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

In summary, it is impossible to determine how far away a laser pointer is by determining only its light. In order to determine how far away the laser pointer is, you would need to add a couple of mirrors and a stopwatch.
  • #1
TimeRip496
254
5
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?
 
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  • #2
Cosmological redshift due to expansion of space?
 
  • #3
TimeRip496 said:
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.
 
  • #4
By adding a couple of mirrors and a stopwatch?
 
  • #5
Blackberg said:
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.
 
  • #7
ToBePhysicist said:
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?
 
  • #8
DaveC426913 said:
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...
 
  • #9
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.
 
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  • #10
DaveC426913 said:
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?
 
  • #11
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
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
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
jerromyjon said:
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?
 
  • #15
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
It wouldn't be much use without knowing the unshifted frequency. You wouldn't know how much it had shifted.
 
  • #17
mrspeedybob said:
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?
 
  • #18
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.
 
  • #19
DaveC426913 said:
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.
 

1. What is the reason behind a laser beam not spreading out?

The reason a laser beam does not spread out is due to its unique properties, including coherence and collimation. Coherence refers to the fact that all the photons in a laser beam have the same wavelength and are in phase, meaning they are all moving in sync. Collimation refers to the beam being tightly focused and parallel, allowing it to travel long distances without dispersing.

2. How is a laser beam different from other types of light?

A laser beam is different from other types of light in several ways. First, it is monochromatic, meaning it consists of only one color or wavelength. Additionally, it is highly directional and can be focused into a tight beam. Finally, it is also highly coherent, meaning all the photons in the beam are moving in sync.

3. Can a laser beam ever spread out?

Yes, a laser beam can spread out over long distances due to factors such as atmospheric conditions or imperfections in the beam itself. However, compared to other types of light, a laser beam will spread out much less due to its unique properties.

4. How is a laser beam created?

A laser beam is created by stimulating atoms or molecules to emit photons in a specific direction. This process is called stimulated emission and is achieved by exciting the atoms with an external energy source, such as electricity or light. The emitted photons then bounce back and forth between two mirrors, amplifying the beam before it is released as a laser beam.

5. What are some applications of laser beams?

Laser beams have a wide range of applications in various fields, including communication, medicine, manufacturing, and entertainment. They are used in technologies such as laser printers, barcode scanners, and fiber optic communication. In medicine, they are used for surgeries, diagnostics, and treatments. In manufacturing, laser beams are used for cutting, welding, and drilling. And in entertainment, they are used in laser light shows and holograms.

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