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[Azkanan]

If you point a laser at the moon, let's say its on the far right side of the moon.
If you then flick your wrist so the laser is pointing at the left side of the moon,

Then the laser beam will have moved (aprox) *20 the speed of light over the surface of the moon.

Wrapped up with diagrams in 60 seconds:

http://youtu.be/lR4tJr7sMPM

You're trolling right?

If you're not: try it with a water hose and you'll see why it doesn't work

I'm not a physicist yet D;

It made sense to me...

[RedKing]

On an unrelated note, I still havent messaged myself from the future yet. Poor show, future me.

Maybe you did, and are missing the cues.

I have to admit, past me is notoriously stupid and makes the worst decisions ever.

Seriously. Past me was a total dumbass. I don't even *want* to talk to that guy. Because then he'll be all like, "Hey who's going to win the Super Bowl this year so I can get rich?" and then if I tell him, I'll be doing all the work but he'll be the one getting rich, thus changing the timestream and making me cease to exist. Ungrateful asshole.

[ChairmanPoo]

You know, when this topic popped up the first time, I reckoned that any messages from my future self might be unpleasant. The reasoning is that I often feel angry about mistakes that took place, say, six months ago, and feel like kicking myself, so logic suggests that the ammount of abuse my future self will want to unload on my present self will be impressive.

[Bauglir]

If you point a laser at the moon, let's say its on the far right side of the moon.
If you then flick your wrist so the laser is pointing at the left side of the moon,

Then the laser beam will have moved (aprox) *20 the speed of light over the surface of the moon.

Wrapped up with diagrams in 60 seconds:

http://youtu.be/lR4tJr7sMPM

You're trolling right?

If you're not: try it with a water hose and you'll see why it doesn't work

I'm not a physicist yet D;

It made sense to me...

The intersection between the moon and the laser isn't an object. No thing in the example moves faster than light. At least given my understanding, although I also recall hearing something about information not being able to move faster than light or something, but I don't really understand why that would be other than that it causes paradoxes so I can't say for sure whether it is relevant. I'm no physicist!

[ChairmanPoo]

I'm no physicist, either. But I'm a physician. 

[Azkanan]

When you turn the laserpen, or whatever, in your hand, I imagine the laser beam a meter in front of you moves at distance X.

But the laser beam's end moves from position right-of-moon to left-of-moon at the relative speed as a meter in front of you. Ergo, doesn't the light at the end of the laser speed across the surface of the moon faster than light?

In retrospect, no. It would just take time, at the speed of light, to move across the surface of the moon - bending the laser beam...

...As you said, with the hosepipe.

You sneaky dwarf, you.

[Kogut]

If you point a laser at the moon, let's say its on the far right side of the moon.
If you then flick your wrist so the laser is pointing at the left side of the moon,

Then the laser beam will have moved (aprox) *20 the speed of light over the surface of the moon.

Wrapped up with diagrams in 60 seconds:

http://youtu.be/lR4tJr7sMPM

You're trolling right?

If you're not: try it with a water hose and you'll see why it doesn't work

I'm not a physicist yet D;

It made sense to me...

DID ANYONE ACTUALLY WATCH THE WHOLE VIDEO?

He clearly explains why this is not truly breaking the speed of light.

TheRinzl3r 3 dni temu playlista: MinutePhysics +57

[Starver]

When you turn the laserpen, or whatever, in your hand, I imagine the laser beam a meter in front of you moves at distance X.

But the laser beam's end moves from position right-of-moon to left-of-moon at the relative speed as a meter in front of you. Ergo, doesn't the light at the end of the laser speed across the surface of the moon faster than light?

In retrospect, no. It would just take time, at the speed of light, to move across the surface of the moon - bending the laser beam...

...As you said, with the hosepipe.

You sneaky dwarf, you.

Yerbutnobut.

Flicking the pen does send the "end spot" across the moon at FTL speeds.  Except that the time for the "movement to start/finish" from your initial motion takes the expected distance that a SoL-type of communication would take from where you are.

It's a superluminal track, indeed, but it conveys no information along its length, and takes standard SoL time (from you, the origin) to even happen..  That's what the hosepipe idea shows.  Two things could become wet within moments of each other (even though the water could never have physically gone from one of the things to the other in such a time, given whatever physical constraints you lay upon that jet of water), but the water is coming from you, and only reaches either/both of them as fast as the water does go.  It's different bits of water for each.  And you might well have switched the water-flow on or off as it was being aimed towards either target, but if the presence/absence of water was the only information each would have, they wouldn't have any way of knowing if the other was getting wet, or if the other wanted to get wet, erected an umbrella, etc.  The "end of the beam", or where the water splashes, just happens to be a description of a continually renewed and independent bit of your transmission, and is not something that can be given a state to 'hold'.



(I deleted a whole "lighthouse illuminating targets at each end of the universe" set of paragraphs above that para. The above was originally the "TL;DR;" conclusion, but became long enough in its own right and said everything I wanted to as well.  Still too long, probably.)






[1] Ignoring the expansion that will be happening, yadayadayada.  Let's just say we have targets set 14 billion LY away, assumed to be in an equivalent frame of reference (which means they're not 'stationary' to the 'local' frame of reference, just as translated from the origin, etc) and forget about the whole expansion issue.

[PTTG??]

The interesting thing is, imagine that the laser is on the moon. You're on a vast plain with a very fast camera.

The laser dot is moved, FTL, from west to east past you. You film it approaching and departing with the fast camera.

What does the dot look like to the camera? What do you actually see, assuming that you could perceive the dot moving?

I presume that you would first see the dot appear in front of you and then split. One dot will be distorted and twice as long as it should be, moving east. One dot will be half as long and will move west.

Assuming that the dot appears to move at 2c from the lunar perspective, the eastward-moving dot will appear to the camera to be moving at 2c, while the westward-moving dot will appear to be 0.5. I'm not certain of this.

Hmmm. I would like to see mythbusters try this. Perhaps they could model it with sound waves.

[Virex]

You would see the dot move at superluminal speed. There would not be any relativistic effects because the dot is not a physical object, it's the result of a bunch of photons reflecting off of a surface and those photons do not behave differently from any other photon.

[Starver]

I'd say (if I understand your setup correctly) that the first glimpse of laser you would have would be the dot at your feet/directly in your eyes, whichever is the spot closest to you.  You'd then see the (delayed by increasing distances[1]) receding 'incoming' lit spot, heading towards its origin point plus the (equally delayed by increasing distances[2]) receding 'outgoing' lit spot.  So, I think that's much as you put it, although I didn't bother to work out if the quoted (apparent) speeds would be correct, beyond the feteneted additions.


But I've been awake for about 22 hours, so my brain may not be understanding things as correctly as it might do.

Virex: I knew someone could write what I wrote earlier in fewer words.  I think you're somewhere about there with your post, lacking only the "You'd have to wait to see the effect" bit, perhaps...


[1] Minus a proportionate amount of time relating to the time prior to your own more direct illumination that the spot had been projected at that point, but never making it a non-delay, so still receding.
[2] And this time, plus a proportionate amount of time relating to the time post your own illumination that the spot ends up being projected at that point, and so receding faster.

[PTTG??]

Virex: Exactly. But it takes time for the photons to bounce off the surface and to your eyes. If you simply put sensors along the path and have them record the time the dot goes by, it will be seen as moving linearly, at 2c, just like the moon. (though the moon people will see the dot move at 2c three seconds (or maybe six?) after they project it.)

But when you're standing there, the light will reach you from the nearby location sooner than it will from the place it was before it reaches you. If that makes sense.

Starver's got it.

[Another]

An interesting special case is if the spot moves at exactly c. Than all the photons from the past reach you simultaneously in a kind of a very bright flash (assuming flat Earth and heightless observer of course).

[Darvi]

Virex: Exactly. But it takes time for the photons to bounce off the surface and to your eyes. If you simply put sensors along the path and have them record the time the dot goes by, it will be seen as moving linearly, at 2c, just like the moon. (though the moon people will see the dot move at 2c three seconds (or maybe six?) after they project it.)

My guess would be that, if the dot moves at 2c and the sensors were all a quantum length apart, not every sensor would actually sense the photons, only every second one. So while the dot dos move at FTL speed, the light doesn't actually touch the surface across the entire length.

[Starver]

How might the wave/particle duality apply, though?