Random question -- if a laser beam did not spread out

[TimeRip496]

Assume I have an ideal laser pointing which light will always move parallel to where the laser pointer is pointing. This mean that no matter how far I stand away from the laser pointer, I will see the same spot of light with the same light and intensity. In that case is it possible to determine how far the laser pointer is by determining only its light?

 

[jerromyjon]

Cosmological redshift due to expansion of space?

 

[Drakkith]

In that case is it possible to determine how far the laser pointer is by determining only its light?

There is not, because there are no changes to the light that would indicate how far away the pointer is.

 

[Blackberg]

By adding a couple of mirrors and a stopwatch?

 

[DaveC426913]

By adding a couple of mirrors and a stopwatch?

Not sure that'd do the trick. By the setup, you're standing at the target of the beam, not at the source.

 

[ToBePhysicist]

Greetings,
If I got you question right, I want to answer you with a question: How did we measure the distance between the Earth and the moon?
Google and Wikipedia will always be there for you.

 

[DaveC426913]

Greetings,
If I got you question right, I want to answer you with a question: How did we measure the distance between the Earth and the moon?
Google and Wikipedia will always be there for you.

As with Blackberg, I think you are missing the point.

If you are standing on the Moon, and the laser is on Earth, could the laser beam tell you anything about the distance to Earth?

 

[ToBePhysicist]

As with Blackberg, I think you are missing the point.

Laughing out loud...That is one easy question...No...
Unless you got the right equipment...

 

[mfb]

There is no ideal laser pointer. The laws of electromagnetism make this impossible, so it is no meaningful to ask what the laws of electromagnetism would predict for such an impossible thing.

With a perfectly planar wave everywhere in space (which would be possible) there would not even be a laser pointer to determine the distance to.

 

[jerromyjon]

If you are standing on the Moon, and the laser is on Earth, could the laser beam tell you anything about the distance to Earth?

Are frequency values precise enough to discern doppler shift from, shall we say, sea level to orbiting on the moon?

 

[mfb]

Frequency measurements are precise enough to see a height difference within a lab (~1m) due to gravitational redshift. So... yes, certainly something you can measure. With just a frequency value there is no way to know the origin of the shift (relative motion, expansion of space, or gravity), however.

 

[jerromyjon]

But if you account for motion and gravity at what distance does the Earth cease to have a measurable effect? I imagine its proportional to the inverse square of the distance? Would there be a gap between here and there or would the Earth still be "dopplering the laser noticeably" as the moon would be having 1/6th the opposite effect?

 

[ToBePhysicist]

Assuming there is such "Ideal Pointer" you have nothing else to measure,...Right!? (nothing is changing but the thing you want to get!)
http://imagine.gsfc.nasa.gov/features/yba/M31-velocity/images/1overrsq.gif
In this case (r) does not make a difference according to your theory.
Pardon me if I am missing something here!

 

[mfb]

But if you account for motion and gravity at what distance does the Earth cease to have a measurable effect? I imagine its proportional to the inverse square of the distance? Would there be a gap between here and there or would the Earth still be "dopplering the laser noticeably" as the moon would be having 1/6th the opposite effect?

I don't understand your questions, sorry. Which effect do you expect where in which setup?

 

[mrspeedybob]

I think he's asking if you could measure gravitational Doppler shift accurately enough to determine the distance to the source of the gravity based on the difference in Doppler shift at 2 altitudes a known distance apart. For example, suppose you measure the Doppler shift over 1 cm and then measure it again over 1 cm, 1 meter higher and see that the 2'nd measurement shows 0.000020000001% less Doppler shift then the 1'st measurement then you would know that the 1'st measurement was at approximately 10,000,000 meters from the center of the Earth while the 2'nd measurement was at approximately 10,000,001 meters from the center. If you know that the laser is directly below you on the surface of the Earth 6,371,000 meters from the center, you would then know your distance from the laser.

 

[DaveC426913]

It wouldn't be much use without knowing the unshifted frequency. You wouldn't know how much it had shifted.

 

[ToBePhysicist]

I think he's asking if you could measure gravitational Doppler shift accurately enough to determine the distance to the source of the gravity based on the difference in Doppler shift at 2 altitudes a known distance apart. For example, suppose you measure the Doppler shift over 1 cm and then measure it again over 1 cm, 1 meter higher and see that the 2'nd measurement shows 0.000020000001% less Doppler shift then the 1'st measurement then you would know that the 1'st measurement was at approximately 10,000,000 meters from the center of the Earth while the 2'nd measurement was at approximately 10,000,001 meters from the center. If you know that the laser is directly below you on the surface of the Earth 6,371,000 meters from the center, you would then know your distance from the laser.

Why don't people be more specific asking questions?

 

[mfb]

The description in post 1 let's me believe the question has nothing to do with redshift at all.
Let's wait for @TimeRip496 to come back to explain the question better.

 

[mrspeedybob]

It wouldn't be much use without knowing the unshifted frequency. You wouldn't know how much it had shifted.

You wouldn't have to know the original frequency, just the frequency at at least 3 points unequal height from the source. In the example I gave I used 4 points for clarity, but I think 3 would actually be enough. If your points are A, B, and C with A being the lowest and C the highest you measure the frequency drop between A & B, then between B & C. Since gravity is less between B & C then it is between A & B there should be less doppler shift. If A,B, & C are far from the gravity then the gravity gradient will be small, if they are close then the gradient will be larger, so by measuring the gradient of the gravity over a known distance you can compute distance to source.