Target Distance and Light Travel: The Impact on Spatial Position

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In summary, the motion of the target being shot at with light in the direction of Earth's orbit does not depend on the Earth being geocentric or on the speed of the observer. The target will remain at rest in its own frame of reference, regardless of the Earth's rotation and orbit.
  • #1
cfrogue
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If there is a target at a distance d and I shoot light at it, does it remain at the same point in space as light proceeds toward it?

For example, if the target in in line with the revolution of the Earth around the sun, will the target move away at 18.55 miles per second as the light proceeds toward it?
 
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  • #2
cfrogue said:
If there is a target at a distance d and I shoot light at it, does it remain at the same point in space as light proceeds toward it?
In general, no. The only case where it would remain at the same distance is if it were at rest in the frame in which the distance was measured.
 
  • #3
r
DaleSpam said:
In general, no. The only case where it would remain at the same distance is if it were at rest in the frame in which the distance was measured.

Naturally, I was assuming we were on the earth.

So, light takes off at c and the Earth moves also and so does the target.

When light is shot at the target, and light proceeds in free space, will the target remain at rest even though the Earth is in orbit around the sun? In other words, will the target move away with the motion of the Earth as the light speeds toward it?
 
  • #4
cfrogue said:
r

Naturally, I was assuming we were on the earth.

So, light takes off at c and the Earth moves also and so does the target.

When light is shot at the target, and light proceeds in free space, will the target remain at rest even though the Earth is in orbit around the sun? In other words, will the target move away with the motion of the Earth as the light speeds toward it?
Are you asking if the target will accelerate or not depending on whether or not we shoot a light at it? Before the light even reaches it?

Can you rephrase? I'm sure that's not what you mean to ask.
 
  • #5
Al68 said:
Are you asking if the target will accelerate or not depending on whether or not we shoot a light at it? Before the light even reaches it?

Can you rephrase? I'm sure that's not what you mean to ask.

The target is on the earth.
 
  • #6
I still don't understand the question. If the target is stationary relative to Earth's surface, then its motion is the same as Earth's surface relative to the sun, ie both orbital and rotational motion. But that has nothing to do with shooting light at it.
 
  • #7
Al68 said:
I still don't understand the question. If the target is stationary relative to Earth's surface, then its motion is the same as Earth's surface relative to the sun, ie both orbital and rotational motion. But that has nothing to do with shooting light at it.

Light is shot at a target.

I assume light shoots.

Now, when light shoots at a target in the direction of the Earth's orbit, will the target move?

If you say no, then the Earth is geocentric.
 
  • #8
cfrogue said:
Light is shot at a target.

I assume light shoots.

Now, when light shoots at a target in the direction of the Earth's orbit, will the target move?

If you say no, then the Earth is geocentric.
Of course the target will continue its existing motion if no force acts on it, but that has nothing to do with light being shot at it.

I still must not understand the question.
 
  • #9
cfrogue said:
r

Naturally, I was assuming we were on the earth.

So, light takes off at c and the Earth moves also and so does the target.

When light is shot at the target, and light proceeds in free space, will the target remain at rest even though the Earth is in orbit around the sun? In other words, will the target move away with the motion of the Earth as the light speeds toward it?
The source will move with its own motion, and the target will move with its own motion. Neither are constrained to any specific value and neither influence the speed of the light pulse.
 
  • #10
cfrogue said:
If you say no, then the Earth is geocentric.
Obviously the Earth is geocentric. That's the definition of geocentric.
 
  • #11
DaleSpam said:
cfrogue said:
If you say no, then the Earth is geocentric.
Obviously the Earth is geocentric. That's the definition of geocentric.
You mean it doesn't depend on whether I say no or not? :eek:
 
  • #12
Al68 said:
You mean it doesn't depend on whether I say no or not? :eek:
Correct. The answer to scenario has nothing to do with whether or not the Earth is geocentric nor does what you say the answer to the scenario is.

The Earth is geocentric by definition. I would think that is blatantly obvious.
 
  • #13
DaleSpam said:
Correct. The answer to scenario has nothing to do with whether or not the Earth is geocentric nor does what you say the answer to the scenario is.

The Earth is geocentric by definition. I would think that is blatantly obvious.
LOL. As far as I can tell the answer to the scenario (what is the motion of the target?), has nothing to do with any light being shot at it as well.

What is the point of the question? Is the target trying to dodge the light? :confused:
 
  • #14
Perhaps the OP is backwardsly asking if the speed of light is dependent on the speed of the observer?

In the scenario given, the target and shooter are stationary with respect to each other. All that other stuff about the Earth's rotation and orbit are irrelevant.
 
  • #15
Al68 said:
As far as I can tell the answer to the scenario (what is the motion of the target?), has nothing to do with any light being shot at it as well.
russ_watters said:
All that other stuff about the Earth's rotation and orbit are irrelevant.
I agree. The second postulate is that light travels at c in vacuum in any inertial frame regardless of any other factors.
 
  • #16
cfrogue said:
Light is shot at a target.

I assume light shoots.

Now, when light shoots at a target in the direction of the Earth's orbit, will the target move?

According to what frame?

In the frame of the Earth, no. The light takes a time of d/c to reach the target.

In the frame of the Sun, then yes. The light proceeds towards the target at c, but the target moves away at the speed of 18.55 miles/sec. Thus the time it takes the light to reach the target is d/(c-18.55mps)
 
  • #17
russ_watters said:
Perhaps the OP is backwardsly asking if the speed of light is dependent on the speed of the observer?

In the scenario given, the target and shooter are stationary with respect to each other. All that other stuff about the Earth's rotation and orbit are irrelevant.

I think you mean the speed of light does not depend on the speed of the light source.

This has been verified by experiments that the speed of light cannot be altered by the motion of the light source.

Now, that being said, from the light source, light emits at c period regardless of the motion of the light source.

Is this correct?
 
  • #18
cfrogue said:
I think you mean the speed of light does not depend on the speed of the light source.

This has been verified by experiments that the speed of light cannot be altered by the motion of the light source.

Now, that being said, from the light source, light emits at c period regardless of the motion of the light source.

Is this correct?
"Motion" is a little ambiguous. It is acceleration that can cause problems. The speed of light is constant in all inertial (non-accelerating) frames - ie, when the source and observer are not accelerating with respect to each other.

Also, I don't like the way "speed of the light source" is worded. Speeds are measured between two points. So hopefully, when you say "speed of the light source", you mean speed of the light source with respect to the target.

So for a concise answer to the OP:
For example, if the target in in line with the revolution of the Earth around the sun, will the target move away at 18.55 miles per second as the light proceeds toward it?
It appears you are talking about a source and target both fixed to the earth. That means the speed of the target with respect to the source is zero, not 18.55 mi/s. That the speed of the target relative to some arbitrary point hanging in outer space is 18.55 mi/s is completely irrelevant.
 
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  • #19
russ_watters said:
"Motion" is a little ambiguous. It is acceleration that can cause problems. The speed of light is constant in all inertial (non-accelerating) frames - ie, when the source and observer are not accelerating with respect to each other.

Also, I don't like the way "speed of the light source" is worded. Speeds are measured between two points. So hopefully, when you say "speed of the light source", you mean speed of the light source with respect to the target.

So for a concise answer to the OP: It appears you are talking about a source and target both fixed to the earth. That means the speed of the target with respect to the source is zero, not 18.55 mi/s. That the speed of the target relative to some arbitrary point hanging in outer space is 18.55 mi/s is completely irrelevant.

Well, let's see if we can clear the light source thing up.

If a rocket rode by the Earth and when "parallel", the Earth and the rocket happen to shoot light in the same direction and parallel, would the light beams be located at the same x-axis distance in space at any time t in whichever frame looked at them?
 
  • #20
russ_watters said:
"Motion" is a little ambiguous. It is acceleration that can cause problems. The speed of light is constant in all inertial (non-accelerating) frames - ie, when the source and observer are not accelerating with respect to each other.

Also, I don't like the way "speed of the light source" is worded. Speeds are measured between two points. So hopefully, when you say "speed of the light source", you mean speed of the light source with respect to the target.

So for a concise answer to the OP: It appears you are talking about a source and target both fixed to the earth. That means the speed of the target with respect to the source is zero, not 18.55 mi/s. That the speed of the target relative to some arbitrary point hanging in outer space is 18.55 mi/s is completely irrelevant.

Acceleration is a problem.

The metrics in an accelerated frame after acceleration is complete are expanded to
x/sqrt( 1 - (v/c)^2) compared to the originating frame.
 
  • #21
Janus said:
According to what frame?

In the frame of the Earth, no. The light takes a time of d/c to reach the target.

In the frame of the Sun, then yes. The light proceeds towards the target at c, but the target moves away at the speed of 18.55 miles/sec. Thus the time it takes the light to reach the target is d/(c-18.55mps)

Is the 18.55 number a pheudo absolute motion number or simply the speed around the sun.

I cannot find anything to validate this.

Of course, the milky way is doing something also.
 
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  • #22
DaleSpam said:
Obviously the Earth is geocentric. That's the definition of geocentric.


Yes, I know.

But the geocentric model of the universe is similar to SR, no?

We are encouraged to consider the frame as the center of the inertial frame universe?
 
  • #23
DaleSpam said:
I agree. The second postulate is that light travels at c in vacuum in any inertial frame regardless of any other factors.

That is what it says, regardless of the motion of the light source.

Thus, for all possible light source motions relative to a frame, light will move at the same speed c in a vacuum of course.
 
  • #24
I don't fully understand the OP's question, but I do know that if you shoot light across a playground roundabout to say some person sitting directly opposite to you, it will miss its target because of the coriolis effect. And since Earth rotates 1/day, you'll definitely miss on any experiment carried out on the surface of Earth.

So you can't just assume that because your target and source are stationary with respect to each other that your 'shoot light' will definitely hit the target.
 
  • #25
cfrogue said:
Thus, for all possible light source motions relative to a frame, light will move at the same speed c in a vacuum of course.
Exactly (relative to an inertial frame).

So your initial concern or question is resolved?
 
  • #26
Forgive me, but I've been reading with interest, and am having difficulty getting it to reconcile.

With a similar example, say we have the sun, Earth and a rocket ship armed with a laser.
The Earth is moving away from the sun at 18.55mps (assume this is in a straight line and ignore Earth's rotation)
The rocket ship is approaching the same distance from Earth as the sun, but is not moving relative to Earth (so will be traveling at 18.55mps). At the instant that the rocket ship and sun line up, the ship fires its laser to Earth and the sun emits its photons.

Now, If I am on Earth awaiting the light from both of these, it would seem that the rockets laser should get to me first as they only have the initial distance to travel.(it hasn't changed as we are not moving relative to each other.) But the light from the Sun had to travel a bit further as Earth is moving away.

Even if I add an extra observer out in space watching the whole thing (say he's still relative to the sun and watches the spaceship and Earth moving past). The instant the photons and laser are fired wouldn't he see the ships laser with more speed to the photons to account for the laser hitting first?

I know i must be wrong with these examples, I am currently thinking it has to do with the simultaneity of when the laser/photons are fired, but can't figure out in my head why and how it works.
 
  • #27
jacksnap said:
I know i must be wrong with these examples, I am currently thinking it has to do with the simultaneity of when the laser/photons are fired, but can't figure out in my head why and how it works.
You are correct, it is almost always due to the relativity of simultaneity. All reference frames agree that the rocket laser arrives first. In some frames it arrives first because it travels a shorter distance, in other frames it arrives first because it was fired first due to the relativity of simultaneity.
 
  • #28
cfrogue said:
Well, let's see if we can clear the light source thing up.

If a rocket rode by the Earth and when "parallel", the Earth and the rocket happen to shoot light in the same direction and parallel, would the light beams be located at the same x-axis distance in space at any time t in whichever frame looked at them?
Yes, if they both struck the same target, each frame would agree that both light beams arrived at the target at the same time, but they would disagree about what specific time that is.
DaleSpam said:
You are correct, it is almost always due to the relativity of simultaneity. All reference frames agree that the rocket laser arrives first. In some frames it arrives first because it travels a shorter distance, in other frames it arrives first because it was fired first due to the relativity of simultaneity.
I have to disagree, DaleSmam, if I understand the question correctly. In each frame, the light from the ship and from the sun (solar flare?) left at the same time, since the events are local. In each frame, they would arrive at the target at the same time, since presumably, there could be an observer in each frame.

What the frames would disagree on is the specific time of the simultaneous arrival of the light beams at the target, and the distance from source to target.
 
  • #29
Al68 said:
In each frame, the light from the ship and from the sun (solar flare?) left at the same time, since the events are local.
Hmm, I understood his post differently. I understood the Earth to be in the middle and the sun and the rocket equidistant and on opposite sides. Perhaps the OP can clarify.
 
  • #30
DaleSpam said:
Hmm, I understood his post differently. I understood the Earth to be in the middle and the sun and the rocket equidistant and on opposite sides. Perhaps the OP can clarify.

If you mean my question, then I meant the rocket and sun are on the same side, practically next to each other.
 
  • #31
DaleSpam said:
Hmm, I understood his post differently. I understood the Earth to be in the middle and the sun and the rocket equidistant and on opposite sides. Perhaps the OP can clarify.

My actual question, since I have thought about it, is this way.

The isotropy of space and the light postulate stipulate that light emits c regardless of any possible motion of the light source.

So, light always emits c.

Now, I have read here it will always be measured c. But, measuring c and emitting c are two different concepts.

So, if it always emits c, and the receiver is somehow moving, then how exactly will it be measured c.

Note, because it emits c, this is not about light speed anisotropy.

Here is how it seems to me.

Light is emitted from a light source with a light receiver located at a distance d.

Light proceeds toward the receiver at c regardless of any possible motion of the light source.

In the mean time, the light source and the light receiver move together with some kind of unknown actual underlying motion since all objects are in some kind of motion.

As the light moves, the light source and light receiver are stationary to one another and so the distance remains d. But as light moves toward the receiver, that receiver actually moves.

Where am I going wrong?
 
  • #32
Al68 said:
Yes, if they both struck the same target, each frame would agree that both light beams arrived at the target at the same time, but they would disagree about what specific time that is.I have to disagree, DaleSmam, if I understand the question correctly. In each frame, the light from the ship and from the sun (solar flare?) left at the same time, since the events are local. In each frame, they would arrive at the target at the same time, since presumably, there could be an observer in each frame.

What the frames would disagree on is the specific time of the simultaneous arrival of the light beams at the target, and the distance from source to target.

You understood my question correctly.
 
  • #33
jacksnap said:
If you mean my question, then I meant the rocket and sun are on the same side, practically next to each other.
Sorry, I misunderstood the scenario. Please ignore my response above, Al68 is correct.
 
  • #34
cfrogue said:
Where am I going wrong?
You are misunderstanding the concept of motion - I addressed this in my previous post.

"Motion" is the change in displacement (with time) of one object with respect to another. In your scenario, the two objects are your emitter and observer and they are motionless with respect to each other. Adding to the problem additional reference frames that the source and target are moving with respect to just plain isn't how "motion" works. Even in Galilean Relativity, the only motion that matters is the motion between the two objects in question. This should be obvious, since any object can and does have an infinite number of different speeds at the same time. The only one of those speeds that matters is the one between investigated in the problem: the one between the source and target.

What differentiates Galilean Relativity from Einstein's Relativity is that in Galileo's, the speed of light was constant with respect to a unversal reference frame. In Einstein's, it is constant with respect to all inertial observers (which includes your emitter and target).

Think about this: if you are on a train that is moving at constant speed and you are playing table tennis, does the motion of the train have any impact on your game?
 
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  • #35
russ_watters said:
You are misunderstanding the concept of motion - I addressed this in my previous post.

"Motion" is the change in displacement (with time) of one object with respect to another. In your scenario, the two objects are your emitter and observer and they are motionless with respect to each other. Adding additional reference frames that the source and target are moving with respect to just plain isn't how "motion" works. Even in Galilean Relativity, the only motion that matters is the motion between the two objects in question. This should be obvious, since any object can and does have an infinite number of different speeds at the same time. The only one of those speeds that matters is the one between investigated in the problem: the one between the source and target.

What differentiates Galilean Relativity from Einstein's Relativity is that in Galileo's, the speed of light was constant with respect to a unversal reference frame. In Einstein's, it is constant with respect to all inertial observers (which includes your emitter and target).

Part of what may be confusing you is your word usage and comprehension is very sloppy, so you are having trouble finding proper meaning in the words you are writing. You would be well served by making an effort to read and write with more precision of wording/meaning.

OK,
So you are saying that light always emits at c and is measured c.

Yes, the emitter and receiver are at a fixed distance d.

Are objects in the universe in some kind of motion?
 

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