Scientists Recreate the Density of a White Dwarf in the Lab

The density of a white dwarf star defies our imagination. A spoonful of white dwarf matter would weigh as much as a car on Earth. Atoms within the star are squeezed so tightly that they are on the edge of collapse. Squeeze a white dwarf just a bit more, and it will collapse into a neutron star. And now, we can recreate the density of a white dwarf within a lab.

Continue reading “Scientists Recreate the Density of a White Dwarf in the Lab”

Why Pulsars Are So Bright

When pulsars were first discovered in 1967, their rhythmic radio-wave pulsations were a mystery. Some thought their radio beams must be of extraterrestrial origin.

We’ve learned a lot since then. We know that pulsars are magnetized, rotating neutrons stars. We know that they rotate very rapidly, with their magnetic poles sending sweeping beams of radio waves out into space. And if they’re aimed the right way, we can “see” them as pulses of radio waves, even though the radio waves are steady. They’re kind of like lighthouses.

But the exact mechanism that creates all of that electromagnetic radiation has remained a mystery.

Continue reading “Why Pulsars Are So Bright”

Could The Physical Constants Change? Possibly, But Probably Not

The world we see around us seems to be rooted in scientific laws. Theories and equations that are absolute and universal. Central to these are fundamental physical constants. The speed of light, the mass of a proton, the constant of gravitational attraction. But are these constants really constant? What would happen to our theories if they changed?

Continue reading “Could The Physical Constants Change? Possibly, But Probably Not”

Can wormholes act like time machines?

Artist illustration of a spacecraft passing through a wormhole to a distant galaxy. Image credit: NASA.

Time travel into the past is a tricky thing. We know of no single law of physics that absolutely forbids it, and yet we can’t find a way to do it, and if we could do it, the possibility opens up all sorts of uncomfortable paradoxes (like what would happen if you killed your own grandfather).

But there could be a way to do it. We just need to find a wormhole first.

Continue reading “Can wormholes act like time machines?”

How Large Can A Planet Be?

Jupiter is the largest planet in the solar system. In terms of mass, Jupiter towers over the other planets. If you were to gather all the other planets together into a single mass, Jupiter would still be 2.5 times more massive. It is hard to understate just how huge Jupiter is. But as we’ve discovered thousands of exoplanets in recent decades, it raises an interesting question about how Jupiter compares. Put another way, just how large can a planet be? The answer is more subtle than you might think.

Continue reading “How Large Can A Planet Be?”

CERN is Planning to Build a Much Larger Particle Collider. Much, Much, Larger.

CERN's Future Circular Collider. Image Credit: CERN

CERN, the European Organization for Nuclear Research, wants to build a particle collider that will dwarf the Large Hadron Collider (LHC). The LHC has made important discoveries, and planned upgrades to its power ensures it will keep working on physics problems into the future. But eventually, it won’t be enough to unlock the secrets of physics. Eventually, we’ll need something larger and more powerful.

Enter the Future Circular Collider (FCC.) The FCC will exceed the LHC in power by an order of magnitude. On January 15th, the FCC collaboration released its Conceptual Design Report (CDR) that lays out the options for CERN’s Future Circular Collider.

Continue reading “CERN is Planning to Build a Much Larger Particle Collider. Much, Much, Larger.”

The Large Hadron Collider has been Shut Down, and Will Stay Down for Two Years While they Perform Major Upgrades

The Compact Muon Solenoid Detector on the LHC. Image Credit: CERN

The Large Hadron Collider (LHC) is getting a big boost to its performance. Unfortunately, for fans of ground-breaking physics, the whole thing has to be shut down for two years while the work is done. But once it’s back up and running, its enhanced capabilities will make it even more powerful.
Continue reading “The Large Hadron Collider has been Shut Down, and Will Stay Down for Two Years While they Perform Major Upgrades”

Another Strange Discovery From LHC That Nobody Understands

New results from ALICE at the Large Hadron Collider show so-called strange hadrons being created where none were expected. As the number of proton-proton collisions (the blue lines) increase, the more of these strange hadrons are seen (as shown by the red squares in the graph). (Image: CERN)

There are some strange results being announced in the physics world lately. A fluid with a negative effective mass, and the discovery of five new particles, are all challenging our understanding of the universe.

New results from ALICE (A Large Ion Collider Experiment) are adding to the strangeness.

ALICE is a detector on the Large Hadron Collider (LHC). It’s one of seven detectors, and ALICE’s role is to “study the physics of strongly interacting matter at extreme energy densities, where a phase of matter called quark-gluon plasma forms,” according to the CERN website. Quark-gluon plasma is a state of matter that existed only a few millionths of a second after the Big Bang.

In what we might call normal matter—that is the familiar atoms that we all learn about in high school—protons and neutrons are made up of quarks. Those quarks are held together by other particles called gluons. (“Glue-ons,” get it?) In a state known as confinement, these quarks and gluons are permanently bound together. In fact, quarks have never been observed in isolation.

A cut-away view of the ALICE detector at CERN’s LHC. Image: By Pcharito – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31365856

The LHC is used to collide particles together at extremely high speeds, creating temperatures that can be 100,000 times hotter than the center of our Sun. In new results just released from CERN, lead ions were collided, and the resulting extreme conditions come close to replicating the state of the Universe those few millionths of a second after the Big Bang.

In those extreme temperatures, the state of confinement was broken, and the quarks and gluons were released, and formed quark-gluon plasma.

So far, this is pretty well understood. But in these new results, something additional happened. There was increased production of what are called “strange hadrons.” Strange hadrons themselves are well-known particles. They have names like Kaon, Lambda, Xi and Omega. They’re called strange hadrons because they each have one “strange quark.”

If all of this seems a little murky, here’s the dinger: Strange hadrons may be well-known particles, because they’ve been observed in collisions between heavy nuclei. But they haven’t been observed in collisions between protons.

“Being able to isolate the quark-gluon-plasma-like phenomena in a smaller and simpler system…opens up an entirely new dimension for the study of the properties of the fundamental state that our universe emerged from.” – Federico Antinori, Spokesperson of the ALICE collaboration.

“We are very excited about this discovery,” said Federico Antinori, Spokesperson of the ALICE collaboration. “We are again learning a lot about this primordial state of matter. Being able to isolate the quark-gluon-plasma-like phenomena in a smaller and simpler system, such as the collision between two protons, opens up an entirely new dimension for the study of the properties of the fundamental state that our universe emerged from.”

Enhanced Strangeness?

The creation of quark-gluon plasma at CERN provides physicists an opportunity to study the strong interaction. The strong interaction is also known as the strong force, one of the four fundamental forces in the Universe, and the one that binds quarks into protons and neutrons. It’s also an opportunity to study something else: the increased production of strange hadrons.

In a delicious turn of phrase, CERN calls this phenomenon “enhanced strangeness production.” (Somebody at CERN has a flair for language.)

Enhanced strangeness production from quark-gluon plasma was predicted in the 1980s, and was observed in the 1990s at CERN’s Super Proton Synchrotron. The ALICE experiment at the LHC is giving physicists their best opportunity yet to study how proton-proton collisions can have enhanced strangeness production in the same way that heavy ion collisions can.

According to the press release announcing these results, “Studying these processes more precisely will be key to better understand the microscopic mechanisms of the quark-gluon plasma and the collective behaviour of particles in small systems.”

I couldn’t have said it better myself.

What are Leptons?

During the 19th and 20th centuries, physicists began to probe deep into the nature of matter and energy. In so doing, they quickly realized that the rules which govern them become increasingly blurry the deeper one goes. Whereas the predominant theory used to be that all matter was made up of indivisible atoms, scientists began to realize that atoms are themselves composed of even smaller particles.

From these investigations, the Standard Model of Particle Physics was born. According to this model, all matter in the Universe is composed of two kinds of particles: hadrons – from which Large Hadron Collider (LHC) gets its name – and leptons. Where hadrons are composed of other elementary particles (quarks, anti-quarks, etc), leptons are elementary particles that exist on their own.

Definition:

The word lepton comes from the Greek leptos, which means “small”, “fine”, or “thin”. The first recorded use of the word was by physicist Leon Rosenfeld in his book Nuclear Forces (1948). In the book, he attributed the use of the word to a suggestion made by Danish chemist and physicist Prof. Christian Moller.

The Standard Model of Elementary Particles. Image: By MissMJ - Own work by uploader, PBS NOVA [1], Fermilab, Office of Science, United States Department of Energy, Particle Data Group, CC BY 3.0
The Standard Model of Particle Physics, showing all known elementary particles. Credit: Wikipedia Commons/MissMJ/PBS NOVA/Fermilab/Particle Data Group
The term was chosen to refer to particles of small mass, since the only known leptons in Rosenfeld’s time were muons. These elementary particles are over 200 times more massive than electrons, but have only about one-ninth the the mass of a proton. Along with quarks, leptons are the basic building blocks of matter, and are therefore seen as “elementary particles”.

Types of Leptons:

According to the Standard Model, there are six different types of leptons. These include the Electron, the Muon, and Tau particles, as well as their associated neutrinos (i.e. electron neutrino, muon neutrino, and tau neutrino). Leptons have negative charge and a distinct mass, whereas their neutrinos have a neutral charge.

Electrons are the lightest, with a mass of 0.000511 gigaelectronvolts (GeV), while Muons have a mass of 0.1066 Gev and Tau particles (the heaviest) have a mass of 1.777 Gev. The different varieties of the elementary particles are commonly called “flavors”. While each of the three lepton flavors are different and distinct (in terms of their interactions with other particles), they are not immutable.

A neutrino can change its flavor, a process which is known as “neutrino flavor oscillation”. This can take a number of forms, which include solar neutrino, atmospheric neutrino, nuclear reactor, or beam oscillations. In all observed cases, the oscillations were confirmed by what appeared to be a deficit in the number of neutrinos being created.

Muons, a type of lepton, shown being produced by the Large Hadron Collider. Credit: CERN
Muons, a type of lepton, shown being produced by the Large Hadron Collider. Credit: CERN

One observed cause has to do with “muon decay” (see below), a process where muons change their flavor to become electron neutrinos  or  tau neutrinos – depending on the circumstances. In addition, all three leptons and their neutrinos have an associated antiparticle (antilepton).

For each, the antileptons have an identical mass, but all of the other properties are reversed. These pairings consist of the electron/positron, muon/antimuon, tau/antitau, electron neutrino/electron antineutrino, muon neutrino/muan antinuetrino, and tau neutrino/tau antineutrino.

The present Standard Model assumes that there are no more than three types (aka. “generations”) of leptons with their associated neutrinos in existence. This accords with experimental evidence that attempts to model the process of nucleosynthesis after the Big Bang, where the existence of more than three leptons would have affected the abundance of helium in the early Universe.

Properties:

All leptons possess a negative charge. They also possess an intrinsic rotation in the form of their spin, which means that electrons with an electric charge – i.e. “charged leptons” – will generate magnetic fields. They are able to interact with other matter only though weak electromagnetic forces. Ultimately, their charge determines the strength of these interactions, as well as the strength of their electric field and how they react to external electrical or magnetic fields.

None are capable of interacting with matter via strong forces, however. In the Standard Model, each lepton starts out with no intrinsic mass. Charged leptons obtain an effective mass through interactions with the Higgs field, while neutrinos either remain massless or have only very small masses.

History of Study:

The first lepton to be identified was the electron, which was discovered by British physicist J.J. Thomson and his colleagues in 1897 using a series of cathode ray tube experiments. The next discoveries came during the 1930s, which would lead to the creation of a new classification for weakly-interacting particles that were similar to electrons.

The first discovery was made by Austrian-Swiss physicist Wolfgang Pauli in 1930, who proposed the existence of the electron neutrino in order to resolve the ways in which beta decay contradicted the Conservation of Energy law, and Newton’s Laws of Motion (specifically the Conservation of Momentum and Conservation of Angular Momentum).

The positron and muon were discovered by Carl D. Anders in 1932 and 1936, respectively. Due to the mass of the muon, it was initially mistook for a meson. But due to its behavior (which resembled that of an electron) and the fact that it did not undergo strong interaction, the muon was reclassified. Along with the electron and the electron neutrino, it became part of a new group of particles known as “leptons”.

In 1962, a team of American physicists – consisting of Leon M. Lederman, Melvin Schwartz, and Jack Steinberger – were able to detect of interactions by the muon neutrino, thus showing that more than one type of neutrino existed. At the same time, theoretical physicists postulated the existence of many other flavors of neutrinos, which would eventually be confirmed experimentally.

The tau particle followed in the 1970s, thanks to experiments conducted by Nobel-Prize winning physicist Martin Lewis Perl and his colleagues at the SLAC National Accelerator Laboratory. Evidence of its associated neutrino followed thanks to the study of tau decay, which showed missing energy and momentum analogous to the missing energy and momentum caused by the beta decay of electrons.

In 2000, the tau neutrino was directly observed thanks to the Direct Observation of the NU Tau (DONUT) experiment at Fermilab. This would be the last particle of the Standard Model to be observed until 2012, when CERN announced that it had detected a particle that was likely the long-sought-after Higgs Boson.

Today, there are some particle physicists who believe that there are leptons still waiting to be found. These “fourth generation” particles, if they are indeed real, would exist beyond the Standard Model of particle physics, and would likely interact with matter in even more exotic ways.

We have written many interesting articles about Leptons and subatomic particles here at Universe Today. Here’s What are Subatomic Particles?, What are Baryons?First Collisions of the LHC, Two New Subatomic Particles Found, and Physicists Maybe, Just Maybe, Confirm the Possible Discovery of 5th Force of Nature.

For more information, SLAC’s Virtual Visitor Center has a good introduction to Leptons and be sure to check out the Particle Data Group (PDG) Review of Particle Physics.

Astronomy Cast also has episodes on the topic. Here’s Episode 106: The Search for the Theory of Everything, and Episode 393: The Standard Model – Leptons & Quarks.

Sources: