Why does laser beam hit the same target when fired?

[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

 

[Nugatory]

@jay t you are posting so quickly that it is clear that you are not taking the time to read and think about the responses you've been getting. That somewhat defeats the purpose of hanging out in one of the very few places on the internet that allows anyone to interact with real professional physicists.

So slow down... take a few hours to read and think about what's already posted. Follow and study the links on light clocks and aberration and other suggestions you've been given and haven't looked at yet.

 

[PeterDonis]

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

It should be obvious that this is false. If it were true, laser pointers wouldn't work.

 

[Ibix]

If it were true, laser pointers wouldn't work.

Good point!

(Sorry.)

 

[jt128]

@jay t you are posting so quickly that it is clear that you are not taking the time to read and think about the responses you've been getting. That somewhat defeats the purpose of hanging out in one of the very few places on the internet that allows anyone to interact with real professional physicists.

So slow down... take a few hours to read and think about what's already posted. Follow and study the links on light clocks and aberration and other suggestions you've been given and haven't looked at yet.

Responses are coming in too quicky. And most are me explaining.

ok...

i'll simplify the question. If this doesnt work. Then i give up. Tell me on which step the logic breaks down.

Analysis below is based on the image. Assume that the distance between the gun and the laser is far enough so that it can be measured.

Step 1
In the image the truck is moving at 5mph. Lasers are fired in quick succession. In this case, both lasers hit the surface of the blue moving target. The target is moving because the truck is moving from left to right at 5mph. I Am not talking about relative frame of reference, i am talking about the actual spot on the blue surface the laser hits. Question for step 1: Will the lasers hit the exact same spot on the blue surface? I say no, but that the different between shot1's actual hit, and shot2's actual hit will be tiny. What do you say? And why?

Step 2
Assume the truck speeds up to 1000 miles per hour.
Question for step 2: Will the lasers hit the exact same spot on the blue surface? I also say no, but that the different will be more noticeable. What do you say? and why?

We can tell the actual spot on the blue surface the laser hits by physically looking at the burn marks.
Also, i say, that the faster the truck moves towards the speed of light, the more noticeable the distance of the burn marks will be.

 

[Sagittarius A-Star]

Responses are coming in too quicky. And most are me explaining.
View attachment 355790
ok...

i'll simplify the question. If this doesnt work. Then i give up. Tell me on which step the logic breaks down.

Analysis below is based on the image. Assume that the distance between the gun and the laser is far enough so that it can be measured.

View attachment 355791

Step 1
In the image the truck is moving at 5mph. Lasers are fired in quick succession. In this case, both lasers hit the surface of the blue moving target. The target is moving because the truck is moving from left to right at 5mph. I Am not talking about relative frame of reference, i am talking about the actual spot on the blue surface the laser hits. Question for step 1: Will the lasers hit the exact same spot on the blue surface? I say no, but that the different between shot1's actual hit, and shot2's actual hit will be tiny. What do you say? And why?

Step 2
Assume the truck speeds up to 1000 miles per hour.
Question for step 2: Will the lasers hit the exact same spot on the blue surface? I also say no, but that the different will be more noticeable. What do you say? and why?

Always the same spot will be hit, regardless the (constant) speed of the truck. Else this experiment could be used to measure an absolute speed of the truck, which is not possible according to the principle of relativity.

 

[Ibix]

And most are me explaining.

Unfortunately, you are explaining you erroneous understanding instead of listening to people who know what they're talking about.

Will the lasers hit the exact same spot on the blue surface?

Of course they will, because the gun and the target are attached to the truck so they're in the exact same relationship both times the trigger is pulled. You've done the exact same thing twice, so they'll hit in the exact same place. You can easily write down the equations of motion using the Lorentz transforms and demonstrate that this is the case.

You continue to explain as if you believe there's some absolute frame of reference in which light moves in a special way. That has been known to be a wrong model for over a century.

 

[jbriggs444]

Responses are coming in too quicky. And most are me explaining.
View attachment 355790
ok...

i'll simplify the question. If this doesnt work. Then i give up. Tell me on which step the logic breaks down.

This matches the interpretation of the problem that I posted back in #24

We have a laser pointed straight up. We have a target directly above the laser. Both are mounted in a cart that is moving.

Two shots are fired in succession. If we pretend that the cart is at rest, we expect that both shots will strike the target dead center.

If we drop the pretense that the cart is at rest, your expectation appears to be that each shot will traverse an "actually vertical" (vertical in the ground rest frame) spatial path which misses the center of the target. You should naturally expect that both shots will miss the target by the same margin.

Your expectation is incorrect due to relativistic aberration as pointed out by others. Both pulses will emerge from the laser at an angle. In the case of a laser gun, this is the result of the relativity of simultaneity applied across the laser wave front. The leading edge of the pulse (the edge in the direction of the cart's travel) will be emitted a fraction of a second after the trailing edge. The result is a slanted wave front and a slanted direction of travel for each laser pulse.

In the case of collimation by use of a black painted vertical rifle barrel the same aberration angle is found because the only laser pulses that can successfully traverse the rifle barrel are ones at that exact same angle -- the angle that leads the target by exactly enough for a dead center hit.

 

[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.

Unfortunately, you are explaining you erroneous understanding instead of listening to people who know what they're talking about.

Of course they will, because the gun and the target are attached to the truck so they're in the exact same relationship both times the trigger is pulled. You've done the exact same thing twice, so they'll hit in the exact same place. You can easily write down the equations of motion using the Lorentz transforms and demonstrate that this is the case.

You continue to explain as if you believe there's some absolute frame of reference in which light moves in a special way. That has been known to be a wrong model for over a century.

is this the point of this forum? that i explain my erroneous understanding, and then try to understand what you guys are saying?

but regarding how you responded, i fired the laser in quick succession so that both lasers are in flight before both hit the blue target (as represented in the image). So in the image, you can see shot2 is directly behind shot1. And both have not yet reached the target yet. How can they both hit the same spot when there is clearly some distance between shot1 and shot2? Also, In multiple iterations of the experiment as the truck moves closer and closer to the speed of light, are you still saying it will hit the same spot?

looking at the image logically on the drawing, it doesnt work in my head. Because you clearly see distance between the 2 shots.

 

[Ibix]

I dont know how to respond to this...

Read the references that people have posted!

 

[jt128]

This matches the interpretation of the problem that I posted back in #24

We have a laser pointed straight up. We have a target directly above the laser. Both are mounted in a cart that is moving.

Two shots are fired in succession. If we pretend that the cart is at rest, we expect that both shots will strike the target dead center.

If we drop the pretense that the cart is at rest, your expectation appears to be that each shot will traverse an "actually vertical" (vertical in the ground rest frame) spatial path which misses the center of the target. You should naturally expect that both shots will miss the target by the same margin.

Your expectation is incorrect due to relativistic aberration as pointed out by others. Both pulses will emerge from the laser at an angle. In the case of a laser gun, this is the result of the relativity of simultaneity applied across the laser wave front. The leading edge of the pulse (the edge in the direction of the cart's travel) will be emitted a fraction of a second after the trailing edge. The result is a slanted wave front and a slanted direction of travel for each laser pulse.

In the case of collimation by use of a black painted vertical rifle barrel the same aberration angle is found because the only laser pulses that can successfully traverse the rifle barrel are ones at that exact same angle -- the angle that leads the target by exactly enough for a dead center hit.

relativistic aberration is about the role of the observer relative to the light source, the direction from which they receive light will appear shifted. Thats what i am not talking about that. I am talking about the actual burn mark on the target. Am i talking nonsense?