Science team shows light is made of particles and waves

[C1ay]

Work completed by a visiting research professor at Rowan University, physics professors and a student from the institution shows that light is made of particles and waves, a finding that refutes a common belief held for about 80 years.

 

Shahriar S. Afshar, the visiting professor who is currently at Boston's Institute for Radiation-Induced Mass Studies (IRIMS), led a team, including Rowan physics professors Drs. Eduardo Flores and Ernst Knoesel and student Keith McDonald, that proved Afshar's original claims, which were based on a series of experiments he had conducted several years ago.

 

An article on the work titled "Paradox in Wave-Particle Duality" recently published in "Foundations of Physics," a prestigious, refereed academic journal, supports Albert Einstein's long-debated belief that quantum physics is incomplete. For eight decades the scientific community generally had supported Niels Bohr's ideas commonly known as the Copenhagen Interpretation of Quantum Mechanics. In 1927, in his "Principle of Complementarity," he asserted that in any experiment light shows only one aspect at a time, either it behaves as a wave or as a particle. Einstein was deeply troubled by that principle, since he could not accept that any external measurement would prevent light to reveal its full dual nature, according to Afshar. The fundamental problem, however, seemed to be that one has to destroy the photon in order to measure either aspects of it. Then, once destroyed, there is no light left to measure the other aspect.

 

"About 150 years ago, light was thought to behave solely as a wave similar to sound and water waves. In 1905, Einstein observed that light might also act as being made out of small particles. Since then physicists found it difficult understanding the full nature of light since in some situations it acts like a particle and in others like a wave," Flores said. "This dual nature of light led to the insight that all fundamental physical objects include a wave and a particle aspect, even electrons, protons and students."

 

Afshar conducted his initial theoretical and experimental work at IRIMS, where he served the privately funded organization as a principal investigator. He later continued his work at the Harvard University Physics Department as a research scholar, where he was able to verify his initial findings before going to Rowan.

 

In 2004, Afshar claimed that he had devised an experiment that challenged Bohr's principle of complementarity. The Rowan team was formed to verify Afshar's claim at extremely low light intensity levels. Afshar, Flores and Knoesel conducted experiments at Rowan that validated Afshar's initial findings for single photons.

 

In this modified double-slit experiment, a laser beam hits a screen with two small pinholes. As a particle, light goes through one of the pinholes. Through a lens system, the light is then imaged onto two detectors, where a certain detector measures only the photons, which went through a particular pinhole. In this way, Afshar verified the particle nature of light. As a wave, light goes through both pinholes and forms a so-called interference pattern of bright and dark fringes.

 

"Afshar's experiment consists of the clever idea of putting small absorbing wires at the exact position of the dark interference fringes, where you expect no light," Knoesel said. "He then observed that the wires do not change the total light intensity, so there are really dark fringes at the position of the wires. That proves that light also behaves as a wave in the same experiment in which it behaves as a particle."

 

The findings of the Afshar experiment were published online on January 23 in the "Foundations of Physics," an international journal devoted to the conceptual bases and fundamental theories of modern physics, biophysics and cosmology, with several distinguished Nobel laureates on its editorial board. The paper version of the manuscript was published in the February 2007 edition and is now available in libraries throughout the world.

 

"The important new contribution is that light carries both wave and particle aspects at all times, and future experiments will further clarify the nature of each component." Afshar said.

 

Flores continued, "It is interesting to note that even after 80 years we can still gain a better understanding about the nature of light using refined measurement techniques and creative ideas and therefore are able add to the vast insights of former scientists."

 

Source: Rowan University

[Boerseun]

Awesomely interesting! Just goes to show, we're still a long way off, but making headway every single day!

[Tormod]

Hm...did the Hypography Politburo (Dept. of Close-Mindedness) actually accept this for publication? Heresy.

[pgrmdave]

So...what does that mean, exactly?

[Pyrotex]

No! It is not heresy!

 

Well, not NOW. Not after the Hypo Quantum Quibbler Team worked so damn hard to "rewire" the underlying structure of the Space-Time Continuum to make this new experiment possible!

 

Whew! And are we ever exhausted! You ever try to thread quarks on a quantum string? In the dark?

 

PS: I think this will go down as one of the KEY DEFINING EXPERIMENTS of the 21st Century! No kidding. I believe we are about to blow our understanding of the quantum realm wide freakin open.

[Tormod]

So...what does that mean, exactly?

 

I mean, seriously, don't we have a hidden agenda to *not* publish anything that a) says something else than MAINSTREAM SCIENCE (ie the TRUTH) and :) tries to look at things from a NEW PERSPECTIVE???

 

What's the world coming to.

[CraigD]

There’s a clear description of Afshar’s experiment at this wikipedia section.

 

My first impression upon reading the hypography article and this Rowan University article was that there was either something wrong with Afshar’s experimental conclusions, or with everything I understood about decoherence, with a strong leaning to former.

 

After reading a more detailed description at wikipedia, and Afshar, Flores, McDonald, and Knoesel’s Foundation of Physics paper (Note that the actual apparatus described in these two descriptions differ substantially, but are, I think, equivalent), I’m convinced that Afshar’s experiment, both the 2001 high-intensity light and the 2004 low-intensity, single photon one, do not challenge complementarity, because they do not actually capture “which way” information for individual photons, but only detector counts of on the order of 10^4 photons – that AFMK call “the integrated flux image”. It is not possible from this data to give a photon-by-photon, left/right account (eg: “LRRLRLLRRRLRRLRLL …(10^4+ characters)… LLRRL”) more accurate than one based on an accurate count of the total number of photons emitted and that number of coin tosses. Were the experiment repeated, and the data necessary for such an account recorded, I believe the interference pattern would disappear (the number of photons counted at the detectors be reduced), as has been demonstrated in various “quantum eraser” experiments performed in the past few decades, such as [quant-ph/9903047] A Delayed Choice Quantum Eraser. My opinion appears to be shared by some of the critics cited in the wikipedia article.

 

I’m at a loss (and restrained by politeness) to explain why apparently educated physicists such as AFMK would not arrive at a similar objection to their offered interpretation, but nonetheless, don’t think these experiments are scientifically significant.

[CraigD]

So...what does that mean, exactly?

Briefly put, quantum physics predicts that

• Even if only a single photon is present within the apparatus at any time, light directed through a blocking device with 2 properly sized and spaced slits will produce an interference pattern of light and dark bands on a detector, such as photographic film, placed behind it.
  
• Detecting with certainty which of the 2 slits each photon passes through (including by the simple method of blocking one slit) will cause the pattern to disappear
  

Afshar et. all claim to have violated the second of these predictions. If this is so, it could have far-reaching implications.

[Pyrotex]

...Afshar et. all claim to have violated the second of these predictions. If this is so, it could have far-reaching implications.

 

I smell Nobel Prize.

[IDMclean]

Doesn't suprise me at all.

 

As I have placed as objection in the past, why must I accept that the photon, electron, etc went through the slit at all?

 

Ah, convient fictions. How scientist love them, and forget to note them.

 

I hope to hear about repeated experiments soon by other scientific groups. Maybe then we can make some headway into the relative-quantum realm.

[CraigD]

I smell Nobel Prize.

I smell publicity for IRIMS (follow that link and see if you agree it has a promotional feel to it) and Rowan University. Coming from a slightly smaller, less frequently renamed, but very publicity-hungry-school-for-teachers turned-college-turned-university, Concord University (Concord College in my day), I sympathize. I was myself a party to a sort of academic “publicity campaign” involving a discrete math and probability paper I coauthored with my professor, which, while less likely of mention for a Nobel, was clearly correct.

[CraigD]

As I have placed as objection in the past, why must I accept that the photon, electron, etc went through the slit at all?

This seems an easy question to answer: because when you cover the slits, no photons, electrons, etc. appear on the other side at all.

I hope to hear about repeated experiments soon by other scientific groups.

The experiment itself is a pretty old one, though AFAIK, the idea of detecting the loss of an interference pattern by masking its dark bands and noting the reduction in light at the detector, rather than detecting the pattern or lack of one at the detector itself, is new, as is detecting the photon’s with a pair of distant detectors, rather than some sort of device near the slit.

 

Record the actual sequence of (very low light level) photon detections, not just their counts, and I predict the detected number of events will drop to a number indicating loss of the interference pattern – a profoundly weird effect, but one that’s appeared in every previous, similar experiment that recorded that information.

[Jay-qu]

I fail to see how this experiment shows both the wave and particle nature of light...

[CraigD]

I fail to see how this experiment shows both the wave and particle nature of light...

As any 2-slit diffraction experiment does, it shows the wave nature of light, because a “interference pattern” of alternating bright and dark bands appears behind the 2-slits. In Afshar’s variation of the experiment, this pattern is detected by placing wires where the dark bands appear, and measuring the intensity of the light with and without the wires to find that it is nearly (98%) unchanged.

 

The experiment doesn’t really show the particle nature of light, but this is a given from other experiments. The classic experiment for this is to place successively darker filters (stacked smoked glass plates, etc.) between a light source and a detector (an electronic photomultiplier, photographic film, etc.) and measure the light gathered. You’ll detect a “notchiness” in the data as it approaches zero, finding that you reach a point where adding filters doesn’t reduce the light, but zeros it. This “final step” depends on the frequency of the light, and represents a single photon. If light were purely a wave, it would be possible to continue reducing its intensity to the limits of your detector’s sensitivity.

 

Afshar and colleagues aren’t attempting to demonstrate the wave/particle nature of light, which has long been well accepted. They’re attempting to demonstrate that both can be detected simultaneously. The principle of complementarity predicts, and past experiments similar to Afshar’s confirm, that this is impossible: when you gather data of exactly which photon went through which slit, the interference pattern disappears. Afshar contends (but I disagree) that he has managed to gather this data without disappearing the pattern.

[Qfwfq]

"Afshar's experiment consists of the clever idea of putting small absorbing wires at the exact position of the dark interference fringes, where you expect no light," Knoesel said. "He then observed that the wires do not change the total light intensity, so there are really dark fringes at the position of the wires. That proves that light also behaves as a wave in the same experiment in which it behaves as a particle."

I utterly fail to see what this adds to the classic experiment. The very fact that one is receiving counts of individual photons from the detector and also seeing the interference means that both aspects are present in the experiment. Nobody would have expected those wires to change the total count anyway. I agree with:

Even if only a single photon is present within the apparatus at any time, light directed through a blocking device with 2 properly sized and spaced slits will produce an interference pattern of light and dark bands on a detector, such as photographic film, placed behind it.

except I wouldn't exactly say "pattern of light and dark bands" but greater and less frequency of counts. For each single photon it's a matter of probability density. Where this probability density is about zero, you don't expect the obstacle to make much difference.