30 years of Chernobyl disaster April 26, 2016Posted by apetrov in Uncategorized.
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30 years ago, on 26 April 1986, the biggest nuclear accident happened at the Chernobyl nuclear power station.
The picture above is of my 8th grade class (I am in the front row) on a trip from Leningrad to Kiev. We wanted to make sure that we’d spend May 1st (Labor Day in the Soviet Union) in Kiev! We took that picture in Gomel, which is about 80 miles away from Chernobyl, where our train made a regular stop. We were instructed to bury some pieces of clothing and shoes after coming back to Leningrad due to excess of radioactive dust on them…
“Ladies and gentlemen, we have detected gravitational waves.” February 11, 2016Posted by apetrov in Uncategorized.
The title says it all. Today, The Light Interferometer Gravitational-Wave Observatory (or simply LIGO) collaboration announced the detection of gravitational waves coming from the merger of two black holes located somewhere in the Southern sky, in the direction of the Magellanic Clouds. In the presentation, organized by the National Science Foundation, David Reitze (Caltech), Gabriela Gonzales (Louisiana State), Rainer Weiss (MIT), and Kip Thorn (Caltech), announced to the room full of reporters — and thousand of scientists worldwide via the video feeds — that they have seen a gravitational wave event. Their paper, along with a nice explanation of the result, can be seen here.
The data that they have is rather remarkable. The event, which occurred on 14 September 2015, has been seen by two sites (Livingston and Hanford) of the experiment, as can be seen in the picture taken from their presentation. It likely happened over a billion years ago (1.3B light years away) and is consistent with the merger of two black holes, of 29 and 46 solar masses. The resulting larger black hole’s mass is about 62 solar masses, which means that about 3 solar masses of energy (29+36-62=3) has been radiated in the form of gravitational waves. This is a huge amount of energy! The shape of the signal is exactly what one should expect from the merging of two black holes, with 5.1 sigma significance.
It is interesting to note that the information presented today totally confirms the rumors that have been floating around for a couple of months. Physicists like to spread rumors, as it seems.
Since the gravitational waves are quadrupole, the most straightforward way to see the gravitational waves is to measure the relative stretches of the its two arms (see another picture from the MIT LIGO site) that are perpendicular to each other. Gravitational wave from black holes falling onto each other and then merging. The LIGO device is a marble of engineering — one needs to detect a signal that is very small — approximately of the size of the nucleus on the length scale of the experiment. This is done with the help of interferometry, where the laser beams bounce through the arms of the experiment and then are compared to each other. The small change of phase of the beams can be related to the change of the relative distance traveled by each beam. This difference is induced by the passing gravitational wave, which contracts one of the arms and extends the other. The way noise that can mimic gravitational wave signal is eliminated should be a subject of another blog post.
This is really a remarkable result, even though it was widely expected since the (indirect) discovery of Hulse and Taylor of binary pulsar in 1974! It seems that now we have another way to study the Universe.
Nobel Prize in Physics 2015 October 6, 2015Posted by apetrov in Uncategorized.
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So, the Nobel Prize in Physics 2015 has been announced. To much surprise of many (including the author), it was awarded jointly to Takaaki Kajita and Arthur B. McDonald “for the discovery of neutrino oscillations, which shows that neutrinos have mass.” Well deserved Nobel Prize for a fantastic discovery.
What is this Nobel prize all about? Some years ago (circa 1997) there were a couple of “deficit” problems in physics. First, it appeared that the detected number of (electron) neutrinos coming form the Sun was measured to be less than expected. This could be explained in a number of ways. First, neutrino could oscillate — that is, neutrinos produced as electron neutrinos in nuclear reactions in the Sun could turn into muon or tau neutrinos and thus not be detected by existing experiments, which were sensitive to electron neutrinos. This was the most exciting possibility that ultimately turned out to be correct! But it was by far not the only one! For example, one could say that the Standard Solar Model (SSM) predicted the fluxes wrong — after all, the flux of solar neutrinos is proportional to core temperature to a very high power (~T25 for 8B neutrinos, for example). So it is reasonable to say that neutrino flux is not so well known because the temperature is not well measured (this might be disputed by solar physicists). Or something more exotic could happen — like the fact that neutrinos could have large magnetic moment and thus change its helicity while propagating in the Sun to turn into a right-handed neutrino that is sterile.
The solution to this is rather ingenious — measure neutrino flux in two ways — sensitive to neutrino flavor (using “charged current (CC) interactions”) and insensitive to neutrino flavor (using “neutral current (NC) interactions”)! Choosing heavy water — which contains deuterium — is then ideal for this detection. This is exactly what SNO collaboration, led by A. McDonald did
As it turned out, the NC flux was exactly what SSM predicted, while the CC flux was smaller. Hence the conclusion that electron neutrinos would oscillate into other types of neutrinos!
Another “deficit problem” was associated with the ratio of “atmospheric” muon and electron neutrinos. Cosmic rays hit Earth’s atmosphere and create pions that subsequently decay into muons and muon neutrinos. Muons would also eventually decay, mainly into an electron, muon (anti)neutrino and an electron neutrino, as
As can be seen from the above figure, one would expect to have 2 muon-flavored neutrinos per one electron-flavored one.
This is not what Super K experiment (T. Kajita) saw — the ratio really changed with angle — that is, the ratio of neutrino fluxes from above would differ substantially from the ratio from below (this would describe neutrinos that went through the Earth and then got into the detector). The solution was again neutrino oscillations – this time, muon neutrinos oscillated into the tau ones.
The presence of neutrino oscillations imply that they have (tiny) masses — something that is not predicted by minimal Standard Model. So one can say that this is the first indication of physics beyond the Standard Model. And this is very exciting.
I think it is interesting to note that this Nobel prize might help the situation with funding of US particle physics research (if anything can help…). It shows that physics has not ended with the discovery of the Higgs boson — and Fermilab might be on the right track to uncover other secrets of the Universe.
Harvard University is to change its name April 1, 2015Posted by apetrov in Uncategorized.
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A phrase from William Shakespeare’s Romeo and Juliet states: “What’s in a name? That which we call a rose By any other name would smell as sweet.” This cannot be any further from the truth in the corporate world. The name of a corporation is its face, so setting a brand requires a lot of work and money. But what happens when something goes wrong? The way to deal with corporate problems often involves re-branding, changing the name and the face of the corporation. It works as customers usually do not check the history of a company before buying its products or using its services. It simply works.
With the Universities today run according to the corporate model, it was only a matter of time until re-branding came to the academic world. And leading Universities, like Harvard, seem to be embracing the model. Since 2013 article in Harvard Crimson, big Universities became a focus of investigations of many leading newspapers and politicians. Harvard, in particular, has been a focus of a brewing controversy. The University with the largest endowment of any university in the world, has got its name associated with the person who was not, in fact, the founder of Harvard University. As reported, in the very recent internal investigation by Harvard Crimson, John Harvard cannot be the founder of the school, because the Massachusetts Colony’s vote had come two years prior to Harvard’s bequest (compare this to Ezra Cornell’s founding of Cornell University). This led several prominent Massachusetts politicians to suggest that the University will be returned to the ownership by the Commonwealth with its name changed to University of Massachusetts, Cambridge. “We have a fantastic University system here in Massachusetts, with the flagship campus in Amherst,” said one of the prominent politicians who preferred not to be named, “Any University in the World would be proud to be a part of it.”
Returning a prominent private University to the ownership by the State is highly unusual nowadays and is probably highly specific to New England. With tightening budgets many states seek to privatize the Universities to remove them from their budget. For instance, there is a talk that a large public Midwestern school, Wayne State University, will soon change its owners and its name. Two prominent figures, W. Rooney and W. Gretzky, are rumored to work on acquiring the University and re-branding it as simply Wayne’s University. And the changes are rumored go even further. An external company Haleburton has already completed an assessment of the University’s strengths. The company noted WSU’s worldwide reputation in chemistry, physics and medicine and its Carnegie I research status, and recommended that the school should concentrate its efforts on graduating hockey, football, basketball and baseball players. “We are preparing our graduates to have highly successful careers. What job in the United States brings more money than the NFL or NHL player?” a member of WSU’s Academic Senate has been quoted in saying. “We are all excited about the change and looking forward to what else future would bring us.”
Data recall at the LHC? April 1, 2014Posted by apetrov in Uncategorized.
Tags: check the date, particle physics, physics
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In a stunning turn of events, Large Hadron Collider (LHC) management announced a recall and review of thousands of results that came from its four main detectors, ATLAS, CMS, LHCb and ALICE, in the course of the past several years when it learned that the ignition switches used to start the LHC accelerator (see the image) might have been produced by GM.
GM’s CEO, A. Ibarra, who is better known in the scientific world for the famous Davidson-Ibarra bound in leptogenesis, will be testifying on the Capitol Hill today. This new revelation will definitely add new questions to already long list of queries to be addressed by the embattled CEO. In particular, the infamous LHC disaster that happened on 10 September 2008, which cost taxpayers over 21Million dollars to fix, and has long suspected been caused by a magnet quench, might have been caused by too much paper accidentally placed on a switch by a graduate student, who was on duty that day.
“We want to know why it took LHC management five years to issue that recall”, an unidentified US Government official said in the interview, “We want to know what is being done to correct the problem. From our side, we do everything humanly possible to accommodate US high energy particle physics researchers and help them to avoid such problems in the future. For example, we included a 6.6% cut in US HEP funding in the President’s 2015 budget request.” He added, “We suspected that something might be going on at the LHC after it was convincingly proven to us at our weekly seminar that the detected Higgs boson is ‘simply one Xenon atom of the 1 trillion 167 billion 20 million Xenon atoms which there are in the LHC!’”
This is not the first time accelerators cause physicists to rethink their results and designs. For example, last year Japanese scientists had to overcome the problem of unintended acceleration of positrons at their flagship facility KEK.
At this point, it is not clear how GM’s ignition switches problems would affect funding of operations at the National Ignition Facility in Livermore, CA.
And the 2013 Nobel Prize in Physics goes to… October 8, 2013Posted by apetrov in Particle Physics, Physics, Science, Uncategorized.
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Today the 2013 Nobel Prize in Physics was awarded to François Englert (Université Libre de Bruxelles, Belgium) and Peter W. Higgs (University of Edinburgh, UK). The official citation is “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider.” What did they do almost 50 years ago that warranted their Nobel Prize today? Let’s see (for the simple analogy see my previous post from yesterday).
The overriding principle of building a theory of elementary particle interactions is symmetry. A theory must be invariant under a set of space-time symmetries (such as rotations, boosts), as well as under a set of “internal” symmetries, the ones that are specified by the model builder. This set of symmetries restrict how particles interact and also puts constraints on the properties of those particles. In particular, the symmetries of the Standard Model of particle physics require that W and Z bosons (particles that mediate weak interactions) must be massless. Since we know they must be massive, a new mechanism that generates those masses (i.e. breaks the symmetry) must be put in place. Note that a theory with massive W’s and Z that are “put in theory by hand” is not consistent (renormalizable).
The appropriate mechanism was known in the beginning of the 1960’s. It goes under the name of spontaneous symmetry breaking. In one variant it involves a spin-zero field whose self-interactions are governed by a “Mexican hat”-shaped potential
It is postulated that the theory ends up in vacuum state that “breaks” the original symmetries of the model (like the valley in the picture above). One problem with this idea was that a theorem by G. Goldstone required a presence of a massless spin-zero particle, which was not experimentally observed. It was Robert Brout, François Englert, Peter Higgs, and somewhat later (but independently), by Gerry Guralnik, C. R. Hagen, Tom Kibble who showed a loophole in a version of Goldstone theorem when it is applied to relativistic gauge theories. In the proposed mechanism massless spin-zero particle does not show up, but gets “eaten” by the massless vector bosons giving them a mass. Precisely as needed for the electroweak bosons W and Z to get their masses! A massive particle, the Higgs boson, is a consequence of this (BEH or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism and represents excitation of the Higgs field about its new vacuum state.
It took about 50 years to experimentally confirm the idea by finding the Higgs boson! Tracking the historic timeline, the first paper by Englert and Brout, was sent to Physical Review Letter on 26 June 1964 and published in the issue dated 31 August 1964. Higgs’ paper, received by Physical Review Letters on 31 August 1964 (on the same day Englert and Brout’s paper was published) and published in the issue dated 19 October 1964. What is interesting is that the original version of the paper by Higgs, submitted to the journal Physics Letters, was rejected (on the grounds that it did not warrant rapid publication). Higgs revised the paper and resubmitted it to Physical Review Letters, where it was published after another revision in which he actually pointed out the possibility of the spin-zero particle — the one that now carries his name. CERN’s announcement of Higgs boson discovery came 4 July 2012.
Is this the last Nobel Prize for particle physics? I think not. There are still many unanswered questions — and the answers would warrant Nobel Prizes. Theory of strong interactions (which ARE responsible for masses of all luminous matter in the Universe) is not yet solved analytically, the nature of dark matter is not known, the picture of how the Universe came to have baryon asymmetry is not cleared. Is there new physics beyond what we already know? And if yes, what is it? These are very interesting questions that need answers.
Higgs mechanism for electrical engineers October 7, 2013Posted by apetrov in Particle Physics, Physics, Science, Uncategorized.
Tags: higgs boson
Since the Higgs boson’s discovery a little over a year ago at CERN I have been getting a lot of questions from my friends to explain to them “what this Higgs thing does.” So I often tried to use the crowd analogy that is ascribed to Prof. David Miller, to describe the Higgs (or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism. Interestingly enough, it did not work well for most of my old school friends, majority of whom happen to pursue careers in engineering. So I thought that perhaps another analogy would be more appropriate. Here it is, please let me know what you think!
Imagine Higgs field as represented by some quantity of slightly magnetized iron filings, i.e. small pieces of iron that look like powder, spread over a table or other surface to represent Higgs field that permeates the Universe. Iron filings are common not only as dirt in metal shops, they are often used in school experiments and other science demonstrations to visualize the magnetic field. It is important for them to be slightly magnetized, as this represents self-interaction of the Higgs field. Here they are pictured in a somewhat cartoonish way:
How can Higgs field generate mass? Moreover, how can one field generate different masses for different types of particles? Let us first make an analogue of fermion mass generation. If we take a small magnet and put it in the filings, the magnet would pick up a bunch of filings, right? How much would it pick up? It depends on the “strength” of that magnet. It could be a little:
…or it could be a lot, depending on what kind of magnet we use — or how strong it is:
If we neglect the masses of our magnets, as we assumed they are small, the mass of the picked up mess with the magnets inside is totally determined by the mass of the picked filings, which in turn is determined by the interaction strength between the magnets and the filings. This is precisely how fermion mass generation works in the Standard Model!
In the Standard Model the massless fermions are coupled to the Higgs field via so-called Yukawa interactions, whose strength is parametrized by a number, the Yukawa coupling constant. For different fermion types (or flavors) the couplings would be numerically different, ranging from one to one part in a million. As a result of interaction with the Higgs field (NOT the boson!) in the form of its vacuum expectation value, all fermions acquire masses (ok, maybe not all — neutrinos could be different). And those masses would depend on the strength of the interaction of fermions with Higgs field, just like in our example with magnets and iron filings!
Now imagine that we simply kicked the table! No magnets. The filings would clamp together to form lumps of filings. Each lump would have a mass, which would only depend on how strong the filings attract to each other (remember that they are slightly magnetized?). If we don’t know how strong they are magnetized, we cannot tell how massive each lamp will be, so we would have to measure their masses.
This gives a good analogy of the fact that Higgs boson is an excitation of the Higgs field (the fact that was pointed out by Higgs), and why we cannot predict its mass from the first principles, but need a direct observation at the LHC!
Notice that this picture (so far) does not provide direct analogy to how gauge bosons (W’s and Z bosons) receive their masses. W’s and Z are also initially massless because of the gauge (internal) symmetries required by the construction of the Standard Model. We did know their mass from earlier CERN and SLAC experiments — and even prior to those, we knew that W’s were massive from the fact that weak interactions are of the finite range.
To extend our analogy, let’s clean up the mess — literally! Let’s throw a bucket of water over the table covered with those iron filings and see what happens. Streams of water would pick up iron filings and flow from the table. Assuming that that water’s mass is negligible, the total mass of those water streams (aka dirty water) would be completely determined by the mass of picked iron filings, just like masses of W’s and Z are determined by the Higgs field.
This explanation seemed to work better for my engineering friends! What do you think?
Another one bites the dust. Or “Super-B? What Super-B?” November 28, 2012Posted by apetrov in Uncategorized.
Tags: charm physics, particle physics, Super-B experiment
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Studies of New Physics require several independent approaches. In the language of experimental physics it means several different experiments. Better yet, several accelerators that have detectors that study similar things, but produce results with different systematic and statistical uncertainties. For a number of years that was how things were: physicists searched for New Physics in high-energy experiments where new particles could be produced directly (think TeVatron or LHC experiments), or low-energy, extremely clean measurements that explored quantum effects of heavy new physics particles. In other words, New Physics could also be searched for indirectly.
As a prominent example of the later approach, detectors BaBar at SLAC (USA) and Belle at KEK (Japan) studied decays of copiously produced B-mesons in hopes to find glimpses of New Physics in quantum loops. These experiments measured many Standard Model-related parameters (in particular, confirming the mechanism of CP-violation in the Standard Model) and discovered many unexpected effects (like new mesons containing charmed quarks, as well as oscillations of charm mesons). But they did not see any effects that could not be explained by the Standard Model. A way to go in this case was to significantly increase luminosity of the machine, thereby allowing for very rare processes to be observed. Two super-flavor factories (those machines are really like factories, churning out millions of B-mesons) were proposed, the Belle-II experiment at KEK and a new Super-B factory at the newly-created Cabibbo Lab in Frascatti, Italy. I have already written about the Cabibbo Lab.
It appears, however, that Italian government decided today that it cannot fund the Super-B flavor factory. Tommaso Dorigo reported it in his blog this morning. Here is more hard data: there is a press release (in Italian) from the INFN that basically tells you that “economic conditions… were incompatible with the costs of the project evaluated.” Which is another way of saying that Italian government is not going to fund it. This follows by the news from the PhysicsWorld saying the same thing.
Many physicists have been expressing doubts that the original Super-B plan, which was, in my opinion, very bold, could be executed within the proposed time frame. Yet, physicists pressed on… that is until this morning’s announcement. Reality of our world sets in — there is not enough money for basic research…
So, is Higgs finally here? July 4, 2012Posted by apetrov in Uncategorized.
Today is a big day at CERN. There are two collaborations that presented their latest results on the search of the Higgs boson. Did they finally discover the Higgs boson?
Let’s first figure out what it all means? What two collaborations? What is Higgs boson? And, most importantly, what do we mean by “discovered”?
First things first. The two collaborations that I’m talking about are CMS and ATLAS, two huge detectors and hundreds of professors, postdocs and graduate students working to get and work the data that come out of it. The collaborations looked at almost three years (although 2010 does not really count and 2012 is still going — but with fantastic pace) and found signals of the Higgs boson, a particle that was predicted to be there in the minimal Standard Model.
Why is that we need Higgs boson and why is it there? The Standard Model of particle physics is described by its symmetries — or the symmetry group (SU(2)xU(1)) under which matter contents transform. This symmetry tells us how particles interact — and in fact, that makes Standard Model quite a constrained system. So the introduction of this symmetry is very important. However, this symmetry also tells us that all particles that are described there should be massless! What should one do? The idea is to break that symmetry, of course. The problem is how to break that symmetry. One cannot simply add symmetry-breaking terms (that would wreck the whole original setup), one has to do it indirectly. So the idea was proposed to introduce a field that interacts with all fields that are present in the Standard Model. That field also interacts with itself and forms a condensate (i.e. provides non-zero value for energy density of the vacuum) once, roughly speaking, the temperature of the Universe after the Big Bang drops below certain value. This mechanism gives mass to both electroweak gauge bosons (particles that represent weak force) and quark and leptons. The mechanism itself was first proposed in 1962 by Philip Warren Anderson. The model of spontaneous symmetry breaking was independently developed in 1964 by three groups, Robert Brout and Francois Englert; Peter Higgs; and Gerald Guralnik, C. R. Hagen, and Tom Kibble. The particle that manifests this effect is the famous Higgs boson. Read more about it here. I’ll talk more about it in my later posts.
CMS went first (which is a bit unusual, as Atlas, for some alphabetical reason, would always be first to deliver their talk). The talk was delivered by Joe Incandela, a CMS spokesperson. A gist of their talk is that they looked at several possible decay channels of the Higgs boson. First, Higgs can decay to two photons (H → γγ). They see a significant bump at mH = 125 GeV, but only in the combination if different reconstruction techniques. The overall significance is over 4 sigma. Next, they talked about H → ZZ channel. This channel is tougher, as they need to reconstruct Z’s that decay in different decay channels. Now, if they combine their data in H → γγ and in H → ZZ they find that statistical significance for signal that the Higgs is there at 5 sigma. However, once they combine all data, especially the H → ττ channel, their combined statistical significance goes slightly down to 4.9 sigma. This is just below discovery by the standards of Physical Review Letters, a very influential physics journal. But this is in very significant.
The next talk was by ATLAS. Fabiolla Gianotti, ATLAS spokesperson, gave that talk. They also see excess in H → γγ channel, but their statistical significance in that channel is lower, 4.5 sigma. Also, they include the so-called look-elsewhere effect — and then their statistical significance goes down to 3.5 sigma. Then, she discussed the H → ZZ channel. They see the excess with 3.4 sigma significance at mH about 125 GeV. Now, the combined results have excess at mH = 126.5 GeV with (local) statistical significance of 5.0 sigma. This is a discovery.
In passing, ATLAS also see a bump in their 4-lepton channel at approximately 90 GeV. Still not clear what it is….
This discovery is very significant. It tells us that our ideas on how electroweak symmetry is broken are at least partially correct. This is also the first truly elementary particle discovered since the Z-boson. There are still many questions, both experimental and theoretical, about the analyses presented today at CERN. What is going on with the H → ττ channel? Is it really a Standard Model Higgs boson? Or some other scalar particle. We’ll sure to study those things indeed.
P.S. The theorists who described the effects were there and not only were acknowledged by the experimental speakers, but also got to say a couple of words at the end.
Why advertisers should know physics June 12, 2012Posted by apetrov in Uncategorized.
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Some years ago, when I decided to start blogging, I wrote about an interesting advertisement that Ford Motor Company put up in all major magazines (it was even placed on the side of their headquarters in Dearborn, MI):
Here is what I said in that post of mine from several years ago: “What is interesting about this ad is the equations that this lady is writing — they look like the equations from the famous Peskin and Schroeder’s book on Quantum Field Theory (QFT), equations describing renormalization of phi^4 theory! How did Ford get a hold of them?
As it turns out, I happen know the answer. This ad was made by a company that is headquartered in Detroit — I have a business card of one of the authors of this ad!
What happened is that a couple of months ago I was sitting in my office at Wayne State University, looking over my QFT notes that I’m supposed to teach next Fall. A guy showed up at my door and asked to “write down a complicated-looking equation.” Now, that’s not a usual question that I get when I sit in my office during the lunchtime! He quickly explained that he works for this advertisement company (called JWT) and they were contracted by Ford to produce a series of ads that should highlight the talent of Ford engineers and at the same time appeal to young people. (He showed me a prototype of an ad with that girl sitting next to the blackboard.) So his boss sent him to the closest university (which happen to be WSU, we are located 5 min down Woodward Avenue from their office) to fish out a “complicated equation.” The rest is simple — I use Peskin and Schroeder as a main text for my graduate QFT course, so that list of equations was indeed about renormalization of phi^4 theory… I must add that I received no monetary (or any other) compensation…
Amazing, isn’t it?”
The reason I re-post part of that old post is the following. I recently went to Florida to participate in CIPANP-2012 conference (I’ll post my impressions of this conference later this week). Now, Kennedy Space Center is on Cape Canaveral in Florida, so I rented a car and went to visit that marvelous place. The place is truly amazing! Lots of things to see. The place is still making history: I visited it just a couple of days after the historic launch of the Space X‘s Dragon capsule.
I also visited a gift shop and bought the following souvenir there:
See how many mistakes they got in there? And it’s not “rocket science”, it’s freshman physics! Quite embarrassing… Clearly, people from that JWT advertising agency in the example above take their job responsibilities much more seriously.
See that NASA seal in the upper left corner? Since I am sure that NASA scientists know physics, I take it as indication that they never visit their gift shop.