define measurement of length

[swaha]

i want to know how to measure length in relativistic mechanics. i mean if length is some times of a particular value of length ie what we call standard length why does it change/ appear to change when someone moves with speed comparable to the speed of light.

i got the answer that since the concept of simultanity changes length seems different in two different reference frames one moving with a velocity not zero relative to the other. my question is what length has got to do with simultanity?

[swansont]

i want to know how to measure length in relativistic mechanics. i mean if length is some times of a particular value of length ie what we call standard length why does it change/ appear to change when someone moves with speed comparable to the speed of light.

i got the answer that since the concept of simultanity changes length seems different in two different reference frames one moving with a velocity not zero relative to the other. my question is what length has got to do with simultanity?

 

Because the speed of light is constant, what you measure for a length will depend on your reference frame. Measurements depend on light (or other signals) getting to a measurement device. Simultaneity is similarly dependent on the constant speed of light — it means there is no absolute simultaneity. These are different aspects of relativity which derive from the same principle of constant light speed.

[swaha]

well then please explain how is actually length measured?

suppose 2 people have 2 rods of same length when they were at rest. when one starts moving the other sees the length of the rod different. y?

[swansont]

There are a number of ways you can measure it, but in some way it will involve light traveling from the object to the detector. It could be picture taken as the object passes by a ruler — light travels to the detector and is recorded, which is why simultaneity is an issue. Was the measurement of the front and back simultaneous? No — that depends on the frame of reference.

[gokul.er137]

simultaneity - by itself in science means that things that happen simultaneous is not viewed the same by all observers. if you are sitting in a bus, with respect to a person inside it, you are stationary. with respect to a person outside the moving bus, you are moving. with respect to you, that person is moving. but in a vectorially opposite direction. this by itself is the best example of the loss of simultaneity. the reason for this is because, the reference frames of observers are entirely different.

 

but the speed of light must be a constant in all frames, irrespective of the velocity or other physical properties. so, in order to maintain a universality of the speed of light, the physical dimensions and the time dimensions are distorted. but again the length contraction does not lead to a change in the measurement of the length of that particular object in its frame. that is because it is purely associated with the the moving frame.so to say, in order to maintain a universality of the speed of light, the basic dimensions bend and curve. more so, even gravity succumbs to the constancy of the speed of light.

[swaha]

what if we have a continuous source of light? how do we know which light signal to observe. 2 signals may start at different times to reach us. pls explain.