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# Physics/How far can a laser beam in space travel?

Question
Hi! I checked that the most powerful laser in the world can produce a 1,25 petawatt laser beam, so how far would this laser beam be able to travel in space? I know space does not produce diffraction so the beam should be able to travel longer than in atmosphere but the ''vacuum'' of space is also a bad conductor of heat. So how quickly would the beam lose 50% of its energy and when would it be at 25% and 1% and finally at 0,1%?

You may or may not be aware that interstellar space has dust in it -- not much, but enough to cause a measurable loss of light as photons travel through it. This loss is called the "interstellar extinction."

http://en.wikipedia.org/wiki/Extinction_%28astronomy%29

Note that the degree to which light is lost as it travels in the interstellar medium depends on the wavelength. The peta-watt laser I found info about has a wavelength of 1057 nm, or 1.057 microns, 1/λ also about 1.
http://texaspetawatt.ph.utexas.edu/laser-capabilities.php

The chart in the first URL shows that A(λ), for 1/λ = 1, is about .1 A(V), where A(V) is the ratio of visual extinction to path length, or a loss of about 1.8 magnitudes in intensity for every kiloparsec the light has traveled.
Thus, in the wavelength for this laser, there will be a loss of .18 magnitudes in intensity for every kiloparsec of travel.

Historical reasons have resulted in astronomers expressing change of intensity in terms of change in magnitude. An increase in stellar brightness of 1 magnitude is equal to a decrease of 2.5 in light intensity. Thus, an increase of .18 in magnitude would mean a decrease in intensity of about 18%.

THUS, a beam of light of wavelength equal to about 1 micron, traveling through inter-stellar space, would have 82% of the intensity it had 1 kiloparsec previously. I'll leave it to you to calculate the numbers from there.

As astronomical calculations are not my specialty, this is all I will say on this subject.

Physics

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Have been fascinated by physical laws ever since I learned, at age seven, that magnets work under water. My study continued through college and has not ceased even after I retired.

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B.A. in Physics (with honors) from University of California at Berkeley.M.A. in Physics (with honors) from University of Texas Austin.