An interesting thought...

[Benm]

Thats not the question though - its what a beam would look like if emitted from a fast rotating planet, lighthouse style. As far as i know, such scenarios dont look like spirals to observers in the real world. Obviously this is not to be confused with galaxies that actually are spiral in shape, and are observed as they are and should be.

 

[Meatball]

Don't get me wrong, I am quite a SF fan, and wonderer myself (Ph.D in wondering around)

I find the 'Is the space infinite or finite' thread much more discussable...

I don't get your surface travel faster than speed of light you pointed out, though.
Care to enlighten me, so to speak?

Its actually very simple. If you take a pointer and flick the dot across the wall very quickly, one can say that the dot traveled across the wall X m/s.

If you step back, further from the wall, and flick the laser again at the same speed in your hand, the dot goes across the wall in the same manner, except a lot faster.

The further away you step from the wall, the faster the dot moves.

If it's a gaussian beam profile, the wave front can be spread out, or flattened out the faster the dot moves.

So if a laser is shone into the sky, and the user flicks the beam from one star to another, the dot (or the wavefront itself) can travel between the stars at a velocity greater than light speed.

The wave front's vectors can change faster than electromagnetic waves can propagate, is how it was explained to me.

 

[Eudaimonium]

You could not simply 'observe' the light beam in such foggy space ,

The light from the beam still needs to travel to you in order for you too see it.

However, light has it's own speed, so does the water from the hose.
If you move the hose fast, the water appears bent,
So will the light beam when imagined (not viewed) on grand scale.

Kinda tough to explain your thought on such subject, isn't it?

 

[pullbangdead]

Thats your problem right there, you cannot magically see those photons from any angle where they do not travel towards you. This could be 'remedied' by placing the eath in some kind of mist or fog, where you get scattering that would allow you to observe the whole ordeal from the 'side'.

The question would still be if it would look like a spiral from that perspective. I'm inclined to think that it would not, since the beams would actually point out in a straight line, and still be observable as such. They might very well look like 'pieces of a pie' radiating out, but there would be no bending to observe.

Yes, naturally there's no way to observe this spirally-shape. But, in our thought experiment, if we had a magical method of instantaneously detecting the position of all emitted photons, then they would appear as a spiral shape.

But then, we don't need to detect them, we know where they're going and how fast they're going to get there. We can plot positions of photons at different times. If you plot the positions of all photons emitted after 2 full rotations of the planet, the plot will be a spiral shape that goes all the way around twice. The photons will be traveling in straight lines, and if you plot the position of a given photon at different times, that will be straight lines heading away from the planet. But if you plot the positions of all photons at a single time, you would see a spirally shape in their locations. A photon wouldn't travel in a spiral, but the positions of all photons would form a spiral.

Of course you can't "see" this without a magical thought experiment, and it doesn't come out as a perfect shape without a magical light source and neglecting several other things. But the photons would be there, in locations that formed a spiral when plotted from above.

And really this is a good thing to kindof realize: in human perception, light is instant, there's a beam and it's all the way from here to there instantly. But in reality, and observably in a long enough time/length scale, photons are traveling at a fixed speed, and so they do get effects like this, where a photon emitted earlier is farther away from the planet than one that was emitted later in a different direction.

 

[Benm]

The problem is, that its nothing like water spraying from a hose thats rotating around. There is no 'drag' that actually bends the light beams.

You could argue that they would appear bent when observing from the planet, but that isn't actually correct. What happens is that the planet rotates and the beams would -appear- to be bent. This is assuming you can instantly detect the position of any photon regardless of its distance, obviously violating the concept that information cannot travel faster than light.

These relativistic things are often mind boggling, and no classical equivalent is suited to describe them.

 

[Eudaimonium]

The problem is, that its nothing like water spraying from a hose thats rotating around. There is no 'drag' that actually bends the light beams.

You could argue that they would appear bent when observing from the planet, but that isn't actually correct. What happens is that the planet rotates and the beams would -appear- to be bent. This is assuming you can instantly detect the position of any photon regardless of its distance, obviously violating the concept that information cannot travel faster than light.

These relativistic things are often mind boggling, and no classical equivalent is suited to describe them.

You could not see that beam bent, but it is (in such case).

And you last sentence... man, agreed 100% !:bowdown:

 

[Bluefan]

What happens is that the planet rotates and the beams would -appear- to be bent. This is assuming you can instantly detect the position of any photon regardless of its distance, obviously violating the concept that information cannot travel faster than light.

This is somewhat what I think. The beam may appear bent, the light still travells forward. I think it really looks a lot like a rotating garden hose, there the water always goes straight away from the source, a single drop doesn't bend. But because each drop is sent out at a different direction after eachother, plotting a line through all of them seems bend, even though no water travels that curve.

Just take 3 ultrafast lasers and sent 3 pulses away, separated by a constant angle and delay. You can plot a bend line through all 3 pulses that really doesn't do anything. Placing photodiodes in that bend line will make them detect pulses at the same time. No relativistics needed, everything in the reference frame.

 

[Eudaimonium]

That is what I'm tryign to say ALL along, I just didn't come up with better expression to describe the looks of the beam, other than 'bent', I didn't mean it is bent literally, I mean that is looks bent (plotted line of all photons), get it?

Thanks for clearing stuff up here!

 

[Benm]

Reference is key here though. When you are -on- the spinning planet, the reflections will appear to be bent into a spiral. But this is not so because they are bent, it is only because the planet rotates between observing reflections at close range and at larger distances.

This effect isnt all that obvious in most practical situations - just a second of will put your beam beyond the distance to the moon. On the other hand, if you were to point a laser at mars in the position you see it, the light will completely miss mars. This is because of two factors: you are looking at a delayed image of mars by a few minutes, and also because the light takes the same amount of time to get there. If you wanted to hit mars, you would have to point to its orbital position ahead of what you are seeing, calculating in -twice- the time light takes to travel there.