FAQ
It is currently Sun Sep 24, 2017 1:25 am


Author Message
anticorncob28
Post  Post subject: Does red light travel faster than blue light?  |  Posted: Tue Feb 28, 2017 3:52 am
User avatar

Joined: Thu May 05, 2016 10:31 pm
Posts: 42
Location: Nebraska, USA

Offline
If not, then please explain what is wrong with the following reasoning.
Blue light has a shorter wavelength than red light. Therefore to make the same displacement the blue light has to curve more and travel a greater total distance.
For example, the curve y = sin(3x) from x = 0 to x = 10 is longer than the curve y = sin(x) from 0 to 10, so a beam of light with shorter wavelength needs more time to go 10 units.

_________________
"Climate change is the canvas on which the history of the 21st century will be painted."
-Mark Lynas, Six Degrees: Our Future on a Hotter Planet


Top
marnixR
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Tue Feb 28, 2017 8:41 am
User avatar

Joined: Thu Aug 04, 2011 8:35 pm
Posts: 4848
Location: Cardiff, Wales

Offline
tell me: why should speed have anything to do with wave length ?
it's almost like saying that the vibration on your steering wheel determines the speed of your car

_________________
"Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
"Someone is WRONG on the internet" (xkcd)


Top
anticorncob28
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Tue Feb 28, 2017 9:17 pm
User avatar

Joined: Thu May 05, 2016 10:31 pm
Posts: 42
Location: Nebraska, USA

Offline
So the answer is no. But I don't think you understand why I thought the answer might be yes.
marnixR wrote:
tell me: why should speed have anything to do with wave length ?
it's almost like saying that the vibration on your steering wheel determines the speed of your car

About the car. Let x represent the horizontal velocity and y vertical velocity. Say x is 60 (mph). Then most of the car has vertical velocity 0, so the total velocity is sqrt(60^2 + 0^2) = 60.
Now about the steering wheel. If it's vibrating up and down, at some points its vertical velocity will be some non-zero number y. The total velocity of the steering wheel is sqrt(60^2 + y^2) > 60 while it is moving up and down.
With a beam of light, its horizontal speed in a vacuum is c, right?
But if light has a wavelength and bobs up and down, it also has a non-zero vertical velocity y. The total speed is sqrt(c^2 + y^2) > c, which is a problem.
I'd figure that a photon must travel at a speed less than c horizontally so its total speed remains c. For shorter wavelengths the photon moves faster up and down and thus must move slower horizontally so its total speed is c.

So how is this wrong?

_________________
"Climate change is the canvas on which the history of the 21st century will be painted."
-Mark Lynas, Six Degrees: Our Future on a Hotter Planet


Top
PhDemon
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Tue Feb 28, 2017 9:42 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 499
Location: Newcastle-upon-Tyne

Offline
I'm a bit drunk and haven't analysed your thought experiment in detail but how would your thought experiment apply to photons? Could the issue be with treating light purely as a classical wave? It isn't. It's a bit more complicated than that...

_________________
"The big trouble with dumb bastards is that they are too dumb to believe there is such a thing as being smart"

- Kurt Vonnegut, The Sirens of Titan


Top
Chrispen Evan
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Tue Feb 28, 2017 11:10 pm
User avatar

Joined: Thu Sep 22, 2016 8:17 pm
Posts: 27

Offline
Photons don't travel with a sinusoidal motion.


Top
PhDemon
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Tue Feb 28, 2017 11:20 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 499
Location: Newcastle-upon-Tyne

Offline
That's why I asked the question in my above post... But after rereading the OP I'm not convinced that
Quote:
Blue light has a shorter wavelength than red light. Therefore to make the same displacement the blue light has to curve more and travel a greater total distance


is correct. Can you explain why you think more curvature equates with more distance? Surely the distance travelled by the wave is the same regardless of how many peaks and troughs there are in that distance. In QM and classical physics the curvature of a wave(function) is related to its energy not the distance travelled.

_________________
"The big trouble with dumb bastards is that they are too dumb to believe there is such a thing as being smart"

- Kurt Vonnegut, The Sirens of Titan


Top
marnixR
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Tue Feb 28, 2017 11:47 pm
User avatar

Joined: Thu Aug 04, 2011 8:35 pm
Posts: 4848
Location: Cardiff, Wales

Offline
I think I have some idea of the thinking behind the OP - if you were driving your car in a series of big S-curves, then you would cover more miles for the same amount of straight distance covered than if you did smaller S-curves

imo that's a faulty analogy

_________________
"Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
"Someone is WRONG on the internet" (xkcd)


Top
PhDemon
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Wed Mar 01, 2017 12:06 am

Joined: Wed Jun 26, 2013 10:44 am
Posts: 499
Location: Newcastle-upon-Tyne

Offline
I agree, there's no hypothetical particle moving up and down through the peaks and troughs of the wave, you cannot equate the length of the path such a particle would make to the distance travelled by the wave.

_________________
"The big trouble with dumb bastards is that they are too dumb to believe there is such a thing as being smart"

- Kurt Vonnegut, The Sirens of Titan


Top
marnixR
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Wed Mar 01, 2017 8:55 am
User avatar

Joined: Thu Aug 04, 2011 8:35 pm
Posts: 4848
Location: Cardiff, Wales

Offline
tbh, I have the suspicion that entities like the photon or the electron, since they exhibit properties of both particle and wave, are neither

hence we can't predict their behaviour by comparing them with other entities that are clearly pure particle or wave

_________________
"Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
"Someone is WRONG on the internet" (xkcd)


Top
PhDemon
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Wed Mar 01, 2017 9:06 am

Joined: Wed Jun 26, 2013 10:44 am
Posts: 499
Location: Newcastle-upon-Tyne

Offline
marnixR wrote:
tbh, I have the suspicion that entities like the photon or the electron, since they exhibit properties of both particle and wave, are neither

hence we can't predict their behaviour by comparing them with other entities that are clearly pure particle or wave


This is true (and a point I've made a few times here), treating or describing a quantum object as a classical wave or a classical particle is sometimes fine as an approximation but it often (usually!) gives the wrong answer! I've used the invented term "wavicle" before to emphasize quantum objects are neither particles or waves but something completely different that has properties associated with both...

_________________
"The big trouble with dumb bastards is that they are too dumb to believe there is such a thing as being smart"

- Kurt Vonnegut, The Sirens of Titan


Top
anticorncob28
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Wed Mar 01, 2017 7:49 pm
User avatar

Joined: Thu May 05, 2016 10:31 pm
Posts: 42
Location: Nebraska, USA

Offline
My question has been answered; thank-you to all.

_________________
"Climate change is the canvas on which the history of the 21st century will be painted."
-Mark Lynas, Six Degrees: Our Future on a Hotter Planet


Top
me_again
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 2:06 pm

Joined: Fri Jun 09, 2017 9:54 am
Posts: 9

Offline
Hello,

According to Einstein, in his theory of General Relativity *
the speed of light varies with the gravitational potential (and is constant given a constant potential).

And lightrays radially further away from the earth, for example, will exibit gravitational redshift.

Hence that redshifted light travels faster at that height, locally, compared to the earth surface.


* Relativity - The Special and General theory - page 90


Best.


Top
me_again
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 2:41 pm

Joined: Fri Jun 09, 2017 9:54 am
Posts: 9

Offline
Page 90 in the 100 year Anniversary edition that is.


Top
me_again
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 4:10 pm

Joined: Fri Jun 09, 2017 9:54 am
Posts: 9

Offline
Here's the quote from chapter 22


" In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the
velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity
and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can
only take place when the velocity of propagation of light varies with position. Now we might think that as a
consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust.
But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited
domain of validity ; its results hold only so long as we are able to disregard the influences of gravitational fields on
the phenomena (e.g. of light). "


Top
me_again
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 5:45 pm

Joined: Fri Jun 09, 2017 9:54 am
Posts: 9

Offline
Here's an explanation

https://physics.stackexchange.com/quest ... onal-field

But the last paragraph is the tricky part.

Indeed, if you stand say 100 meters higher instead of on the earth surface, then the relative difference with the clock at say 1000 meters will be different.
But: The ticking rate at 1000 meters is always the same, there is only one outcome of the experiment, with the clock at 1000 meters having only one result, it will show 5 minutes past 12 for instance. Doesn't matter where you stand in the universe, that clock will indicate 5 minutes past 12 after the experiment. Hmm..


Top
iNow
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 8:23 pm
User avatar
Original Member
Original Member

Joined: Thu Aug 04, 2011 11:40 pm
Posts: 5700
Location: Iowa

Offline
You're misunderstanding it. You need to abandon the notion that the speed of light varies. It doesn't. It's constant, and that's true for all observers regardless of frame.

What varies in these differing gravitational potentials is time, and even that changes based on the observer. There is no preferred frame of reference. What you seem to be missing is the relativity of simultaneity.

Keep in mind too that light is blueshifted when it falls into a gravity well but then redshifts when it climbs out. Time and length contraction follow the same relation.

_________________
iNow

"[Time] is one of those concepts that is profoundly resistant to a simple definition." ~C. Sagan


Top
me_again
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 9:17 pm

Joined: Fri Jun 09, 2017 9:54 am
Posts: 9

Offline
What can we misunderstand from Einstein's statement ?

" A curvature of rays of light can
only take place when the velocity of propagation of light varies with position. " - 1916, see source above


Top
PhDemon
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 9:34 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 499
Location: Newcastle-upon-Tyne

Offline
Einstein is not an oracle (this is science not religion), physics has moved on since he died, navel gazing about what he meant in random quotations does not give any insights into current thinking on the subject...

_________________
"The big trouble with dumb bastards is that they are too dumb to believe there is such a thing as being smart"

- Kurt Vonnegut, The Sirens of Titan


Top
janus
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 9:48 pm

Joined: Mon Dec 28, 2015 7:11 pm
Posts: 36

Offline
me_again wrote:
What can we misunderstand from Einstein's statement ?

" A curvature of rays of light can
only take place when the velocity of propagation of light varies with position. " - 1916, see source above


And INow said"

Quote:
You're misunderstanding it. You need to abandon the notion that the speed of light varies.


"speed" and "velocity" are two separate concepts, as velocity refers to both the speed and direction of movement.
So while an object cannot change speed without changing velocity, it can change velocity without changing speed (by changing direction).

Thus the speed of light can be a constant while its velocity can change, And Inow's statement and the quote from Einstein are in accord with each other.


Top
me_again
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Fri Jun 09, 2017 10:29 pm

Joined: Fri Jun 09, 2017 9:54 am
Posts: 9

Offline
Indeed Janus, when light is passing by closely to the sun for instance, it changes direction (spacial curvature). But experimental evidence, as Einstein predicted, shows that you need to double that effect to get the correct doubled bending angle. And that recquires an additional effect, namely the part of the beam front closest to the sun going slower than the part situated further (or 'higher').

So to comply to the experimental evidence, the velocity has to change by means of a change in direction, accompanied by a slowing down.

Would you say that is correct ?


Top
me_again
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Wed Jun 14, 2017 7:54 am

Joined: Fri Jun 09, 2017 9:54 am
Posts: 9

Offline
Anybody ?


Top
iNow
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Thu Jun 15, 2017 12:36 pm
User avatar
Original Member
Original Member

Joined: Thu Aug 04, 2011 11:40 pm
Posts: 5700
Location: Iowa

Offline
Your question isn't clear to me.

me_again wrote:
when light is passing by closely to the sun for instance, it changes direction (spacial curvature). But experimental evidence, as Einstein predicted, shows that you need to double that effect to get the correct doubled bending angle.

Do you mean double the effect of gravity on light? Something else?

me_again wrote:
And that recquires an additional effect, namely the part of the beam front closest to the sun going slower than the part situated further (or 'higher').

What do you mean by beam front? Also, speed is constant, and velocity would be same within the same beam (same vector).

me_again wrote:
So to comply to the experimental evidence, the velocity has to change by means of a change in direction, accompanied by a slowing down.

Relative to whom? What reference frame are you focused upon?

_________________
iNow

"[Time] is one of those concepts that is profoundly resistant to a simple definition." ~C. Sagan


Top
me_again
Post  Post subject: Re: Does red light travel faster than blue light?  |  Posted: Thu Jun 15, 2017 1:31 pm

Joined: Fri Jun 09, 2017 9:54 am
Posts: 9

Offline
iNow,

I've added my responses between the lines.


"Your question isn't clear to me.

me_again wrote:
when light is passing by closely to the sun for instance, it changes direction (spacial curvature). But experimental evidence, as Einstein predicted, shows that you need to double that effect to get the correct doubled bending angle.

Do you mean double the effect of gravity on light? Something else?

>> Double the bending angle.
Before 1915 the bending angle was only predicted * 50% of the experimental result which came later.

*: both by Newtonian theory and by Einstein.


me_again wrote:
And that recquires an additional effect, namely the part of the beam front closest to the sun going slower than the part situated further (or 'higher').

What do you mean by beam front? Also, speed is constant, and velocity would be same within the same beam (same vector).

>> To check Einstein's 1911 paper page 904, which used that idea but came up 50% short. At that time the spacial curvature was not part of his proposal yet.
>>Related also to the concept of space as an optical medium (Huygens), where light slows down in the region of higher density.

me_again wrote:
So to comply to the experimental evidence, the velocity has to change by means of a change in direction, accompanied by a slowing down.

Relative to whom? What reference frame are you focused upon? "

From the center of the sun :
-change in direction due to spacial curvature : 50%
-the part of the beam front closest to the center travels slower than the part further away, for the reason given above, resulting in an additional change in direction : another 50%

Thus complying to experimental evidence.
_________________
iNow


Top
Display posts from previous:  Sort by  
Print view

Who is online
Users browsing this forum: No registered users and 0 guests
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
Jump to:   
cron

Delete all board cookies | The team | All times are UTC


This free forum is proudly hosted by ProphpBB | phpBB software | Report Abuse | Privacy