Expert: Philip Stahl - 9/8/2010

Question
Hello-
If I have a microwave and a laser, and I point them up and into space, assuming no real interference or anything in the way for a few light years, how far will the beams go on each.
Could it be said that the beams might go on forever?
hypotheical.
Mark

Answer
Hello,

You propose an interesting hypothetical, however it's not hypothetically realistic! (In this regard, all hypotheticals must at least have some remote grounding in reality!)  In this case, there will still be interstellar matter, dust along the path which will surely absorb most or all of the microwave emissions.

As for the laser, its beam width will be expected to diffuse the farther it propagates. I would surmise long before it reaches a distance of 1 ly it will be so diffused as to be indistinguishable from the background of space. In addition, bear in mind that it will conform to the inverse square law for radiation, which states that the intensity will diminish as the inverse square of the distance.

If a laser exhibits say, 100,000 lumens at a distance of 10 m, then even at a distance of 500 m (50 times farther than 10m)  its intensity will have been reduced to:


(100,000)L/ (50)^2 = 100,000 L/ 2500 = 40L

At one light year (~ 10^16 m or 10^15 times farther than 10m) you can work out the new intensity as essentially nil. (e.g. 10^-25 L)

Meanwhile, the beam width will plausibly have spread across a vast area, rendering it so diffuse as to be unidentifiable.

Beams "going on forever" to me, has to mean at least there is a feasible empirical or experimental way to detect them - either one- say by the time they reach a few light years- far less 20, 500 or 100,000. I am certainly not aware of any.