# The Naked Scientists Forum

### Author Topic: Why do photons have the same speed regardless of the velocity of their source?  (Read 12859 times)

• First timers
• Posts: 6
• M4574H_0F_D154574H
##### Why do photons have the same speed regardless of the velocity of their source?
« on: 10/03/2016 19:24:55 »
I learned that light travels at the same given speed no matter if a light source is static in a fixed place or the source is moving towards you in that same space.

What is it that decelarates a photon that is emitted from an object moving toward you with say half the speed of light to come back to exactly the fixed speed of light.

in other words, the photon would leave the lightsource at half the speed of light in order to arive at your position with the exact lightspeed , or am i seeing this wrong?

So 'something' must slow it down otherwise it would arive at one and a half times lightspeed.
What is this 'something'?
« Last Edit: 12/03/2016 10:35:12 by chris »

#### evan_au

• Neilep Level Member
• Posts: 4131
• Thanked: 249 times
##### Re: Why do photons emmited from a moving source and a static source have same speed
« Reply #1 on: 10/03/2016 20:15:07 »
The fact that the speed of light in a vacuum is measured the same by all observers is really hard to get your head around!

Einstein showed that when you travel at really high speeds (like half the speed of light, in your example), your measurement of length and your measurement of time changes. He later showed that time distortion also occurs in a gravitational field.

They change in such a way that you always measure the speed of light in your laboratory to be c≈3x108 m/s.

So the light does not change it's speed in the universe; your perception of the universe changes.

See: http://en.wikipedia.org/wiki/Lorentz_transformation

Note that although light does not change its speed, different observers will measure the frequency and energy of the photons to be different.

The following users thanked this post: NeT-HeaD

• First timers
• Posts: 6
• M4574H_0F_D154574H
##### Re: Why do photons emmited from a moving source and a static source have same speed
« Reply #2 on: 11/03/2016 02:08:37 »
hmm.
I'm not any closer to understanding this and get the relativity between spacetime and light.

In my example i was not traveling at any speed . simply observing a photon reachin me from a static fixed point(lets say our sun) and a photon reachin me from a towards me moving object(lets say a meteorite like object reflecting or emmiting a foton towards me while the object travels at half the speed of light)

i cannot see how both photons have the same speed unless the  one emmited from the moving meteorite was somehow slowed down by something.
Anyway , tnx for the answer , you gave me some more background that i didn't have before :)

All in all i could be looking at this all wrong maybe.

A photos is a partical moving in space and time . Maybe the properties of space and time just simply don't alow for faster
travel. or movement. Or maybe it isn't moving at al because it's proppelled towards me but maybe i'm atracting it towards me instead and it really is a matter of perceptual dynamics.

I need more coffee...
« Last Edit: 11/03/2016 02:20:27 by NeT-HeaD »

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1921
• Thanked: 125 times
##### Re: Why do photons emmited from a moving source and a static source have same speed
« Reply #3 on: 11/03/2016 15:47:48 »
Try not to think of the photons as tiny bullets fired from the atom. Think more of vibrations. If you move your hand up and down on the surface of a pond the vibrations will create waves. The same thing happens with light and sound, but the speed that those vibrations travel depends on the medium they are travelling through, not on the speed of the object that produced the vibrations.
So, for an aircraft on the ground or flying towards you the speed of the sound is entirely dependant on the properties of the air and is the same for a stationary plane or one coming towards you at 500mph. It's not that the sound slows down, it's just that it wasn't moving until the instant it was generated.
However, there is a big difference between sound and light. If an aircraft is stationary and we move towards it we will measure the speed of the sound of its engine to be the speed of sound in air minus our own speed. This is because we are also moving in the medium ie air, so we are moving relative to the air which contains the sound waves.
With light however, there is no medium that the light is moving through, so if you move towards a light source you will still measure the same speed of light as if you we're stationary . People used to think there was such a medium and called it aether.
I'm afraid this is a very simplified explanation, skips a lot of detail and doesn't cover all the issues, but I hope it gives you something to visualise.
« Last Edit: 11/03/2016 15:51:05 by Colin2B »

The following users thanked this post: NeT-HeaD

• First timers
• Posts: 6
• M4574H_0F_D154574H
##### Re: Why do photons emmited from a moving source and a static source have same speed
« Reply #4 on: 11/03/2016 20:44:03 »
Tnx Colin,

The theories and comparison of light and sound , and the doppler effect when soundsourses(or receivers) move were allready known to me.

I'm simply(well , not simple at all, but as a figure of speech) trying to understand why lightspeed is a fixed given absolute limit when it comes to moving through space.
Nothing can move faster (according to einsteins and other theories) and i want to know what it is that makes everything hold up to this fact.
As you said, in space is nothing that can 'vibrate' (or be modulated) but still a lightsource does do exactly that. Light is indeed a 'movement' in a particular frequency and amplitude but what it is what 'vibrates' and gets 'modulated' to transport the properties of light so it becomes visible at every distance from the emiting source blows my mind. simply cannot figure it out.
I must remind everyone here that i am no scientist or not even a student. i only found this matter interesting all of my life but always got put off by loads of 'shop talk'i cannot grasp.

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1921
• Thanked: 125 times
##### Re: Why do photons emmited from a moving source and a static source have same speed
« Reply #5 on: 11/03/2016 23:21:21 »
I must remind everyone here that i am no scientist or not even a student. i only found this matter interesting all of my life but always got put off by loads of 'shop talk'i cannot grasp.

No, I should thank you for trying.
This problem took the best brains hundreds of years to work out what was happening and even now there are gaps in the detail.
I think you come close to the real problem when you ask 'what vibrates'. This is a trap most of us fall into, we are so used to thinking of waves in water or sound waves in air that we think there must be some thing which vibrates. The truth is that it is the magnetic and electric fields which vibrate and they don't need anything to support them, they work anywhere even in a total vacuum.
The area you are tackling is not an easy one, but don't let that put you off trying, you are asking all the right questions.

The following users thanked this post: NeT-HeaD

• First timers
• Posts: 6
• M4574H_0F_D154574H
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #6 on: 20/03/2016 12:49:14 »
Tnx colin,

It took some time of letting go of this mater to get a 'flash' of insight. I now understand why doppler doesn't apply to lightsourses that move.

It's simple really. The moment a photon leaves its source it starts traveling at lightspeed and because relative to this lightspeed  this source is a point of origin that sits  still  and is actually static because the emmited photons speed is already the maximum and constant. there's is no acceleration. light emmision is instant. and there lies the clue.
i think.

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1921
• Thanked: 125 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #7 on: 20/03/2016 13:19:23 »
It's simple really. The moment a photon leaves its source it starts traveling at lightspeed and because relative to this lightspeed  this source is a point of origin that sits  still  and is actually static because the emmited photons speed is already the maximum and constant. there's is no acceleration. light emmision is instant. and there lies the clue.
i think.
That's pretty much it.

Only complication is that Doppler shift does apply to moving sources. Although the light has the constant speed once emitted, with a moving source the light waves are squished in the direction of motion, so the wavelength shortens, the frequency increases and the colour becomes bluer. Opposite if the source moves away from a receiver.

#### jeffreyH

• Global Moderator
• Neilep Level Member
• Posts: 3929
• Thanked: 55 times
• The graviton sucks
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #8 on: 20/03/2016 13:44:40 »
If we think of the universe as a fixed 3 dimensional grid with each cell having the dimensions (x, y, z) and where x=y=z we then have a universe divided into cubes. Light can move from one of these cubes to another in L amount of time where we have L as representing a direct relationship to the speed of light. Nothing else in the universe can move this fast and so the time taken for anything else to change cubic position has to be less than L. At the very smallest scales this is what happens. Only we cannot directly observe it. To our intuitive senses at our macroscopic scale it doesn't make sense. Like much of quantum mechanics at the microscopic scale.

#### timey

• Neilep Level Member
• Posts: 1309
• Thanked: 8 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #9 on: 20/03/2016 13:52:44 »
Only complication is that Doppler shift does apply to moving sources. Although the light has the constant speed once emitted, with a moving source the light waves are squished in the direction of motion, so the wavelength shortens, the frequency increases and the colour becomes bluer. Opposite if the source moves away from a receiver.

However...  The Pound Rebka experiment shows that a blueshift, and a redshift of lights wavelength occurs when the emitter, and the receiver of the light source are both held """static""" relative to each other!!!
« Last Edit: 20/03/2016 13:58:22 by timey »

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1921
• Thanked: 125 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #10 on: 20/03/2016 14:52:46 »
Only complication is that Doppler shift does apply to moving sources. Although the light has the constant speed once emitted, with a moving source the light waves are squished in the direction of motion, so the wavelength shortens, the frequency increases and the colour becomes bluer. Opposite if the source moves away from a receiver.

However...  The Pound Rebka experiment shows that a blueshift, and a redshift of lights wavelength occurs when the emitter, and the receiver of the light source are both held """static""" relative to each other!!!
But I was talking about Doppler shift due to movement.

#### timey

• Neilep Level Member
• Posts: 1309
• Thanked: 8 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #11 on: 20/03/2016 15:21:03 »
Well yes - of course Colin!

And isn't it interesting that a representation of Doppler shift can perhaps be found in the redshift, blueshift phenomenon between a light source emitter and receiver that are indeed held static relative to each other?

#### JoeBrown

• Sr. Member
• Posts: 151
• Thanked: 4 times
• Does everything simple always gotta be so complex?
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #12 on: 20/03/2016 19:22:50 »
Well yes - of course Colin!

And isn't it interesting that a representation of Doppler shift can perhaps be found in the redshift, blueshift phenomenon between a light source emitter and receiver that are indeed held static relative to each other?

Doppler shift doesn't occur when source/reception are static.  That's not what the experiment showed, nor was it the intention.  They were verifying Einstein / gravity shift.  They used relative doppler motion/shift to cancel the effect of gravity shift.

#### JoeBrown

• Sr. Member
• Posts: 151
• Thanked: 4 times
• Does everything simple always gotta be so complex?
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #13 on: 20/03/2016 19:39:07 »
*I think* the basic premise of light speed in relativity boils down to this:

The act of acceleration and motion affect observers and emitters time dilation.  This time dilation cancels the speed differential to that of light speed.

Tho that doesn't explain why it is what it is, it kinda explains how it works that way.  Tho I'm not an expert on relativity.  I'm still trying to figure out how Albert came up w/that, imagining he was running alongside a light corpuscle...  I've imagined similar and seen nothing of the sort.
« Last Edit: 20/03/2016 22:09:23 by JoeBrown »

#### timey

• Neilep Level Member
• Posts: 1309
• Thanked: 8 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #14 on: 20/03/2016 20:28:56 »
Well yes - of course Colin!

And isn't it interesting that a representation of Doppler shift can perhaps be found in the redshift, blueshift phenomenon between a light source emitter and receiver that are indeed held static relative to each other?

Doppler shift doesn't occur when source/reception are static.  That's not what the experiment showed, nor was it the intention.  They were verifying Einstein / gravity shift.  They used relative doppler motion/shift to cancel the effect of gravity shift.

Firstly, what an experiments intention is has no bearing on the use of its results.

You are stating that a relative Doppler  motion/shift was used to cancel out gravity shift...

Therefore if the blueshift redshift effects between a light sources emitter and receiver that are held static relative to each other can be cancelled out by relative Doppler motion/shift, then there is indeed a representation of a Doppler shift within the redshift blueshift observations within gravity shift.

You are forgetting that the Pound Rebka was initiated with a view to measuring gravitational shift with respect to time dilation considerations.  The gravitational field itself experiences time dilation.  It's not just a case of SR time dilation with regards to moving objects... you know!

#### JoeBrown

• Sr. Member
• Posts: 151
• Thanked: 4 times
• Does everything simple always gotta be so complex?
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #15 on: 20/03/2016 22:07:20 »
Firstly, what an experiments intention is has no bearing on the use of its results.
It wasn't the intention to show doppler shift occurs in static relative motion, nor was it shown.  It showed that gravitational shift matched the predicted (expected) results.

Which was the point.  Doppler shift doesn't occur w/out differences in relative motion. That's how it is understood and it is how it works.

Those are the facts.  I intended to point out your previous statement was interestingly inaccurate.

Quote
You are forgetting that the Pound Rebka was initiated with a view to measuring gravitational shift with respect to time dilation considerations.  The gravitational field itself experiences time dilation.  It's not just a case of SR time dilation with regards to moving objects... you know!

I didn't forget...  Tho I'm still no expert.

I omitted reference to gravity, to provide a simple(r) answer to initial question.  Why over complicate?  Is it prudent to use general when special relativity achieves the desired result?
« Last Edit: 20/03/2016 22:17:32 by JoeBrown »

#### timey

• Neilep Level Member
• Posts: 1309
• Thanked: 8 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #16 on: 20/03/2016 22:51:58 »
Yup - you are correct.  It wasn't the intention to show doppler shift occurs in static relative motion... But, in that a Doppler shift cancelled out the """relative motion""" of the redshift blueshift observations, it 'is' shown, whether intended or not, that a representation of the Doppler shift is apparent within the gravitational shift of redshift blueshift observations between a light source emitter and receiver that are held """static""" relative to each other...  And this 'is' interesting!

Yup -  The experiment did show that gravitational shift matched the predicted (expected) results with regards to a clock ticking faster in elevation.

Nope - I am not over complicating matters.  Call it GR or SR at your will and want.  Fact is, according to currently held theory, you get time dilation for objects travelling at relativistic speeds, and you get time contraction in a reducing gravitational field.

#### jeffreyH

• Global Moderator
• Neilep Level Member
• Posts: 3929
• Thanked: 55 times
• The graviton sucks
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #17 on: 21/03/2016 08:22:12 »
If it is even meaningful we could have the fastest rate of time equalling 1. In other words 100%. This would have to occur at infinity outside the influence of gravity. Since infinity is not on the number line nothing can reach it. This is exactly why there is no fixed background. Time will never run at full speed and neither will it completely stop as both states are absolutes that can never be reached.

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1921
• Thanked: 125 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #18 on: 21/03/2016 08:49:15 »
Time will never run at full speed and neither will it completely stop as both states are absolutes that can never be reached.
What happens for the free falling observer Jeff? I assume time runs at 100%. As she falls (pushed off the tower by those who disagree with her theory) she looks up and sees the source receding- red shifted, looking down she realises the detector is moving up against the light waves - blue shifted. Her view is that there is no gravity, just acceleration and all  effects at top and bottom of tower are due to doppler shift.

#### puppypower

• Hero Member
• Posts: 561
• Thanked: 43 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #19 on: 21/03/2016 11:45:19 »
The simplest explanation for why photons always move at the same speed, regardless of reference, is the speed of light is the ground state of the universe. This can be easily inferred. In our universe, there is a net conversion of mass to energy; fusion, and not a net conversion of energy to mass. This reflects that the net direction of potential in the universe, is the lowering mass potential toward an energy based ground state. All the forces of nature give off energy, as one of their lower potential products.

As a visual analogy, say we assume sea level is the ground state for the surface of the earth, since all the water goes there. It does not matter where you begin as your reference; hill, mountain or cloud. They will use the same ground state. There may be different potentials with this ground state; reflected in the wavelength of the energy, but all water goes to sea level.

If you look at gravity, gravity will cause mass to clump causing local space-time to contract. If we contract space-time to the limit of a point-instant, we will get the speed of light reference. Gravity is a reflection of space-time lowering potential back to the speed of light reference; toward the point-instant. This point-instant limit; speed of light reference, is achieved by the black hole.

If you were traveling at the speed of light, and looked out at the universe, the universe would appear like a point-instant.  That being said, the only wavelength of energy/photons you would be able to see would be infinite wavelength energy. The reason is anything with a smaller wavelength than infinite, will have a wavelength that is a fraction of a point in size. This is not mathematically possible by definition. A point is a small as you can get by definition. This implies all energy, with shorter wavelength than infinite, will need to lower potential by gaining wavelength, to reach the ground state; all energy will need to red shift.

All roads lead to the speed of light ground state in their own ways. This is why all references, will see and make use of the exact same ground state.

The advantage of using the C ground state, instead of an inertial zero state, is you can start the universe earlier than with the singularity of the big bang. The reason is the singularity sets a potential with the C ground state. The singularity defines a state of highest potential that needs to expand. It needs to expand to deal with the energy and needs to clump to deal with the mass. It also needs forces to deal with other states of matter.

I won't get into this, since such a discussion diverges to much from the topic at hand by starting earlier than the inertial time=0; state of highest potential.

#### timey

• Neilep Level Member
• Posts: 1309
• Thanked: 8 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #20 on: 21/03/2016 17:55:23 »
What happens for the free falling observer Jeff? I assume time runs at 100%. As she falls (pushed off the tower by those who disagree with her theory) she looks up and sees the source receding- red shifted, looking down she realises the detector is moving up against the light waves - blue shifted. Her view is that there is no gravity, just acceleration and all  effects at top and bottom of tower are due to doppler shift.

Thanks for the push Colin!

Whaaaaargh....  I'm faaaaallliiiing!

...at a rate of 9.8m/s2 acceleration...
As I look behind me, I can see blue shifted light rushing towards me, but from the point that it passes me I cannot see it redshift towards the earth from my position.  My position in the field is changing, but as the light is travelling at the speed of light, it's position in the field changes faster than mine.

Alternatively, the second time you pushed me off the tower, (clearly I hadn't yet learned my lesson ;) )...  light is being sent way from Earth.  I look up behind me as I fall, I cannot see the light redshift away behind me... but looking downwards, up till the point that the light passes my position in the field, I can see it redshift towards me.

Now, then... having been pushed off the tower twice, I think I can be forgiven for a bit of wonky thinking, or could it perhaps be that I have become en-light-ened?

My observations when falling where as such:
Firstly:
A Doppler shift was used to cancel out the redshift and blueshift effects, thereby proving that these effects of redshift and blueshift exist as a result of the changes in the gravitational field.
That as a direct result of the observation above in relation to the light source emitter and receiver being held static relative to each other, there is proven a representation of Doppler shift within the relative motion of light via changes in the gravity field.
That both of the above considered in relation to the constant speed of light and the 'constant distance' between ground and top of tower of the experiment, shows that something is a little amiss.  Light should not have any such Doppler shift relative motion properties over a constant distance, travelling at a constant speed!
And... that taking into account that the light will be travelling at the speed of light under the currently thought remit of the minuscule time drifts occurring within the changes in the gravitational field - 'does not' in fact describe the relative motion of a Doppler shift!

Secondly:
That I was falling at a rate of 9.8m/s2.
That the first time I was pushed off the tower, my mobile phone fell out of my pocket... and in that I arranged my mass as to be as lacking in air resistance as my phones mass, I thought I noticed my phone and I arrived on the ground at the same time.
That the second time you pushed me off the tower Colin, with great presence of mind, to say so, I picked your pocket of your phone and repeated the experiment.  Your phone and my phone, (both smashed to smithereens now of course) did not weigh the same, in weight or worth.  Mine was heavier, yours more expensive!  But, again, I noticed that your phone and I arrived on the ground at the same time.

Why does the light not accelerate at 9.8m/s2?  Answer:  Because the speed of light is constant.
Are we expecting that the light will have taken a couple of a billionth of a second longer time to travel the distance on account of the gravity field slowing time down?  Answer:  Yes we do.
We have attributed light with mass equivalent to its energy.  We state that the gravitational field robs the light of energy when leaving a gravitational field, ie: redshift.  So... what exactly is replenishing lights energy when we observe a blueshift?

The energy of mine and the phones falling is easily explained due to potential energy, via the fact of my mass and my position, or changing position, within the gravitational field.

The fact of the lights increase in energy found in its falling towards a gravitational field is described by adding relativistic mass.  But, by this remit, as my mass, or the mass of the phones falls towards earth it must also increase.  Again... we come back to the question as to why lights mass does not 'accelerate' at 9.8m/s2, like mine does?

Looking at the energy of my mass, we can see that as I fall, the potential energy that my mass experiences is reducing as I fall into the gravitational field.  Therefore, the energy that light experiences as a result of the adding of relativistic mass, also will 'reduce' as it falls towards earth.  But light energy does not reduce as it gets closer to a gravitational field.  This constitutes a breach of logic, unless it is the increase in inherent mass energy as the relativistic mass is increased that describes the increase in energy.  It should be noted, that adding relativistic mass to light as its energy 'increases' would not subject the light to any more, or any less than an acceleration of 9.8m/s2!
Yet light is not subject to any 'further' acceleration of its own speed under the influence of a gravitational field.

However... I observed that a Doppler shift cancelled out the redshift blueshift observations from top and bottom of tower perspectives.  These perspectives of the light source emitter and receiver having been held static relative to each other!!!
This alerts me to these 5 considerations:
a) That there is a relative motion in light representing a Doppler shift that does not incorporate any relative motion of distance.
b) That on the basis of there being a relative motion in light representing a Doppler shift that did not incorporate any relative motion of distance, that redshift considerations are not necessarily indicative of an expanding universe.
c) That the relative motion of Doppler shift found in light that is apparent over a 'constant distance' does not add up to gravitational shift, plus time drift consideration, plus the aspect of potential energy reduction in light caused by decreasing distance between its mass and the mass of the earth as it enters further into the gravitational field.  (or at least not without rendering distance as a variable.)
d) That if I were to look upon an increasing gravitational field increasing the rate of time... that the fact of a Doppler shift cancelling out redshift blueshift frequencies indicates that light, travelling at the speed of light, is accelerated towards a gravitational field via an increase in the rate of time that directly relates to the increase in a gravitational field via a representation of Doppler shift.  That light is simply getting its energy directly from the gravitational field, it's frequency is indicative of the rate of time, and it's additions or reductions in wavelength are not 'distance' related, but 'time' related.
e) That perhaps there is no acceleration of gravity, perhaps it is instead only an acceleration of time.

So... what is happening when I am falling?  The difference between light and I being that light has no mass... and, more importantly, it is in fact proven that my clock will run faster in a reduced gravitational field, not slower.  As me and my clock fall, it's rate of time will register ever so slightly slower.
Ok, my mass is subject to gravity potential, but if light 'gets' its energy from a gravitational field, do I also 'get' energy from the gravitational field?  And if I do, then does this effect the status of my e=mc2 inherent mass energy?  ie: having rid ourselves of the idea of relativistic mass for light, does my mass 'change' in a gravitational field, changing my inherent mass energy?

If I state my inherent mass energy as constant, and add on the aspect of potential energy, plus gravitational field energy... given that this describes an 'energy level' that will produce frequency and wave length, does my atomic clocks caesium atoms inherent mass energy, plus the potential energy, plus the gravitational field energy that it is experiencing at elevation, describe the fact of the frequency of its cycles being increased?
Can the aspect of the kinetic energy of a moving body of 'mass' be deducted from the additions of inherent mass energy with potential energy and gravitational field energy, to then describe a slowing of time for both co-moving, and more obviously, non co-moving objects?

And finally...  If so, then could the phenomenon of time itself be 'energy' related?

#### alancalverd

• Global Moderator
• Neilep Level Member
• Posts: 4728
• Thanked: 155 times
• life is too short to drink instant coffee
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #21 on: 21/03/2016 19:00:29 »
Keep it simple.

Doppler shift is due to relative speed of source and receiver. Gravitational shift is due to relative gravitational potential of source and receiver. They are not the same phenomenon but have the same effect on wavelength, so you can use one to measure the other.

As for the original question, there is no "why". It just happens that an awful lot of observations become interconsistent if you assume that c is constant for all observers, and as far as we can ascertain, it is. The task of teaching physics is to explain why everything else is as it is, given only that c is constant.

#### timey

• Neilep Level Member
• Posts: 1309
• Thanked: 8 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #22 on: 21/03/2016 19:55:39 »
Keeping it simple...

Doppler shift is due to relative speed of source and receiver.  But... if there is no relative speed between source and receiver, then any representation of Doppler shift 'must' be due to relative motion within the propagation of light within a gravitational field.

Keeping it simple...

Gravitational shift is due to relative gravitational potential of source and receiver. Doppler shift and gravitational shift are not the same phenomenon but have the same effect on wavelength, so you can use one to measure the other...  But... If there is no relative expansion of distance via a gravitational shift, there is 'again' no relative motion for a Doppler shift to occur in.

Keeping it simple...

How can lights frequency be cancelled out by a Doppler shift between a light source emitter and a light source receiver that are both held static of motion relative to each other - and consist of a 'constant' distance apart?

#### evan_au

• Neilep Level Member
• Posts: 4131
• Thanked: 249 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #23 on: 21/03/2016 20:15:43 »
Quote
How can lights frequency be cancelled out by a Doppler shift between a light source emitter and a light source receiver that are both held static of motion relative to each other - and consist of a 'constant' distance apart?
If two objects are not moving relative to each other, their Doppler shift is zero.
The only way for Doppler shift to equal gravitational shift is if the gravitational shift is also zero.
Gravitational shift is zero if the two observers are at the same gravitational potential, eg both standing on the surface of the Earth.

Quote
Doppler shift and gravitational shift are not the same phenomenon but have the same effect on wavelength, so you can use one to measure the other
This is true in one direction only.

If you observe the moving light source from multiple directions (or watch it as it passes you), you will see that gravitational shift is cancelled in one direction, but not others.

The reason is that the Doppler effect can produce red and blue shift. However, gravitational frequency shift produces the same shift whether the object is moving towards you or away from you. This is called Relativistic Beaming.

#### timey

• Neilep Level Member
• Posts: 1309
• Thanked: 8 times
##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #24 on: 21/03/2016 20:36:02 »
Evan - my point is that within the Pound Rebka, a Doppler shift was used to cancel out the frequency of redshift from bottom of tower to top of tower.  And again to cancel out the frequency of blueshift from top of tower to bottom.

There was no relative speed between the top of tower and bottom of tower in either instance.  There was also no relative expansion or contraction of the distance between top of tower and bottom of tower in either instance.

Yet - a Doppler shift cancelled the blueshift redshift frequencies between the 2 'static in motion' and 'constant in distance' locations... How did it do this?

#### The Naked Scientists Forum

##### Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #24 on: 21/03/2016 20:36:02 »