On Monday, October 1, 2012, Tom Roberts wrote in sci.physics.relativity:
> Wavelength is not an intrinsic property of light, so it cannot be
> discussed independent of how it is measured. But it is clear that in vacuum the
> light ray itself is unchanged as it propagates.
> Differently moving observers will measure different wavelengths for a given
> light ray, because their MEASURING INSTRUMENTS are oriented differently in
> spacetime, and such a measurement inherently PROJECTS the light ray onto the
> measuring instrument.
Honest Roberts,
Do you mean that the orientation of the MEASURING INSTRUMENT varies with the speed of the observer? This orientation remains unchanged so long as the speed of the observer remains constant?
Stephen Hawking contradicts Tom Roberts: Wavelength is an INTRINSIC property - it can already be changed before the interaction with the observer:
http://www.amazon.com/Brief-History-Time-Stephen-Hawking/dp/0553380168 Stephen Hawking, "A Brief History of Time", Chapter 3: "...we must first understand the Doppler effect. As we have seen, visible light consists of fluctuations, or waves, in the electromagnetic field. The wavelength (or distance from one wave crest to the next) of light is extremely small, ranging from four to seven ten-millionths of a meter. The different wavelengths of light are what the human eye sees as different colors, with the longest wavelengths appearing at the red end of the spectrum and the shortest wavelengths at the blue end. Now imagine a source of light at a constant distance from us, such as a star, emitting waves of light at a constant wavelength. Obviously the wavelength of the waves we receive will be the same as the wavelength at which they are emitted (the gravitational field of the galaxy will not be large enough to have a significant effect). Suppose now that the source starts moving toward us. When the source emits the next wave crest it will be nearer to us, so the distance between wave crests will be smaller than when the star was stationary. This means that the wavelength of the waves we receive is shorter than when the star was stationary. Correspondingly, if the source is moving away from us, the wavelength of the waves we receive will be longer. In the case of light, therefore, means that stars moving away from us will have their spectra shifted toward the red end of the spectrum (red-shifted) and those moving toward us will have their spectra blue-shifted."
> Stephen Hawking contradicts Tom Roberts: Wavelength is an INTRINSIC property - it can already
> be changed before the interaction with the observer:
You cannot read, Pentcho.
Hawking did not in any way say that wavelength is intrinsic, and I defy you to explain how you drew the conclusion from what he said that it is intrinsic.
In fact, if you read it again, you will see that he said just the opposite.
On Thu, 04 Oct 2012 16:41:45 -0500, Big Dog wrote:
> On 10/4/2012 3:04 PM, Pentcho Valev wrote:
>> Stephen Hawking contradicts Tom Roberts: Wavelength is an INTRINSIC
>> property - it can already be changed before the interaction with the
>> observer:
> You cannot read, Pentcho.
> Hawking did not in any way say that wavelength is intrinsic, and I defy
> you to explain how you drew the conclusion from what he said that it is
> intrinsic.
> In fact, if you read it again, you will see that he said just the
> opposite.
> On Thu, 04 Oct 2012 16:41:45 -0500, Big Dog wrote:
>> On 10/4/2012 3:04 PM, Pentcho Valev wrote:
>>> Stephen Hawking contradicts Tom Roberts: Wavelength is an INTRINSIC
>>> property - it can already be changed before the interaction with the
>>> observer:
>> You cannot read, Pentcho.
>> Hawking did not in any way say that wavelength is intrinsic, and I defy
>> you to explain how you drew the conclusion from what he said that it is
>> intrinsic.
>> In fact, if you read it again, you will see that he said just the
>> opposite.
> Where exactly did he said that, please cite
Here:
Stephen Hawking, "A Brief History of Time", Chapter 3: "...we must first understand the Doppler effect. As we have seen, visible light consists of fluctuations, or waves, in the electromagnetic field. The wavelength (or distance from one wave crest to the next) of light is extremely small, ranging from four to seven ten-millionths of a meter. The different wavelengths of light are what the human eye sees as different colors, with the longest wavelengths appearing at the red end of the spectrum and the shortest wavelengths at the blue end. Now imagine a source of light at a constant distance from us, such as a star, emitting waves of light at a constant wavelength. Obviously the wavelength of the waves we receive will be the same as the wavelength at which they are emitted (the gravitational field of the galaxy will not be large enough to have a significant effect). Suppose now that the source starts moving toward us. When the source emits the next wave crest it will be nearer to us, so the distance between wave crests will be smaller than when the star was stationary. This means that the wavelength of the waves we receive is shorter than when the star was stationary. Correspondingly, if the source is moving away from us, the wavelength of the waves we receive will be longer. In the case of light, therefore, means that stars moving away from us will have their spectra shifted toward the red end of the spectrum (red-shifted) and those moving toward us will have their spectra blue-shifted."
> > On Thu, 04 Oct 2012 16:41:45 -0500, Big Dog wrote:
> >> On 10/4/2012 3:04 PM, Pentcho Valev wrote:
> >>> Stephen Hawking contradicts Tom Roberts: Wavelength is an INTRINSIC
> >>> property - it can already be changed before the interaction with the
> >>> observer:
> >> You cannot read, Pentcho.
> >> Hawking did not in any way say that wavelength is intrinsic, and I defy
> >> you to explain how you drew the conclusion from what he said that it is
> >> intrinsic.
> >> In fact, if you read it again, you will see that he said just the
> >> opposite.
> > Where exactly did he said that, please cite
> Here:
> Stephen Hawking, "A Brief History of Time", Chapter 3: "...we must first
> understand the Doppler effect. As we have seen, visible light consists
> of fluctuations, or waves, in the electromagnetic field. The wavelength
> (or distance from one wave crest to the next) of light is extremely
> small, ranging from four to seven ten-millionths of a meter. The
> different wavelengths of light are what the human eye sees as different
> colors, with the longest wavelengths appearing at the red end of the
> spectrum and the shortest wavelengths at the blue end. Now imagine a
> source of light at a constant distance from us, such as a star, emitting
> waves of light at a constant wavelength. Obviously the wavelength of the
> waves we receive will be the same as the wavelength at which they are
> emitted (the gravitational field of the galaxy will not be large enough
> to have a significant effect). Suppose now that the source starts moving
> toward us. When the source emits the next wave crest it will be nearer
> to us, so the distance between wave crests will be smaller than when the
> star was stationary. This means that the wavelength of the waves we
> receive is shorter than when the star was stationary. Correspondingly,
> if the source is moving away from us, the wavelength of the waves we
> receive will be longer. In the case of light, therefore, means that
> stars moving away from us will have their spectra shifted toward the red
> end of the spectrum (red-shifted) and those moving toward us will have
> their spectra blue-shifted."
To all:
Constant distance:constant wavelength. Both moving along the same
line or not moving. Unfortunately, in a 'vacant space' without a
reference, we cannot discern such movement if it existed. Maybe that
can be called an intrinsic wavelength. But we cannot directly measure
a wavelength. We infer it from measured frequency (passing crest to
crest in time) and the assumption that the speed of light is a
constant.
A crank wrote:
> But we cannot directly measure a wavelength. We infer it from
> measured frequency (passing crest to crest in time) and the
> assumption that the speed of light is a constant.
> xxein
This will come as quite a revelation to, among others, the hundreds
of undergraduates each year who measure laser light wavelength in
the lab without measuring its frequency at all.
On Thu, 04 Oct 2012 17:50:51 -0500, Big Dog wrote:
> On 10/4/2012 5:29 PM, Pete Weber wrote:
>> On Thu, 04 Oct 2012 16:41:45 -0500, Big Dog wrote:
>>> On 10/4/2012 3:04 PM, Pentcho Valev wrote:
>>>> Stephen Hawking contradicts Tom Roberts: Wavelength is an INTRINSIC
>>>> property - it can already be changed before the interaction with the
>>>> observer:
>>> You cannot read, Pentcho.
>>> Hawking did not in any way say that wavelength is intrinsic, and I
>>> defy you to explain how you drew the conclusion from what he said that
>>> it is intrinsic.
>>> In fact, if you read it again, you will see that he said just the
>>> opposite.
>> Where exactly did he said that, please cite
> Here:
> Stephen Hawking, "A Brief History of Time", Chapter 3: "...we must first
> understand the Doppler effect. As we have seen, visible light consists
> of fluctuations, or waves, in the electromagnetic field. The wavelength
> (or distance from one wave crest to the next) of light is extremely
> small, ranging from four to seven ten-millionths of a meter. The
> different wavelengths of light are what the human eye sees as different
> colors, with the longest wavelengths appearing at the red end of the
> spectrum and the shortest wavelengths at the blue end. Now imagine a
> source of light at a constant distance from us, such as a star, emitting
> waves of light at a constant wavelength. Obviously the wavelength of the
> waves we receive will be the same as the wavelength at which they are
> emitted (the gravitational field of the galaxy will not be large enough
> to have a significant effect). Suppose now that the source starts moving
> toward us. When the source emits the next wave crest it will be nearer
> to us, so the distance between wave crests will be smaller than when the
> star was stationary. This means that the wavelength of the waves we
> receive is shorter than when the star was stationary. Correspondingly,
> if the source is moving away from us, the wavelength of the waves we
> receive will be longer. In the case of light, therefore, means that
> stars moving away from us will have their spectra shifted toward the red
> end of the spectrum (red-shifted) and those moving toward us will have
> their spectra blue-shifted."
He says exactly you are wrong, and that the wavelength
remains unchanged along lights journey through empty space
> Constant distance:constant wavelength. Both moving along the same line
> or not moving. Unfortunately, in a 'vacant space' without a reference,
> we cannot discern such movement if it existed. Maybe that can be called
> an intrinsic wavelength. But we cannot directly measure a wavelength. > We infer it from measured frequency (passing crest to crest in time) and
> the assumption that the speed of light is a constant.
> xxein
you might get nobelized for your apparatus, what do you use?
This is just plain false. For well over a century astronomers have used diffraction gratings to directly measure wavelength.
> We infer it from measured frequency (passing crest to
> crest in time) and the assumption that the speed of light is a
> constant.
Nonsense. Indeed it is only within the past few decades that it has been possible to measure the frequency of a light ray. Of course we have been able to do that for radio waves for nearly a century.
> On 10/5/2012 7:33 AM, Pete Weber wrote:
>> [about a quote from Hawking]
>> He says exactly you are wrong, and that the wavelength
>> remains unchanged along lights journey through empty space
> He says that in the case where the source is not moving relative to us.
No, he does not say that. He says "the wavelength of the
waves we receive will be the same as the wavelength at which they are
emitted". He does NOT say the wavelength "remains unchanged along lights journey" (or anything close to that). He is clearly discussing our MEASUREMENT of the light's wavelength, and not any sort of "intrinsic wavelength of the light". Because, of course, light does not have an intrinsic wavelength; wavelength is a RELATIONSHIP between the light ray and a measuring instrument.
This is quite obvious in relativity. Hawking clearly understand it, but all too many people around here do not.
> On 10/5/12 10/5/12 8:08 AM, Big Dog wrote:
>> On 10/5/2012 7:33 AM, Pete Weber wrote:
>>> [about a quote from Hawking]
>>> He says exactly you are wrong, and that the wavelength
>>> remains unchanged along lights journey through empty space
>> He says that in the case where the source is not moving relative to us.
> No, he does not say that. He says "the wavelength of the
> waves we receive will be the same as the wavelength at which they are
> emitted".
He also says "This means that the wavelength of the waves we receive is shorter than when the star was stationary. Correspondingly, if the source is moving away from us, the wavelength of the waves we receive will be longer."
There is no distinction between any imagined "actual" wavelength and the measured wavelength in this statement. If you are making a contextual distinction between THE wavelength and THE MEASUREMENT OF THE wavelength, then it is best not to project that distinction onto Hawking's writing. Granted, Hawking is writing here a book aimed at laypeople with nothing more than a high school background in physics, and so he is deliberately not being very precise in his language. But this is precisely why one shouldn't use writings in this category for reference, because one can easily interpret its statements without constraint and without justification.
> He does NOT say the wavelength "remains unchanged along lights
> journey" (or anything close to that). He is clearly discussing our
> MEASUREMENT of the light's wavelength, and not any sort of "intrinsic
> wavelength of the light". Because, of course, light does not have an
> intrinsic wavelength; wavelength is a RELATIONSHIP between the light ray
> and a measuring instrument.
> This is quite obvious in relativity. Hawking clearly understand it, but
> all too many people around here do not.
On Fri, 05 Oct 2012 08:18:41 -0500, Tom Roberts wrote:
> On 10/5/12 10/5/12 8:08 AM, Big Dog wrote:
>> On 10/5/2012 7:33 AM, Pete Weber wrote:
>>> [about a quote from Hawking]
>>> He says exactly you are wrong, and that the wavelength remains
>>> unchanged along lights journey through empty space
>> He says that in the case where the source is not moving relative to us.
> No, he does not say that. He says "the wavelength of the waves we
> receive will be the same as the wavelength at which they are emitted".
> He does NOT say the wavelength "remains unchanged along lights journey"
> (or anything close to that). He is clearly discussing our MEASUREMENT of
> the light's wavelength, and not any sort of "intrinsic wavelength of the
> light". Because, of course, light does not have an intrinsic wavelength;
> wavelength is a RELATIONSHIP between the light ray and a measuring
> instrument.
> This is quite obvious in relativity. Hawking clearly understand it, but
> all too many people around here do not.
clearly here you do not,
he says that the wavelength would _measures_ doppler shifted for an observer _in motion_ wrt an emitter
this only is right if the wavelength travels unshifted and
_measures_ unshifted for a stationary observer wrt a source
"Obviously the wavelength of the waves we receive will
be the same as the wavelength at which they are emitted"
On Fri, 05 Oct 2012 09:15:49 -0500, Big Dog wrote:
> On 10/5/2012 8:18 AM, Tom Roberts wrote:
>> On 10/5/12 10/5/12 8:08 AM, Big Dog wrote:
>>> On 10/5/2012 7:33 AM, Pete Weber wrote:
>>>> [about a quote from Hawking]
>>>> He says exactly you are wrong, and that the wavelength remains
>>>> unchanged along lights journey through empty space
>>> He says that in the case where the source is not moving relative to
>>> us.
>> No, he does not say that. He says "the wavelength of the waves we
>> receive will be the same as the wavelength at which they are emitted".
> He also says "This means that the wavelength of the waves we receive is
> shorter than when the star was stationary. Correspondingly, if the
> source is moving away from us, the wavelength of the waves we receive
> will be longer."
> There is no distinction between any imagined "actual" wavelength and the
> measured wavelength in this statement. If you are making a contextual
> distinction between THE wavelength and THE MEASUREMENT OF THE
> wavelength,
sure it is, and is not imagined, wavelengths are real
you cant have a travelling unreal imaginary wavelength!
> then it is best not to project that distinction onto
> Hawking's writing. Granted, Hawking is writing here a book aimed at
> laypeople with nothing more than a high school background in physics,
> and so he is deliberately not being very precise in his language. But
> this is precisely why one shouldn't use writings in this category for
> reference, > because one can easily interpret its statements without
> constraint and without justification.
On Thursday, October 4, 2012 5:41:41 PM UTC-4, Big Dog wrote:
> On 10/4/2012 3:04 PM, Pentcho Valev wrote:
> > Stephen Hawking contradicts Tom Roberts: Wavelength is an INTRINSIC property - it can already
> > be changed before the interaction with the observer:
> You cannot read, Pentcho.
> Hawking did not in any way say that wavelength is intrinsic, and I defy
> you to explain how you drew the conclusion from what he said that it is
> intrinsic.
Wavelength of sodium source is intrinsic 589 nm in all frames.
Incoming soidum light from a moving sodium source becomes a new
light source in the observer's frame and the observer's grating defines a new wavelength for this new light source. This means that the speed of incoming sodium light is source/observer dependent.
On Friday, October 5, 2012 9:18:41 AM UTC-4, tjrob137 wrote:
> On 10/5/12 10/5/12 8:08 AM, Big Dog wrote:
> > On 10/5/2012 7:33 AM, Pete Weber wrote:
> >> [about a quote from Hawking]
> >> He says exactly you are wrong, and that the wavelength
> >> remains unchanged along lights journey through empty space
> > He says that in the case where the source is not moving relative to us.
> No, he does not say that. He says "the wavelength of the
> waves we receive will be the same as the wavelength at which they are
> emitted". He does NOT say the wavelength "remains unchanged along lights
> journey" (or anything close to that). He is clearly discussing our MEASUREMENT
> of the light's wavelength, and not any sort of "intrinsic wavelength of the
> light". Because, of course, light does not have an intrinsic wavelength;
> wavelength is a RELATIONSHIP between the light ray and a measuring instrument.
Wavelength of any sodium source is intrinsic 589 nm in all frames.
Incoming soidum light from a moving sodium source becomes a new
light source in the observer's frame and the observer's grating defines a new wavelength for this new light source. This means that the speed of incoming sodium light is source/observer dependent.
> On 10/5/2012 8:18 AM, Tom Roberts wrote:
>> On 10/5/12 10/5/12 8:08 AM, Big Dog wrote:
>>> On 10/5/2012 7:33 AM, Pete Weber wrote:
>>>> [about a quote from Hawking]
>>>> He says exactly you are wrong, and that the wavelength
>>>> remains unchanged along lights journey through empty space
>>> He says that in the case where the source is not moving relative to us.
>> No, he does not say that. He says "the wavelength of the
>> waves we receive will be the same as the wavelength at which they are
>> emitted".
> He also says "This means that the wavelength of the waves we receive is shorter
> than when the star was stationary. Correspondingly, if the source is moving away
> from us, the wavelength of the waves we receive will be longer."
> There is no distinction between any imagined "actual" wavelength and the
> measured wavelength in this statement.
I disagree. He carefully mentions "the wavelength of the waves WE RECEIVE" and "the wavelength AT WHICH THEY ARE EMITTED" [emphasis mine]. That's why I said 'He is clearly discussing our MEASUREMENT of the light's wavelength, and not any sort of "intrinsic wavelength of the light".'
While I agree that books written for general audiences are not good reference material, in this particular case Hawking is being rather precise -- he makes no mention of wavelength "possessed by the light", but only of wavelength AS RECEIVED and AS EMITTED. This is an appropriate distinction, because in modern physics, light does not have an intrinsic wavelength, but measurements of wavelength obtain definite values.
For a massive (timelike) object, its proper length is an
intrinsic property. There is no analogous quantity for light.
Stated geometrically: the displacement 4-vector between an
object's front and back is spacelike, and its norm is the
object's proper length. For a light wave, the displacement
4-vector between successive wavecrests is null, so its norm
is not useful as a measurement. But any observer can make a
measurement of these lengths by projecting those displacement
4-vectors onto a length-measuring instrument.
You carefully avoid the question in my original message:
Do you mean that the orientation of the MEASURING INSTRUMENT varies with the speed of the observer? This orientation remains unchanged so long as the speed of the observer remains constant?
> Do you mean that the orientation of the MEASURING INSTRUMENT varies with the
> speed of the observer? This orientation remains unchanged so long as the
> speed of the observer remains constant?
The way your question is phrased implicitly assumes some sort of "absolute speed". But there is no such thing in relativity.
Differently moving observers' instruments are oriented differently in spacetime. In Minkowski spacetime, the orientation of a clock remains unchanged as long as its 4-velocity remains unchanged (i.e. it is moving inertially). But in other manifolds this need not hold (though it's not clear the relevant concepts have meaning). For rulers, non-rotation must be added.
> Yes or no, Honest Roberts?
If you insist on "yes or no", then I must respond: mu, to UNASK the question.
