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

1. Mar 12, 2015

### 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?

2. Mar 12, 2015

### jerromyjon

Cosmological redshift due to expansion of space?

3. Mar 12, 2015

### Drakkith

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

4. Mar 14, 2015

### Blackberg

By adding a couple of mirrors and a stopwatch?

5. Mar 14, 2015

### DaveC426913

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

6. Mar 15, 2015

### ToBePhysicist

7. Mar 15, 2015

### DaveC426913

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?

8. Mar 15, 2015

### ToBePhysicist

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

9. Mar 15, 2015

### Staff: Mentor

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.

10. Mar 15, 2015

### jerromyjon

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

11. Mar 15, 2015

### Staff: Mentor

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.

12. Mar 15, 2015

### 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?

13. Mar 15, 2015

### 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 [Broken]
In this case (r) does not make a difference according to your theory.
Pardon me if I am missing something here!

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14. Mar 15, 2015

### Staff: Mentor

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

15. Mar 15, 2015

### 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.

16. Mar 15, 2015

### DaveC426913

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

17. Mar 15, 2015

### ToBePhysicist

Why don't people be more specific asking questions?

18. Mar 15, 2015

### Staff: Mentor

The description in post 1 lets 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.

19. Mar 16, 2015

### mrspeedybob

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.