I Why does laser beam hit the same target when fired?

[jt128]

Why does a laser beam hit the same target when fired?

Based on the speed of the rotation of the speed of earth, and the precise-ness of todays' equipments, we can place a lazer and a target several meters apart and tell distance moved up to microns to millimeters apart. So why does the photons of a fired lazer beam hit the same spot every time?

once the beam leaves the lazer, it is unaffected by relative movement of the gun affixed to the earths surface, or the movement of the earth itself therefore: Frame of reference doesnt apply here, because the light is unaffected by any relative movements of the other bodies.

Therefore, since the lazer moves in a straight path, while the earth with its affixed target moves, then why does the photons of the lazer hit the same spot without the measuring devices detecting a distance apart with 2 successive shots?

(also, keep in mind this is a simplified question because truly, theres movement of the earths rotation, then rotation around the sun, then movement around our galaxy at high speeds etc.. so the movement of the target is really quite a lot)

 

[DaveC426913]

Why does a laser beam hit the same target when fired?

The short answer is: because it has been calibrated to do so. If, when the experiment is set up, the laser's target is off by any detectable amount - for whatever reason - it is adjusted until that discrepancy goes away.

But let's look a little closer:

Have you done any calculations to show how much deflection we should expect to see from, say, the rotation of the Earth? (rough, order-of-magnitude calcs would be sufficient for now.)

A table top laser setup is maybe a couple metres across.
How long does it take for light to travel 2 metres?
How much does the Earth rotate in that length of time?

Follow up: how wide is a typical collimated laser beam?
How would be go about measuring deflection, taking into account margins of error?

 

[PeroK]

Based on the speed of the rotation of the speed of earth, and the precise-ness of todays' equipments, we can place a lazer and a target several meters apart and tell distance moved up to microns to millimeters apart. So why does the photons of a fired lazer beam hit the same spot every time?

The laser is pointing at the same spot every time. If nothing changes within the laser set-up, then it will hit the target at the same point every time.

once the beam leaves the lazer, it is unaffected by relative movement of the gun affixed to the earths surface, or the movement of the earth itself therefore: Frame of reference doesnt apply here, because the light is unaffected by any relative movements of the other bodies.

I'm not sure what you mean by that.

Therefore, since the lazer moves in a straight path, while the earth with its affixed target moves, then why does the photons of the lazer hit the same spot without the measuring devices detecting a distance apart with 2 successive shots?

I'm not sure why you would expect anything else. The speed of light is an invariant, but you can point a laser at any target you like.

(also, keep in mind this is a simplified question because truly, theres movement of the earths rotation, then rotation around the sun, then movement around our galaxy at high speeds etc.. so the movement of the target is really quite a lot)

The first postulate of special relativity is effectively that all motion is relative. There is no such thing as absolute motion through space. The motion of the Earth relative to the Sun, galactic centre etc. is irrelevant. What is relevant is local effects, like a minor earthquake, which could change shake the laser equipment and/or target during an experiment.

 

[DaveC426913]

The first postulate of special relativity is effectively that all motion is relative. There is no such thing as absolute motion through space.

"Based on the speed of the rotation of the speed of earth..."

 

[jt128]

The short answer is: because it has been calibrated to do so. If, when the experiment is set up, the laser's target is off by any detectable amount - for whatever reason - it is adjusted until that discrepancy goes away.

But let's look a little closer:

Have you done any calculations to show how much deflection we should expect to see from, say, the rotation of the Earth? (rough, order-of-magnitude calcs would be sufficient for now.)

A table top laser setup is maybe a couple metres across.
How long does it take for light to travel 2 metres?
How much does the Earth rotate in that length of time?

Follow up: how wide is a typical collimated laser beam?
How would be go about measuring deflection, taking into account margins of error?

It was calibrated to do so? I do not understand what you meant by this.
In the setup, the gun aims at a single point. There isn't any calibration. Its just a simple laser gun aimed at a single target.

Concerning, the questions you have about calculations. Is it necessary for this question? We know:
1. the speed of light.
2. the speed of the rotation of the earth.
3. the sensitivity of the equipment.

We can calculate everything precisely to get the distance needed to observe a micro-meter change in distance of both shot lasers.

My question is getting at something more fundamental. If the photons are truly "free" from Earth's reference frame once they leave the laser, and the target is moving with Earth, shouldn't we see a measurable difference between shots?

 

[PeroK]

If the photons are truly "free" from Earth's reference frame once they leave the laser, and the target is moving with Earth, shouldn't we see a measurable difference between shots?

A reference frame is a system of coordinates. It's not something physical. You can't be "free" of a reference frame.

I suspect you are misunderstanding something fundamental about the nature of light.

 

[jt128]

The laser is pointing at the same spot every time. If nothing changes within the laser set-up, then it will hit the target at the same point every time.

But why?

Let me make the question a bit more unrealistically extreme so you can see what im getting at.
Assume light moves at the pace of a snail. And the target is attached to a running puppy. The light will miss the bulls eye everytime.

And why? because the a puppy is faster than a snail. Now back to reality, the laser moves much faster the speed of the rotation of the earth, but given that the puppy (the target) is still moving, albeit slower, there should be a tiny (but measurable with today's kind of equipment) deviation between the point where the photons should exactly hit, and where it actually hit.

But there isnt one. I'm asking why?

 

[Ibix]

Therefore, since the lazer moves in a straight path, while the earth with its affixed target moves, then why does the photons of the lazer hit the same spot without the measuring devices detecting a distance apart with 2 successive shots?

Why would you expect a different result if you do the same experiment twice in a row?

 

[Ibix]

The light will miss the bulls eye everytime.

...and in exactly the same way. So if you set it up to hit once it'll hit every time.

I think you are thinking that there's some absolute rest frame and light moves isotropically only in that frame. That's essentially the ether model that Michelson and Morley tested and falsified in the late 19th century.

 

[jt128]

Why would you expect a different result if you do the same experiment twice in a row?

I didnt do the same experiment twice in a row. I did the experiment once.
The experiment is me literally firing 2 shots in quick succession at a target from a laser gun.

1. we know the earths rotation speed
2. the sentisitivity of the equipment
3. thus we know the distance to put the laser from the target
4. thus we know the speed at which to fire the successive shots.

its a single experiment. not two.

 

[PeroK]

Let me make the question a bit more unrealistically extreme so you can see what im getting at.
Assume light moves at the pace of a snail. And the target is attached to a running puppy. The light will miss the bulls eye everytime.

Exactly. It will miss every time. It's not like the first snail will hit the puppy and the subsequent snails will miss.

And why? because the a puppy is faster than a snail. Now back to reality, the laser moves much faster the speed of the rotation of the earth, but given that the puppy (the target) is still moving, albeit slower, there should be a tiny (but measurable with today's kind of equipment) deviation between the point where the photons should exactly hit, and where it actually hit.

Yes. But, once you have calibrated your laser and target so that it hits the bullseye, it will hit every time. Unless the rotation of the Earth changes. It's the same experiment every time.

 

[Ibix]

its a single experiment. not two

You fired two shots from an identical setup. Whether you call this two experiments or one is irrelevant - you did the same thing twice. Why would you expect different reslts?

 

[PeroK]

I didnt do the same experiment twice in a row. I did the experiment once.
The experiment is me literally firing 2 shots in quick succession at a target from a laser gun.

1. we know the earths rotation speed
2. the sentisitivity of the equipment
3. thus we know the distance to put the laser from the target
4. thus we know the speed at which to fire the successive shots.

its a single experiment. not two.

Whether you call that one experiment or two, it's two separate pulses of light. Unless something changes, both will hit the same spot. If you fire at a moving target (moving relative to you) you have to adjust your aim. The same is true of a light pulse. But, once you have adjusted your aim, you'll hit the same spot every time.

 

[jt128]

You fired two shots from an identical setup. Whether you call this tw experiments or one is irrelevant - you did the same thing twice. Why would you expect different reslts?

Because the target is moving.
If you fired two successive shots without moving the gun. But the target is moving, then it will not hit the same spot. Agree?

 

[jt128]

Whether you call that one experiment or two, it's two separate pulses of light. Unless something changes, both will hit the same spot. If you fire at a moving target (moving relative to you) you have to adjust your aim. The same is true of a light pulse. But, once you have adjusted your aim, you'll hit the same spot every time.

I am not adjusting my aim. My aim is in the same spot. But the target is moving. The target's movement is the "something that changes". Therefore, it should not hit the same spot.

 

[PeroK]

Because the target is moving.
If you fired two successive shots without moving the gun. But the target is moving, then it will not hit the same spot. Agree?

Yes, but in your experiment, both the laser and the target have the same relative motion each time.

 

[Ibix]

Because the target is moving.

And it has the exact same motion each time and starts with the exact same position with respect to the laser. So the mechanics of the shot are identical. If you hit once, you'll hit twice

 

[jt128]

Yes, but in your experiment, both the laser and the target have the same relative motion each time.

How does the laser and the target have the same relative motion? the laser (light) once it leaves the gun does not move from its trajectory. It is unaffected by any relative movement. If shot at target A, it will continue going towards target A's original position. The Real target A however, is moving.

Established by Einstein's Special Theory of Relativity.

Light (electromagnetic radiation) always travels at the same speed (and in this case direction) in vacuum (approximately 299,792,458 meters per second), regardless of:
1. The motion of the source
2. The motion of the observer

 

[PeroK]

How does the laser and the target have the same relative motion? the laser (light) once it leaves the gun does not move from its trajectory. It is unaffected by any relative movement. If shot at target A, it will continue going towards target A's original position.

If you fire a laser at target A and target A is moving laterally relative to you, then you'll miss every time. But, if you adjust your aim to take account of the relative motion (this means aiming slightly where the target is moving to), then you will hit the target every time.

The relative motion between the bottom of a tall building and the top of a tall building is constant. Yes, they move relative to each other, but that relative motion remains constant.

PS Imaging the laser is at the bottom of the building at the target at the top.

 

[Pencilvester]

How does the laser and the target have the same relative motion? the laser (light) once it leaves the gun does not move from its trajectory. It is unaffected by any relative movement. If shot at target A, it will continue going towards target A's original position. The Real target A however, is moving.

I think you’re confusing yourself by unnecessarily introducing a “target”. Try reformulating your scenario simply with a laser that leaves a scorch mark on a blank piece of paper that is at rest relative to the laser (and presumably the ground). You are thinking that if you fire the laser twice without changing the aim of the laser that it will make two distinct scorch marks? Why?

 

[Ibix]

Light (electromagnetic radiation) always travels at the same speed in vacuum (approximately 299,792,458 meters per second), regardless of:
1. The motion of the source
2. The motion of the observer

Speed is independent of source, not velocity. In a frame where the laser is moving laterally (for example) the beam also has lateral motion.

You keep worrying about the moving target and forgetting that the laser is also moving, so you gave the exact same setup for both shots.

 

[Nugatory]

Because the target is moving.

It makes no sense to say "moving" without saying what the motion is relative to. In your setup the target is not moving relative to the source.

 

[jt128]

And it has the exact same motion each time and starts with the exact same position with respect to the laser. So the mechanics of the shot are identical. If you hit once, you'll hit twice

No. You wont.

--[beam 2]--> [some distance] --[beam 1]--> [Moving Target]

Beam 1 will hit the target at X.
Beam 2 will hit the target at X + Y.

 

[jbriggs444]

(the target) is still moving, albeit slower, there should be a tiny (but measurable with today's kind of equipment) deviation between the point where the photons should exactly hit, and where it actually hit.

But there isnt one. I'm asking why?

Let me try to interpret the question in a way that makes it sensible.

We [think we] have a stationary laser and a stationary target. Our lab is sitting on the equator. The laser is pointing due south. We set up a target directly to the south of the laser. We fire the laser and it strikes the target dead center.

Naively, this makes sense. But what about the rotation of the earth? If the beam was travelling due south, and the target was moving, shouldn't the beam have to lead the target to ensure a dead center hit?

Yes, indeed, if we drop our pretense that our lab is stationary and adopt the idea that it was actually moving to the east at 1000 miles per hour or so then a light pulse fired by the moving laser will follow an eastward-slanted path, even though the laser is pointed due south.

If we look carefully at the internals of the laser, we can see that this is a result of the relativity of simultaneity. The leading edge of the laser will emit the pulse a fraction of a second later than the trailing edge. (Leading clocks lag). The result is a slanted wave front and a pulse that travels at an angle.

If one collimates the beam with a different mechanism, the aberration angle will be the same, though the mechanical explanation may be different.

 

[jt128]

It makes no sense to say "moving" without saying what the motion is relative to. In your setup the target is not moving relative to the source.

The source is on the ground. A ground mounter laser gun. The target (some distance away) is also on the ground. They are both affixed to the ground of a moving earth. Therefore the target is moving relative to the source.

The laser beam however, once fired, is not.

 

[jbriggs444]

The laser beam however, once fired, is not.

Yes, it is. If you modulate the beam into pulses, each pulse follows a diagonal path.

 

[Ibix]

The laser beam however, once fired, is not.

This is wrong, and it's the source of all your confusion. As I noted above, it's essentially a classical ether model, one which was falsified even before relativity was discovered.

 

[jt128]

Yes, it is.

established by Einstein's Special Theory of Relativity.

Light (electromagnetic radiation) always travels at the same speed/direction in vacuum (approximately 299,792,458 meters per second), regardless of:
1. The motion of the source
2. The motion of the observer

So the light is not moving relative to the source and target.

 

[PeroK]

How does the laser and the target have the same relative motion? the laser (light) once it leaves the gun does not move from its trajectory. It is unaffected by any relative movement. If shot at target A, it will continue going towards target A's original position. The Real target A however, is moving.

Established by Einstein's Special Theory of Relativity.

Light (electromagnetic radiation) always travels at the same speed (and in this case direction) in vacuum (approximately 299,792,458 meters per second), regardless of:
1. The motion of the source
2. The motion of the observer

Light has an invariant speed. So what? That doesn't mean that you can't hit something with a laser.

 

[DrGreg]

Maybe this animation from a 10-year old post will help.

Even though the ceiling of the train is moving relative to the track, the ball hits the same point on the ceiling every time.

 

[Nugatory]

Light (electromagnetic radiation) always travels at the same speed in vacuum (approximately 299,792,458 meters per second), regardless of:
1. The motion of the source
2. The motion of the observer

Same speed, yes. But not necessarily the same direction. The angles between the light beam and the path of the moving source/target will not be the same when using different frames in which the source and the target (at rest relative to one another) are moving.

For a clear example, google for "light clock", see how the path between source and target and back again is a zigzag using the frame in which the light clock is moving, straight back and forth using the frame in which the light clock is at rest.

(Edit: and look at the animation posted by @DrGreg, which landed while I was composing this post)

 

[jbriggs444]

established by Einstein's Special Theory of Relativity.

Light (electromagnetic radiation) always travels at the same speed/direction in vacuum (approximately 299,792,458 meters per second), regardless of:
1. The motion of the source
2. The motion of the observer

So the light is not moving relative to the source and target.

A pulse of light moves at c relative to the source.
It moves at c relative to the target.
It moves.

 

[Ibix]

So the light is not moving relative to the source and target.

No! Actually listen to what we are telling you! Your understanding of what Einstein meant is simply wrong.

That is why we cannot detect what you think we should - because your prediction is wrong.

 

[jt128]

Maybe this animation from a 10-year old post will help.

Even though the ceiling of the train is moving relative to the track, the ball hits the same point on the ceiling every time.

I'll no longer respond to responses like this. Sorry but its wasting time.
We both know a mass-ed ball, is different to mass-less light.

 

[jt128]

No! Actually listen to what we are telling you! Your understanding of what Einstein meant is simply wrong.

That is why we cannot detect what you think we should - because your prediction is wrong.

Ok. so just to clarify, youre saying that the mass-less light, move the same way a ball with mass moves, when thrown inside a moving train?

 

[Ibix]

I'll no longer respond to responses like this. Sorry but its wasting time.
We both know a mass-ed ball, is different to mass-less light.

Not in any way that's relevant to this discussion. I repeat - your mental model of what relativity says about light is wrong. You can't correct it until you accept that, at least provisionally.

 

[Sagittarius A-Star]

I'll no longer respond to responses like this. Sorry but its wasting time.
We both know a mass-ed ball, is different to mass-less light.

The animation is correct also for a light-pulse. Light has momentum.

 

[Borg]

If someone departs the starting gate from a roller coaster on two different occasions, they end up in the same end point each time because of the physical forces acting on the car as it travels on the coaster. The only difference here is that light is just affected by gravity. Would you expect a roller coaster car to end up someplace else for the reasons that you specified?

 

[Ibix]

Ok. so just to clarify, youre saying that the mass-less light, move the same way a ball with mass moves, when thrown inside a moving train?

Faster, and all frames will agree the speed, but basically yes.

 

[jbriggs444]

Faster, and all frames will agree the speed, but basically yes.

Importantly, although all frames will agree on the speed of a light pulse, not all frames will agree on the angle of the path traversed by that light pulse.

 

[PeroK]

Ok. so just to clarify, youre saying that the mass-less light, move the same way a ball with mass moves, when thrown inside a moving train?

Yes. Just a bit faster!

 

[Nugatory]

Ok. so just to clarify, youre saying that the mass-less light, move the same way a ball with mass moves, when thrown inside a moving train?

Maybe better to think of it as saying that the straight lines through space corresponding to the path of the light and the path of the ball transform in the same way when viewed using different frames? They're just straight lines.

You may want to find a light clock animation, as I suggested above. You will continue to confuse yourself until you understand and can build on that simplest case.

 

[jt128]

I dont know how to respond to this...
I thought that the direction of light is not influenced by the direction of its source.

keep in mind, I not talking about the following:
--> "When a light source is moving relative to an observer, the direction of the light appears altered due to the relative motion."

I am instead talking about the actual direction. Not observed direction. I believe it is not affected by the direction of the source. Are you saying I am wrong?

 

[PeroK]

Ok. so just to clarify, youre saying that the mass-less light, move the same way a ball with mass moves, when thrown inside a moving train?

This is the basis of Einstein's "light clock". See here, for example.

https://www.einstein-online.info/en/spotlight/light-clocks-time-dilation/

 

[Ibix]

I am instead talking about the actual direction.

What do you think is the "actual" direction? How would you measure this?

 

[jbriggs444]

I am instead talking about the actual direction. Not observed direction. I believe it is not affected by the direction of the source. Are you saying I am wrong?

You will have to define what you mean by "observed direction" and "actual direction". I can guess what you mean by "observed direction" but have no clue about what "actual direction" could mean.

Observed direction: The direction of the spatial path traversed by a light pulse, as judged from the observer's rest frame.

Actual direction: Something else.

 

[Dale]

My question is getting at something more fundamental. If the photons are truly "free" from Earth's reference frame once they leave the laser, and the target is moving with Earth, shouldn't we see a measurable difference between shots?

I am pretty unclear about what you are asking. What is different between the shots? Have you changed anything about the setup between the shots? If everything is the same between the shots then what would lead you to expect that anything would be different?

I thought that the direction of light is not influenced by the direction of its source.

The speed of light is not influenced by the source. The direction certainly can be. How else could you point with a laser pointer?

Edit:

I am not adjusting my aim. My aim is in the same spot. But the target is moving. The target's movement is the "something that changes". Therefore, it should not hit the same spot.

I just saw this. Motion is relative, so if you are moving the target relative to the laser then it will hit in different locations.

It is hard to tell since you do not say what the target is moving relative to. Since motion is relative, just saying "the target is moving" is an incomplete statement. You need to say what it is moving relative to.

 

[Nugatory]

I am instead talking about the actual direction. Not observed direction. I believe it is not affected by the direction of the source. Are you saying I am wrong?

There is no such thing as an "actual direction", for the same reason that there is no such thing as an "actual velocity" - they are both always relative to something else, or more precisely "frame-dependent".

 

[Sagittarius A-Star]

I am instead talking about the actual direction. Not observed direction. I believe it is not affected by the direction of the source.

The direction is frame-dependent. This is called aberration.

 

[PeroK]

The direction is frame-dependent. This is called aberration.

... which was one of the clues that led to the special theory of relativity in the first place:

https://en.wikipedia.org/wiki/Relativistic_aberration