Another (interesting) thought experiment about light speed and its limits (or lack thereof)

A person on Earth has a super strong laser. The (hypothetical) laser is so strong that the person can project a bright dot of light anywhere, say, on another planet, many light years away.

So..the person on Earth points the laser at, say, at a planet or object, near, example, the star Betelgeuze in Orion.

The person then "swipes" the laser beam only a few degrees across the night sky - the laser dot races in a few seconds from the planet around Betelgeuze to the another star or planet on an entirely different section of the night sky.

The "real" distance between those two points on the sky is...say....200 light years.

Observer somewhere in outer space who don't know about the man on Earth with the laser see the light spot, says it's some kind of object/light of unknown origin.

They track the light and measure that it in fact traverses a distance of 200 light years in a mere seconds. (We assume they can do this, for example they could have observing stations around the universe and were able to follow and track the light on its way from the first planet to the second. That the light is a "projection" they don't know).

So..those people say they saw an object which moved at "impossible" speed, coming to the conclusion that it moved a lot faster than light speed.

Your thoughts?

reply to post by flexy123


You have to factor in the time it takes for light to reach whatever you are pointing at in the first place. If you shine a laser at something 200 light years away its going to take 200 years for the light to reach it. If you then quickly shift it to another point in the sky thats still the same distance from you its going to take another 200 years for the light to leave your laser and hit the second target. So it would take 200 years for the dot to catch up to the new target.

Dont forget There is only a finite number of photons emitting from any one light source at time. So the dot of light would effectively vanish during the movement as not every point in between the two objects would even get hit with photons from the laser because there might be more points in between than there are photons emitted during the sweep.

Try thinking of it like a hose squirting out water , if you suddenly shift the position of the nozzle in an arc then the stream of water takes some time to shift to the new position. Also if you move it very quickly not every point in between will get wet, there will be gaps of dryness.

reply to post by flexy123


When I move a light beam, it is not, as was already stated a solid object moving at speed. It is a spray of photons with their directional axis being reoriented. So you might see the light move from point a to point b faster than light, but this is not the same as moving faster than light.

Heres a quick pic i made that might help. Its not perfect and in reality the curve would be more severe but you should be able to get the idea.



The beam would look bent but the photons are still only traveling in straight lines.

If you point at an object 1 lightyear away, it would take two years for you to see the reflected light on the object (the "spot"). Suppose you then sweep 1 degree to the left or right, where another object in the sky is. If that object is as far away as the first one, it will again take 2 years for the light to travel forth and back, before you can observe the spot on the 2nd object. If the 2nd object is light years away, supposedly more to the background, then you have to add twice that distance before you can see the spot again.

Suppose our universe is like a glass sphere and an observer can watch this light. It doesn't change the speed of light when the observer is in another position or even outside our universe. Assuming time is experienced at the same rate as we do, it will take just as long for the observer to see the 2nd spot.

Even though is takes only a second to point it in the right direction, the light still has to travel the distance before being able to observe it.