Target Distance and Light Travel: The Impact on Spatial Position

[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?

 

[russ_watters]

Simple, the Earth is moving somehow or not moving.

I am guessing it is moving. How about you?

From where I'm sitting right now the Earth is not moving. Of course, when I drove home from work yesterday, it was moving. Do you understand that? "Moving" and "not moving" actually aren't fundamentally different from each other. 0 and 60 are both just numbers. Right now, sitting on my couch, the Earth has a speed of 0. But when driving home from work, it was 60.

 

[russ_watters]

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?

You could respond to the examples given already that explain why if it did exist, we could detect it. Ignoring the proof doesn't make it go away.

[there is also a sticky on that very request at the top of the SR forum page]

 

[cfrogue]

You could respond to the examples given already that explain why if it did exist, we could detect it. Ignoring the proof doesn't make it go away.

What?

Are you claiming it is impossible to detect?

How do you prove this?

Can I see the proof?

Otherwise, you have no choice but to assume a frame moves around in some unknown way.
This is simple logic.

 

[cfrogue]

From where I'm sitting right now the Earth is not moving. Of course, when I drove home from work yesterday, it was moving. Do you understand that? "Moving" and "not moving" actually aren't fundamentally different from each other. 0 and 60 are both just numbers. Right now, sitting on my couch, the Earth has a speed of 0. But when driving home from work, it was 60.

I have the relative motion thing figured out.

While you drove home, you were at rest and the Earth was moving relative to you.

When Apollo went to the moon, the spaceship did not move, the Earth did.

Yes, I have this figured out.

 

[Dale]

Simple, the Earth is moving somehow or not moving.

I am guessing it is moving. How about you?

You are guessing that it is moving relative to what?

 

[russ_watters]

What?

Are you claiming it is impossible to detect?

No, you are claiming that it exists, but is impossible to detect. *I* (Ie, the laws of physics) claim that it doesn't exist and experiments prove it.

How do you prove this?

Can I see the proof?

Examples, and where to find more, have been given. If you have specific questions about specific ones, ask. But you need to put some effort into this yourself.

Otherwise, you have no choice but to assume a frame moves around in some unknown way.
This is simple logic.

That's not logic, it's gibberish.

Lets try some logic, though: if you don't believe that something can be shown to exist (absolute motion/rest), why do you still believe it does? Isn't that illogical?

I have the relative motion thing figured out.

While you drove home, you were at rest and the Earth was moving relative to you.

When Apollo went to the moon, the spaceship did not move, the Earth did.

Yes, I have this figured out.

Clearly you don't or you wouldn't have asked the questions above.

 

[cfrogue]

You are guessing that it is moving relative to what?

Well, the Earth is moving relative to the sun which is moving relative to the milky way.

So, if I shot a laser at a target on the earth, would I hit it perfectly?

 

[russ_watters]

Well, the Earth is moving relative to the sun which is moving relative to the milky way.

Ok...

So, if I shot a laser at a target on the earth, would I hit it perfectly?

If you have good aim, sure.

 

[cfrogue]

No, you are claiming that it exists, but is impossible to detect. *I* (Ie, the laws of physics) claim that it doesn't exist and experiments prove it. Examples, and where to find more, have been given. If you have specific questions about specific ones, ask. But you need to put some effort into this yourself.

That's not logic, it's gibberish.

Lets try some logic, though: if you don't believe that something can be shown to exist (absolute motion/rest), why do you still believe it does? Isn't that illogical?
Clearly you don't or you wouldn't have asked the questions above.

*I* (Ie, the laws of physics) claim that it doesn't exist and experiments prove it.

How do you prove something does not exist?

May I see the proof?

 

[cfrogue]

Ok... If you have good aim, sure.

Well, you line up the target, the Earth is moving with a rotation and then in its orbit around the sun.

Then the milky way moves.

How do you know where the target will be when the light reaches it?

Are you assuming the target is at absolute rest?