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Does time go slower underwater?
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hamster_nz:

--- Quote from: Circlotron on July 12, 2020, 08:57:54 am ---Light travels fastest in a vacuum. When it passes through water it goes slower and causes refraction. Seeing it goes slower in water, would not time also pass more slowly in water to an observer outside the water?

--- End quote ---

Water has as refractive index of 1.333. So if my math is right that makes the speed of light about 75% of that in a vacuum (which is pretty much the same as air).

Even with a few tonnes of of water (cubic meters) it won't make an appreciable difference to time - according to Wikipedia...


--- Quote ---To illustrate then, without accounting for the effects of rotation, proximity to Earth's gravitational well will cause a clock on the planet's surface to accumulate around 0.0219 fewer seconds over a period of one year than would a distant observer's clock

--- End quote ---

So just roughly banging numbers about. A few tonnes of water might make 0.000 000 000 000 000 000 001 of a second's difference, over a year.

Rick Law:

--- Quote from: EEVblog on July 13, 2020, 03:29:00 am ---Technically the light itself does not slow down in water, it's the combined electromagnetic wave effect caused by the material:
...
...[video snipped away for quote]

--- End quote ---
I've seen that guy(in the video) in other presentations, he is rather good.

Whether how light slow down or not is an interesting point to bring up, but how it slow down really doesn't alter the fact that it did slow down.  That however brings up a couple of cans of worms that is very interesting.  I was debating whether I should bring those complication up - or not, but with that video, that gives me a reason to get into it.  (I actually had passion about the first can of worm once upon a time -- way before SI's definition of Time quantity became real)

First can of worms

At the time Eisenstein came up with time dilation and speed of light being a important factor, Time was not we defined as Time today in the SI unit  --  that is why I included the SI definition of time in my first reply.

In Eisenstein's days, EM wave (light) propagation was understood to be electric field inducing a magnetic field which in-turn induce an electric field which in-turn induce a magnetic field, so on, so the EM wave propagates.  Fine, we accept that.  So, how fast one field induce the next how that next field induce another next, so on, takes time, so there is a speed limit, fine, we accept that.  And with that light/EM wave and time is rather closely linked as other theories developed, and in particular, how Eisenstein think of time since relativity is the one that changed how we view time.

When SI-Unit for Time was introduced, whether that definition "the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom" preserved fully (ie: quantitative vs qualitative) the nature of time (and its impact on other accepted theories) is an interesting discussion that some still consider an open one after SI unit was published.  Whether that was a finished discussion, or was an abandoned discussion, is unknown to me.  So, not wanting to touch the mutability of the definition of time, I just inserted the definition, and added a jab about some experimental physics there would interesting.

Second can of worms

The other complication that I hand-waved over is velocity.  Velocity alone doesn't cause time dilation, acceleration does.  We write our stuff talking about velocity, (common writing about twins paradox where one twin travels and one twin doesn't travel for example), we pretend: under the cover that without acceleration you don't get velocity.  But in fact, it is the acceleration that one twin must do (and the non-traveling twin doesn't) that caused the time difference.

Eating up the worms

The pretending is because the deeper one digs, the more difficult it is to explain the dirt that one digs up.  So, staying with generally accepted principal and brushing over the deeper complications (while staying true to observed result) is the way around having a 100 page explanation that no one wants to read anyway.  (Or trying to explain the Twin Paradox's explanation is actually wrong, it is not velocity, it is acceleration...  One would be arguing with a thousand people a thousand times over.)

So as long as the "light travel slower in water" matches the observed result, that's fine.  But digging deeper will always uncover more interesting facts.  Some of the facts may even be Nobel Prize material, who knows.

newbrain:

--- Quote from: Rick Law, abridged by newbrain on July 13, 2020, 05:43:55 am ---At the time Eisenstein....
In Eisenstein's days....
how Eisenstein think of time.... since relativity is the one that changed how we view time.

Some of the facts may even be Nobel Prize material, who knows.

--- End quote ---
I think an Oscar would have been more appropriate for Eisenstein - though they probably shared the same barber...
I'm surprised that the director of immortal masterpieces such as Battleship Potemkin or Alexander Nevskij would have such insights on time and EM waves propagation!
About time dilation, it can be promptly experienced by anyone watching his movies...

On a more serious note, the two minutephysics' videos below are a good explanation of the twin paradox, and why velocity is not the cause of the elapsed time difference when the twins reunite.
 
and
Rick Law:

--- Quote from: hamster_nz on July 13, 2020, 05:41:26 am ---...
So just roughly banging numbers about. A few tonnes of water might make 0.000 000 000 000 000 000 001 of a second's difference, over a year.

--- End quote ---

When in practice it doesn't make a difference, why waste resources dealing with it.  So it is perfectly reasonable to ignore such differences.  I do understand the different perspective here.

To someone practical (Engineer), the different is tiny and can be ignored (most of the time).  But to a theoretician (Physicist), when it is not exact, it is not the same.

Rather like telling a mathematician to accept that 2+2=almost 4 but not exactly 4.  It is kind of earth shaking.

I think the main reason it bugs Physicist is:

Prior to time dilation, the thought was "Time is absolute", there is a universal clock governing progression.  Now we realized there isn't a "Master Clock" for the universe.  The common understanding was "Space is the arena, and Time is what enabled things to happen in that arena".  Time dilation, however small, shattered that idea.

Coupled that with Heisenberg's Uncertainty Principal, Planck's discovery of Planck's Constant (which kicked started Quantum Mechanics), wave-particle duality...  Those earth-shaking discoveries and theories threw the proverbial monkey wrench at the foundation of the Physics.  Physicist before that time frame thought they knew how the universe works, just the tedious job of tidying up loose ends left.  Today, it is hard to find a Physicist who would say they fully understood how the universe really works.

"If you think you understand quantum mechanics, you don't understand quantum mechanics."   Richard P. Feynman.

No, we really don't know what is going on here...  Some theories seem to work, but they don't all work  together with each other.  They just work within their domain.  It is a fascinating but frustrating universe we live in.
free_electron:
nope. much faster. an hour of scubadiving always feels it's over waay too fast ...
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