Is the brightness of light invariant?

I couldn't resist being a smart ass.

 

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ha...neither could I.....

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From what I understand [which may very well be a flawed understanding]

Most SRT gendankens are self justifying simply because they use two relative variables ie; time and distance.
To create a gendanken that elliminates the use of relative time [ time dilation] should prove interesting and probably more conclusive.

Say for example a gendanken that looks a bit like this:

Two ships
A and B
One lIght source Star C

A is stationary to light source C and positioned at a distance of 10 ly from C.

Ship B is flying past Shp A at a velocity of 0.8c [ according to Ship A's observer]

When Ship B is at the same distance from Light source C according to ship A's observer how bright is the light source according to ship A and B at the exact moment when the ships are at the same distance according to Ship A.


What I think:

Ship B is in contracted space and will see the light source as brighter than what ship A would see at that particular distance. [ no mention of timed data sets so far]

Now if this is correct assessment then there poses a problem for what the observer inside ship B sees and records about his ships in-board lights does it not?

 

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No, I don't think so - just off the top of my head. Let's say that the frequency was .00001sec, time on the spaceship is the same for the observer on B and the light on B. The frequency couldn't change, so the wavelength couldn't change.

 

Ahhhh!!! but I am not talking about frequency or Wave length. Which is the whole point I guess.
I am talking about wattage or candlepower, strength of light. or how many photons are hitting the ship at a given moment in time and not over time but at that given moment of time....[that infinitely small moment of time between the future and past]

The number of photons arriving simultaneously? or maybe the strength of the wave...not sure how to describe it...sorry...

 

maybe the concentration of photons at any given moment. A snapshot of that concentration. without any regard for the passage of time no matter how small.

 

but this is precisely what I was saying. If wavelength and frequency didn't change, the number of photons and the strength of the photons didn't change either for observer on B and lights on B no matter how small the passage of time.

 

Maybe if we look at it in another way.

You are onboard ship B.

There are two stars relatively stationary regarding each other, One of those stars is in line of vector. the other is perpedicular to that vector. At this point in space both stars are the same distance away from your ship.

Both stars are emitting a similar strength of light at this distance.

The star that you are vectored for is contracted distance thus is brighter, However the star that is located perpedicular to that vector appears to be the same brightness as if you were stationary relative to that star.

So the observer looks around and he sees two stars the front one is brighter than the one to the right even though both stars emit the samer wattage of light.

If space is vector contracted his distance to the front star is reduced.

So how would this effect the strength or brightness of the lights aboard his ship using the same criteria. Forward and perpendicular brightness should be different yes?

The number of photonic concentration must change surely if the brightness is to increase as you get closer to that light source?

How does frequency apply to an infinitely small moment of time?

Frequency can only be a time duration artifact yes? But if we are not considering time passage how can frequency be applicable or even relevant?

 

Maybe I need to clarify:

Say we are looking at white light. we are 100ly away...is that white any different if we are only 1 ly away [ in both cases we are stationary relative to the source when recording our brightness figure]??

 

If you're asking me if the intensity will be different it's obviously yes. If you're asking me if the individual photons will be different the answer is no.

 

I am sorry but I must be missing some very fundamental understanding here.

I have assumed that light gets brighter not because of frequency but more concentration. I am not sure how frequency can effect brightness without changing the colour.

I think I need to clarify this point better before extending to SRT gendankens...

 

What I mean by this is that if you were to compare one photon 1 ly with one from much further, they would be exactly the same. But obviously you would get many more photons from the nearer source.

 

No problem then, we are on the same uhm....wave length....ha

 

So we have a 100 mm square plate reflecting the light source from 10 Ly we then move our 100 mm square plate so that it is 1 Ly distance.

At each location the plate is stationary with the light source.

The colour of the light reflected remains the same regardless of distance. However the brightness of the light is different. It appears much stronger at the closer position.

Why is this so?

The thinking:
As light propagates outwards from the source the brightness or intensity of the light energy weakens because the amount of energy involved per given segment of circumfurence is reducing relative to the increase in that segments length. So our energy is spread more thinly the further out we go.

So I suggest that in physics this must have a particular lable. Is it strength, brightness or luminescence or intensity or what?

I do not beleive it is of value to place a number of photons per square mm as I don't think a photon can be quantified in that manner.

So how is this aspect of light described normally?

we have frequency, wave length and what else?

 

I think that Radiant intensity (power per unit solid angle) might be what you have in mind.

Be sure not to confuse it with Radiant flux, Radiant energy, Radiance, Irradiance, or any other Radiometry or Photometry measure .

 

so the term watts is adequate 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? However I am not sure.

Could a location from the light source be considered as a location on a virtual sphere?

However thanks for that Pete, Radiant intensity or wattage sounds great.

 

I’d set up a scenario whereby a distant source released two identical long pulses of light towards Earth and then the source vanished leaving the two beams of identical light travelling next to each other through space.

Then I’d give two astronauts a rocket each and provide each one with a stopwatch.

I’d instruct them to start the clock exactly when they saw the light first enter the rocket and stop it when it vanished. I’d also ask each astronaut to use a device to collect 1 seconds worth of light to power some sort of device.

I then set them both off on their trip. One goes along the line of the first beam and the other goes along the second beam. The astronauts don’t realise that they are travelling at very different speeds to each other but when they have finished they return to Earth and compare results.

One astronaut discovers that he has collected different information to the other and thus concludes that their velocities relative to the beams of light MUST have been different.

 

So it is considered that lights speed is invariant to all observers.

Is lights radiant intensity also invariant?

Is there an answer?
Is it worth asking the question?

 

I get the impression that for SRT to function fully, the answer must be that radiant intensity must be invariant and if this is the case I wonder how this is possible?

 

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.