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

[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?

 

[jt128]

Read the references that people have posted!

ok.. maybe i just dont understand. I am reading it and responding. But i do not understand what you are saying. You guys are saying terms like "relativistic aberration" and frames of reference, when i am not talking about that. I am instead talking about the literal actual recorded burn mark on the blue target. We are not talking about the same thing.

Thats why i posted an updated question here: https://www.physicsforums.com/threa...e-same-target-when-fired.1068091/post-7144622

 

[Ibix]

You guys are saying terms like "relativistic aberration" and frames of reference, when i am not talking about that

Indeed - that's why you go wrong.

I am talking about the actual burn mark on the target.

Yes, so are we. You just aren't understanding our explanations because you aren't taking time to think.

Look up the light clock. It is literally this experiment, but analysed correctly.

Am i talking nonsense?

Yes, I'm afraid so. Go and look up the light clock.

 

[Sagittarius A-Star]

But i am talking about the literal actual recorded burn mark on the blue target.

The truck driver can regard his truck to be at rest. The burn mark on the blue target does not care, how fast the street moves with constant speed in backward direction.

 

[jbriggs444]

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?

The short answer is that yes, you are talking nonsense.

Relativistic aberration means that there can be a disagreement about the angle of the path traversed by a light pulse. An observer in the rest frame of a vertically pointed source will see a vertical path. An observer in a frame relative to which the source is moving will see a diagonal path for the same pulse.

Neither observer is incorrect. Neither interpretation is an illusion. Both interpretations turn out to be coordinate relative statements. Both interpretations are consistent with the invariant fact that the center of the target is hit by both shots.

 

[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.

The short answer is that yes, you are talking nonsense.

Relativistic aberration means that there can be a disagreement about the angle of the path traversed by a light pulse. An observer in the rest frame of a vertically pointed source will see a vertical path. An observer in a frame relative to which the source is moving will see a diagonal path for the same pulse.

see what i mean?

1. there can be a disagreement about the angle of the path traversed by a light pulse
2. An observer in the rest frame of a vertically pointed source will see a vertical path.
3. An observer in a frame relative to which the source is moving will see a diagonal path for the same pulse.

this has nothing to do with what i am asking.
I am not talking about the traversed path.
I am not even talking about the observer

I just checked out the light clock experiment. It also does not explain what i am talking about.
that experiment basically shows that when something is moving very fast, time actually slows down for it compared to things that aren't moving. and it demonstrated this by bouncing light between mirrors, which showed that light has to travel a longer path when the clock is moving and it forced time to slow down since light always travels at the same speed.

This has nothing to do with the question i asked. If i am talking nonsense, then can you explain to me from the image above how the 2 light pulses in flight (some distance APART) on the way to the blue target can hit the same position, if the target is moving to the right also at near the speed of light?

 

[Ibix]

it demonstrated this by bouncing light between mirrors,

Moving mirrors, which it hits every time in the same place.

 

[Ibix]

The truck driver can regard his truck to be at rest. The burn mark on the blue target does not care, how fast the street moves with constant speed in backward direction.

@jay t - the above is the simplest statement of why you must be wrong. If the truck is at rest, the lasers will hit in the same place. But by the principle of relativity, the truck can always treat itself as at rest whatever anyone else says its velocity is. So the lasers will always hit in the same place

 

[jt128]

Moving mirrors, which it hits every time in the same place.

man... smh. i give up.
the mirror experiment was proving some else. It has ONE light beam bouncing between 2 mirrors to prove time dilation.

I am proving something else, with 2 beams, with distance between them checking to see if they hit the same spot. The very fact that there is DISTANCE between them means they wont hit the same spot. Is distance irrelevant now?

Thats like two runners running on the same track to a finish line, and you trying to convince me that both runners reach the same time even though both runners are running a constant speed and they are some distance apart one behind each other

 

[Ibix]

the mirror experiment was proving some else. It has ONE light beam bouncing between 2 mirrors to prove time dilation.

What do you imagine is the difference between one beam bouncing twice and two beams a short time apart?

 

[jbriggs444]

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

You thought wrong.

The speed of a light pulse is not influenced by the motion of its source.
The direction of a light pulse can be. That's relativistic aberration in action.

1. there can be a disagreement about the angle of the path traversed by a light pulse

Yes indeed.

2. An observer in the rest frame of a vertically pointed source will see a vertical path.

Yes indeed.

3. An observer in a frame relative to which the source is moving will see a diagonal path for the same pulse.

Yes indeed.

this has nothing to do with what i am asking.

I am not talking about the traversed path.
I am not even talking about the observer

You are talking, I suppose, about the position of the burnt spot on the target then?

We can explain that position in two ways.

1. Adopt the frame of reference of the cart. The shots each traverse a vertical path and land dead center on the target.

2. Adopt the frame of reference of the ground. The shots each traverse a diagonal path and land dead center on the target.

Either way, the invariant fact of the matter is that the target is hit dead center.

This has nothing to do with the question i asked. If i am talking nonsense, then can you explain to me from the image above how the 2 light pulses in flight (some distance APART) on the way to the blue target can hit the same position, if the target is moving to the right also at near the speed of light?

Let us take a careful look at the image.

You show a snapshot of a gun at the bottom, two shots directly above it and a target lined up above all three.

If we watch the scenario evolve from a vantage point mounted to the cart, we see the two shots proceed vertically upward to strike the target dead center.

If we watch the scenario evolve from a vantage point mounted on the ground, we see the two shots proceed diagonally upward to strike the target dead center.

Either way, the target is struck dead center.

The scorch marks on the center of the target are invariant facts of the matter. This means that they are the same regardless of the reference frame that one chooses (if any).

The path taken by the pulses are coordinate dependent descriptions of the scenario. Different frames of reference will describe the same trajectories as following different spatial paths.

 

[sdkfz]

...
This has nothing to do with the question i asked. If i am talking nonsense, then can you explain to me from the image above how the 2 light pulses in flight (some distance APART) on the way to the blue target can hit the same position, if the target is moving to the right also at near the speed of light?

I think it would help you to consider a "long" time between the light pulses. Say the second is an hour after the first (or maybe just keep shooting pules for an hour).

Where would the last pulse hit? The back wall of the truck? Would that pulse "aim" at something where the truck was an hour ago?

You've mentioned twice that the target is fixed to the truck, but so is the laser. An hour after the first shot, that laser is still (like the target) fixed and aimed where it was.

 

[Sagittarius A-Star]

can you explain to me from the image above how the 2 light pulses in flight (some distance APART) on the way to the blue target can hit the same position, if the target is moving to the right also at near the speed of light?

The 2 light pulses are not moving exactly vertically. Both have also a horizontal velocity component of $v$, like the gun and the target.

 

[jt128]

What do you imagine is the difference between one beam bouncing twice and two beams a short time apart?

Ok. now i believe you just dont understand my question.

Can this happen?

--------------FinishLine x
--R2----R1-----FinishLine x + 1
--------------FinishLinex + 2
--------------FinishLinex + 2

Runner 1 is in front of runner 2. Both are running at constant speed.
Can both reach the finish line at the same time? The answer is no right?
And if the finishline is scrolling from top to bottom, then R1, FinishLine X + 1 while runner 2 will reach FinishLine x + 2 right?

 

[PeroK]

I think it would help you to consider a "long" time between the light pulses. Say the second is an hour after the first (or maybe just keep shooting pules for an hour).

Where would the last pulse hit? The back wall of the truck? Would that pulse "aim" at something where the truck was an hour ago?

You've mentioned twice that the target is fixed to the truck, but so is the laser. An hour after the first shot, that laser is still (like the target) fixed and aimed where it was.

That's a great point. In my tall building scenario if you waited 12 hours, then the laser and target would have moved half of the Earth's circumference. So, the last light pulse would have to come out of the wrong end of the laser and tunnel thru the Earth to get to where the target was half a day ago!

 

[PeroK]

Ok. now i believe you just dont understand my

We understand everything!

 

[jt128]

We understand everything!

Than can you answer
1. will both runners reach the finish line at the same time?
2. with both runners reach the same spot of the finish line?

 

[phinds]

@jay t , the relativity of motion has been well understood for 800 years (Google Galilean relativity) and your refusal to understand it is now looking like trolling.

I suggest that you refrain from posting any further in this thread until you have taken some time and gone back over all of the responses you have gotten and really try to understand them. In other words, remember the first law of holes ... when you find yourself in a hold STOP DIGGING.

 

[Ibix]

Ok. now i believe you just dont understand my question.

I understand your question perfectly. You're firing two consecutive shots from a laser and asking if the second hits the burn mark the first one made.

Everybody except you says they will. Everybody except you has explained in various different ways why this must be so, and is pointing out the contradictions in your position. You are relying on some supposed "actual direction" that you can't or won't define.

I don't understand your runners analogy, it's true. I can't work out which way what is moving.

 

[jt128]

I understand your question perfectly. You're firing two consecutive shots from a laser and asking if the second hits the burn mark the first one made.

Everybody except you says they will. Everybody except you has explained in various different ways why this must be so, and is pointing out the contradictions in your position. You are relying on some supposed "actual direction" that you can't or won't define.

I don't understand your runners analogy, it's true. I can't work out which way what is moving.

My runners analogy is the same as the laser analogy.
Except
1. i replaced a gun with a starting line.
2. I replaced a target with a finish line
3. I replaced shot1 and shot2 with runners running at constant speed.

So my simple question to you is this: will both runners reach the finish line at the same time?

 

[jt128]

@weirdoguy i can ask it to you as well. Will both runners reach the finish line at the same time?

 

[jbriggs444]

Ok. now i believe you just dont understand my question.

Can this happen?

--------------FinishLine x
R2 R1-----FinishLine x + 1
--------------FinishLinex + 2
--------------FinishLinex + 2

This is a really bad drawing.

Runner 1 is in front of runner 2. Both are running at constant speed.

You seem to be describing a problem using plain old non-relativistic physics. There is nothing wrong with that.

You have two runners, one in front of the other on a flat surface. There is a finish line in front of them. Painted on the finish line are numbers: 1, 2, 3, 4, etc.

You ask whether both runners can reach the finish line at the same time.

Of course not. Runner 1 will cross first. Runner 2 will cross later.

If the finish line is not scrolling, both runners will cross at the number 1, perhaps. But you have stipulated that the finish line is scrolling horizontally from right to left (across the runner's fields of view).

If the runners are running straight at the finish line, then runner 1 will cross at the number 1 and runner 2 will cross at the number 2, perhaps.

If the runners are each running along a diagonal path but are lined up on a path that leads straight toward the finish line then they can both cross at the number 1.

The line on which the runners are lined up need not match the direction in which they are each running.

 

[PeroK]

Than can you answer
1. will both runners reach the finish line at the same time?
2. with both runners reach the same spot of the finish line?

A line's a line and x is a point. X can be a point on the finishing line, but x is not a line.

Speed is speed and velocity is velocity. Speed has no direction.

These things I understand. I also understand that after 75+ posts you've burned your mental bridges and there's no way back. You've travelled into the dark, and there is no way back to the light.

I'll end my participation in this increasingly pointless thread at that point.

 

[jt128]

This is a really bad drawing.


You seem to be describing a problem using plain old non-relativistic physics. There is nothing wrong with that.

You have two runners, one in front of the other, crossing across a flat surface. There is a finish line in front of them. Painted on the finish line are numbers: 1, 2, 3, 4, etc.

You ask whether both runners can reach the finish line at the same time.

Of course not. Runner 1 will cross first. Runner 2 will cross later.

If the finish line is not scrolling, both runners will cross at the number 1, perhaps. But you have stipulated that the finish line is scrolling horizontally from right to left (across the runner's fields of view).

If the runners are running straight at the finish line, then runner 1 will cross at the number 1 and runner 2 will cross at the number 2, perhaps.

If the runners are each running along a diagonal path but are lined up on a path that leads straight toward the finish line then they can both cross at the number 1.

The line on which the runners are lined up need not match the direction in which they are each running.

thanks. for at least seeing what i am trying to say.
we agree they wont cross the same time.
and we also agree that if the finishing line is scrolling, they wont reach the same point. omg thanks!!!

so all i am doing is replacing runners with light beams, the start point with a gun, and the end point with a target. And this is why my brain hurts when you tell me it cant work the same.

 

[Nugatory]

In the image the truck is moving at 5mph...

Relative to what? The problem statement is incomplete until you've specified this.
But as long as the truck is at rest relative to the laser and we aren't changing the direction the laser points, repeated laser shots will hit the same spot. If the truck is moving relative to the laser then repeated laser shots will hit different spots (unless we re-aim the laser to compensate for the truck being in a different position relative to the laser between shots).

We are now 85 posts into this thread and you are still making the mistake of not considering what the speeds are relative to, first pointed out in #3.

 

[jbriggs444]

thanks. for at least seeing what i am trying to say.
we agree they wont cross the same time.
and we also agree that if the finishing line is scrolling, they wont reach the same point. omg thanks!!!

No. I did not agree with that.

 

[jt128]

No. I did not agree with that.

...which point do you agree with.
1. runners will not reach the same time
2. if finishline is scrolling, runners will not reach the same point.

 

[Ibix]

will both runners reach the finish line at the same time?

No.

My money says you're going to go "ha ha! So they can't cross in the same place then!". That, of course, depends on where they crossed the start line and whether they move diagonally to keep pace with the scrolling. Note that if they crossed the same painted spot on the start line and it is scrolling at the same speed as the finish line, they didn't start at the same place either. And that

 

[jbriggs444]

...which point do you agree with.
1. runners will not reach the same time
2. if finishline is scrolling, runners will not reach the same point.

It depends on the direction the runners are running. Or the direction the light pulses are travelling. Both of which, in turn, depend on the frame of reference that we adopt.

I am about ready to follow @PeroK out the door.

 

[PeterDonis]

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

And you have already been told, multiple times, that you thought wrong. If you want a simple demonstration of why this must be wrong, read my post #52.

 

[jt128]

It depends on the direction the runners are running. Or the direction the light pulses are travelling. Both of which, in turn, depends on the frame of reference that we adopt.

can we stop this.. based on all what ive been explaining from since beginning to end with images included, it clear that i am talking about a straight line.

why can you agree the obvious? Is it that hard? If two people are running to a finish line at constant speed, then of course they wont reach the same time. And if the finishline is scrolling, then of course they wont reach the same point.

Why are you being so difficult with an obvious question? Is not like you will lose anything man. omg.

 

[PeterDonis]

can we stop this

Sure. I've just done so, by closing this thread.

why can you agree the obvious?

We all agree on what is obvious to us, who understand special relativity: that the light pulses in your original question will both hit the same target at the same place, since the light source and the target are at rest relative to each other.

You are the one who continues to be unable to agree to this obvious fact.

If two people are running to a finish line at constant speed, then of course they wont reach the same time.

Yes, we all agree on that.

And if the finishline is scrolling, then of course they wont reach the same point if the runners choose the direction of their run relative to the ground, rather than relative to the scrolling finish line.

Note the bolded addition to your statement in the quote above. It is crucial. Your logic has that bolded addition as an unstated assumption. And that unstated assumption, while you can declare that it is true for your runner example, is not true for your original question about the light source and the target. As your original question was stated, both the light source and the target are "scrolling", and the light's motion, viewed in the "ground" frame (the frame which, in your runner example, is not "scrolling"), will "scroll" right along with the source and the target. That is what the principle of relativity says, and countless experiments have shown that the principle of relativity is correct on this point.

So if your "logic" tells you anything else, your "logic" is wrong. At this point everyone else in this thread has spent more than enough effort in trying to explain why your "logic" is wrong. Enough is enough. At this point you're on your own. This thread is closed.

Why are you being so difficult with an obvious question? Is not like you will lose anything man. omg.

This kind of attitude is not going to help you at all. When you are the one who is wrong, accusing others who are right, and who have been very patient in trying to explain to you why they are right and you are wrong, of being "difficult" is only going to make things worse.

If you ask, how do I know we're right and you're wrong, see what I said above about what countless experiments have shown. (And in my post #52, as I've already noted, I gave you a simple everyday phenomenon that demonstrates that a key element of what you think you understand is simply wrong.)

Thread closed.