MinutePhysics Special Relativity Chapter 2

If you missed Chapter 1, check it out here.

Here's Chapter 2.



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How People Got Time Dilation Wrong

While we are still waiting for Minute Physics to continue with its Special Relativity series, Fermilab's Don Lincoln has a nice video on the things that many people got wrong with SR's time dilation concepts. I see these misconception often, so this might be quite beneficial to those who do not have a formal lesson in SR. Heck, I think even physics students might benefit watching this.



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Lawrence Krauss Hit By Sexual Misconduct Allegations

Oh dear. I knew it was going to happen that someone from within the physics community will be hit by such allegations (hey, we are all still human after all and not immune to doing such nasty actions). But it is still a bit surprising and disappointing when such allegations happens to someone whose writings I've enjoyed over the years.

I'm sure this will work through the legal system, and I'm not going to comment anymore than that. I think most, if not, all of us here in the US who work either in the academic settings or at a govt. lab had gone through training or classes on sexual harassment. In my case, I've had gone through several of these, including training on Title IX, etc.. etc.

This is an issue we all have to face, and we are now starting to see how pervasive it really is.

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New Measurement of Hubble Constant Brings New Puzzle

The most extensive measurement of the Hubble constant based on observations made by the Hubble telescope (how appropriate) has revealed a discrepancy between its value and those made earlier by ESA's Planck satellite.

Planck’s result predicted that the Hubble constant value should now be 67 kilometers per second per megaparsec (3.3 million light-years), and could be no higher than 69 kilometers per second per megaparsec. This means that for every 3.3 million light-years farther away a galaxy is from us, it is moving 67 kilometers per second faster. But Riess’s team measured a value of 73 kilometers per second per megaparsec, indicating galaxies are moving at a faster rate than implied by observations of the early universe.

The Hubble data are so precise that astronomers cannot dismiss the gap between the two results as errors in any single measurement or method. “Both results have been tested multiple ways, so barring a series of unrelated mistakes,” Riess explained, “it is increasingly likely that this is not a bug but a feature of the universe.”

The arXiv version of the paper can be found here.

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The Dark Life Of The Higgs Boson

I decided to modify a bit the title of the Symmetry article that I'm linking to, because in that article, the possible link between the Higgs boson and dark matter is made. This allows for the study of the decay of the Higgs to be used to detect the presence of dark matter.

The Standard Model not only predicts all the different possible decays of Higgs bosons, but how favorable each decay is. For instance, it predicts that about 60 percent of Higgs bosons will transform into a pair of bottom quarks, whereas only 0.2 percent will transform into a pair of photons. If the experimental results show Higgs bosons decaying into certain particles more or less often than predicted, it could mean that a few Higgs bosons are sneaking off and transforming into dark matter.

Of course, these kinds of precision measurements cannot tell scientists if the Higgs is evolving into dark matter as part of its decay path—only that it is behaving strangely. To catch the Higgs in the act, scientists need irrefutable evidence of the Higgs schmoozing with dark matter.

So there you have it.

If you are not up to speed on the discovery of the Higgs (i.e. you've been living under a rock for the past few years), I've mentioned a link to a nice update here.

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Observation of 3-Photon Bound States

They seem to be making a steady and impressive success along this line.

A new paper in Science[1] has shown an impressive result of the possibility of causing 3 different photons to be "bound" or entangled with one another after traversing through a cold rubidium atom gas.

In controlled experiments, the researchers found that when they shone a very weak laser beam through a dense cloud of ultracold rubidium atoms, rather than exiting the cloud as single, randomly spaced photons, the photons bound together in pairs or triplets, suggesting some kind of interaction — in this case, attraction — taking place among them.

Now, without going overboard with the superlatives, it must be stressed that this does not occur in vacuum, i.e. 3 photons just don't say hi to one another and decide to hang out together. The presence of the cold rubidium gas is essential for a photon to bound with one of the atoms to form a polariton:

The researchers then developed a hypothesis to explain what might have caused the photons to interact in the first place. Their model, based on physical principles, puts forth the following scenario: As a single photon moves through the cloud of rubidium atoms, it briefly lands on a nearby atom before skipping to another atom, like a bee flitting between flowers, until it reaches the other end.

If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton — a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the atoms remain where they are, while the photons exit, still bound together. The researchers found that this same phenomenon can occur with three photons, forming an even stronger bond than the interactions between two photons.

This has almost the same flavor as the "attraction" between two electrons in a superconductor to form the bound Cooper pairs, which requires a background of lattice ion vibration or virtual phonons to mediate the coupling.

So photons can talk to one another, and in this case, 3 of them can hang out together. They just need a matchmaker as an intermediary, since they are just way too shy to do it on their own.

And with that sugary concoction, I think I need more coffee this morning.

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[1] Q-Y Liang et al., Science v.359, p.783 (2018).

Light From A Single Strontium Atom

The image of light from a single strontium atom in an atom trap has won the Engineering and Physical Sciences Research Council photography competition.

You can see a more detailed photo of it on Science Alert.

Unfortunately, there is a bit of misconception going on here. You are not actually seeing the single strontium atom, because it highly depends on what you mean by "seeing". The laser excites the single strontium atom, and then the strontium atom relaxes and releases energy in the form of light. This is the light that you are seeing, and it is probably a result of one or more atomic transition in the atom, but certainly not all of it.

So you're seeing light due to the atomic transition of the atom. You are not actually seeing the atom itself, as proclaimed by some website. This is the nasty obstacle that the general public has to wade through when reading something like this. We need to make it very clear when we report this to the media on what it really is in no uncertain terms, because they WILL try to sensationalize it as much as they can.

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What's So Important About The g-2 Experiment?

If it is covered in CNN, then it has to be a big-enough news. :)

I mentioned earlier that the g-2 experiment at Fermilab was about to start (it has started now), which is basically a continuation and refinement of what was done several years ago at Brookhaven. In case the importance of this experiment escapes you, Don Lincoln of Fermilab has written a piece on the CNN website on this experiment and why it is being done.

If you are not in science, you need to keep in mind this important theme: scientists, and definitely physicists, like it A LOT when we see hints at something that somehow does not fit with our current understanding. We like it when we see discrepancies of our results with the things that we already know.

This may sound odd to many people, but it is true! This is because this is why many of us get into this field in the first place: to explore new and uncharted territories! Results that do not fit with our current understanding give hints at new physics, something beyond what we already know. This is exploration in the truest sense.

This is why there were people who actually were disappointed that we saw the Higgs, and within the energy range that the Standard Model predicted. It is why many, especially theorists working on Supersymmetry, are disappointed that the results out of the LHC so far are within what the Standard Model has predicted.

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Shedding Light On Radiation Reaction

This is basically an inverse Compton scattering. The latest experiment that studies this has been getting a bit of a press, because of the sensationalistic claims of light "stopping" electrons in their tracks.

A review of the experiment, and the theory behind this, is sufficiently covered in APS Physics, and you do get free access to the actually paper itself in PRX. But after all the brouhaha, this is the conclusion we get:

The differing conclusions in these papers serve as a call to improve the quantum theory for radiation reaction. But it must be emphasized that the new data are too statistically weak to claim evidence of quantum radiation reaction, let alone to decide that one existing model is better than the others. Progress on both fronts will come from collecting more collision events and attaining a more stable electron bunch from laser-wakefield acceleration. Additional information could come from pursuing complementary experimental approaches to observing radiation reaction (for example, Ref. [7]), which may be possible with the next generation of high-intensity laser systems [8]. In the meantime, experiments like those from the Mangles and Zepf teams are ushering in a new era in which the interaction between matter and ultraintense laser light is being used to investigate fundamental phenomena, some of which have never before been studied in the lab.

I know that they need very high-energy electron beam, but the laser wakefield technique that they used seem to be providing a larger spread in energy than what they can resolve:

Both experiments obtained only a small number of such successful events, mainly because it was difficult to achieve a good spatiotemporal overlap between the laser pulse and the electron bunch, each of which has a duration of only a few tens of femtoseconds and is just a few micrometers in width. A further complication was that the average energy of the laser-wakefield-accelerated electrons fluctuated by an amount comparable to the energy loss from radiation reaction.

I suppose this is the first step in trying to sort this out, and I have no doubt that there will be an improvement in such an experiment soon.

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Therapeutic Particles

No, this is not some mumbo-jumbo New Age stuff.

While this technique has become more common, and there are already several places here in the US that are researching this, this is a nice article to introduce to you the current state-of-the-art in using charged particles in medicine, especially in treating and attacking cancer. It appears that the use of carbon ions is definitely catching up in popularity over the current use of protons.

When you read this article, pay attention to the fact that this is an outcome of our understanding of particle accelerators, that this is a particle accelerator applications, and that high-energy physics experimental facilities are often the ones that either initiated the project, or are hosting it. So next time someone asks you the practical applications of particle accelerators or particle physics, point to this.

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MInutePhysics Special Relativity Chapter 1

Here is Chapter 1 of MinutePhysics attempt at a series to teach Special Relativity to those of you who are not physicists. You might want to subscribe to it if this is of any interest to you.



Note sure if one needs to build that contraption that is shown at the end of the video, though. :)

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