Sunday, August 18, 2019

Big Bang Disproved?!

Of course not! But this is still a fun video for you to watch, especially if you are not up to speed on (i) how we know that the universe is expanding and (ii) the current discrepancy in the measurement of the Hubble constant via two different methods.

But unlike politics or social interactions, discrepancies and disagreement in science are actually welcomed and a fundamental aspects of scientific progress. It is how we refine and polish our knowledge into a more accurate form. As Don Lincoln says at the end of the video, scientists love discrepancies. It means that there are more things that we don't know, and more opportunities to learn and discover something new.


Wednesday, August 14, 2019

Relativisitic Length Contraction Is Not So Simple To See

OK, I actually had fun reading this article, mainly because it opened up a topic that I was only barely aware of. This Physics World article describes the simple issue of length contraction, but then delves into why OBSERVING this effect, such as with our own eyes, is not so simple.

If the Starship Enterprise dipped into the Earth’s atmosphere at a sub-warp speed, would we see it? And if the craft were visible, would it look like the object we’re familiar with from TV, with its saucer section and two nacelles? Well, if the Enterprise were travelling fast enough, then – bright physicists that we are – we’d expect the craft to experience the length contraction dictated by special relativity.

According to this famous principle, a body moving relative to an observer will appear slightly shorter in the direction the body’s travelling in. Specifically, its observed length will have been reduced by the Lorentz factor (1–v2/c2)1/2, where v is the relative velocity of the moving object and c is the speed of light in a vacuum. However, the Enterprise won’t be seen as shorter despite zipping along so fast. In fact, it will appear to be the same length, but rotated.

You might not have heard of this phenomenon before, but it’s often called the “Terrell effect” or “Terrell rotation”. It’s named after James Terrell – a physicist at the Los Alamos National Laboratory in the US, who first came up with the idea in 1957. The apparent rotation of an object moving near the speed of light is, in essence, a consequence of the time it takes light rays to travel from various points on the moving body to an observer’s eyes.
You can read the rest of the explanation and graphics in the article. Again, this is not to say that your "pole-in-barn" exercise that you did in relativity lessons is not valid. It is just that in that case, you were not asked what you actually SEE with your eyes when that pole is passing through the barn, and that your pole is long and thin, as opposed to an object with a substantial size and width. The notion that such object will be seen with our eyes flat as a pancake is arguably may not be true here.


Thursday, August 08, 2019

RIP J. Robert Shrieffer

I'm sad to hear the passing of a giant in our field, and certainly in the field of Condensed Matter Physics. Nobel Laureate J. Robert Schrieffer has passed away at the age of 88. He is the "S" in BCS theory of superconductivity, one of the most monumental theories of the last century, and one of the most cited. So "complete" was the theory that, by early 1986, many people thought that the field of superconductivity has been fully "solved", and that nothing new can come out of it. Of course, that got completely changed after that.

Unfortunately, I wasn't aware of his predicament during the last years of Schrieffer's life. I certainly was not aware that he was incarcerated for a while.

Late in life, Dr. Schrieffer’s love of fast cars ended in tragedy. In September 2004, he was driving from San Francisco to Santa Barbara, Calif., when his car, traveling at more than 100 miles per hour, slammed into a van, killing a man and injuring seven other people.

Dr. Schrieffer, whose Florida driver’s license was suspended, pleaded no contest to felony vehicular manslaughter and apologized to the victims and their families. He was sentenced to two years in prison and released after serving one year.

Florida State placed Dr. Schrieffer on leave after the incident, and he retired in 2006.

I've met him only once while I was a graduate student, and he was already at Florida State/NHML at that time. His book and Michael Tinkham's were the two that I used when I decided to go into superconductivity.

Leon Cooper is the only surviving members left of the BCS trio.


Tuesday, August 06, 2019

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.


Saturday, August 03, 2019

Einstein's Blunder Explained

This, actually, is a good and quick summary of the Einstein cosmological equation by Minute Physics. You'll get a brief history of the cosmological constant, and how it came back to life.