Quantum Mechanics and the Double-Slit Experiment

The double-slit experiment continues to be of interest with respect to quantum mechanics, even after so many years. I've mentioned about this many times, with this one being the most relevant here to this particular post. And note that I made that blog entry back in 2013!

This time, Don Lincoln of Fermilab has released a video on the topic of the double-slit experiment and how it is relevant to QM.

BTW, has he lost weight? If he has, I hope it is on purpose and not due to an illness.

In any case, watch the video and check out the link that I gave. This issue doesn't look like it will be resolved anytime soon unless some new experiment comes up.

Zz.

Combining The Best Of Both Worlds

This is a fascinating and important advancement in the physics of light sources. It seems that it has been shown experimentally how one can get the short, intense light pulses that one gets from a FEL source, and combine it with the repetition that one gets from a synchrotron light source.

Now a Sino-German team has shown that a pattern of pulses can be generated in a synchrotron radiation source that combines the advantages of both systems. The synchrotron source delivers short, intense microbunches of electrons that produce radiation pulses having a laser-like character (as with FELs), but which can also follow each other closely in sequence (as with synchrotron light sources). 

Another review of this work, from Nature where it was published, can be found here.

While this is an important step, it really is a proof-of-principle experiment, and it requires a bit more experimental work to show that this can be viable.

Although this paper represents a crucial step towards generating high-power, small-bandwidth light pulses in a particle accelerator, steady-state microbunching has not yet been demonstrated. Deng et al. have shown that, after one turn in the synchrotron, the microbunched beam can produce coherent radiation. The next challenge is to prove that this scheme can achieve such a feat over many turns. This will be difficult to accomplish experimentally for at least three reasons.

But if this can be demonstrated, a lot of things that are done at a FEL can be performed even more at an "ordinary" synchrotron light source, a facility that is a lot more plentiful.

An important point that I want to point out here is that, these are all "tools" that allow us to study things. Without these tools, we have no ability to experimentally detect, see, or measure things. It enables us to do things that we could not do before. So the advancement in science, technology, medicine, etc, depend on not only having these tools, but also the continual improvement of these tools. Advancement in science requires all of these things to occur to able to explore more difficult and complex ideas and scenarios.

This advancement in accelerator-based light source has nothing to do with high-energy physics. In fact, if you look at the type of applications that are being mentioned, there's nothing about particle physics at all!

.....on an accelerator that could extend the capabilities of these machines even further, potentially yielding applications in a next-generation chip-etching technology called extreme-ultraviolet lithography and an advanced imaging method known as angle-resolved photoemission spectroscopy.

So once again, this is my continuing attempt at trying to make people aware that "accelerators" do not automatically mean "particle collider" or "high energy physics". In fact, the majority of particle accelerators in this world are not involved in high energy physics experiments.

Zz.

DESI Begins

A new eye on the sky is about to add to our knowledge of dark energy.



It's interesting that in the list of funding agencies, NASA is absent. This goes to show you that many of these research activities that seem to be "astronomy-related" are not the sole domain of NASA. In fact, the area of particle-astrophysics is more closely related to particle physics than astronomy.

The video didn't clarify explicitly that in looking at the "spectrum" of light from each of these celestial bodies, one gets the radial velocity of these bodies with respect to us (i.e. via the amount of redshift), not its distance from us. That last piece of information can only be "deduced" using the radial velocity and the Hubble equation, i.e. the Hubble constant, a number that is still being refined.

Still, this new telescope is going to be quite exciting in revealing more of the mysteries of dark energy.

Zz.

Light Drags Electrons Backward?

As someone who was trained in condensed matter physics, and someone who also worked in photoemmission, light detectors, and photoelectron sources, research work on light interaction with solids, and especially with metallic surfaces, is something I tend to follow rather closely.

I've been reading this article for the past few days and it gets fascinating each time. This is a report on a very puzzling photon drag effect in metals, or in this case, on gold, which is the definitive Drude metal if there is any. What is puzzling is not the photon drag on the conduction electron itself. What is puzzling is that the direction of the photon drag appears to be completely reversed between the effect seen in vacuum versus in ambient air.

A review of the paper can be found here. If you don't have access to PRL, the arXiv version of the paper can be found here. So it appears as if that, when done in vacuum, light appears to push the conduction electrons backward, while when done in air, it pushes electrons forward as expected.

As they varied the angle, the team measured a voltage that largely agreed with theoretical expectations based on the simple light-pushing-electrons picture. However, the voltage they measured was the opposite of that expected, implying that the current flow was in the wrong direction. It’s a weird effect," says Strait. “It’s as if the electrons are somehow managing to flow backward when hit by the light.”

Certainly, surface effects may be at play here. And those of us who have done photoemission spectroscopy can tell you all about surface reconstruction, even in vacuum, when a freshly-cleaved surface literally changes characteristics right in front of your eyes as you continually perform a measurement on it. So I am not surprised by the differences detected between vacuum and in-air measurement.

But what is very puzzling is the dramatic difference here, and why light appears to push the conduction electrons one way in air, and in the opposite direction in vacuum. I fully expect more experiments on this, and certainly more theoretical models to explain this puzzling observation.

This is just one more example where, as we apply our knowledge to the edge of what we know, we start finding new mysteries to solve or to explain. Light interaction with matter is one of the most common and understood phenomena. Light interaction with metals is the basis of the photoelectric effect. Yet, as we push the boundaries of our knowledge, and start to look at very minute details due to its application in, say, photonics, we also start to see the new things that we do not expect.

It is why I always laugh whenever someone thinks that there is an "end of physics". Even on the things that we think we know or things that are very common, if we start to make better and more sensitive measurement, I don't doubt that we will start finding something else that we have not anticipated.

Zz.

Why Does Light Bend When It Enters Glass?

Don Lincoln tackles another "everyday" phenomenon. This time, he tries to give you an "explanation" on why light changes direction when it goes from one medium to another, and why some of the more popular explanation that have been given may be either incomplete, or wrong.



Certainly, any undergraduate physics student would have already dealt with the boundary conditions using Maxwell's equations, so this should be entirely new. However, he skipped rather quickly something that I thought was not handled thoroughly.

The continuity of the parallel component of E to the boundary is fine. However, Lincoln argued that the reason why the perpendicular component of the F field is shorter in glass is due to the polarization of the material, and thus, the sum of the light's E-field and the E-field from the polarization will cause the net, resultant E-field to be shorter.

But if the material's polarization can affect the perpendicular component, why doesn't it also affect the parallel component? After all, we assume that the material is isotropic. This, he left out, and at least to me, made it sound that the parallel component is not affected. If this is so, why?

Zz.

Cerenkov Radiation

Don Lincoln tackles the origin of Cerenkov radiation this time. This is the case where a body travels faster than light in a medium.

This is not purely academic. This is how we detect certain particles, such as neutrinos. Those photodetectors in, say, SuperKamiokande, are detecting these Cerenkov radiation. In fact, if you look in a pool of water of nuclear fuel rods, the blue light is the result of Cerenkov radiation.

So here's a chance for you to learn about Cerenkov radiation.



Zz.

Don't Go To The Movies With A Physicist?

OK, no one tell any of my friends that, or I'll be going to the movie alone from now on.

This article interviews professors Maxim Sukharev and Michael Dugger of the Applied Physics Lab at Arizona State University on the physics that they noticed in the movies. The article focuses on light, as in lasers, since these scientists are experts on them.

“Lightsabers? I don’t know what those are supposed to be,” said Dugger in puzzlement, as the two settled into Siskel and Ebert mode. “If that’s a laser, particles of light would never just stop abruptly like that."

“Of course, if you see somebody on the big screen with a Russian accent doing science, that person will turn out to be a bad character,” Sukharev said with a chuckle. He completed a doctorate in the Department of High-Power Lasers in the General Physics Institute of the Russian Academy of Sciences in Moscow. “But what’s really laughable to me is when a spacecraft is shown speeding through the vacuum of deep space and yet we hear, ‘Zoom, zoom.’ 

I'm not that critical of the scientific mistakes or outrageous applications of science in the movies. They are, after all, fiction. But I can suspend my disbelief only so much, and if a movie takes too many liberties and transgression against science, then the movie is not longer that credible, because one can just make things up without regards to anything.

I can't wait for Avengers 4!

Zz.

Two Quantum Properties Teleported Simultaneously

People all over the net are going ga-ga over the report on the imaging of the wave-particle behavior of light at the same time. I, on the other hand, am more fascinated by the report that two different quantum properties have been teleported simultaneously for the very first time.

The values of two inherent properties of one photon – its spin and its orbital angular momentum – have been transferred via quantum teleportation onto another photon for the first time by physicists in China. Previous experiments have managed to teleport a single property, but scaling that up to two properties proved to be a difficult task, which has only now been achieved. The team's work is a crucial step forward in improving our understanding of the fundamentals of quantum mechanics and the result could also play an important role in the development of quantum communications and quantum computers. 

 See if you can view the actual Nature paper here. I'm not sure how long the free access will last.

Zz.