Friday, September 30, 2016

Dark Matter Biggest Challenge

A very nice article on Forbes' website on the latest challenge in understanding Dark Matter.

It boils down to on why in some cases, Dark Matter dominates, while in others, it seems that everything can be satisfactorily explained without using it. It is why we continue to study this and why we look for possible Dark Matter candidates.  There is still a lot of physics to be done here.


Monday, September 26, 2016

10 Years Of Not Even Wrong

Physics World has a provocative article and podcast to commemorate the 10-year anniversary of Peter Woit's devastating criticism of String Theory in his book "Not Even Wrong".

Not Even Wrong coincided with the publication of another book – The Trouble with Physics – that had a similar theme and tone, penned by Woit’s friend and renowned physicist Lee Smolin. Together, the two books put the theory and its practitioners under a critical spotlight and took string theory’s supposed inadequacies to task. The books sparked a sensation both in the string-theory community and in the wider media, which until then had heard only glowing reports of the theory’s successes. 

Interestingly enough, the few students that I've encountered who told me that they want to go into String Theory have never heard or were not aware of Woit's book. I can understand NOT WANTING to read it, but to not even be aware of it and what it is about sounds rather .... naive. This is a prominent physicist who produced a series of undeniable criticism of a particular field of study that you want to go into. Not only should you be aware of it, but you need to read it and figure it out.

It is still a great book to read even if it is 10 years old now.


Friday, September 23, 2016

Without Direction, or Has No Prefered Direction?

This is why popular news coverage of science can often make subtle mistakes that might change the meaning of something.

This UPI news coverage talks about a recent publication in PRL that studied the CMB and found no large-scale anisotropy in our universe. What this means is that our universe, based on the CMB, is isotropic, i.e. the same in all direction, and that our universe has no detectable rotation.

However, instead of saying that, it keeps harping on the idea that the universe "has no direction". It has directions. In fact, it has infinite directions. It is just that it looks the same in all of these directions. Not having a preferred direction, or being isotropic, is not exactly the same as "having no direction".

If you read the APS Physics article accompanying this paper, you'll notice that such a phrase was never used.

I don't know. As a layperson, if you read that UPI news article, what impression does that leave you? Or am I making a mountain out of a mole hill here?


Wednesday, September 21, 2016

Recap of ICHEP 2016

If you missed the recent brouhaha about the missing 750 GeV bump, here is the recap of ICHEP conference held recently in Chicago.


Tuesday, September 20, 2016

We Lost Deborah Jin

Wow! I didn't see this one coming.

I just read the news that Deborah Jin, someone who I consider to be a leading candidate to win the Nobel Prize, has passed away on Sept. 15 after a battle with cancer. Her work on the ultra-cold Fermionic gasses was groundbreaking, and she should have been awarded the Nobel Prize a long time ago!

Nearly two decades ago, Jin and her then PhD student Brian DeMarco were the first researchers to observe quantum degeneracy in a sufficiently cooled gas of fermionic atoms. They were the first to demonstrate the creation and control of such an ultracold "Fermi gas", which has since provided us with new insights into superconductivity and other electronic effects in materials. You can read this 2002 feature written by Jin on "A Fermi gas of atoms"

CRAP! We have lost another good one, and well before her time! Deepest condolences to her family and friends.

Edit: Here's the press release from JILA about this.


Monday, September 19, 2016

What Happen When A Law Professor Tries To Use The Physics Of Climate Change

Usually, something like this doesn't have a happy ending. This happened in a congressional hearing by Ronald Rotunda of Chapman University’s Fowler School of La.

But during the hearing, Rotunda picked an odd example of such a dissenter — Jerry Mitrovica, a Harvard geoscientist whose work has shown that when, in a warming world, you lose massive amounts of ice from Greenland or Antarctica, sea level actually plunges near these great ice sheets, but rises farther away from them. The reason is gravity: Ice sheets are so massive that they pull the ocean towards them, but as they lose mass, some of the ocean surges back across the globe.

We have covered this idea extensively in the past, including by interviewing Mitrovica. He has found, for instance, that if the West Antarctic ice sheet collapses, the United States would experience much worse sea level rise than many other parts of the world, simply because it is so distant from West Antarctica. “The peak areas are 30 to 35 percent higher,” Mitrovica told me last year.

But if Greenland melts, pretty much the opposite happens — the Southern hemisphere gets worse sea level rise. And if both melt together, they might partially offset one another.

Rotunda appears to have misinterpreted Mitrovica’s important insight as reflecting a contrarian perspective on climate change.

It is always a bad idea when a person, testifying as an "expert", does not understand the source that person is using, and then had the gall to tell a physicist questioning the conclusion to "read his article".


Friday, September 16, 2016

Another Case Where Electrons "Attract" Each Other

This is from a couple of months ago (hey, I'm kinda slow nowadays!), but what the hey....

So we all know (at least, I hope we all do) that the basic mechanism in conventional superconductors is the formation of Cooper Pairs. This is where two electrons in the material form an attractive coupling, which simply means that two electrons attraction each other. Despite the Coulomb repulsion between the two electrons, this attraction is due to the fact that the electrons live in a sea of phonons (lattice vibrations) that are formed by the positive ions of the material (or crystal lattice). So these phonons are the "glue" that bind these electrons together. Without them, two isolated electrons do not attract one another, obviously.

Now it seems that a long-proposed alternative mechanism of electron attraction has been confirmed. This time, two electrons attract each other not due to phonons, but due  to the repulsion of other electrons surrounding them. This is significant, and different than the phononic mechanism because this time, it is purely electronic in nature.

The original theoretical idea of such mechanism was first proposed by William Little[1], and the first experiment indicating its validity has been shown by Hamo et al,[2]. Certainly, there is an impetus to show if such electronic coupling could be a mechanism that leads to superconductivity. So far, none has been found. The Hamo et al., experiment decided to not deal with such complexity and just try to investigate first if such coupling would occur in the first place. So they did a nano-scale engineering design to show such a thing.

Very, very clever!

The big hurdle next is to find a material that can exhibit a similar effect.


[1] W.A. Little, Phys. Rev. 134, 1416 (1964),
[2] A. Hamo et al., Nature 535, 395 (2016).

Tuesday, September 13, 2016

The Difference Between Ghosts And Dark Matter

A rather interesting piece that I stumbled upon on the NPR website. The author is trying to explain why, since both ghosts and dark matter can't be "seen", why wouldn't scientists believe in ghosts while a large percentage of physicists believe in the existence of dark matter.

So how do physicists and astronomers get away with claiming the existence of cosmic ghosts (dark matter and dark energy) when they would probably roll their eyes at descriptions of the more terrestrial haunted-house kind?

The answer is data, its prevalence and its stability.

There are literally thousands of studies now of those rotating-too-fast galaxies out there — and they all get the same, quite noticeable result. In other words, data for the existence of dark matter is prevalent. It's not like you see the effect once in a while but then it disappears. The magnitude of the result — meaning its strength — also stays pretty consistent from one study to the next. The same holds true for studies of dark energy.
We need to make something VERY CLEAR here, especially for non-scientists. While we do not know exactly what this dark matter is made of, or we don't know what it is, we have already a very clear set of parameters of its CHARACTERISTICS. Based on what we have already observed and measured, there are QUANTITATIVE properties of these so-called dark matter.

This is important because of two reasons: (i) there are no such definitive behavior, characteristics, and quantitative description of "ghosts", and (ii) these quantitative properties allow us to make measurements and rule out unsuitable candidates that to not fit into what we already know.

This article is similar to the public science event that I attended several years ago in which Dan Hooper of Fermilab/UofC described the science of ghosts. Back then, he too included the possibility on whether ghosts can be made up of dark matter, and based on what we know about dark matter and what people have described what ghosts can do, he concluded that ghosts cannot be made up of dark matter.

So no. Ghosts and Dark Matter are not in the same league.


Direct Measurement of the Density Matrix

It is often a source of irony. We teach, or learn, something by dealing with the simplest case first, devoid of complexities. Only after that do we start learning more complex situations.

Yet, in many cases, it is extremely difficult to duplicate, in practice, this simplest case. Quantum mechanics is one such example. While we learn about QM at the intro level by looking at the case of an infinite potential well, a finite potential well, 1-electron central force, etc... trying to actually get a clear experiment on this is actually quite difficult. This is because we have to isolate the system that we want to measure from the rest of the world, so that only the simplest, most fundamental parameters are involved in the experiment.

This is one such case. The experimenters claims to finally being able to directly measure the elements of a density matrix. Yet, if you have done any amount of QM, you would have seen this density matrix in your QM classes. I remember encountering it when I was using Merzbacher's text. So this is a classic, text-book item that we are all taught in school. Yet, it is not an easy thing to measure directly, till now. This is possible due to advancements in the so-called "weak measurement" that have previously produced Bohn's Pilot-Wave-like results.

Still, it is nice to know that what you learn in those textbooks are actually correct! :)


Thursday, September 08, 2016

Drilling In The Importance Of Units, Via A Banana Bread

For some odd reasons, intro physics students somehow can't get it into their heads the importance of having units in their answers whenever they are called for. And they also don't tend to see why carrying their units during a calculation can help them check if they are doing things correctly.

I've had constant issues with students not including units in their solutions. Even after I emphasize its importance, and even after I explain why it is important, I still constantly get answers, even in exams, of just a number without any units for length, speed, etc...

So one day, at the beginning of a new session, I decided to try something that I hope would be memorable to the students. I normally have between 10 to 20 students in a class. So on the first day of class, I baked a loaf of banana bread (I am actually a decent baker) and brought it to class. I cut the up into enough pieces for all the students, warning them that it has nuts and not gluten free, in case there are students with such allergies.

I let them enjoy the banana bread and then, I told them I baked it. They were impressed. Then I said that I'll give them the recipe if they want to bake this on their own. This is what I gave them:

  • 12 butter
  • 1 sugar
  • 2 eggs, beaten
  • 4 bananas, finely crushed
  • 1 12 flour
  • 1 baking soda
  • 12 salt
  • 12 vanilla
I then included the rest of the instruction, but that is not relevant here.

Of course, within a minute, I had students telling me that this recipe is not complete and vague. "What is half butter and one sugar?" someone asked.

"Oh, you mean that you need the UNITS of measure for these ingredients to make the banana bread?" I innocently asked. "Does that mean that without knowing if these are 1/2 cup, or 1 teaspoon, or 1 tablespoon, these number really a vague and meaningless?" I continued.

That was when the students realized that they just had a lesson. The lesson here being that the need to know the units of measure is (i) necessary and that (ii) it isn't exclusive only to physics, and that we all have been using these units of measure everyday without realizing it. Without such units, a lot of things won't make sense.

I can't claim that this exercise was effective, but I did notice that I saw a significantly lower occurrences of missing units in the students homework and exams. The few times that this did occur, the only comment that I wrote next to the number with the missing units was "Banana bread!"