measuring distance

[Mart]

I want to measure the distance to a mirror.

I send a pulse of light to it and measure how long it takes for the round trip. I can then calculate the distance.

While I'm waiting for the light to return am I correct in saying that the distance between myself and the mirror has no definite value?

[5614]

I think it's safe to say that there is a definitive value, as you are standing x meters from the mirror, it is more a case of that fact that you do not know it.

[Mart]

Who would know it and how?

[losfomot]

I want to measure the distance to a mirror.

I send a pulse of light to it and measure how long it takes for the round trip. I can then calculate the distance.

While I'm waiting for the light to return am I correct in saying that the distance between myself and the mirror has no definite value?

 

 

I am going to venture an answer...

 

If the mirror is moving (or could move) relative to you- then even when your pulse of light returns and gives you an answer, it is possible that the mirror has already moved and that answer is no longer valid. I don't think this means it has no definite value though, just an indeterminate value. If the motion was uniform, the distance at any given moment could be worked out mathematically.

 

If you and the mirror are known to be stationary relative to eachother, then it doesn't matter when you perform the measurement, the distance WILL have a definite value... consider that you could perform your measurement, record the answer and then perform the measurement again, and again. Each time the answer will be the same. So you could only wonder about the definite value of the distance the first time you perform the measurement. Every subsequent time you will know the answer before the light pulse returns. You will look back and realize that there is no way you could have gotten a different answer the first time you made the measurement and conclude that the distance DID HAVE a definite value before the light pulse returned... you just didn't know what that value was yet.

 

So, overall, I would say NO, you are not correct in saying that.

[Mart]

Originally Posted by Losfomot

If you and the mirror are known to be stationary relative to each other, then it doesn't matter when you perform the measurement, the distance WILL have a definite value... consider that you could perform your measurement, record the answer and then perform the measurement again, and again.

They could only be known to be stationary relative to each other in retrospect. That's to say I would have evidence about the past. The problem is : can I have the same sort of evidence about the present distance of the mirror?

[losfomot]

They could only be known to be stationary relative to each other in retrospect. That's to say I would have evidence about the past. The problem is : can I have the same sort of evidence about the present[/b'] distance of the mirror?

 

 

O...K.... no, I guess you can't. You will only know for sure in retrospect, after taking the measurement. But this does not mean that the distance does not have a definite value.

 

The only way your original statement could be correct is if you define the term 'definite value' as being any value that you yourself know ahead of time.

 

To define the term in such a way is incorrect (IMO) and a bit egotistical (also IMO) and would obviously lead to your original statement being correct.

 

 

If a tree fell in the woods, and nobody was there to here it, would it make a sound?

 

Of course it would, just like a distance has a definite value, whether someone has taken the time to measure it or not.

[Mart]

Originally Posted by Losfomot

If a tree fell in the woods, and nobody was there to here it, would it make a sound?

 

Of course it would, just like a distance has a definite value, whether someone has taken the time to measure it or not.

If there is a definite value to the distance and no one has measured it how is that definite distance established and maintained? Wouldn't some type of interaction be necessary?

 

If a tree falls in the woods, and nobody is there to hear it then it makes vibrations but it doesn't make a sound. Sound needs somebody - unless you allow that trees and rocks hear sounds.

[doomtiki]

I know this is the classical physics forum, but you can never know how far you are from the mirror. The uncertainty principle will allow you to accurately measure how far you are from the mirror, but you would not know it's velocity so it could be moving away from you causing relativistic length dilation. Also, in alternate histories the mirror could be different lengths away from you.

[swansont]

I know this is the classical physics forum, but you can never know how far you are from the mirror. The uncertainty principle will allow you to accurately measure how far you are from the mirror, but you would not know it's velocity so it could be moving away from you causing relativistic length dilation. Also, in alternate histories the mirror could be different lengths away from you.

 

You could measure a Doppler shift of the reflected light to measure the velocity to some precision.

[losfomot]

If there is a definite value to the distance and no one has measured it how[/b'] is that definite distance established and maintained? Wouldn't some type of interaction be necessary?

 

established and maintained? Why would the distance need to be established and maintained in order to be real? Interaction? why? The distance is there, and has a definite value. If you want to know what that value is then go ahead and measure it. You are not going to get an instantaneous value, but you will still get a definite value. If the object is moving it will take a few seperate measurements to figure out how fast and in what direction the object is moving, and then you will even know ahead of time where it will be.

 

If the mirror happens to change its velocity in a random manner, then your pretty much out of luck on successfully predicting how far away it is. However, an inability to measure a distance does not mean that the distance does not have a definite value... (for the umpteenth time) you just don't know what that definite value is.

 

[i']If a tree falls in the woods, and nobody is there to hear it then it makes vibrations but it doesn't make a sound. Sound needs somebody - unless you allow that trees and rocks hear sounds.[/i]

 

You can call it sound or you can call it vibrations, it comes down to the same thing. You can leave a recorder in the woods and play back the vibrations/sound. Personally, I am sure that plants CAN detect vibrations/sound.

 

 

It all comes down to what you define the term 'definite value' as meaning.

 

Why don't you rephrase the question without using that term.

[Mart]

If I've got this right, according to Losfomot distances exist independently of measurement. Until a distance is measured by someone its value remains unknown (but knowable). On this thesis, when we measure we are attempting to ascertain a pre-existing quantity. This suggets a universe in which everybody's distance to every other body exists and has some specific value. Due to limitations (finite speed of light. etc) we need time to get these numbers. Also when I move from home to work all these distances are being instantaneously updated even to the farthest reaches of space.

[YT2095]

wouldn`t something like a LIDAR that the police use to catch you speeding be any use?

or maybe a Theodolite(sp?)

[losfomot]

If I've got this right, according to Losfomot distances exist independently of measurement.

 

Right.

 

Until a distance is measured by someone its value remains unknown (but knowable)

 

Not necessarily knowable. Someone may not have the means to measure said distance. The distance still exists though.

 

On this thesis, when we measure we are attempting to ascertain a pre-existing quantity. This suggets a universe in which everybody's distance to every other body exists and has some specific value.

 

I feel like a trap is being set.

 

Due to limitations (finite speed of light. etc) we need time to get these numbers.

 

sure.

 

Also when I move from home to work all these distances are being instantaneously updated even to the farthest reaches of space.

 

I would change 'being instantaneously updated' to 'constantly changing' and then tentatively agree.

[Mart]

Originally Posted by Losfomot

I would change 'being instantaneously updated' to 'constantly changing' and then tentatively agree.

What specific reservations do you have about instant updating of a distance?

[losfomot]

What specific reservations do you have about instant updating of a distance?

 

Again, it's your terminology. The term 'being instantaneously updated' implies (or, rather, could imply) that someone or something has knowledge or information that is being updated instantaneously. This is not the case.

 

Yes, if the target or source is moving, the instantaneous distance between them is constantly changing. Because information can not travel FTL, you cannot know the instantaneous distance instantaneously.

[Mart]

Losfomot . . . in an earlier post you said "The distance is there, and has a definite value." Any idea where this distance value is stored (for want of a better term) other than in the physical distance (which has yet to be measured)?

[losfomot]

Losfomot . . . in an earlier post you said "The distance is there, and has a definite value." Any idea where this distance value is stored (for want of a better term) other than in the physical distance (which has yet to be measured)?

 

Yes, it's stored in the archives of a secret temple in the delta quadrant of the Andromeda galaxy. I can't be any more specific than that because, well, as I said, it's a secret.

[Mart]

Keep it safe.