Why does laser beam hit the same target when fired?

[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.