Target Distance and Light Travel: The Impact on Spatial Position

[cfrogue]

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?

 

[Dale]

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.

 

[cfrogue]

r

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?

 

[Al68]

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.

 

[cfrogue]

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.

 

[Al68]

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.

 

[cfrogue]

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.

 

[Al68]

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.

 

[Dale]

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.

 

[Dale]

If you say no, then the Earth is geocentric.

Obviously the Earth is geocentric. That's the definition of geocentric.

 

[Al68]

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?

 

[Dale]

You mean it doesn't depend on whether I say no or not?

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.

 

[Al68]

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?

 

[russ_watters]

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.

 

[Dale]

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.

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.

 

[Janus]

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)

 

[cfrogue]

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?

 

[russ_watters]

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.

 

[cfrogue]

"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?

 

[cfrogue]

"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.

 

[cfrogue]

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.

 

[cfrogue]

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?

 

[cfrogue]

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.

 

[YellowTaxi]

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.

 

[Dale]

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?

 

[jacksnap]

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.

 

[Dale]

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.

 

[Al68]

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.

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.

 

[Dale]

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.

 

[jacksnap]

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.

 

[cfrogue]

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?

 

[cfrogue]

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.

 

[Dale]

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.

 

[russ_watters]

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?

 

[cfrogue]

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?

 

[russ_watters]

Are objects in the universe in some kind of motion?

With respect to some objects, yes, with respect to other objects, no.

 

[cfrogue]

With respect to some objects, yes, with respect to other objects, no.

OK, I see. You are saying there is only relative motion.

But, light always emits at c correct?

 

[russ_watters]

Both correct.

 

[cfrogue]

Both correct.

So, when you are in a stationary frame, is that frame at absolute rest?

 

[russ_watters]

So, when you are in a stationary frame, is that frame at absolute rest?

You are always stationary with respect to yourself and there is no such thing as absolute rest.

 

[cfrogue]

You are always stationary with respect to yourself and there is no such thing as absolute rest.

OK, so you are stationary in a frame and there is no such thing as absolute rest.

Thus, the frame has some kind of motion, but it is not known.


Is this correct?

 

[Dale]

Thus, the frame has some kind of motion, but it is not known.

Why wouldn't it be known? The motion of any given inertial frame wrt any other given inertial frame is well-defined.

 

[russ_watters]

OK, so you are stationary in a frame and there is no such thing as absolute rest.

You are stationary with respect to your frame of reference, yes.

Thus, the frame has some kind of motion, but it is not known.


Is this correct?

That's very oddly worded. As I said above:

...since any object can and does have an infinite number of different speeds at the same time.

So an object has an infinite number of different speeds, depending on what you are measuring its speed with respect to. But I don't know why you would say it isn't known. Lots of them can be known.

 

[cfrogue]

You are stationary with respect to your frame of reference, yes. That's very oddly worded. As I said above: So an object has an infinite number of different speeds, depending on what you are measuring its speed with respect to. But I don't know why you would say it isn't known. Lots of them can be known.

Well, a frame is at absolute rest or is moving.

You said absolute rest does not exists. So, I guess a frame moves.

What if no other frame exists locally and you are not able to determine relative motion.

Does this mean the frame is at absolute rest or is it moving in some unknown way?

 

[russ_watters]

Well, a frame is at absolute rest or is moving.

You said absolute rest does not exists. So, I guess a frame moves.

What if no other frame exists locally and you are not able to determine relative motion.

Does this mean the frame is at absolute rest or is it moving in some unknown way?

You seem to be implying that if a frame is not at absolute rest, it has an absolute motion. That's not correct. It has been said many times in this thread, in different ways:

All motion (or lack thereof) is relative. It is measured between two objects/frames of reference.

If you have no other frame of reference to measure an object's speed against, then you can say nothing about its speed.

I'm not sure why we are having so much trouble explaining this to you, but you may want to read the wiki on the concept of motion: http://en.wikipedia.org/wiki/Motion_(physics )

 

[cfrogue]

You seem to be implying that if a frame is not at absolute rest, it has an absolute motion. That's not correct. It has been said many times in this thread, in different ways:

All motion (or lack thereof) is relative. It is measured between two objects/frames of reference.

If you have no other frame of reference to measure an object's speed against, then you can say nothing about its speed.

So, if a frame is in the universe and the nearest object is billions of light years away, you have no frame of reference in an reasonable time reference, is the frame moving somehow or is it at absolute rest?

Certainly, you would have no idea at all since there is no mechanical way to detect absolute motion, but does that lack of ability to detect it imply it does not exist?

 

[russ_watters]

So, if a frame is in the universe and the nearest object is billions of light years away, you have no frame of reference in an reasonable time reference, is the frame moving somehow or is it at absolute rest?

Whether it is easy or difficult to measure the speed of an object with respect to another doesn't really have any bearing on how the laws of physics work. I'm not sure how many times you need to see this in order for it to sink in: there is no such thing as absolute rest.

Perhaps the problem is you simply choose not to believe that this is how the universe works? That's what this implies:

Certainly, you would have no idea at all since there is no mechanical way to detect absolute motion, but does that lack of ability to detect it imply it does not exist?

No, it is what we can detect that implies absolute motion/rest does not exist.

Again, think of the table tennis on a train example. Despite the fact that we can measure the ping pong table both stationary and moving, it has no impact on the play of the game.

 

[Dale]

Well, a frame is at absolute rest or is moving.

How do you conclude this?

 

[cfrogue]

How do you conclude this?

Simple, the Earth is moving somehow or not moving.

I am guessing it is moving. How about you?

 

[cfrogue]

Whether it is easy or difficult to measure the speed of an object with respect to another doesn't really have any bearing on how the laws of physics work. I'm not sure how many times you need to see this in order for it to sink in: there is no such thing as absolute rest.

Perhaps the problem is you simply choose not to believe that this is how the universe works? That's what this implies: No, it is what we can detect that implies absolute motion/rest does not exist.
Again, think of the table tennis on a train example. Despite the fact that we can measure the ping pong table both stationary and moving, it has no impact on the play of the game.

Absolute motion is not detectable. That is a fact or we would not be having this conversation.

How do you prove that means it does not exist? How do you prove an object has no motion unless there is another to compare it to?

May I see this proof?