Observational Effects of "FTL" spotlight from laser pointer?

[AndromedaRXJ]

So from what I understand, it's possible to have a very powerful laser pointer where you point at an arbitrarily large and far away surface and make the spotlight appear to move at faster than light. E.g. you can be holding the laser in your hand and you simply flick your wrist.

My question is, what does an observer on that surface see? Will the observer see the spotlight arrive at one location before it leaves its initial location? Can there be any observer that will see this? Does the image of the spotlight continuously appearing on the surface, simply count as a continuous series of events which can be seen in any order since they're not causally connected?

 

[Ibix]

My question is, what does an observer on that surface see?

He sees a light flash on for the brief fraction of a second your laser pointer is pointing at him. He can phone a friend at another location to see if he saw it too, but such communications will be at or below lightspeed.

Does the image of the spotlight continuously appearing on the surface, simply count as a continuous series of events which can be seen in any order since they're not causally connected?

Yes. They have a common cause in the past, but one part of the flash does not cause the next.

 

[DrStupid]

Will the observer see the spotlight arrive at one location before it leaves its initial location?

That depends on the definition of "initial location".

Does the image of the spotlight continuously appearing on the surface, simply count as a continuous series of events which can be seen in any order since they're not causally connected?

No, they do not need to appear continuously. Depending on the conditins they can suddenly appear or disappear and there can also be more than one spotlight at once. But you are right in regard to the order of the events.

 

[PeroK]

So from what I understand, it's possible to have a very powerful laser pointer where you point at an arbitrarily large and far away surface and make the spotlight appear to move at faster than light. E.g. you can be holding the laser in your hand and you simply flick your wrist.

My question is, what does an observer on that surface see? Will the observer see the spotlight arrive at one location before it leaves its initial location? Can there be any observer that will see this? Does the image of the spotlight continuously appearing on the surface, simply count as a continuous series of events which can be seen in any order since they're not causally connected?

If a light goes on next to you at time $t=0$ and, one second later, a light goes on a distance of, say, 2 light-seconds from you, then what is special about that? Nothing moved faster than light.

Or, if the two lights go on simultaneously, what is special about that? Nothing moved at infinite speed.

The limit of the speed of light means that all particles move at sub-light speeds, but it does not mean that all events must be timelike separated.

 

[AndromedaRXJ]

That depends on the definition of "initial location".

The location of the spotlight before the person holding the laser pointer flicks his wrist.

 

[CWatters]

The spot of light might appear to be moving across the surface of the target but it's not. At all times light only moves from the laser to the target, no light moves across the surface of the target.

 

[AndromedaRXJ]

He sees a light flash on for the brief fraction of a second your laser pointer is pointing at him. He can phone a friend at another location to see if he saw it too, but such communications will be at or below lightspeed.

I mean if he's observing the spotlight, not the person flashing the laser. Would the spotlight be like a tachyon where you can't see it approaching?

 

[DrStupid]

The location of the spotlight before the person holding the laser pointer flicks his wrist.

That means that different observers do not need to agree about this position.

 

[AndromedaRXJ]

The spot of light might appear to be moving across the surface of the target but it's not. At all times light only moves from the laser to the target, no light moves across the surface of the target.

I know that. I'm just wondering what it would actually look like to observe this spotlight. I know nothing is actually moving faster than light.

 

[PeroK]

I know that. I'm just wondering what it would actually look like to observe this spotlight. I know nothing is actually moving faster than light.

It would look like any other spotlight!

 

[CWatters]

To see the spot moving across the target there must be light reflected from the target to the observer. That light can only travel at the speed of light. So an observer might find himself illuminated by the source dirrectly before he sees the spot appear elsewhere on the target eg before he sees reflected light.

 

[AndromedaRXJ]

It would look like any other spotlight!

But what would it look like if the spotlight "moves" across the surface faster than light?

That means that different observers do not need to agree about this position.

Okay that's interesting. So how would that happen?

And can you elaborate further on how there would be more than one spotlight at once?

 

[PeroK]

... if the target is angled slightly, then you can arrange things so that the light illuminates all of the target simultaneously (in the rest frame of the target).

 

[DrStupid]

And can you elaborate further on how there would be more than one spotlight at once?

A simple example is a fast rotating laser. It would emit a spiral of light. When this spiral hits a surface there would appear a spot which separates in two spots moving away from each other.

Edit: Using a fast rotating mirror would be better than rotating the laser itself:

 

[PeroK]

But what would it look like if the spotlight "moves" across the surface faster than light?


Okay that's interesting. So how would that happen?

And can you elaborate further on how there would be more than one spotlight at once?

As far as the target is concerned photons impact different points of the target at different times. There is nothing moving across the target. Any single observer will have to wait until photons hitting the target are reflected to their location. It's not even possible for the observer to say or know that all the light came from the same source. They might be able to calculate that. But, it would just look like any old photons hitting the target.

As my above post, with an angled target you could have the spotlight rotate from left to right, but the target be illuminated from right to left.

As a gross example, the right hand end of the target could be only half the distance from the spotlight that the left hand end is.

 

[CWatters]

But what would it look like if the spotlight "moves" across the surface faster than light?

Depends where you are observing from.

If you are at the target the spot will be moving so fast that everywhere will appear to be illuminated at the same time.

If you had some sort of high speed camera to record and play back what happens then...

If the spot is moving slow enough it will appear to move across the surface "normally".

If the spot is moving faster than light its possible for the spot to appear to be moving backwards.

 

[AndromedaRXJ]

If the spot is moving faster than light its possible for the spot to appear to be moving backwards.

How does this happen? Is this analogous to observing a hypothetical tachyon?

 

[PeroK]

How does this happen? Is this analogous to observing a hypothetical tachyon?

No. It's analogous to an object moving in the opposite direction.

 

[DrStupid]

How does this happen?

The direction is as frame depedent as the order of the events.

 

[AndromedaRXJ]

The direction is as frame depedent as the order of the events.

What frame would you have to be into see it like this though? Is this what the stationary observer at the surface sees? Or does it have to be a different frame?

And this might be a stupid question, but say the observer knows the nature of the source and knows that the source is flicking the laser in a particular direction (perhaps the person with the laser tells him what he'll do way before hand), would the observe be right in simply disagreeing with the order of events that he sees if it contradicts his previous knowledge of what the source is doing? (he sees the spotlight move backwards, but knows the person holding the laser isn't flicking the laser in that direction).

 

[PeroK]

What frame would you have to be into see it like this though? Is this what the stationary observer at the surface sees? Or does it have to be a different frame?

And this might be a stupid question, but say the observer knows the nature of the source and knows that the source is flicking the laser in a particular direction (perhaps the person with the laser tells him what he'll do way before hand), would the observe be right in simply disagreeing with the order of events that he sees if it contradicts his previous knowledge of what the source is doing? (he sees the spotlight move backwards, but knows the person holding the laser isn't flicking the laser in that direction).

Consider this set-up. You have a machine that fires tennis balls at, say, $10m/s$. You have three friends lined up: the first is in front of you $100m$ away, the second is at $45$ degrees to his right and nearer ($50m$ away). The last is on your right only $10m$ away.

At $t=0$, you fire a ball at the first friend. He catches it at $t = 10s$.

You swivel the machine and at $t = 1s$ you fire a ball at the second friend. He catches it at $t = 6s$.

You swivel the machine again and at $t = 2s$ you fire a ball at the third friend. He catches it at $t = 3s$.

Now, in your analysis, something truly extraordinary and tachyon-like has happened. The third ball (which was fired two seconds after the first) was caught (hit the target) first! And the first ball to be fired hit the target last. You see this as something moving faster than light or going backwards in time. The virtual ball didn't just move from friend one to friend three faster than light, but backwards in time.

Whereas, I see it as three different balls being fired in different directions and being caught at different times, with no tachyon-like behaviour and nothing to get excited about. Nothing moved from friend one to friend three or vice versa and nothing moved more than $10m/s$.

 

[DrStupid]

And this might be a stupid question, but say the observer knows the nature of the source and knows that the source is flicking the laser in a particular direction (perhaps the person with the laser tells him what he'll do way before hand), would the observe be right in simply disagreeing with the order of events that he sees if it contradicts his previous knowledge of what the source is doing? (he sees the spotlight move backwards, but knows the person holding the laser isn't flicking the laser in that direction).

If both observers ar at rest relative to each other they would agree about the order of events. But that doesn't mean that the spot will move in the same direction as the aim of the laser. In my example in #14 there are two spots (intersection of the yellow spiral and the vertical line on the right side) moving in opposite directions.

 

[Nugatory]

would the observe be right in simply disagreeing with the order of events that he sees if it contradicts his previous knowledge of what the source is doing? (he sees the spotlight move backwards, but knows the person holding the laser isn't flicking the laser in that direction).

If the distances are the same, then the events "light hits point A" and "light hits point B" will be spacelike-separated. Thus, the question "which happened first?" is not meaningful as asked - different observers moving at different speeds relative to one another will disagree about the relative ordering of the two events. This is the relativity of simultaneity (google for "Einstein train simultaneity", and look at some of our many many threads on this subject); it is one of the most basic concepts in relativity, and you must understand it before you can take on any harder questions about relativity.

However, it also important to understand that these two events are different from the events "Light for point A leaves laser" and "light for point B leaves laser". Those two events are timelike-separated, and all observers will agree about their relative ordering. Furthermore, all observers will agree that the "light for A leaves laser" event happened before the "light hits point A", and likewise the "light for point B leaves laser" event occurs before the "light hits point B" event.

 

[russ_watters]

Guys, I think most of you are missing the point. This is the point:

To see the spot moving across the target there must be light reflected from the target to the observer. That light can only travel at the speed of light. So an observer might find himself illuminated by the source dirrectly before he sees the spot appear elsewhere on the target eg before he sees reflected light.

Right. Viewed from a distance, we see the spot sweep across the ground/wall/whatever. However, person standing in the sweep would see the initial spot, then the direct laser, then... I think... see it sweeping away from him in both directions at once. But that last part is tough.

 

[PeroK]

Guys, I think most of you are missing the point. This is the point:

Right. Viewed from a distance, we see the spot sweep across the ground/wall/whatever. However, person standing in the sweep would see the initial spot, then the direct laser, then... I think... see it sweeping away from him in both directions at once. But that last part is tough.

The point is that nothing is moving across the surface. It can be arranged for light to be incident upon a surface in any way. Left to right, right to left, simultaneous, random. There are no constraints as nothing is moving across the surface. There is only an illusion of motion or sorts.

 

[DrStupid]

The point is that nothing is moving across the surface.

That depends on the definition of "something" and/or "moving".

 

[AndromedaRXJ]

But that last part is tough.

I think it's the part I'm most interested in too.

 

[Ibix]

What frame would you have to be into see it like this though? Is this what the stationary observer at the surface sees? Or does it have to be a different frame?

It isn't about the frame. It's about where you are.

The simplest case is to use @PeroK's suggestion to have the plane angled so that it is illuminated simultaneously in your frame. Then it's just like the whole length of the path flashes once. What do you see? The light from the nearest point on the beam path reaches you first. Then the light from the next nearest point - which will be the pair of points on each side of the first one. Then the next nearest points - the next pair of points outwards from the nearest one. So you will see flashes going in both directions outward from a single point.

You can analyse this in any frame, and the answer will always be the same for what you actually see. It's just really easy to analyse if we've picked a setup where the flash is simultaneous in your rest frame.

 

[PeroK]

I think it's the part I'm most interested in too.

The problem is that there is nothing of any physical interest here.

 

[PeterDonis]

That depends on the definition of "something" and/or "moving".

The definitions being used for those in this thread should be obvious: they are the definitions that imply that "something" cannot "move" faster than light. Yes, there are other definitions that could be adopted for those words, but they are irrelevant to the discussion in this thread.

 

[russ_watters]

The problem is that there is nothing of any physical interest here.

Why not? You can see it. I find that interesting.

We've had similar threads about sonic booms.

 

[russ_watters]

I think it's the part I'm most interested in too.

Having thought about it a little more, I think what the person sees is:

Start: Origin point only.

Next: When the beam sweeps past him he instantly sees a solid line back to the origin point, that retracts to the origin point, before it and the origin point disappear.

Simultaneously, he sees the point continuing past him at some speed.

...still thinking about what those speeds are.

 

[russ_watters]

The problem is that there is nothing of any physical interest here.

The definitions being used for those in this thread should be obvious: they are the definitions that imply that "something" cannot "move" faster than light. Yes, there are other definitions that could be adopted for those words, but they are irrelevant to the discussion in this thread.

Guys!

This debate about "something" or "nothing" is not helpful. The OP did not introduce these words. The OP wants to know about what the observer sees. Period. Arguing about whether what the observer sees is "something" or "nothing" is not relevant or helpful.

 

[Ibix]

Simultaneously, he sees the point continuing past him at some speed.

...still thinking about what those speeds are.

Mathematically, you just need to note that if you see a point a distance $d$ away from you illuminated at time $t$ then it was actually illuminated at $t_r=t-d/c$. Then you write down the "equation of motion" of the laser spot - i.e. an expression for $x(t_r)$, $y(t_r)$ and $z(t_r)$ - and hence write down $d(t_r)$. That gives you an expression for $t(t_r)$, which may be multivalued - in which case you'll find you see the spot in multiple places.

 

[AndromedaRXJ]

Having thought about it a little more, I think what the person sees is:

Start: Origin point only.

Next: When the beam sweeps past him he instantly sees a solid line back to the origin point, that retracts to the origin point, before it and the origin point disappear.

Simultaneously, he sees the point continuing past him at some speed.

...still thinking about what those speeds are.

I know I asked this already, but so this isn't equivalent to what a tachyon would do? Or rather, suppose a tachyon went the exact same speed as the dot. Would the observation of its motion be the same?