Weird physics question about the speed of light (see below). Serious please.?

Question

Ok, a guy I knew a long time ago told me this one, but said he forgot the answer his physics teach came up with:

You are holding a flashlight (a theoretically perfectly straight and coherent laser), and pointing it at a nebula that is, oh, let's say, 5 light years away. This assumes, of course, you can hold it there for 5 years, but bear with me. The nebula is 3 light years across to your point of view (left to right).

The beam is shining on the leftmost side. You move your wrist, and now the beam is pointing at the rightmost side, 3 light years away from where it just was. 5 years later, the end of the beam catches up with your movement, and now shines on the right side, 3 light years from where it started.

The termination of the beam moved instantly across the nebula, exceeding the speed of light. Is that what would happen? What is wrong with this picture? (the actual physics, not the absence of our theoretical laser). Thanks!

Answer 1

The light isn't actually moving faster than light though. The termination of the beam is not a thing that has mass, it's a logical concept. The light was here, now it has gone away and appears somewhere else. There was no physical object with mass moving across the nebulae at faster than light speed.
At a quantum level you are sending a stream of photons at the nebulae at light speed. as you aim the laser across the nebulae the photons strike different spots on the nebulae in succession. The photons are not themselves moving across the nebulae. You are merley observing the effect of the photons fired at various directions.

Answer 2

Inertial mass (m) is limited to velocity (v) below the speed of light (c). This happens because of the Lorentz Transform L(v) = 1/sqrt(1 - (v/c)^2); so that when v = c, L(v = c) ==> infinity. This results from L(v = c) = 1/0 and anything divided by zero approaches infinity, which is sometimes called a singularity.

As light does not have inertial mass, it is not limited by L(v); so it can travel at, but not exceed, the speed of light. So that L/t = c, where L is the wavelength and t = 1/f the time light travels one wavelength and f is the frequency of that light. In which case, we have c = Lf showing that as the frequency goes up, the wavelength must go down to preserve the constant speed of light c.

What you would see from the nebula 5 ly away is the beam of coherent light move across the sky, but not faster than the speed of light. What you see is predicated on photons striking your retina and those photons take time to travel (five years in your case). Thus, when the first stream of photons reaches you, you'd see them in the left hand end of the beam's travel.

But as later photons reach your eyes the beam would start to move...left to right. But the tangential velocity would not be faster than spol (c) because the photons cannot get there, to your eyes, that fast. Thus, if seen from the outside, the beam would bend backward towards the starting side, as you swept the flashlight from left to right.

Imagine the beam of light to be like a stream of water coming out of a hose. If you move that hose from left to right, the stream of water will lag the motion of the nozzle. But it will eventually catch up once the nozzle (e.g., the flashlight) stops moving.

Answer 3

this is like the oscilloscope trace problem on an old cathode ray tube where a beam of electrons hits a phosphor to make a spot of light. if you display a low frequency, say 1Hz, sinusoidal signal on such a screen you will see a dot of light moving slowly across the screen, once every second. now up the frequency and display one cycle of a 50MHz signal. the spot of light now only has 20nS of time available to traverse the screen, and you seem to have a problem! for if the screen was say 10cm wide, the spot of light would need to move at around 500,000,000 cm s-1 to trace out the waveform, or faster than light.
( of course the explanation is, it is never the same electron which creates the spot of light in a different place on the screen, just as it is never the same photon which hits a different spot in your distant galaxy)

Answer 4

What's wrong is you're thinking of the beam as a solid object like a baseball bat, where a small movement at one end is translated into a large movement at the other that seems instantaneous (it isn't, and you couldn't swing a bat at .9c and expect the far end to move faster than c, either).

A beam of light is just a stream of photons, or waves if you prefer. Once they have left your laser their trajectory is fixed in that direction, and if you move your laser the next group of photons or wavefront will go off in that direction.

Answer 5

well let me recap what u said; u are sending a pointed light laser ray toa distant nebula at 5 lightyears away. the width of the nebula is 3 light years.ok after five years you flick the light to the right end!

you hold the beam steady on the left corner of the nebula (how?, that means you need to keep moving on earth to compensate for its rotation!) or let us assume u are in the space fixed

doyou have the idea of the angle it subtends? (17 deg!) quite an object on sky!

coming back the q, if you are stationary with respect to the nebula, the light will be seen /received at the left corner at the end of the five year, and till the flick is complete, the light will be received in discrete quantum packets and when complete jumps to the right end!

Answer 6

You are trying to impose an absolute sense of
time across things that are very far apart.

"Its all relative". That is, the time in which
an event happens in one place is not the
same time in which it is perceived at another.

In this case, the light of the beam is being
seen in one time frame and then instantly
is not.

In another time frame, 3 light years away,
5 years from now, light starts to be received.

The "thing" that moved was whether or not
you were receiving light. That isn't a thing, but
rather a conceptual view point.

Conceptual view points are ... conceptual ...
and free to move about as freely as your
brain cares to make them move. They certainly
aren't constrained by the speed of light.

Answer 7

a difficulty the following is the top: 'hence ought to take someone decrease back in time'. regardless of if shall we commute on the speed of sunshine, i do not imagine scientists trust that we'd commute backwards in time. the point of relativity is that as you frame of mind the speed of sunshine time outdoors your deliver or gadget slows down, yet no longer that it reverses route. notwithstanding, in case you should commute decrease back in time (ignoring the way you do it for a second), i imagine someone would pass into the former days. i do not see the way it ought to 'technically' be 1700 for someone and not at all for actually everyone else - at the same time as the age of someone is subjective, i.e. what percentage years they in my opinion have experienced, the date must be extra purpose - taken in words of the type of earth's rotations about the daylight which have got here about.