Exploring the Speed of Light and Position in Inertial Reference Frames

In summary, the conversation discusses the concept of measuring the speed of light and its behavior when emitted from a moving source. The Michelson-Morley experiment is mentioned, which showed that the speed of light is constant between any two observers who are stationary. The conversation also delves into the idea of using light detectors arranged in a sphere to measure offsets and delays in the light's movement. Ultimately, the participants conclude that the speed of light should not be measured against an external frame of reference and that the first postulate of Special Relativity states that the laws of the universe are the same for any inertial reference frame.
  • #1
ABHoT
33
0
Be gentle, I have physics textbooks on order ;) which should satisfy my cravings, until then..


I would like to try and stick a pole-in-the-ground so to speak so I can say 'all movement is an offset from here'.

I've read up a little on the Michelson-Morley experiment.

I believe light can travel both through a medium (when on Earth) and without a medium (when in space).

I think I mean using the speed of light outside a medium. I am currently believing that nothing in the universe that emits light is in exactly the same place after it's started emmitting. Wherever the centre of the expanding sphere of light is, is the pole-in-the-ground, and is not where the source is anymore.

If you take 6 light detectors and arrange them somewhere in space in a sphere, or 12 (is there a name for the number series that describes increasing the points on a sphere symmetrically?). In the centre of this sphere is a light source and you switch it on, measure when the front surface of the light wall hits each detector, then switch it off and measure when the back surface of the light wall hits the detectors.

If light, either because of its speed or something else about it, behaves independently of anything elses movement once it's been emitted, would this be be picked up as a delay in the readings hitting some of the sensors?

I am wondering what would be the minimum speed of our solar system/galaxy/planet/group-of-detectors for an offset to be detected, or what would be the widest sphere for the detectors to be configured into register an offset, or both (assuming there actually ever would be an offset registered).

The following are the results I'm imagining:

a) The front surface of the light hits all detectors simultaneously. Shortly after so does the back surface.
b) The front surface of the light hits all detectors simultaneously. Shortly after the back surface hits the detectors (or stops hitting the detectors, rather) in an offset sort of way.
c) Both the front and back surface spheres of the light wall register delays of the same 'shape' and timing.
d) Both the front and back surface spheres of the light wall register a delay of the same shape, the back surface registering an even larger offset but in the same direction.
e) The front surface hits the detectors in an offset way and the back surface stops hitting the detectors but is offset in a different direction(!)

I am not sure if this is an imaginary repeat of the Micheson-Morley experiment, which answer I should expect or what they ought to mean.
 
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  • #2
ABHoT said:
If light, either because of its speed or something else about it, behaves independently of anything elses movement once it's been emitted, would this be be picked up as a delay in the readings hitting some of the sensors?
That's not what SR predicts nor what the MMx shows. They predict/show that the speed of light between any two observers that are stationary wrt each other is always constant. Therefore, there will never be any delay.

Think about it this way: the MMx apparatus could be mounted inside your sphere and provide the results for your experiment.
 
  • #3
Thx.
russ_watters said:
...the speed of light between any two observers that are stationary wrt each other is always constant.
Forgive me if this is obvious: This sounds like it implies then that if observer or object A is moving towards observer (or object) B at speed x and object A turns it light on, the light will exceed the constant c and travel towards object B at speed x + c(?)
 
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  • #4
ABHoT said:
Thx.Forgive me if this is obvious: This sounds like it implies then that if observer or object A is moving towards observer (or object) B at speed x and object A turns it light on, the light will exceed the constant c and travel towards object B at speed x + c(?)

That is not the implication, and a brief perusal of Einstein's 1915 paper(s) will explain that nicely.
 
  • #5
ABHoT said:
Thx.Forgive me if this is obvious: This sounds like it implies then that if observer or object A is moving towards observer (or object) B at speed x and object A turns it light on, the light will exceed the constant c and travel towards object B at speed x + c(?)
Though you haven't explicitly said so, you're introducing an external frame of reference from which to measure the speed of light. That's not how you measure speed. The speed is measured by taking the distance between A and B and dividing by the time for the the light to traverse it. That's it. You're taking that speed and adding the speed that the whole device is moving relative to an external reference frame. That's not what speed is.

That the speed isn't measured against an external frame of reference is the point of the first posulate of SR and what the MMx shows. The first postulate says that the laws of the universe are the same for any inertial reference frame. That means s=d/t regardless of what inertial frame of reference you are in.
 
  • #6
russ_watters said:
...the speed of light between any two observers that are stationary wrt each other is always constant.
I am thinking a clittle clearer now.

I have two sticks in space with light bulbs on the ends, say bulbs A & B on one stick, bulbs C & D on the other. Both sticks are straight and are the same length.

I hold both sticks upright, next to each other lining up bulbs A & C at the top and bulbs B, D at the bottom. I switch on bulbs A & C together and simultaneously drop the first stick so it is falling (or it was already moving and just flying past the other stick but the bulbs were level when the lights were switched on).

Light from A should reach B at the same time that light from C reaches D because both the top bulbs are stationary wrt to their bottom bulbs, both are level when it started and both sticks are the same length.

But stick one is falling and these are really long sticks, so by the time light reaches bulb B from A the sticks should no longer be level. To my thinking, at some point light from bulb A on stick one ought to pass bulb D on stick two before light from C has.

But how can this be if everything was equal and level at the start? Does SR make this possible?
 
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  • #7
ABHoT said:
I am thinking a clittle clearer now.

I have two sticks in space with light bulbs on the ends, say bulbs A & B on one stick, bulbs C & D on the other. Both sticks are straight and are the same length.

I hold both sticks upright, next to each other lining up bulbs A & C at the top and bulbs B, D at the bottom. I switch on bulbs A & C together and simultaneously drop the first stick so it is falling (or it was already moving and just flying past the other stick but the bulbs were level when the lights were switched on).

Light from A should reach B at the same time that light from C reaches D because both the top bulbs are stationary wrt to their bottom bulbs, both are level when it started and both sticks are the same length.

But stick one is falling and these are really long sticks, so by the time light reaches bulb B from A the sticks should no longer be level. To my thinking, at some point light from bulb A on stick one ought to pass bulb D on stick two before light from C has.

But how can this be if everything was equal and level at the start? Does SR make this possible?

It is the Relativity of Simultaneity. http://www.pitt.edu/~jdnorton/Goodies/rel_of_sim/index.html
 
  • #8
so light from bulbs A and C WILL both reach D at the same time, and will both reach bulb B at the same time but slightly later? and if I measured the distance between A (or C) when the light started to separately B and D when the light reaches each resp., d/t will show the same speed for light for A --> B, A --> D, C --> B, C --> D.
 
  • #9
I suppore as both sticks can't occupy exactly the same space there would always be a slight diagonal for A --> D and C --> B meaning still the same speed for light throughout(?) but slightly longer times & distances than A --> B and C --> D.
 
  • #10
ABHoT said:
I am thinking a clittle clearer now.

I have two sticks in space with light bulbs on the ends, say bulbs A & B on one stick, bulbs C & D on the other. Both sticks are straight and are the same length.

I hold both sticks upright, next to each other lining up bulbs A & C at the top and bulbs B, D at the bottom. I switch on bulbs A & C together and simultaneously drop the first stick so it is falling (or it was already moving and just flying past the other stick but the bulbs were level when the lights were switched on).

Light from A should reach B at the same time that light from C reaches D because both the top bulbs are stationary wrt to their bottom bulbs, both are level when it started and both sticks are the same length.

But stick one is falling and these are really long sticks, so by the time light reaches bulb B from A the sticks should no longer be level. To my thinking, at some point light from bulb A on stick one ought to pass bulb D on stick two before light from C has.

But how can this be if everything was equal and level at the start? Does SR make this possible?
That one sentence of mine had a slightly imprecise wording: the requirement is for an inertial (non-accelerating) frame of reference, not merely a stationary one. When you drop the stick, you are adding an acceleration, essentially changing the experimental setup in the middle of the experiment. This can be dealt with, but it isn't the simple test case of SR. SR is for inertial reference frames.
 

What is the speed of light?

The speed of light is a universal constant, represented by the letter "c", which is approximately 299,792,458 meters per second (m/s) in a vacuum. This means that light travels at this speed regardless of the observer's frame of reference.

How was the speed of light first measured?

The first accurate measurement of the speed of light was conducted by Danish astronomer Ole Rømer in the late 17th century using astronomical observations of Jupiter's moon Io. He observed discrepancies in the moon's predicted and observed positions, which led him to calculate the time it took for light to travel from Jupiter to Earth.

Can the speed of light be exceeded?

According to the theory of relativity, the speed of light is the ultimate speed limit for anything in the universe. This means that it is impossible for an object with mass to reach or exceed the speed of light. However, some theories suggest that certain particles called tachyons may travel faster than the speed of light, but they have not been proven to exist.

How does the speed of light affect the measurement of distance?

Due to the finite speed of light, there is a delay in the time it takes for light to travel from an object to the observer. This means that the farther away an object is, the longer it takes for its light to reach us, making it appear as though the object is further away than it actually is. Therefore, the speed of light must be taken into account when measuring distances in the universe.

Does the speed of light change in different mediums?

The speed of light is affected by the medium through which it travels. In a vacuum, light travels at its maximum speed of approximately 299,792,458 m/s. However, in different mediums such as water or air, the speed of light is slower due to interactions with particles in the medium. This is why light appears to bend when passing through water or a prism.

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