# Thread: Is the brightness of light invariant?

1. ## Is the brightness of light invariant?

Say we refer to candle power or wattage of a light source.

Is candle power recorded by the object relative to the velocity of that object ?

In other words does the light get brighter the faster you are moving towards it? [ even after you take length contraction in to account.]

According to relativity and also any experinments done.

Is the brightness of light only determined by distance from the source?

Can Special Relativity Theory accomodate this issue?

Just find this aspect facinating and look forward to some informed responses if possible...

also maybe there is a better way to phrase the question....any one?

2. To increase the power delivered to a surface you have two options, either you can increase the number of photons per time or you can increase the energy of the photons themselves. In the case of a surface moving towards a light source the Doppler effect will increase the energy of the photons (blue shift) thereby increasing the power. This is a classical effect, not just a relativistic effect. In addition, by moving towards the light the surface will hit more photons per unit time, further increasing the power. This is also a classical effect. Now, according to SR, time dilation and length contraction also occur and make it so that the source is seen by the surface as slightly red-shifted, which will decrease the energy of each photon and thereby decrease the total power. This is a relativistic effect, not a classical effect. Overall the classical effects are stronger with the net result being that a surface moving towards a source receives an increased amount of power.

-Dale

3. Originally Posted by DaleSpam
To increase the power delivered to a surface you have two options, either you can increase the number of photons per time or you can increase the energy of the photons themselves. In the case of a surface moving towards a light source the Doppler effect will increase the energy of the photons (blue shift) thereby increasing the power. This is a classical effect, not just a relativistic effect. In addition, by moving towards the light the surface will hit more photons per unit time, further increasing the power. This is also a classical effect. Now, according to SR, time dilation and length contraction also occur and make it so that the source is seen by the surface as slightly red-shifted, which will decrease the energy of each photon and thereby decrease the total power. This is a relativistic effect, not a classical effect. Overall the classical effects are stronger with the net result being that a surface moving towards a source receives an increased amount of power.

-Dale
...which is striking evidence that the surface is moving at a relative but variable velocity to the already emitted beam, is it not?

4. Originally Posted by dav57
...which is striking evidence that the surface is moving at a relative but variable velocity to the already emitted beam, is it not?
Yes, assuming that you know the power of the emitted beam and you know that there was no intervening absorption or scattering. Generally, (e.g. with a distant galaxy as the source) you don't know the emitted power but you do know the emitted frequency. You can then calculate the relative velocity using just the frequency shifts without worrying about the actual power.

-Dale

5. The answer depends much on the context in which you ask the question. In a slight way you may be comparing apples to oranges. For example, Candle power usually refers to the visible spectrum. If the emitter and the receiver are moving very fast relative to each other, you could make the emitter less visible by converting ultra violet to x rays.
Generally candle power refers to intensity and not to wavelength. The only way you can increase intensity at the receiving end (other then focasing light rays like a telescope or gravitational lensing can) is to decrease the distance to the emitter, or increase the emission in the direction of the receptor.
However, two objects moving towards eachother increases the frequency as well as decreasing the wavelength. Increasing the frequency at the receptor is the same as saying that the intensity has increased. So following this logic, relative motion does increase the intensity as well as increasing the energy of each photon or wave.

6. Originally Posted by DaleSpam
Yes, assuming that you know the power of the emitted beam and you know that there was no intervening absorption or scattering. Generally, (e.g. with a distant galaxy as the source) you don't know the emitted power but you do know the emitted frequency. You can then calculate the relative velocity using just the frequency shifts without worrying about the actual power.

-Dale

Type 1A supernovae are almost identical. The reason is that their mass and composition is virtually identical. They occur when a white dwarf star collects enough mass to reach the Chandrasekhar limit...about 1.4 solar masses. At that point, the star collapses and BOOM.

Since their intensity is almost identical, they have turned out to be a good yardstick for measuring the distances to galaxies. And their dopler shift, as noted, then gives us the radial velocity of the galaxy.

7. so how would using intensity of light be useful in terms of SRT gendankens do you think?
Would the results be the same or similar as when we use light velocity?

8. giving it more thought: two objects moving towards eachother increases the frequency as well as decreasing the wavelength. Increasing the frequency at the receptor is the same as saying that the intensity has increased. So following this logic, relative motion does increase the intensity as well as increasing the energy of each photon or wave.

9. Originally Posted by Tortise
giving it more thought: two objects moving towards eachother increases the frequency as well as decreasing the wavelength. Increasing the frequency at the receptor is the same as saying that the intensity has increased. So following this logic, relative motion does increase the intensity as well as increasing the energy of each photon or wave.
So, looking at this logically, if an already emitted beam of light can vary in characteristic from the point of view of an observer who is continually changing their velocity, then surely the observer is changing his velocity RELATIVE to the beam of light. So is this case, doesn't this demonstrate that light speed is NOT invarient?

10. I'm not sure I understand your question. Are you questioning relativity or my logic? If you're questioning relativity then there are others on this forum who could better answer your question. I would only point out that the speed of light in a vacuum is INvariant and the other variables change to suit that fact.

11. Originally Posted by Tortise
I'm not sure I understand your question. Are you questioning relativity or my logic? If you're questioning relativity then there are others on this forum who could better answer your question. I would only point out that the speed of light in a vacuum is INvariant and the other variables change to suit that fact.
I am implying a similarity between sound waves and light waves (although I know this is pretty stupid!)

When an observer of a sound wave changes his velocity with respect to an already emitted wave, he records a frequency change in accordance with the Doppler Effect. Therefore, the fact that the observer notices a doppler effect impies a change in relative velocity between the travelling wave and the observer i.e. Sound is NOT invariant.

With regard to light, an observer experiences a frequency change depending on his velocity at that particular moment. If he changes his velocity relative to the already emitted beam, the frequency changes.

This, I believe, is a clue that the observer's speed with respect to the beam of light is NOT invariant, but rather changes just like sound.

12. Oh, I must have misunderstood when you said
So is this case, doesn't this demonstrate that light speed is NOT invarient?
But I understand what you're saying now. Your saying you may disagree with the theory of relativity if I understand you correctly.
If it is your position that in some frame of reference that you might measure light in a vacuum slower then normal, then you disagree with relativity.

13. Originally Posted by Tortise
Oh, I must have misunderstood when you said
But I understand what you're saying now. Your saying you may disagree with the theory of relativity if I understand you correctly.
If it is your position that in some frame of reference that you might measure light in a vacuum slower then normal, then you disagree with relativity.
Well, the Classical effect of changing your velocity wrt a beam of light is greater than the relativistic effect, as someone pointed out.

If one changes their velocity wrt an already emitted beam of light and views a change, then surely ANY change can only be down to one thing - the fact that the relative velocity between observer and light beam has varied.

14. Hi dav57,

Can you think of a way to define and measure your velocity with respect to the light beam?

15. Well, I'm not dav57, but why do you think it is a problem to measure your velocity with respect to a beam of light? Simply do an analysis of the continuum spectrum, locate the signature of hydrogen, or any other emission signature you choose, and calculate the Doppler shift. If the light is being emitted by a celestial object many light years distant, any changes in the Doppler shift is due to changes in the velocity of the observer with respect to the object. Choose a star or whatever that emitted the light several, or thousands, of years ago. Any Doppler changes an instellar traveller would record from the chosen source would be due to his own changing velocity relative to the source. Only a moron could believe the star keeps changing its velocity with respect to his 'rest' frame.

16. Scenario: Ship heading to source of light at 0.8c v according to that light source.

The thing that intrigues me is that if an object is travelling at relative v=0.8c it's length has contracted thus it is closer to the source of light it is heading towards. Thus the brightness recorded of that source is increased because the distances involved are less.

However a light source aboard the ship must also get brighter using the same principle. So an Observer aboard the ship should be able to measure his velocity simply by recording the changes in brightness of the light along the vector axis. This however means that the brightness of light along the perpedicular to vector axis should not increase. So we would have a situation where in one direction the same light is brighter yet in another it is not so bright.

If the light source dims as the velocity increases to maintain invariance then the source of light the ship is heading towards must also dim therefore the entire universe of stars becomes less bright. For the ship at 0.8cv

So I see a potential conflict iwthin SRT but can not express it correctly.

As brightness is not a time orientated data set [ Brightness involves distance only and not time??]

17. So I see a potential conflict iwthin SRT but can not express it correctly.
Hold on I'm calling the associated press - we're in the process of proving relativity is wrong!
Or wait more likely we just don't understand it. Before we tell the world we've found a hole in one of the best tested theories ever, why don't we take a seccond and think about this.

Time is elastic so to speak. Time diolation and length contraction explains your problem very well. Think about it. The best scientists in the world have tested this theory with real world experiments. What is more likely that it is hard to understand or that you have succeeded where everyone else has failed?

18. 2inquisitive,

I asked dav57 to define velocity with respect to a beam of light. What you have described is how to determine your speed relative to the emitter of the light beam for a beam with some spectral structure. This is not a definition of velocity with respect to a beam of light. Of course, the point of my question was that "velocity with respect to light" has no simple meaning. Indeed, light is always moving at the same speed with respect to you (so long as you move inertially). Consider the following ideal case: you are next to a monochromatic beam of light, what is your speed with respect to the beam of light?

19. Originally Posted by Tortise
Hold on I'm calling the associated press - we're in the process of proving relativity is wrong!
Or wait more likely we just don't understand it. Before we tell the world we've found a hold in one of the best tested theories ever, why don't we take a seccond and think about this.

Time is elastic so to speak. Time diolation and length contraction explains your problem very well. Think about it. The best scientists in the world have tested this theory with real world experiments. What is more likely that it is hard to understand or that you have succeeded where everyone else has failed?
Actually I am more interested in how SRT manages to accommodate this issue, as I am sure it can.

I am attempting to generate a gendanken to see just how SRT can work with-out the need for timed data sets. [ Or maybe some one else could come up with an appropriate gendanken???]

20. I thnk Physics Monkey knows what I am on about. It isn't easy to contrive this gendanken that removes the need to use time/ distance data sets as a part of it.
But merely distance only....[of course time is necessary but not the priority ]

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