Observational Effects of "FTL" spotlight from laser pointer?

  • #1
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?
 

Answers and Replies

  • #2
Ibix
Science Advisor
Insights Author
2020 Award
7,392
6,473
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.
 
  • #3
2,073
455
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.
 
  • #4
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
16,139
8,169
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.
 
  • #5
That depends on the definition of "initial location".
The location of the spotlight before the person holding the laser pointer flicks his wrist.
 
  • Like
Likes russ_watters
  • #6
CWatters
Science Advisor
Homework Helper
Gold Member
10,532
2,298
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.
 
  • Like
Likes Imager and RandyD123
  • #7
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?
 
  • #8
2,073
455
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.
 
  • #9
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.
 
  • #10
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
16,139
8,169
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!
 
  • #11
CWatters
Science Advisor
Homework Helper
Gold Member
10,532
2,298
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.
 
  • Like
Likes russ_watters and anorlunda
  • #12
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?
 
  • #13
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
16,139
8,169
... 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).
 
  • #14
2,073
455
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 seperates in two spots moving away from each other.

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

9xX49ud.gif


 

Attachments

Last edited:
  • Like
Likes Dale
  • #15
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
16,139
8,169
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.
 
Last edited:
  • #16
CWatters
Science Advisor
Homework Helper
Gold Member
10,532
2,298
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.
 
  • #17
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?
 
  • #18
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
16,139
8,169
How does this happen? Is this analogous to observing a hypothetical tachyon?
No. It's analogous to an object moving in the opposite direction.
 
  • #19
2,073
455
How does this happen?
The direction is as frame depedent as the order of the events.
 
  • #20
The direction is as frame depedent as the order of the events.
What frame would you have to be in to 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).
 
  • #21
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
16,139
8,169
What frame would you have to be in to 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##.
 
  • #22
2,073
455
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.
 
  • #23
Nugatory
Mentor
13,230
6,091
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.
 
  • #24
russ_watters
Mentor
20,240
6,812
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.
 
  • #25
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
16,139
8,169
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.
 

Related Threads on Observational Effects of "FTL" spotlight from laser pointer?

  • Last Post
2
Replies
25
Views
4K
  • Last Post
Replies
3
Views
868
  • Last Post
Replies
13
Views
3K
Replies
15
Views
820
Replies
3
Views
669
Replies
77
Views
10K
  • Last Post
Replies
6
Views
2K
Replies
5
Views
412
Top