whole thing.

The problem is that all measurements are local.  You can't "see the whole thing."  Rather, you see whatever information reaches your local position.  If you're at the right place at the right time you can measure the entire thing as a sum of local measurements.

#### jeffreyH

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##### Re: Can we prove light is affected by gravity?
« Reply #41 on: 30/01/2014 00:51:50 »
As an example of this thinking, if you are relating all your measurements of a wave function from a position halfway up the rising edge of a peak you don't see it in the same way as removing yourself and viewing from a distance. Your calculations are much less complex when you can see the whole thing.

The problem is that all measurements are local.  You can't "see the whole thing."  Rather, you see whatever information reaches your local position.  If you're at the right place at the right time you can measure the entire thing as a sum of local measurements.

It's a big problem. On a related note I did see a page on a modified double slit experiment that appeared to cast doubt on wave particle duality. I would like to know whether this is valid or pseudoscience. The link is here.

http://arxiv.org/ftp/arxiv/papers/1007/1007.5323.pdf

The thing is if the conclusions here are valid they tie in with some of my ideas on waves. I would be interested in opinions of the validity of this article.

#### JP

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##### Re: Can we prove light is affected by gravity?
« Reply #42 on: 30/01/2014 15:26:37 »
That article looks very poorly written and the results look very poorly quantified.  It would take a lot of effort to dig into the meat of the paper, since it provides no descriptions of what the expected diffraction pattern would be using a standard model and what the difference in the measured model is.  The difference looks to be within experimental error (indeed, the author notes that the measured values are within the "uncertainty relation" for the fringe visibility/which way information).  His claims seem to be that there are some qualitative differences and that he suspects with better equipment he could prove quantum mechanics wrong.  Given that it is relatively straightforward to compute the diffraction patterns in this case and then to compare them quantitatively to experiments, I'd relegate this to a well-intentioned paper by someone who doesn't understand the physics or math involved.  And given that he's trying to overturn 200 years of wave theory of light, he needs more than well-intentioned hand-waving to do so.

By the way, this experiment is classical.  Although light is photons, these effects can be understood and modeled by classical waves and Maxwell's equations, so finding a violation with theoretical predictions would invalidate Maxwell's equations.

#### jeffreyH

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##### Re: Can we prove light is affected by gravity?
« Reply #43 on: 31/01/2014 02:15:22 »
That article looks very poorly written and the results look very poorly quantified.  It would take a lot of effort to dig into the meat of the paper, since it provides no descriptions of what the expected diffraction pattern would be using a standard model and what the difference in the measured model is.  The difference looks to be within experimental error (indeed, the author notes that the measured values are within the "uncertainty relation" for the fringe visibility/which way information).  His claims seem to be that there are some qualitative differences and that he suspects with better equipment he could prove quantum mechanics wrong.  Given that it is relatively straightforward to compute the diffraction patterns in this case and then to compare them quantitatively to experiments, I'd relegate this to a well-intentioned paper by someone who doesn't understand the physics or math involved.  And given that he's trying to overturn 200 years of wave theory of light, he needs more than well-intentioned hand-waving to do so.

By the way, this experiment is classical.  Although light is photons, these effects can be understood and modeled by classical waves and Maxwell's equations, so finding a violation with theoretical predictions would invalidate Maxwell's equations.

Well I'm going to avoid that one then. Thanks for the opinion. I noticed all the spelling errors but thought it might be a bad translation. I have a sine wave function in excel that I'm playing with and can now produce the basic De Broglie wave and some other interesting results. I also have methods for sine wave mixing. It may be interesting to delve into the interference patterns.

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##### Re: Can we prove light is affected by gravity?
« Reply #43 on: 31/01/2014 02:15:22 »