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#1 Ira

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Posted 25 July 2013 - 05:39 PM

http://www.extremete...y-for-a-minu...

/Ira

#2 davidpitre

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Posted 25 July 2013 - 07:23 PM

What caught my eye was:
"These atomic spins can maintain coherence (data integrity) for around a minute". Pretty cool.

#3 EJN

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Posted 25 July 2013 - 07:53 PM

Articles like this do not explain the difference between phase velocity
& group velocity - even if the group velocity = 0 the phase velocity
still equals c (in that medium).

http://en.wikipedia..../Group_velocity
http://en.wikipedia..../Phase_velocity

#4 Charlie B

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Posted 26 July 2013 - 09:58 AM

Phase and group velocity do not seem to be relevant to this experiment. The experimenters stored a light image, which is information, in the crystal's molecular spin states. Although, the article implies that light was stopped, the actual feat seemed to be maintaining the spin states for 60 seconds.

Regards,
Charlie B

#5 TL2101

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Posted 26 July 2013 - 12:50 PM

Is this the same result as happens in the Bose-Einstein condensate?

#6 Charlie B

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Posted 26 July 2013 - 05:17 PM

Is this the same result as happens in the Bose-Einstein condensate?



I don't think so. The BE condensate is at (almost) the lowest possible quantum state for the atoms and no information is imprinted. This appears to be a defined state, not the lowest.

Charlie B

#7 Pess

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Posted 28 July 2013 - 08:51 AM

Phase and group velocity do not seem to be relevant to this experiment. The experimenters stored a light image, which is information, in the crystal's molecular spin states. Although, the article implies that light was stopped, the actual feat seemed to be maintaining the spin states for 60 seconds.

Regards,
Charlie B


I really hate the way they report this in the news.

Light always travels at 'c'. Its average speed can appear to be 'slowed' because when photons travel through a substrate they are constantly stopped and re-emitted by the atoms within the substrate.

The photons dash at the speed of 'c' from atom to atom but held up as they dissappear (absorbed by an atom) and re-created again. This destruction and recreation takes time and that is why the overall speed of the photons measured appears to have slowed. A Photon, during the dash between atoms, always travels at 'c'.

Now it isn't that exciting that information is preserved. Every time we look through a window we are seeing photons impressed with the same information they entered the opposite side with.

What IS impressive is that scientists were able to create a medium where the absorption/recreation of photons occurred so slowly that it took a minute for the photons to come out the other side WHILE STILL PRESERVING the information the photons entered with!

That's the amazing part!

But any casual reader of these articles comes away with the impression that the photons were somehow brought to a standstill.

On a separate note, I was talking to a group of high school students about physics the other day. What I like to do is take a complex subject and try and put it in simple terms that a high school student might understand.

I was stumped by a question: Why don't photons have rest mass? I started to stumble around explaining Gauge theory and Higgs Boson interactions with magnetic fields...etc etc. But their eyes glazed over.

Anyone have a good, simple explanation for why photons have no rest mass?

Pesse (I can c clearly now) Mist

#8 llanitedave

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Posted 28 July 2013 - 09:49 AM

Because they never get tired.

#9 derangedhermit

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Posted 28 July 2013 - 10:57 AM

i've forgotten how long it takes a photon from a star's core, say our Sun, to pop out at the surface, but it is a very long time. I wonder what the average speed is? Much less than highway speeds, I think. Maybe less than walking speed.

#10 llanitedave

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Posted 28 July 2013 - 03:10 PM

Photons from the Sun's core don't make it directly to the surface. They are generated as gamma rays in the core, and as energy gets absorbed and re-emitted in the random walk outward, it gets emitted as new photons of progressively lower energy levels. If I remember right, the energy takes at least tens of thousands of years to dissipate outward, but it's not because any individual packet is traveling slowly. Also, since it's emitted each time in a random direction, as often as not it gets sent back inward again.

#11 StarWars

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Posted 28 July 2013 - 03:49 PM




Interesting subject!! :D


I have a flashlight and when the switch is pressed to the on position there is a beam of light.
When switched to the off position the beam of light is gone. :scratchhead:

Where did the light go... :question:


I bet there is a big difference between LED produced light and light from the sun!! :cool:

#12 Ira

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Posted 28 July 2013 - 04:32 PM

i've forgotten how long it takes a photon from a star's core, say our Sun, to pop out at the surface, but it is a very long time. I wonder what the average speed is? Much less than highway speeds, I think. Maybe less than walking speed.


Last I heard, it takes 10,000 years.

/Ira

#13 ColoHank

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Posted 28 July 2013 - 05:51 PM

I have a flashlight and when the switch is pressed to the on position there is a beam of light.
When switched to the off position the beam of light is gone.

Where did the light go...



Isn't it absorbed by whatever the light eventually strikes (a wall, dust particles in the atmosphere, etc.) and converted to heat energy?

#14 Ira

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Posted 28 July 2013 - 08:14 PM



Interesting subject!! :D


I have a flashlight and when the switch is pressed to the on position there is a beam of light.
When switched to the off position the beam of light is gone. :scratchhead:

Where did the light go... :question:


I bet there is a big difference between LED produced light and light from the sun!! :cool:


Your light is off toward the outer reaches of the cosmos. Whatever part of it isn't blocked by matter, interstellar dust or other junk will just keep on travelling forever at the speed of c. You can never catch up to it.

/Ira

#15 scopethis

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Posted 29 July 2013 - 07:38 AM

when light is reflected off of a mirror, is it traveling at its original speed?

#16 Ira

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Posted 29 July 2013 - 09:44 AM

when light is reflected off of a mirror, is it traveling at its original speed?


Why wouldn't it be? It's only when travelling in a medium other than the vacuum of space that its speed decreases. It's reflecting off the mirror, not travelling through it. Although, if it's a second surface mirror it will travel a few mm through the glass before reflecting, so its speed will slow then. But when it exits the glass it should be travelling again at c, or whatever c is in the earth's atmosphere.

/Ira

#17 Mister T

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Posted 29 July 2013 - 10:59 AM

So since atoms are mostly empty space, :question:when light is traveling through the atmosphere how far does the average photon go before it hits something that will slow it down?

#18 llanitedave

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Posted 29 July 2013 - 12:39 PM

It's not the nucleus that the light will be hitting, it's the electron shell, so the empty space surrounding the nucleus doesn't really matter.

#19 Qwickdraw

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Posted 29 July 2013 - 05:10 PM

isnt this closer to converting light energy to another type of energy and then using that stored energy to generate new light rather then actually storing light?

#20 Pess

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Posted 30 July 2013 - 03:54 PM

when light is reflected off of a mirror, is it traveling at its original speed?


Why wouldn't it be? It's only when travelling in a medium other than the vacuum of space that its speed decreases. It's reflecting off the mirror, not travelling through it. Although, if it's a second surface mirror it will travel a few mm through the glass before reflecting, so its speed will slow then. But when it exits the glass it should be travelling again at c, or whatever c is in the earth's atmosphere.

/Ira



Well, actually light always travels at 'c'...no exceptions. Even through any medium the photons are traveling at 'c'. It is just a medium has atoms and electrons that absorb the photons and then re-emit a new photon. This absorption and re-emitting takes a tad bit of time so if you measure the average photon speed it 'appears' the medium slowed the photons. But atom to atom the photons always travel at 'c'.

In regards to a mirror...the photons are absorbed and reemitted in the same direction of travel they came in on so the original photons that struck the mirror are converted to energy when they are absorbed by the atoms of the mirror...this energy is dumped by the atoms in the form of a new photon.
.

Pesse (Welcome to quantum mechanics.) Mist

#21 Pess

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Posted 30 July 2013 - 05:14 PM

.

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#22 Ira

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Posted 30 July 2013 - 06:32 PM

So why, then, does a lens bend light?

/Ira

#23 Pess

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Posted 30 July 2013 - 08:28 PM

So why, then, does a lens bend light?

/Ira


The new photon can be re-admitted along varying vectors. The atoms in a mirrored surface (or, more correctly, their electrons) can send them straight backwards so you can see your reflection. Or they can re-emit them along the same vector as glass or air does. Or they can slightly shift the vector and so the image appears shifted.

The photons can even be significantly changed such as a laser hitting a rock with the result of photons being re-emitted at infrared wavelengths (heat)

In summary, photons travel in a straight line, they travel at velocity 'c' until they hit something at which point they cease to exist and their energy is transferred into what stops them.

Pesse (cool beans) Mist

#24 ColoHank

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Posted 30 July 2013 - 08:47 PM

In regards to a mirror...the photons are absorbed and reemitted in the same direction of travel they came in on so the original photons that struck the mirror are converted to energy when they are absorbed by the atoms of the mirror...this energy is dumped by the atoms in the form of a new photon.



Most of them, anyway. Some are absorbed and converted to heat energy. Thus, reflectance is always less than 100%, the amount of loss determined by the composition and quality of the reflective surface and the wavelength of the light. Also, the re-emitted photons aren't bounced back in the same direction from whence the originals came. If they were, a person could look at a mirror from any angle and still see his or her image. Reflected photon(s) will follow a path that's equal in incidence to the optic normal but opposite in direction to the incoming photon(s).

#25 StarWars

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Posted 31 July 2013 - 12:37 AM




Interesting subject!! :D


I have a flashlight and when the switch is pressed to the on position there is a beam of light.
When switched to the off position the beam of light is gone. :scratchhead:

Where did the light go... :question:


I bet there is a big difference between LED produced light and light from the sun!! :cool:


Your light is off toward the outer reaches of the cosmos. Whatever part of it isn't blocked by matter, interstellar dust or other junk will just keep on traveling forever at the speed of c. You can never catch up to it.

/Ira



Sounds logical!!






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