[guest39538]

If we have 2 light  bulbs that are identical and both 100w, both light bulbs , A and B start off next to each other.


AB


We then expand the length between the light bulbs

A→B

A→→B

A→→→B

At what radius does A not observe B and B not observe A any more?

[chris]

What do you mean by being invisible to one another? Are you referring to the density of photons in any given patch of space?

[guest39538]

What do you mean by being invisible to one another?

Invisible is not really the word, they relatively can not ''see'' each other because as the distance increases apart , relatively they visually contract .

I drew the question for help you understand the question.

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Are you referring to the density of photons in any given patch of space?

I am referring to light intensity diminishing over a radius.

[Colin2B]

At what radius does A not observe B and B not observe A any more?

You also have to define what you mean by observe, as you don't use the word in the same way as physicists.

[guest39538]

At what radius does A not observe B and B not observe A any more?

You also have to define what you mean by observe, as you don't use the word in the same way as physicists.

Too see...(I know the light bulb has no eyes),

[evan_au]

Setting up the Problem...
If A and B are "point sources", then they will "never" intercept any photons from another source, and so the maximum distance is almost zero. So let's assume that both A and B have a finite area in which they intercept photons.

If A and B are both light sources, then A & B don't have to be very far apart before the glow and reflection of their own light source overpowers the low levels of light received from their distant counterpart. So let's assume that they are in a perfect vacuum (no reflection or scattering of light), and they are both emitting light of a different frequency (so they can separate their own light from light emitted by their counterpart).

If A and B have other light sources around them, then A & B don't have to be very far apart before the glow and reflection of these other light sources overpowers the low levels of light received from their distant counterpart. So let's assume that any other light sources are much fainter and farther away than the counterpart they are trying to detect, and don't emit significant light at the frequencies emitted by A and B.

Let's say that A has to detect at least 10 photons from the direction of B before A can declare that it has detected B.

A Solution?
Let's say that A and B are far enough apart that it takes 1 second to collect 10 photons.

Then, if you double the distance between A and B, then it will take 4 times as long to detect 10 photons (by the inverse square law).

There is no limit to how far away A and B can be apart, and still "see" each other, provided both A and B are very patient, and their light doesn't fail.

[guest39538]

There is no limit to how far away A and B can be apart, and still "see" each other, provided both A and B are very patient, and their light doesn't fail.

And in a visual sense of the human eye an observer with a 100w flash light  walks backwards along a very long railway track shining the light at the observer, at what distance does the observer not observe the walker any more?

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[chiralSPO]

And in a visual sense of the human eye an observer with a 100w flash light  walks backwards along a very long railway track shining the light at the observer, at what distance does the observer not observe the walker any more?

As evan laid out in his post, there are many factors that can interfere with the observation.
There is the issue of clarity of the medium between the source and observer (visibility/foggy/there is a forest in the way...)
There is the issue of competing light sources. Our eyes automatically adjust the brightness of our whole surrounding, so something that would be blindingly bright in the middle of the night might barely be noticeable in the mid afternoon at the beach in the middle of the summer. I can see a single 100 W bulb >10 km away on a clear night out in the countryside, but there is no way I could notice the same bulb during the day at that distance.

There is also the issue of the curvature of the Earth. A very bright lightbulb might in principle be visible from 200 km away, but if both the light and the observer are only 2m above the ground, the ground will be in the way.

[Bored chemist]

If you use a big enough telescope you can, in principle, see a lamp  from as far as you like.

[guest39538]

And in a visual sense of the human eye an observer with a 100w flash light  walks backwards along a very long railway track shining the light at the observer, at what distance does the observer not observe the walker any more?

As evan laid out in his post, there are many factors that can interfere with the observation.
There is the issue of clarity of the medium between the source and observer (visibility/foggy/there is a forest in the way...)
There is the issue of competing light sources. Our eyes automatically adjust the brightness of our whole surrounding, so something that would be blindingly bright in the middle of the night might barely be noticeable in the mid afternoon at the beach in the middle of the summer. I can see a single 100 W bulb >10 km away on a clear night out in the countryside, but there is no way I could notice the same bulb during the day at that distance.

There is also the issue of the curvature of the Earth. A very bright lightbulb might in principle be visible from 200 km away, but if both the light and the observer are only 2m above the ground, the ground will be in the way.

The answers seem to be avoiding the actual question I have asked and the answers are not factors I asked to take into consideration. My drawing shows a dark clear night and an x-axis with no curvature of the earth, it is a hypothetical question and you are adding content to the question.   Your answers thus far have only  been related to light magnitude and no mention of the relative visual contraction of the bulb, how can you see a bulb over a great distance with the eye if the bulb has relatively visual contracted to a visual 0 point source . 

[chiralSPO]

And in a visual sense of the human eye an observer with a 100w flash light  walks backwards along a very long railway track shining the light at the observer, at what distance does the observer not observe the walker any more?

As evan laid out in his post, there are many factors that can interfere with the observation.
There is the issue of clarity of the medium between the source and observer (visibility/foggy/there is a forest in the way...)
There is the issue of competing light sources. Our eyes automatically adjust the brightness of our whole surrounding, so something that would be blindingly bright in the middle of the night might barely be noticeable in the mid afternoon at the beach in the middle of the summer. I can see a single 100 W bulb >10 km away on a clear night out in the countryside, but there is no way I could notice the same bulb during the day at that distance.

There is also the issue of the curvature of the Earth. A very bright lightbulb might in principle be visible from 200 km away, but if both the light and the observer are only 2m above the ground, the ground will be in the way.

The answers seem to be avoiding the actual question I have asked and the answers are not factors I asked to take into consideration. My drawing shows a dark clear night and an x-axis with no curvature of the earth, it is a hypothetical question and you are adding content to the question.   Your answers thus far have only  been related to light magnitude and no mention of the relative visual contraction of the bulb, how can you see a bulb over a great distance with the eye if the bulb has relatively visual contracted to a visual 0 point source .

The bulb won't contract to 0 until you're infinitely far away...

[guest39538]

The bulb won't contract to 0 until you're infinitely far away...

That is not true, I go night fishing regular, I have observed you statement is false by evidence of observation.   you are clearly mistaken.


Please feel free to try the experiment using a pen torch and an open field.

[chiralSPO]

The bulb won't contract to 0 until you're infinitely far away...

That is not true, I go night fishing regular, I have observed you statement is false by evidence of observation.   you are clearly mistaken.


Please feel free to try the experiment using a pen torch and an open field.

Your observations while fishing are due to "the added content" we talked about earlier that you then brushed off...

And in a visual sense of the human eye an observer with a 100w flash light  walks backwards along a very long railway track shining the light at the observer, at what distance does the observer not observe the walker any more?

As evan laid out in his post, there are many factors that can interfere with the observation.
There is the issue of clarity of the medium between the source and observer (visibility/foggy/there is a forest in the way...)
There is the issue of competing light sources. Our eyes automatically adjust the brightness of our whole surrounding, so something that would be blindingly bright in the middle of the night might barely be noticeable in the mid afternoon at the beach in the middle of the summer. I can see a single 100 W bulb >10 km away on a clear night out in the countryside, but there is no way I could notice the same bulb during the day at that distance.

There is also the issue of the curvature of the Earth. A very bright lightbulb might in principle be visible from 200 km away, but if both the light and the observer are only 2m above the ground, the ground will be in the way.

The answers seem to be avoiding the actual question I have asked and the answers are not factors I asked to take into consideration. My drawing shows a dark clear night and an x-axis with no curvature of the earth, it is a hypothetical question and you are adding content to the question.   Your answers thus far have only  been related to light magnitude and no mention of the relative visual contraction of the bulb, how can you see a bulb over a great distance with the eye if the bulb has relatively visual contracted to a visual 0 point source .

[guest39538]

]





Your observations while fishing are due to "the added content" we talked about earlier that you then brushed off...

I have not brushed off ambient light etc, the point you are missing is that yes the light extends to infinite but the visual contraction of the source doe's not.  For example you can not see dust particles in the air no more than a few feet away from you.

[chiralSPO]

]





Your observations while fishing are due to "the added content" we talked about earlier that you then brushed off...

I have not brushed off ambient light etc, the point you are missing is that yes the light extends to infinite but the visual contraction of the source doe's not.  For example you can not see dust particles in the air no more than a few feet away from you.

You CAN see the dust, you just don't NOTICE it. And you can see specks of dust from across the room with a bright flashlight in a darkened room.

[guest39538]

You CAN see the dust, you just don't NOTICE it. And you can see specks of dust from across the room with a bright flashlight in a darkened room.

You are twisting the question. 


Sam observes the sky, he notices nothing for a while then notices a tiny speck in the sky getting nearer to him, as the speck gets nearer the speck relatively expands , when it gets close to identify the dimensions Sam notices its an aeroplane, as the aeroplane passes above Sam he can observe the aeroplanes rest length, as the aeroplane passes him , he notices the aeroplane starts to relatively contract and shrink back down to a speck before eventually becoming dimensionless.

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[guest39538]

O dimension of light

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added-

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added-

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n=0xyz

[guest39538]

All pics courtesy of google and edited.


Relativity and visual  length contraction .

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[guest39538]

Sorry for going slightly off key, notice in this video at 4.19 the near side trains length expands.  It looks like it stretches it is quite cool.

[chiralSPO]

This becomes a question about the resolution of the human eye, not one of general physical principles. If you cared about how close something has to be for our eye to make out its dimensions accurately, you should have asked a question about peoples eyes from the very start. Posing the question as two light-emitting objects without eyes in makes it sound like a question of the fundamental properties of light. Which is it?