[NeT-HeaD (OP)]

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'?

[evan_au]

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.

[NeT-HeaD (OP)]

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...

[Colin2B]

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.

[NeT-HeaD (OP)]

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.

Tnx for your attempt.
 
 

[Colin2B]

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.

Tnx for your attempt.

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.

[NeT-HeaD (OP)]

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]

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]

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]

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!!!

[Colin2B]

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]

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]

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]

*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.

[timey]

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.

P.S.  In answer to your second post:

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]

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.

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?

[timey]

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]

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]

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]

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.