Is the brightness of light invariant?

[Quantum Quack]

No. Radiant intensity will follow a similar pattern to doppler shift - an approaching light source will have a higher radiant intensity than a stationary light source at a given distance. A receding light source will have a lower radiant intensity at that distance.

Pete why would radiant intensity increase relative to velocity?

[This is central to this whole thread I might add]

Is it because the length is contracted or it is because the light applies a pressure? or is it both?

 

[Quantum Quack]

I think you are going to say that it is associated with the frequency increase of decrease relative to velocity and of course that would be doppler shift yes?

How ever how does doppler shift effect radiant intensity?

 

[Pete]

so the term watts is adequate

Watts per steradian. Watts would be a measure of radiant flux.
Actually, perhaps irradiance is what you're after. That's Watts per square meter. The radiance of a source doesn't decrease with distance (although it might be different in different directions, eg a torch). The irradiance decreases with the square of the radius.

although I am not sure exactly what "solid angle" means as the language used is beyond my meger physics vocab.

Possibly sold angle would mean a 3 dimensional segment of the surface of an imaginary or virtual sphere?

You got it.
Just as an angle subtends an arc of a circle, so a solid angle subtends a portion of a sphere.
There are 2 pi radians to a circle;There are 4 pi steradians to a sphere.
A radian is the angle subtended on a circle of radius r by an arc of length r; A steradian is the solid angle subtended on a sphere of radius r by a surface portion of area r².

 

[Pete]

Pete why would radiant intensity increase relative to velocity?

If you're approaching a light source, you're collecting more light per unit time as you approach.

Is it because the length is contracted or it is because the light applies a pressure? or is it both?

Try neither. Although you could derive length contraction from the effect, I think.

 

[Quantum Quack]

so you end up with a curved square segment - like a parachute [ the oblong controled variety ]

And yes it does sound like irradiance is the term. So the same questions apply I guess using irradiance.

Is irradiance invariant to all observers?

 

[Pete]

How ever how does doppler shift effect radiant intensity?

Higher frequency light packs more energy per photon. But that's not the only reason for the increase - you're also collecting more photons per unit time if you're approaching the light source.

 

[Quantum Quack]

hmmmm...need thinking time.....

 

[dav57]

...you're also collecting more photons per unit time if you're approaching the light source.

Pete, I'll give you three guesses as to what I'm about to say to that

 

[Quantum Quack]

It sounds like we are creating a puddle of photons doesn't it?
Do they hang around long enough to do that I wonder?

 

[Prosoothus]

Pete,

But that's not the only reason for the increase - you're also collecting more photons per unit time if you're approaching the light source.

If you're approaching a light source, wouldn't the speed of light remain constant for you, and therefore you'd be collecting the same amount of photons/second?

 

[Quantum Quack]

Pete,



If you're approaching a light source, wouldn't the speed of light remain constant for you, and therefore you'd be collecting the same amount of photons/second?

It is obvious that there is some confusion, as you have pointed out.

This is a constant cause of vexation regarding the subject of invariance. Where by some times it appears it is and sometimes it appears it isn't.

More so because of misinformation or poor assessments.

I would think that lights invariance were true photons would not collect in any different way regardless of your velocity. Simply because as far as the observer is concerned he is at rest relative to the lights velocity. if he is at rest then he can not collect extra photons and would simply witness the light to be always the same velocity..

This is why I am attempting to remove the time data recording from the situation and attempt to deal with "radiant intensity" or "irrandiance" or what ever that is a recorded measurment that does not involve time in the data aquisition.

As mentioned earlier because there are two relative variables involved [ time/distace] in SRT one variable will always justify the other, which they should I might add.

However if we take away one of those variables such as time I think the SRT picture becomes clearer.

Differences in brightness [ irradiance ] can not be avoided by claiming a time factor such as dilation.

Now as I have been asking this question off and on for quite awhile 12 months or so and thus far have recieved no adequate response I can only assume that this aspect of light has not been the subject of thorough questioning and experiment.

There should be after 100 years of research be a very clear assessment available to answer the question. In fact my naive thread starter should have been obvious as to what I was intending. Yet no one has presented an understanding of the issues involved which leads me to believe it has not been explored much by any one.

So I shall persist and maybe with some help we can clear up this issue for future reference.

 

[Pete]

It sounds like we are creating a puddle of photons doesn't it?

I don't know what you mean.

If you're approaching a light source, wouldn't the speed of light remain constant for you, and therefore you'd be collecting the same amount of photons/second?

No... The speed of light is not how many photons pass by per second, it's how fast each photon is going.

 

[Pete]

I would think that lights invariance were true photons would not collect in any different way regardless of your velocity. Simply because as far as the observer is concerned he is at rest relative to the lights velocity. if he is at rest then he can not collect extra photons and would simply witness the light to be always the same velocity.

Your terminology is confused, QQ, which makes it hard to figure out what you mean. "At rest relative to the light's velocity" doesn't make sense. Rest is relative to objects or reference frames. Ie the observer is at rest relative to themself.

This is why I am attempting to remove the time data recording from the situation and attempt to deal with "radiant intensity" or "irrandiance" or what ever that is a recorded measurment that does not involve time in the data aquisition.

Good luck with that. Radiant intensity, irradiance, and any other measure of brightness all involve power (watts), which is energy/time (joules/second).

Now as I have been asking this question off and on for quite awhile 12 months or so and thus far have recieved no adequate response I can only assume that this aspect of light has not been the subject of thorough questioning and experiment.

Oh please! Asking questions off and on on an Internet discussion board is hardly a literature review.

 

[dav57]

....The speed of light is not how many photons pass by per second, it's how fast each photon is going.

So wait a minute, Pete, the light beam has been emitted and travels through space, right?. Each photon is then "collected" by a moving observer.

When the observer increases his velocity, he collects more photons and thus we deduce that to do this (and being as HE is the one who has altered his speed) the relative speed between beam of light and observer MUST have changed.

So, you're right in what you say above, but I ask you how else can an observer collect more photons from a beam of light unless he alters his velocity relative to the beam?

This, to me, is a crucial clue that light is NOT invariant.

Physics Monkey, where are you? I answered your question (although probably wrong) and you've disapeared!

 

[Pete]

When the observer increases his velocity, he collects more photons and thus we deduce that to do this (and being as HE is the one who has altered his speed) the relative speed between beam of light and observer MUST have changed.

It doesn't follow. What are you assuming to reach this conclusion?

You could try it in reverse -
Let's say you're counting photons from two sources, without knowing anything about how those sources are moving.
Let's say that you note that every photon passes you by at the same speed.
Let's also say that count more photons per second from source A than source B.
What would you conclude about the photon streams?

 

[dav57]

It doesn't follow. What are you assuming to reach this conclusion?

You could try it in reverse -
Let's say you're counting photons from two sources, without knowing anything about how those sources are moving.
Let's say that you note that every photon passes you by at the same speed.
Let's also say that count more photons per second from source A than source B.
What would you conclude about the photon streams?

Pete, you're talking about moving sources again. And, yes, in this case, the Doppler shift is created by the fact that the source was moving BEFORE it emitted the photons.

I'm not talking about that scenario. I'm talking about the scenario of two already emitted beams of light followed by the death of the sources. Only then can we explore the possibilities of why a moving observer collects more photons per unit time from a similar beam than a stationary observer.

Please see the "rocket" scenario above, which Physics Monkey hasn't yet replied to.

 

[Pete]

Pete, you're talking about moving sources again.

No. I'm talking about an observer receiving two streams of photons, and seeing what they conclude about the streams (not the sources). Come on, dav, think about it. For me. Please?

 

[dav57]

No. I'm talking about an observer receiving two streams of photons, and seeing what they conclude about the streams (not the sources). Come on, dav, think about it. For me. Please?

Based on the fact that two identically emitted streams of photons from the same source can have their photons "collected" by two different rockets travelling at different speeds. Right? That's one rocket travelling up one stream and the other travelling up the other.

Now, what I don't understand is how the faster rocket can "collect" more and more photons per second the faster it goes, than the slow rocket. Such that when they arrive back at Earth the astronauts compare teh amount of "power" they received from the light beam in a given amount of time.

And why when the fast rocket accelerated and decelerated, did the power he received from the beam increase / decrease?

I can't help but conclude that the rockets alter their speed "RELATIVE" to the beam of light. Yes, I agree this is the Doppler shift but I question: Why a Doppler shift and conclude that the Doppler shift is a clue, sorry - EVIDENCE, that light is NOT invariant.

This leads me to believe that the fact that the perceived wavelength change (being inverse to the frequency) as you increase your speed is providing some kind of illusion that light is in fact invariant.

If the rocket's speed causes the Doppler shift then you must question why there is a Doppler shift in the first place if the relative velocity between rocket and beam remains unchanged.

 

[Pete]

Slow down, dav. Focus on the basics.

An observer watches two particle streams.
The observer notices that every particle passes by at speed v.
The observer notices that in stream A, n particles pass each second.
The observer notices that in stream B, m particles pass each second.

What does the observer conclude about the particle density in the two streams?

 

[Pete]

Reviewing posts, I have a suspicion that an old confusion is muddying the waters.

If we consider a moving observer, then we are clearly considering a reference frame other than the observer's rest frame.

This means that in that frame the speed of the observer relative to the light will not be c. Ie the rate at which the distance between the observer and any given photon changes will be something other than 3e8 m/s.

Is that clear to everyone? It expect it is, but some posts seem to hint otherwise.