Refraction Index & Relativity

[Zipper730]

This probably sounds relatively silly or outright retarded, but I'm curious about the refraction index and relativity.: From what I remember reading about relativity was that as you get closer to the speed of light, time slows down. With refractivity, the speed of light varies depending on the medium in which it travels.

In water, it's something like 75% what it is in a vacuum: How come there is no measurable time-dilation? Is it only because you're traveling so slow (4.6-4.7 mph, which is how fast I could go from one side of the pool to another) that the change my speed relative to light is so low as to be insignificant (4.6-4.7 mph vs 186300 mp/s), or other?

 

[pbehn]

This probably sounds relatively silly or outright retarded, but I'm curious about the refraction index and relativity.: From what I remember reading about relativity was that as you get closer to the speed of light, time slows down. With refractivity, the speed of light varies depending on the medium in which it travels.

In water, it's something like 75% what it is in a vacuum: Since time doesn't seem to slow down when you're underwater (if it did, you'd appear to be moving in slow motion once inside the water, and to you, everything would appear to be wizzing by), why doesn't relativity seem to apply here?

If you can swim underwater at the speed of light you cant drown.

 

[Zipper730]

You know what I meant...

 

[pbehn]

You know what I meant...

Not really, 75% of a number too big to imagine is still too big to imagine. Early attempts at measuring the speed of light involved people opening and closing shutters in the dark across valleys, they just measured how fast they could open and shut their shutters.

 

[swampyankee]

This probably sounds relatively silly or outright retarded, but I'm curious about the refraction index and relativity.: From what I remember reading about relativity was that as you get closer to the speed of light, time slows down. With refractivity, the speed of light varies depending on the medium in which it travels.

In water, it's something like 75% what it is in a vacuum: How come there is no measurable time-dilation? Is it only because you're traveling so slow (4.6-4.7 mph, which is how fast I could go from one side of the pool to another) that the change my speed relative to light is so low as to be insignificant (4.6-4.7 mph vs 186300 mp/s), or other?

Three-quarters of the speed of light is still 225,000,000 m/s; we can't get anything macroscopic anywhere near that speed.
In any case, what counts is speed of light in a vacuum.

 

[pbehn]

Three-quarters of the speed of light is still 225,000,000 m/s; we can't get anything macroscopic anywhere near that speed.
In any case, what counts is speed of light in a vacuum.

I was told at school that a wheelbarrow full of matchsticks contains about 1 million, so 225 wheelbarrows gives you the meters covered in one second. We can calculate with these numbers and represent them but they are actually beyond comprehension in actual size.

 

[ThomasP]

The speed of light (c) is 299,792,458 m/s, in a vacuum, or in a solid.

The idea that light travels at different speeds in different substances is incorrect.

The misunderstanding arises from the behavior of photons in a solid/liquid/gas, and/or gravitational field.

In a vacuum, light travels in a straight line unless influenced by gravity. If there is no net gravitational influence, the time it takes for a photon to travel from a point A to a point B 299,792,458 m distant will be exactly 1 sec.

If there is an object directly between points A and B the photon cannot travel in a straight line, but will instead be absorbed and emitted by the object.

If the object is massive enough the photon will initially have to be travelling in a direction that would miss point B, a trajectory that will pass close to the edge of the object. When the photon approaches the object it will noticeably begin to change direction toward the object, and if the initial trajectory is just right the photon will miss the object but curve around the object and intersect point B. (This phenomenon is the basis for the idea of curved space) Since the photon has to travel a greater distance than if it had traveled in a straight line, the time it takes to travel from point A to point B will therefor take more than 1 sec.

When a photon enters a body of water it does not travel in a straight line, but instead curves around the atoms and molecules, affected by the gravitational pull of the atoms.

It is also absorbed-emitted by atoms (the number of times in proportion to the thickness of the water layer) usually being emitted in a direction other than the one it entered the water with. The absorption-emission process (ie the absorbing-emitting electrons jumping from lower-to-higher-to-lower energy states) takes time to occur.

The accumulation of increased distance due to the gravitational effects of the atoms/molecules, and the time the multiple absorption-emission process takes, increases the time it takes for the photon to travel from point A to point B.

Hence you have the APPEARANCE of the speed of light being slower in water than in vacuum.

Think of it like a Roomba traveling to the other side of a very large room, across an uneven floor (ie gravity/curved space), bouncing off of furniture and changing direction (ie absorbing-emitting), but eventually reaching the opposite side. If the speed of the Roomba is .25 mph and the straight-line distance from point A to B is .25 mile, the distance traveled will be significantly greater than .25 mile, and the time taken will be significantly greater than 1 hour.