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Author Topic: Do photons gain energy when falling within a gravitational field.?  (Read 8179 times)

Offline MikeS

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The generally accepted answer is yes, they become blue-shifted.

Light travels at a constant speed therefore it can neither accelerate or de-accelerate.  If it can't accelerate or de-accelerate then it can not gain or loose energy.

The observed blue shift is nothing more than observing photons from a frame of reference that is time dilated.

There is no such thing as photons gaining or loosing energy due to a gravitational field.  It's an illusion, they only appear to.

Cat amongst the pigeons?


 

Offline JP

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Mike, the energy of a photon doesn't have to do with it's speed.  They do gain energy when falling due to gravity because they gain momentum.
 

Offline MikeS

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Mike, the energy of a photon doesn't have to do with it's speed.  They do gain energy when falling due to gravity because they gain momentum.

Momentum in classical mechanics is the product of mass and velocity.
zero x velocity = zero.  No gain of momentum
 

Offline Bored chemist

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The effect is real and was measured some time ago.
http://prl.aps.org/abstract/PRL/v4/i4/p163_1
A blue shifted photon really does have more energy. I can in principle extract that energy (for example I could have it warm up some water)and I get more energy from the bluer photon than the redder one.

"If it can't accelerate or de-accelerate then it can not gain or loose energy."
Not true, the weights in my grandfather clock move slowly downwards to drive the clock, but they don't accelerate or decelerate.
The pasta I had for lunch will provide me with energy without changing its velocity.

You can't apply classical physics to relativistic problems.

 

Offline MikeS

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The effect is real and was measured some time ago.
http://prl.aps.org/abstract/PRL/v4/i4/p163_1
A blue shifted photon really does have more energy. I can in principle extract that energy (for example I could have it warm up some water)and I get more energy from the bluer photon than the redder one.

"If it can't accelerate or de-accelerate then it can not gain or loose energy."
Not true, the weights in my grandfather clock move slowly downwards to drive the clock, but they don't accelerate or decelerate.
The pasta I had for lunch will provide me with energy without changing its velocity.

You can't apply classical physics to relativistic problems.



I don't agree it only appears to from a dilated time reference frame.  It's all relative.  I agree that a blue shifted photon when measured will appear to have more energy but that's because of time dilation.  The measurement is not instantaneous, it takes time.

I could have said it can't change its velocity which implies the same thing. 
Question.  How can a photon, a massless object gain or loose energy?


I was talking about photons not grandfather clocks or the energy contained in pasta. 
 

Offline Bored chemist

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"Question.  How can a photon, a massless object gain or loose energy?"
Well, one way it can gain energy is to fall down under gravity.
The question is surely, how can a massless particle have any energy at all?
The answer is, I don't know, but my solar powered calculator shows that they do.
 

Offline Phractality

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Mike, the energy of a photon doesn't have to do with it's speed.  They do gain energy when falling due to gravity because they gain momentum.

Momentum in classical mechanics is the product of mass and velocity.
zero x velocity = zero.  No gain of momentum


For a particle with rest mass, momentum is simply p = mv, and force is the time rate of change of momentum, f = dp/dt = d/dt(mv) = m(dv/dt) = ma. However, at non-relativistic speeds, f = ma = m(dv/dt) ≠ ma. Increasing speed near the speed of light increases the mass; so dp = mdv + vdm; dp/dt = m(dv/dt) + v(dm/dt) ≠ ma. However, f = dp/dt remains valid for particles with rest mass at all speeds. When talking about speeds near c, you must forget about f = ma and use the correct formula, f = dp/dt.

Einstein's general relativity formulas are written for Minkowski space-time. The fundamental difference between that and Euclidean space is that Minkowski redefined a straight line is as the path of light. In Euclidean space, gravity bends light, so light accelerates without gaining speed, because the velocity changes perpendicular to its direction of travel. In Minkowski space-time, gravity does not bend light; the velocity of light remains constant, so there is no acceleration.

I don't know why, but the concept of force is not part of general relativity; if it were, it would have to be f = dp/dt, and that would be valid for photons as well as for particles with rest mass. I also don't know why photons are considered to have zero mass in general relativity. The formula E = mc˛, yields m = E/c˛, which makes perfect sense to me.

Gravity changes the momentum of a photon, and the rate of change of momentum is force, so a photon does feel a gravitational force in a gravity field. That force is in the direction of the gradient of gravitational potential in Euclidean space, but in Minkowski space-time it is in the direction of the photon's travel. The velocity in general relativity is constant, so dv/dt = 0, and f = m(dv/dt) + v(dm/dt) = c(dm/dt). The force is the rate of change of mass of the photon times the speed of ligth. Work is force times the parallel component of the distance, so work is being done on the photon by gravity, and that is why the photon gains energy. That's not how it's usually explained in general relativity, but the result is the same. 
 

Offline MikeS

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This is a coincidence from a another thread but I think it does answer the question.
http://www.thenakedscientists.com/forum/index.php?action=post;quote=362480;topic=40248.0;num_replies=12;sesc=ea4dab58f5f23a5a4bd1176a58656db5
This was posted by
CPT ArkAngel
17/07/2011 18:32:13 »
and the links are
http://iopscience.iop.org/1063-7869/42/10/A04

http://www.itep.ru/theor/persons/lab180/okun/em_13.pdf

It would seem that photons do not gain energy when falling within a gravitational field.
 

Offline simplified

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This is a coincidence from a another thread but I think it does answer the question.
http://www.thenakedscientists.com/forum/index.php?action=post;quote=362480;topic=40248.0;num_replies=12;sesc=ea4dab58f5f23a5a4bd1176a58656db5
This was posted by
CPT ArkAngel
17/07/2011 18:32:13 »
and the links are
http://iopscience.iop.org/1063-7869/42/10/A04

http://www.itep.ru/theor/persons/lab180/okun/em_13.pdf

It would seem that photons do not gain energy when falling within a gravitational field.

Maybe,but then a matter and an antimatter create less energy in gravitational field and one Joule of energy in gravitational field is less than one Joule in space without gravitational field. :-\
 

Offline simplified

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I have confused something.Your great work has confused all.We are wrong.Our eyes measure length of wave without using of time.They see contraction of length of wave in gravitational field.This is main thing,which proves increasing of energy of photon in gravitational field. Calculations with using of frequency just confirm it.Though mainstream  is wrong ,proving relative time,in this case mainstream is right ::)
 

Offline MikeS

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This is a coincidence from a another thread but I think it does answer the question.
http://www.thenakedscientists.com/forum/index.php?action=post;quote=362480;topic=40248.0;num_replies=12;sesc=ea4dab58f5f23a5a4bd1176a58656db5
This was posted by
CPT ArkAngel
17/07/2011 18:32:13 »
and the links are
http://iopscience.iop.org/1063-7869/42/10/A04

http://www.itep.ru/theor/persons/lab180/okun/em_13.pdf

It would seem that photons do not gain energy when falling within a gravitational field.

Maybe,but then a matter and an antimatter create less energy in gravitational field and one Joule of energy in gravitational field is less than one Joule in space without gravitational field. :-\


Simplified

Sorry about the delay in replying but it took me a while to understand what I think you were saying.

One Joule It is equal to the energy expended (or work done) in passing an electric current of one ampere through a resistance of one ohm for one second.
http://en.wikipedia.org/wiki/Joule

Ampere
In practical terms, the ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time with 6.241 × 1018electrons, or one coulomb per second constituting one ampere.[3]

http://en.wikipedia.org/wiki/Ampere

Consider the following experiment:-
At the top of a very high tower we have a resistance of one ohm connected through an amp meter.  This is connected to a power source at the bottom of the tower through a pair of wires.  In the circuit at the bottom of the tower is another amp meter.  Let us assume the circuit (apart from the resistance) to be lossless.

We adjust the power source to give a steady current of one ampere according to the amp meter at the bottom of the tower.  When we observe the amp meter at the top of the tower it is  showing less than one ampere.  In a simple series circuit, the current is required to be the same everywhere.    Where is this discrepancy coming from?  Time, at the bottom of the tower is dilated in comparison to the top of the tower.  Electricity flows around the circuit (in a sense) at the speed of light.  As a second at the top of the tower is shorter than a second at the bottom of the tower less electrons flow past a certain point (at the top of the tower) per second.

Energy is the capacity of a physical system to perform work.  As it takes time to perform work, the time factor becomes important.  One Joule of energy is one Joule of energy within its own reference frame but can appear to be more or less from a different reference frame. (That could be worded better)

“one Joule of energy in gravitational field is less than one Joule in space without gravitational field.”

The wording is confusing.
One Joule of energy in a gravitational field is less energy than would be required to be one Joule of energy outside of a gravitational field.
One Joule outside of a gravitational field is more energy than would be required for it to be one Joule of energy within a gravitational field.

Aside
Gravity by dilating time reduces useful (edit) energy and is the universes main source of entropy.
« Last Edit: 16/08/2011 08:31:30 by MikeS »
 

Offline MikeS

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I have confused something.Your great work has confused all.We are wrong.Our eyes measure length of wave without using of time.They see contraction of length of wave in gravitational field.This is main thing,which proves increasing of energy of photon in gravitational field. Calculations with using of frequency just confirm it.Though mainstream  is wrong ,proving relative time,in this case mainstream is right ::)

Our eyes see color, which is proportional to frequency.  The frequency of a photon does not vary but the ‘passage’ of time does.  What appears to us as an increase in energy (the blue shift) is caused by time being dilated deeper within the gravity well.  As a second is slower more cycles arrive in the ‘longer second’ giving the appearance of an increase in frequency.

When measuring the energy of a photon, if allowance is made to correct for time dilation/contraction, it will be seen that the energy is exactly the same.
« Last Edit: 15/08/2011 14:02:14 by MikeS »
 

Offline MikeS

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An object falling within a gravitational field does not 'gain' energy.  Some of its Gravitational potential energy is converted into Kinetic energy in the fall which leaves it with a lower GPE at the end of the fall.  The KE plus the lower GPE add up to the original GPE.  Energy has been conserved.

If an object does not gain energy when falling within a gravitational field why should photons 'falling' within a gravitational field be any different?
The 'apparent' energy gain is offset by time dilation which is equivalent to the same energy reduction as gained by the falling photons. 


Personally, I think that the idea that an object (and photons) gains energy when falling within a gravitational field is a left over from thinking about gravity from a Newtonian perspective.  When applied to anything travelling at the speed of light Newtonian gravity is insufficient to explain what is happening.

Consider the above experiment
At the top of a very high tower we have a resistance of one ohm connected through an amp meter.  This is connected to a power source at the bottom of the tower through a pair of wires.  In the circuit at the bottom of the tower is another amp meter.  Let us assume the circuit (apart from the resistance) to be lossless.

We adjust the power source to give a steady current of one ampere according to the amp meter at the bottom of the tower.  When we observe the amp meter at the top of the tower it is  showing less than one ampere.  In a simple series circuit, the current is required to be the same everywhere.    Where is this discrepancy coming from?  Time, at the bottom of the tower is dilated in comparison to the top of the tower.  Electricity flows around the circuit (in a sense) at the speed of light.  As a second at the top of the tower is shorter than a second at the bottom of the tower less electrons flow past a certain point (at the top of the tower) per second.


As I understand it Newtonian gravity can not explain the difference in current in the circuit in different gravitation potentials.  It cannot explain it because electricity (effectively) travels at the speed of light.  If Newtonian gravity should not be applied to anything travelling at the speed of light then it should not be applied to light itself.
 

Offline simplified

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"Gravity by dilating time reduces energy" ≠ "When measuring the energy of a photon, if allowance is made to correct for time dilation/contraction, it will be seen that the energy is exactly the same." ;)
 

Offline MikeS

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"Gravity by dilating time reduces energy" ≠ "When measuring the energy of a photon, if allowance is made to correct for time dilation/contraction, it will be seen that the energy is exactly the same." ;)

Perhaps badly worded.  Gravity by dilating time reduces useful energy.  Gravity ties up energy rendering it unusable.
For example for a given current in a gravitational field there will be more electrons passing a given point per second than in a non gravitational field (as a second is longer in a gravitational field).  Gravity 'ties' up energy. Entropy is higher in a gravitational field.
« Last Edit: 15/08/2011 18:39:15 by MikeS »
 

Offline MikeS

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"An object in free-fall is in actuality inertial, but as it approaches the planetary object the time scale stretches at an accelerated rate, giving the appearance that it is accelerating towards the planetary object when, in fact, the falling body really isn't accelerating at all. This is why an accelerometer in free-fall doesn't register any acceleration; there isn't any."
http://en.wikipedia.org/wiki/Equivalence_principle

This is why a photon does not gain energy 'falling'within a gravitational field.  A free falling accelerator proves it.
 

Offline simplified

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"An object in free-fall is in actuality inertial, but as it approaches the planetary object the time scale stretches at an accelerated rate, giving the appearance that it is accelerating towards the planetary object when, in fact, the falling body really isn't accelerating at all. This is why an accelerometer in free-fall doesn't register any acceleration; there isn't any."
http://en.wikipedia.org/wiki/Equivalence_principle

This is why a photon does not gain energy 'falling'within a gravitational field.  A free falling accelerator proves it.
Electron and positron have potential energy far to mass.Then they turn into photon.The photon travels to mass.If the travel does not increase energy of the photon,then you lose this potential energy. :P
« Last Edit: 02/09/2011 17:08:50 by simplified »
 

Offline imatfaal

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"An object in free-fall is in actuality inertial, but as it approaches the planetary object the time scale stretches at an accelerated rate, giving the appearance that it is accelerating towards the planetary object when, in fact, the falling body really isn't accelerating at all. This is why an accelerometer in free-fall doesn't register any acceleration; there isn't any."
http://en.wikipedia.org/wiki/Equivalence_principle

This is why a photon does not gain energy 'falling'within a gravitational field.  A free falling accelerator proves it.
Electron and positron have potential energy far to mass.Then they turn into photon.The photon travels to mass.If the travel does not increase energy of the photon,then you lose this potential energy. :P
trouble is with that is that they turn into 2 or 3 photons.  but it is a good argument none the less
 

Offline MikeS

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Electron and positron have potential energy far to mass.Then they turn into photon.The photon travels to mass.If the travel does not increase energy of the photon,then you lose this potential energy. :P

The energy liberated when they annihilate exactly equals the energy of their creation and this includes the extra energy needed to create them at a higher gravitational potential.
 

Offline simplified

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The energy liberated when they annihilate exactly equals the energy of their creation and this includes the extra energy needed to create them at a higher gravitational potential.
Kinetic energy created by mass is relative thing even for motionless observers in different gravitational fields. So? :-\
« Last Edit: 04/09/2011 18:58:29 by simplified »
 

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