# Rotating a laser beam faster than light?

1. Aug 20, 2008

### sinebar

This is probably just nuts but lets say you switch on a laser, point it to the sky and leave it on for billions of years until the beam has traveled billions of light years. Then you rotate the laser 360 degrees in lets say 1 RPM. Does the laser spot that is now billions of light years away from the axis of rotation complete the rotation in 1 minute? If so then the laser spot that is now billions of light years from the axis of rotation would have to travel much faster than light right?

2. Aug 20, 2008

### mgb_phys

A mathematical point can travel faster than light, relativity says you can't send information faster than light.
You can sweep a laser accross the moon faster than light but it doesn't transfer information from one side of the moon to the other (it's known as the lighthouse paradox).
You can even (in theory) make the point where a pair of scissor blades meet go faster than light, if you close them fast enough.

Last edited: Aug 20, 2008
3. Aug 20, 2008

### psycovic23

4. Aug 20, 2008

### mgb_phys

It's just what the OP described.
A lighthouse beam sweeps around in say 1second.
So at a distance of 1mile the beam covers 6.3miles in 1s (ie. 2pi*r)
Obviously as r increases the circumference increases but it still only takes 1 second.
So at some distance (30,00miles) it will go >186,000 miles around the circumference in 1 second so faster than light.

But the beam moving from point to point doesn't carry any information so no violation of relativity.

5. Aug 20, 2008

### Topher925

Kind of a stupid question but can someone define what exactly is meant by information in this context.

6. Aug 20, 2008

### Staff: Mentor

Right, but the spot on the screen is not a single entity. It is a succession of different spots formed by different bundles of light as they hit the screen. Each bundle travels from the laser to the screen at speed c. Nothing actually travels from one point on the screen to the next.

7. Aug 20, 2008

### mgb_phys

It's very simple, anything you didn't know before!
So turningthe lighthouse on/off sends information, on=team A won the superbowl, off=team B. So the light from the light house can't reach you faster than light because you would know who won. But a sweeping beam can't take any information from one side of the moon to the other because there is nothing anyone can do at the left point to change the beam that will arrive at the right point.

But suppose you had a very long solid metal arm that you swung around in an arc, someone could stick a post-it note on the end as it went past them - which the next other person could read.
So this proves that you can't just put a long metal rod on the lighthouse and move it fast enough for the end to go faster than light.

8. Aug 20, 2008

### Landru

People are confusing a moving laser dot with something tangible. The dot is made up of photons shooting outwards from the light source. The dot at starting point is not made up of the same photons as the dot at the end, and the idea that the dot at the start and the dot at the end are one in the same is a product of our imagination. My cats get confused over this very same thing.

9. Aug 21, 2008

### sinebar

What if we replaced the laser with a metal disc with a radii of a billion light years and rotated it at 1 RPM. Would the outer edge of the disk be rotating faster than light?

10. Aug 21, 2008

### HallsofIvy

This is at one with the old question about taking a rigid rod with length several light years and the "pushing" one end a centimeter. Doesn't the other end move a centimeter at the same time? Couldn't you use that to send a message several light years in an instant?

The answer is that what that tells us is that there is no such thing as a "rigid" object, even theoretically, in relativity. The motion of the end of the rod will move as a wave, at a speed determined by the elasticity of the rod but less than the speed of light, up the rod.

Similarly, your disk will distort so that the outer edge is not moving at the same speed as the center. You can't rotate a huge sphere at 1 rpm.

11. Sep 28, 2008

### simondupnik

I guess that the underlying but un-articulated point in most of the questions is this: do the photons, exiting from the rotating source of light, have any tangential velocity, in addition to the outward radial speed of light. Ie, are they travelling radially as the traditional view has it, or in some 'Catherine wheel', bent spoke trajectories? I read that some recent experiments with the lighthouse nuclear emissions (from the rotating samples) DO register angular shift between irradiation and emission.

12. Sep 28, 2008

### Staff: Mentor

:rofl:

13. Nov 15, 2008

### robbie7730

In fact the light beam spot wont move faster than light, the light beam will curve as you move it, so if you pointed it at moon as suggested earlier, the spot cant move faster than light even if the angular movement of laser on earth does point toward a different place on the moon faster than light could move to that spot.

So if the spot is an enormous distance in space on a surface of massive object, say a light year away, then the laser is moved just a few degrees, an observer on that planet would not see the laser spot move for a year, the beam of light would actually move in a curve shape with the spot lagging behind. If it did move instantly at that distance then information could be transfered from that observer to another one a light year from him by rapidly moving laser spot from one to the other in a digital sequence etc.

Another example would be point at moon again, turn it on, but move away from moon in less than a second, if spot moves with laser at any distance like a straight rod then the laser spot would not hit moon, but that would`t be the case

14. Nov 15, 2008

### pallidin

The light beam does not actually "curve", but I do know what you are suggesting and you are correct.

Why some insist that a laser from earth can sweep the surface of the moon faster than light is beyond me. It just doesn't happen, can't happen and never will.

15. Nov 15, 2008

### mgb_phys

Not entirely sure what robbie is trying to say.
The important point is that the point of contact of the laser with the moon can move faster than 'c' but the point of contact is just a mathematical notion - there is nothing actually there. No photon ismoving faster than 'c'.

A simpler example might be to consider two cars driving to the east and west coast from the centre of the USA. They both drive the same distance at the same speed and arrive within a millisecond of each other. So the 'point of arrival' has traveled accros the
USA (3000mi) in 1ms - much faster than light.

16. Nov 15, 2008

### pallidin

Well, I certainly agree with that.

Last edited by a moderator: Nov 15, 2008
17. Nov 15, 2008

### GOD__AM

While I understand part of your point, your explanation is flawed. Using the example of the target being one light year away, if the laser starts to rotate yes it will take one year to notice this on the receiving end. At that point though the spot will start to appear to move faster than the speed of light at the receiving end.

However it's already pointed out that a light spot is just an interaction. Light hits atoms and is reflected. I have used this example before, and I think it gives a good picture of what is happening. Imagine a line of light bulbs, and a switching system that turns them on then off in quick succession. We can rig this line of bulbs so that the time the first bulb blinks to the time the last bulb blinks is greater than the speed of light across the line of bulbs. The light would appear to travel faster that light speed (left to right as it were), but in reality there is no movement of the bulbs.

Substitute the bulbs for the atoms reflecting the light, and the switching system for the laser pointer.

I'm guessing by curved light you are referring to the type of situation where a rotating water sprinkler makes a curved stream of water. That's a fair comparison but the stream will still hit the receiver and cause interactions with atoms that propagate faster than the speed of light given the situation described here.

18. Nov 15, 2008

### pallidin

Wow, I can't believe how hard it is for me to understand some things, even though I'm not entirely brain-dead. Maybe in time and through proper education/discourse I will begin to grasp these concepts.

19. Nov 15, 2008

### gonegahgah

Interesting idea Halls.
Something like this should actually be testable shouldn't it?
You don't need a rod several light years in length.
You simply need a rod of any length; the longer the better for measurement sake.
Measure its length.
Put two sensors at that length apart.
Connect the two sensors via the same amount of wire to a central recorder which records time of contact. As the contacts are stationary with respect to each other this is no problem.
Then use the rod with contact pad to press a message against the first sensor which, because of the rod, is translated to the second sensor via a second contact.
All corresponding contacts are then sent to the central recorder.
Then simply compare the duration between the first contacts received and the second contacts received and divide this by the length of the rod to get the speed of transmission.

Has such an experiment ever been done?

20. Nov 15, 2008

### mgb_phys

Yes it's called measuring the speed of sound in a material.
It's done everytime you ultrasound test a weld or metal pipe.

21. Nov 15, 2008

### gonegahgah

Hi mgb.

Is that a response to my post? If it is - I am perplexed how - could you please explain how you example bears any relationship to the example I described?

Thanks

22. Nov 16, 2008

### gonegahgah

Sorry, maybe this depicts example better.

+---------------(PC)----------------+ (2 sensors (+) connected to time recorder (PC) in centre)
<-|_______________________________|__ (rod with two contacts (|) and handle on right)

The length between contacts is measured to be equal to length between sensors while all are stationary.
Move rod via handle on right towards left till rightmost contact makes contact with rightmost sensor. The leftmost contact should then make contact with the leftmost sensor.
These contact occurances, both left and right, travel to the recorder via the same length of wire, and their time received is recorded.
Take the time difference between the right and left contact occurances and divide this by the length of the rod to get transmission time.

They don't need to be light years long. Anything like this been done?

23. Nov 16, 2008

### schroder

As Halls has explained, there is no such thing as a rigid rod; it is a concept only. You can have a concept of a rigid rod, and you can have a concept of matter and/or information being sent at faster than light speed, but neither exist in reality. When you move one end of the rod, in reality, that movement must be transmitted to the other end by molecular interactions. The molecules themselves may be very nearly elastic, but the interactions between molecules is in the world of heat. The collisions are not elastic and it takes time for the movement of one end to be transmitted to the other end. That mechanical wave actually travels at the speed of sound, much less than the speed of light.
As for the the laser beam on the moon or in deep space, this has been recently discussed in another thread, but apparently there is still confusion on this issue. It is just the angular velocity, or phase velocity of light that is exceeded in these cases, never is the linear velocity of light exceeded. Of course, you Can transmit information using this angular velocity, but you Cannot transmit that information faster than the speed of light itself. The line of lights mentioned by another poster is a good example. Yes, you can program a long string of lights so it would appear that the turning on was advancing along the line at FTL speed. But you could not do this by controlling the lights from a single point at one end of the chain. You would need to preprogram them to act independently of one another, but giving the appearance of acting together in a string. Hence, no useful information is being sent that the lights did not already have!

24. Nov 16, 2008

### gonegahgah

Hi Schroder

I'm not saying it will prove you boys wrong. What I am saying is that it should be very testable; rather than being just another 'thought experiment'.

I'm not aiming this at you but I really don't care if a rigid rod is a concept or not. I'm saying that it is a provable matter - one way or the other - that can be verified without the need to have a rod that is several light years long; and without the need to leave it in the concept basket.

Less talk; more experimental data. That's what I would like. It often amazes me how little data there seems to be when there are so many things going on that would provide data. eg. Robot missions to other planets.

Are the clocks on these robots ticking at different speeds while in space and while on other planets? Do robots return home younger than they would have if they had stayed home?
I've read that the clocks on the shuttles slow down due to higher relative speed but that calculations with the satellites (what are they called) that we use to pinpoint our location make allowance mainly for the lesser gravity which speeds up time and higher relative speed that slows down time (minutely in comparison); so that mainly a faster clock is actually taken into account. So which is it: Shuttles slower clocks or navigation satellites faster clocks?

That's truly getting off the track but the main thing is that it is easy to rattle off concepts to fit any idea. What I find most frustrating is a lack of experimental data when there should be so much around. Only little fragments can be found.

So rather than rigid rods being wrong by assumption; it should be simple enough to verify that statement by experiment.

25. Nov 16, 2008

### Staff: Mentor

This might be news to you, but measuring the speed of a pulse through a rod can easily be done. (And has been, a zillion times.) Here's a student lab experiment version: http://www.picotech.com/experiments/speed_in_solid/speed_in_solid.html"
The physics experimental literature is vast. Just because you are just learning about certain features of the world does not mean that "experimental data doesn't exist".

Last edited by a moderator: Apr 23, 2017