23 replies to this topic

[behavioralist]

Where time is change, and light, which is rooted in space-time and not in mass, is the fastest change, what is the unit of time?

Or, how many "frames per second" is one unit of light moving, as opposed to light not moving.

And, if we "stop-action" light, do we find that all quantum activity has stopped?

[Taun]

I thought absolute 0 was as cold as it was possible to get... 0 degrees Kelvin ( -273 degrees C or -459 degrees F)...
At 0 Kelvin doesn't all molecular motion/action cease? With no heat energy at all only quantuum mechanical motion?

[sepulchrave]

Taun, on 28 November 2012 - 06:11 PM, said:

I thought absolute 0 was as cold as it was possible to get... 0 degrees Kelvin ( -273 degrees C or -459 degrees F)...
At 0 Kelvin doesn't all molecular motion/action cease? With no heat energy at all only quantuum mechanical motion?

You are correct. The sentence you refer to was a bit poorly worded.

Space itself doesn't have a temperature; temperature is a property of objects. A cloud of dust in interstellar space does have a temperature, as does a radiation field. The cosmic microwave background has a temperature of almost 3 K, so it is quite possible to get a billion times colder than that (i.e., 0.000000003 K).

Also, the OP should have provided a link to the story.

[TheLastLazyGun]

http://www.news.harv...-stoplight.html

[StarMountainKid]

I thought that the unit of Planck time is the shortest unit of time, 10 ^ -43 seconds. This is the length of time it takes light to travel one Planck length, 10 ^ -35 meters.

I'm not sure this is relevant to the OP.

[Insanity]

StarMountainKid, on 30 November 2012 - 06:49 PM, said:

I thought that the unit of Planck time is the shortest unit of time, 10 ^ -43 seconds. This is the length of time it takes light to travel one Planck length, 10 ^ -35 meters.

I'm not sure this is relevant to the OP.

You are not incorrect.
It is defined as the amount of time for light to travel, in a vacuum, a Planck length.
Theoretically this is the smallest time measurement that would be possible, as due to our current understanding of the laws of physics we can not measure nor detect any changes for lengths of times less then this.

http://en.wikipedia.org/wiki/Planck_time

Edited by Insanity, 30 November 2012 - 08:03 PM.

[keithisco]

I always found that experiment interesting because it confounds the idea that C is a Constant. It also confounds Energy / Mass Equivalence and one of the (erroneous) platforms of modern, received, establishment physics (i.e. Einstein).

In fact, the experiment poses more questions than answers:

1.  What is the mechanism for transferring light "energy" to the surrounding cloud of atoms? Is it absorption?
2. If the above absorption IS the method then what was the mechanism for transferring back that energy from the cloud of atoms (in precisely the correct amount) to   permit energetic light to escape the chamber?
3. Were they really observing and recording what they thought they were - instrumentation and detection are crucial in validation and verification. How was the measurement of speed actually performed - sure, the math is very simple distance travelled  over time, but on such a small scale I would like to know the detection methods (haven't found them yet, but still looking)
4. How can we be certain that there was not an element of "Confirmation Bias" in their published account.
5. Finally: Shouldn't a beam of light lose amplitude (and thus luminosity) rather than speed of propogation in this experimental set-up?

Just a point to ponder: Why does "Light" have the ability to propogate at the "Speed of Light" forever with no apparent energy source to draw on - makes my head hurt also.

Edited by keithisco, 04 December 2012 - 12:42 PM.

[sepulchrave]

keithisco, on 04 December 2012 - 12:34 PM, said:

I always found that experiment interesting because it confounds the idea that C is a Constant. It also confounds Energy / Mass Equivalence and one of the (erroneous) platforms of modern, received, establishment physics (i.e. Einstein).

No it doesn't.

keithisco, on 04 December 2012 - 12:34 PM, said:

1.  What is the mechanism for transferring light "energy" to the surrounding cloud of atoms? Is it absorption?

Yes. Or rather ``coherent absorption''. See the 7th paragraph in the ``Stopping Cold'' section of the article linked by TheLastLazyGun in post #5 of this thread.

keithisco, on 04 December 2012 - 12:34 PM, said:

2. If the above absorption IS the method then what was the mechanism for transferring back that energy from the cloud of atoms (in precisely the correct amount) to   permit energetic light to escape the chamber?

The cloud of atoms gains energy from the light. Because the mixture is so cold, there is no ``thermalization'' of this energy.

Normally when atoms absorb light the atoms may either (1) go into a higher energy electronic state or (2) go into a higher energy thermal state (i.e. vibrate or move faster). Because this substance is so cold (and being actively cooled), only option (1) is available.

In the absence of thermal scattering, and for an isolated gas of atoms (as compared to a solid, for example) electronic excitations are perfectly lossless.

So once the stabilizing field (i.e. the second laser) is switched off, the gas decays back to the lowest energy level by releasing exactly the same photons as were originally absorbed.

keithisco, on 04 December 2012 - 12:34 PM, said:

3. Were they really observing and recording what they thought they were - instrumentation and detection are crucial in validation and verification. How was the measurement of speed actually performed - sure, the math is very simple distance travelled  over time, but on such a small scale I would like to know the detection methods (haven't found them yet, but still looking)

I think their observations were accurate.

This is the paper detailing Hau's ``stopping light'' experiment: C. Liu et al., Nature 409 490-493 (2001).

keithisco, on 04 December 2012 - 12:34 PM, said:

4. How can we be certain that there was not an element of "Confirmation Bias" in their published account.

That is always a risk. But manipulating light in this way has a long (and repeatable) history, see the wiki on metamaterials for example.

keithisco, on 04 December 2012 - 12:34 PM, said:

5. Finally: Shouldn't a beam of light lose amplitude (and thus luminosity) rather than speed of propogation in this experimental set-up?

It didn't ``lose speed'', not really. The report is a bit misleading.

The light was absorbed and then, at a later time, an identical beam of light was emitted.

In fact, the light did lose quite a bit of amplitude (the absorption-emission process is not perfect). In all cases the emitted luminosity was less than 20% of the absorbed luminosity.

keithisco, on 04 December 2012 - 12:34 PM, said:

Just a point to ponder: Why does "Light" have the ability to propogate at the "Speed of Light" forever with no apparent energy source to draw on - makes my head hurt also.

I don't think it is unreasonable.

Light doesn't interact with anything other than by being absorbed or redirected (scattering). If it experiences no friction, why should it ever slow down?

[keithisco]

Sepulchrave: thanks for your replies - always interesting and well considered. leaving aside my first, somewhat contentious, belief that C is little more than a mis-interpretation by Einstein....

from the article, and what you have added in clarification of said article: the atomic cloud is cooled to just above the Bose - Einstein Condensate point (Ground State for said atom I presume, but this is not made clear in the article) and is continuously cooled during the experimental process.

it follows that this allows the absorption of a Photon. This absorption would raise the energy level of the atom - but, and I am only guessing, the increase in energy is somehow held in check by the cooling to prevent the intrinsic increase in energy level from immediately re-transmitting the Photon?? Is it the Control Coherent Light (i.e. Laser being switched back on) that actually allows the Atoms energy level to increase to the point where the absorbed Photon is then transmitted?

Rather than "exactly the same photon" is released is it not more accurate to say that a Photon is transmitted carrying the Original Information, because the original photon was absorbed? if in fact this is correct, then the data is actually held within the matrix of the atom, and that mechanism is not discussed. It may be no more than a Quantum effect of the specific Atom, being cooled to a specific temperature, that will always emit a Photon when excited at precisely the correct phase and wavelength.

Following on from the claims in the article, I do not recall seeing what the Control in the Experiment was.

For myself, one of the controls would be to maintain the cooling, (after the Probe Beam had been emitted, and stored) and not switching on the Control Laser at all, and determining at what point the system fails to maintain the Energy level of the Atoms to prevent Photon emission and consequent detection by the Probe.

Another would be raisng the temperature of the Atom Cloud and recording the effect on the probe.

"Doping" the Atom cloud with atoms having different properties, and recording the effects

Changing the frequency of the Coherent Control source.

Quite possibly all of these Controls have been performed, but a paper should (IMO) contain all of these elements.

As an Addendum: Light DOES interract variously depending on the wavelength, for instance within the Spectrum of White Light there are the 2 extremes of UV and IR which cause cellular change in human (and most fauna) epidermis

[sepulchrave]

keithisco, on 04 December 2012 - 07:27 PM, said:

Sepulchrave: thanks for your replies - always interesting and well considered. leaving aside my first, somewhat contentious, belief that C is little more than a mis-interpretation by Einstein....

from the article, and what you have added in clarification of said article: the atomic cloud is cooled to just above the Bose - Einstein Condensate point (Ground State for said atom I presume, but this is not made clear in the article) and is continuously cooled during the experimental process.

it follows that this allows the absorption of a Photon. This absorption would raise the energy level of the atom - but, and I am only guessing, the increase in energy is somehow held in check by the cooling to prevent the intrinsic increase in energy level from immediately re-transmitting the Photon?? Is it the Control Coherent Light (i.e. Laser being switched back on) that actually allows the Atoms energy level to increase to the point where the absorbed Photon is then transmitted?

I think so. The control laser prevents the excited atoms from releasing the originally absorbed energy.

keithisco, on 04 December 2012 - 07:27 PM, said:

Rather than "exactly the same photon" is released is it not more accurate to say that a Photon is transmitted carrying the Original Information, because the original photon was absorbed? if in fact this is correct, then the data is actually held within the matrix of the atom, and that mechanism is not discussed. It may be no more than a Quantum effect of the specific Atom, being cooled to a specific temperature, that will always emit a Photon when excited at precisely the correct phase and wavelength.

You are correct; it is the original information. The mechanism for holding this information is quite simple, since a photon is quite simple: a photon has only an energy (proportional to the wavelength and the frequency) and a polarization. An electron in the atom would be promoted to a higher energy level by exactly the energy of the photon, and the spin (or magnetic moment) of the atom could change to reflect the atom's polarization (or spin). Both of these changes are deviations from the ground state, so once the control laser is turned off the atom goes back to the ground state by releasing the energy and magnetic moment in the form of a photon.

Since the system is so cold and so controlled, none of these quantities (the absorbed energy or magnetic moments) could change in between the absorption and emission event, so the emitted photon will have the same information as the absorbed photon.
(Of course as their actual data shows, this only happens 20% or less of the time; the system is not at absolute zero and the control laser isn't perfect so 80% of the absorbed photons get scrambled.)

keithisco, on 04 December 2012 - 07:27 PM, said:

Following on from the claims in the article, I do not recall seeing what the Control in the Experiment was.

For myself, one of the controls would be to maintain the cooling, (after the Probe Beam had been emitted, and stored) and not switching on the Control Laser at all, and determining at what point the system fails to maintain the Energy level of the Atoms to prevent Photon emission and consequent detection by the Probe.

Another would be raisng the temperature of the Atom Cloud and recording the effect on the probe.

"Doping" the Atom cloud with atoms having different properties, and recording the effects

Changing the frequency of the Coherent Control source.

Quite possibly all of these Controls have been performed, but a paper should (IMO) contain all of these elements.

I agree that all of these things are useful controls.

I sort of disagree that the paper should contain all of those elements... in my professional opinion (I am studying physics, I have published several papers and been part of the peer-review process for several others) a good paper is short and direct.

In this paper, I would say that the first control you discuss (leaving the control laser off for longer and longer) has been adequately explored, as you can see in Figure 2d. It is fair to argue that the author's attempt to claim an exponential dependence of pulse transmission vs time might not be correct, but I think everyone will agree that after 1.5 ms there will be almost no pulse transmitted - i.e. they system has completely thermalized (or scrambled) the input pulse.

I agree they could mention other avenues of exploration, but I wouldn't be surprised if two of the controls you mention (temperature of the atom cloud and frequency of the control laser) had already been examined by someone in references 1 through 11 (although I haven't checked these myself, so I could be wrong); at the very least the authors make clear they are building on previous research.

keithisco, on 04 December 2012 - 07:27 PM, said:

As an Addendum: Light DOES interract variously depending on the wavelength, for instance within the Spectrum of White Light there are the 2 extremes of UV and IR which cause cellular change in human (and most fauna) epidermis

Yes, but that is because the absorption and scattering cross-sections of various materials are dependent on the wavelength of light.

Light is a quantization of the electromagnetic field, but light itself can't experience any force because it doesn't couple to any fundamental forces (it has no mass, no charge, etc.).

Absorption or scattering of light can even transmit momentum, but this is more of a ``contact force''; an emergent property rather than a fundamental one.

A beam of light can only be ``slowed'' in the sense that the scattering and absorption of individual photons has the net effect of slowing down the energy propagation of an ensemble of photons. Each individual photon is still zipping along at c in the spaces between atoms, since outside of direct contact no force can act on the photon.

[keithisco]

sepulchrave, on 05 December 2012 - 12:06 AM, said:

I think so. The control laser prevents the excited atoms from releasing the originally absorbed energy.


You are correct; it is the original information. The mechanism for holding this information is quite simple, since a photon is quite simple: a photon has only an energy (proportional to the wavelength and the frequency) and a polarization. An electron in the atom would be promoted to a higher energy level by exactly the energy of the photon, and the spin (or magnetic moment) of the atom could change to reflect the atom's polarization (or spin). Both of these changes are deviations from the ground state, so once the control laser is turned off the atom goes back to the ground state by releasing the energy and magnetic moment in the form of a photon.

Since the system is so cold and so controlled, none of these quantities (the absorbed energy or magnetic moments) could change in between the absorption and emission event, so the emitted photon will have the same information as the absorbed photon.
(Of course as their actual data shows, this only happens 20% or less of the time; the system is not at absolute zero and the control laser isn't perfect so 80% of the absorbed photons get scrambled.)


I agree that all of these things are useful controls.

I sort of disagree that the paper should contain all of those elements... in my professional opinion (I am studying physics, I have published several papers and been part of the peer-review process for several others) a good paper is short and direct.

In this paper, I would say that the first control you discuss (leaving the control laser off for longer and longer) has been adequately explored, as you can see in Figure 2d. It is fair to argue that the author's attempt to claim an exponential dependence of pulse transmission vs time might not be correct, but I think everyone will agree that after 1.5 ms there will be almost no pulse transmitted - i.e. they system has completely thermalized (or scrambled) the input pulse.

I agree they could mention other avenues of exploration, but I wouldn't be surprised if two of the controls you mention (temperature of the atom cloud and frequency of the control laser) had already been examined by someone in references 1 through 11 (although I haven't checked these myself, so I could be wrong); at the very least the authors make clear they are building on previous research.


Yes, but that is because the absorption and scattering cross-sections of various materials are dependent on the wavelength of light.

Light is a quantization of the electromagnetic field, but light itself can't experience any force because it doesn't couple to any fundamental forces (it has no mass, no charge, etc.).

Absorption or scattering of light can even transmit momentum, but this is more of a ``contact force''; an emergent property rather than a fundamental one.

A beam of light can only be ``slowed'' in the sense that the scattering and absorption of individual photons has the net effect of slowing down the energy propagation of an ensemble of photons. Each individual photon is still zipping along at c in the spaces between atoms, since outside of direct contact no force can act on the photon.

Good answers, thanks Sepulchrave!!

[behavioralist]

keithisco, on 04 December 2012 - 12:34 PM, said:

I always found that experiment interesting because it confounds the idea that C is a Constant. It also confounds Energy / Mass Equivalence and one of the (erroneous) platforms of modern, received, establishment physics (i.e. Einstein).

In fact, the experiment poses more questions than answers:

1.  What is the mechanism for transferring light "energy" to the surrounding cloud of atoms? Is it absorption?
2. If the above absorption IS the method then what was the mechanism for transferring back that energy from the cloud of atoms (in precisely the correct amount) to   permit energetic light to escape the chamber?
3. Were they really observing and recording what they thought they were - instrumentation and detection are crucial in validation and verification. How was the measurement of speed actually performed - sure, the math is very simple distance travelled  over time, but on such a small scale I would like to know the detection methods (haven't found them yet, but still looking)
4. How can we be certain that there was not an element of "Confirmation Bias" in their published account.
5. Finally: Shouldn't a beam of light lose amplitude (and thus luminosity) rather than speed of propogation in this experimental set-up?

Just a point to ponder: Why does "Light" have the ability to propogate at the "Speed of Light" forever with no apparent energy source to draw on - makes my head hurt also.

The last Point: light keeps moving but not getting longer. There are a steady number of photons of the  light. I Think you didn't express the question you actually had in mind.

[behavioralist]

Do we know for a fact that no black hole is accelerating light past c? In other words, that a BH contains no temporal anomaly?It's probably quite naive of me, but I tend to imagine that a BH can be a Place where light "continues" just as if it were in space rather than "crashing into the orb" after the initial crowding to fit through the smallest possible door. Time suggests a possibility of a direction where falling never ends.

Edited by behavioralist, 09 December 2012 - 03:02 PM.

[sepulchrave]

behavioralist, on 09 December 2012 - 02:57 PM, said:

Do we know for a fact that no black hole is accelerating light past c? In other words, that a BH contains no temporal anomaly?

Technically we don't know much about black holes.

We know that there really are regions of space that appear to have an immense gravitational pull but are otherwise invisible.

And we know that according to the Theory of General Relativity it is possible for a sufficiently dense object to form a gravitational singularity.

We don't really know that the former is a consequence of the latter, and without a full conformal field theory that merges Quantum Mechanics with gravity we can't really prove it, but there is good evidence to suggest that the ``black holes'' we see in telescopes are the gravitational singularities suggested by General Relativity.

With that in mind, in General Relativity, light can't really be ``accelerated'' by anything. (Even in the sense that ``acceleration'' can mean a change in direction; light always travels along geodesics so technically near a black hole the light doesn't change direction; rather space-time does.)

So to the extend that black holes are described by General Relativity as a gravitational singularity, no... there is no temporal anomaly or ``faster than c'' light.

If General Relativity is not an appropriate (or a complete) description of a black hole then... I don't know.

Are you familiar with the distortion of time and space caused by gravity, especially in regards to inside and outside the event horizon of a black hole?
(In particular, do you understand the meaning of the first image in the wiki page on event horizons?) If not, it might help answer some of your questions.

[keithisco]

sepulchrave, on 09 December 2012 - 06:05 PM, said:

Technically we don't know much about black holes.

We know that there really are regions of space that appear to have an immense gravitational pull but are otherwise invisible.

And we know that according to the Theory of General Relativity it is possible for a sufficiently dense object to form a gravitational singularity.

We don't really know that the former is a consequence of the latter, and without a full conformal field theory that merges Quantum Mechanics with gravity we can't really prove it, but there is good evidence to suggest that the ``black holes'' we see in telescopes are the gravitational singularities suggested by General Relativity.

With that in mind, in General Relativity, light can't really be ``accelerated'' by anything. (Even in the sense that ``acceleration'' can mean a change in direction; light always travels along geodesics so technically near a black hole the light doesn't change direction; rather space-time does.)

So to the extend that black holes are described by General Relativity as a gravitational singularity, no... there is no temporal anomaly or ``faster than c'' light.

If General Relativity is not an appropriate (or a complete) description of a black hole then... I don't know.

Are you familiar with the distortion of time and space caused by gravity, especially in regards to inside and outside the event horizon of a black hole?
(In particular, do you understand the meaning of the first image in the wiki page on event horizons?) If not, it might help answer some of your questions.

To begin with this phrase:

"And we know that according to the Theory of General Relativity it is possible for a sufficiently dense object to form a gravitational singularity"

We dont actually know that this theory (.much more a hypothesis than a theory) is correct, so Scientifically speaking you must NOT say that we "know" anything from a theory, but that "it is surmised"

"Time and Space Distortion"  is very much unproven, in fact it only has Einstein's and Hawking's backing... these people are not Gods in the pursuit of Science, they are theoriticians, the Math propounded is flawed, as has been shown many times  ("C" magically reintroduced in a new term "Y", AFTER "C"  has been negated)