Francis Halzen was awarded the 2026 Nobel Prize in Physics, for IceCube!

 IceCube PI Francis Halzen, professor of physics at the University of Wisconsin, Madison has been awarded the 2026 Nobel Prize in Physics, for "“for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin.”  Francis has been a tireless advocate for IceCube, starting in the 1990's, when the idea of a large ice-based neutrino detector was really a laughing matter; few people took it seriously.   This was a simple matter: convince his fellow physicists, numerous committee and funding agencies that the idea was practical, gather one collaboration to produce a proof-of-principle: AMANDA, and a larger one that could be an industrial-scale experiment like IceCube.  Although Francis was not an experimentalist, he was able to bring together a diverse group of scientist, engineers and computer scientists with the necessary skills. IceCube required two key technical development.

The first was drilling deep holes in the ice.  This was most efficiently done using hot water to melt a hole down through the ice, but IceCube required far higher drilling capacity than any previous experiment.  This required a new drill, the development of which often seemed more akin to oil prospecting than astrophysics.   UW Madison and its Physical Sciences Laboratory were responsible for the drilling.  

 

 The second problem was a data readout that would preserve the details (time structure and amplitude) of the light pulses.  Most of this problem fell to Lawrence Berkeley National Laboratory.   The solution was to put the analog and digitizing electronics in the ice, attached to the photo-multiplier tubes, and send slow digital data to the surface.  The challenges here involved reliability (since the modules would be inaccessible after deployment), low-power operation, and a very wide temperature range, down to -55 degrees C.  In essence, we needed to build 5,160 satellites, but with a fraction of the budget that NASA had.  The required tradeoffs made for many interesting discussions, but the result was outstanding; over 98% of the optical modules are still working, with many having spent 20 years in the ice. 

 The final (and perhaps largest) challenge was funding.  Even after construction was approved, there were many people who were skeptical of (a) whether the IceCube detector would work, or (b) whether we would see anything.  Francis deserves enormous credit for convincing the NSF to fund IceCube, and the NSF deserves enormous credit for listening and taking what must surely have been seen as a risk.  We are all better off for it. 

 

 

3I/ATLAS - not just the fastest object in our solar system, but the oldest

I've previously written about 3I/ATLAS, a visitor from outside that arrived in our solar system last year - here and here.  Although it is gone now - from the inner solar system (it is now beyond Jupiter, and getting farther every day) and - mostly - from our consciousness, but new information is still coming out. 

A new study finds that 3I/ATLAS may be 10 to 12 billion years old.  In comparison, our solar system is only 4.6 billion years old - a relative newcomer. 

The study used infrared measurements from the James Webb Space Telescope (such as the picture above) to measure the ratio of deuterium (heavy hydrogen, containing one neutron and one proton) to regular hydrogen (just one proton).  The researchers found that it was almost 1%  - more than ten times higher than in comets that are confined to our solar system.   It also contained a higher ratio of carbon-12 (6 protons + 6 neutrons) to carbon-13 (6 protons + 7 neutrons) than is typically found in objects in our solar system. This is evidence that it formed in a 'metal-poor' environment.  n. b.  Astronomers define any object heavier than helium to be a 'metal.'  They also concluded that 3I/ATLAS formed at very low temperatures, less than 30 Kelvin (-405 Fahrenheit).

The measured carbon ratio is consistent with formation in the early universe, when there were less heavy elements around than there are today, while the high level of deuterium points to formation in a cold environment.  Both of these connections are not trivial, relying on more complex comparisons; for more details, please read the paper, which is open access. 

What do subatomic particles look like?

"What do subatomic particles look like?" sounds like a pretty basic question.  But, in fact, it is not.  Many physicists would considered it ill-posed, since 'look' presupposes observation with visible light, something that is impossible, since, by any measure, these particles are far smaller than the wavelength of light.  Instead, observing subatomic particles requires far more energetic probes.  And, quantum fluctuations mean that they are ever-changing; instead of an instantaneous shape, we can only hope to observe, at best, the average shape, plus some measure of the fluctuations.   Like many larger objects, the way that they look depends on how one looks at them.

Consider the proton, the central nucleus of a hydrogen atom.   When we ask what it looks like, we can expect to learn about its size and hopefully shape.  Is it spherical?  How does its density change in going from the center to its periphery?   This begets an important question: density of what?  There are many different ways to characterize a proton.  One could measure the charge distribution, mass density, or at internal pressure, or look at where the quarks and/or gluons are within it.  One could also probe quarks or gluons within a specific momentum (Bjorken-x) range. 

Or, one could ask where its 'protonness' is.  More generally, one can ask where its baryon number is.  Baryon number is what makes protons and neutrons what they are, and distinguishes them from mesons.  In the valence quark model, protons have three quarks (as in the left drawing above). When one adds in gluons and evanescent quark-antiquark pairs, it looks more like the drawing on the right.  Then one can define baryons as having three more quarks than antiquarks.  But, this is not the only possibility.  However, sophisticated mathematics - the requirement of local gauge invariance of the proton  - points toward the existence of a baryon junction, comprising a nonperturbative localized configuration of gluons which is connected to these three quarks.  This model makes some predictions that seem to be borne out experimentally.   Another consequence is that the protonness is likely to be concentrated in the center of the nucleus.

We can probe this using a reaction that is sensitive to baryon number.  Backward photoproduction (also known as u-channel photoproduction) is one such reaction.  In it, a photon interacts with a proton, producing a meson, plus the original baryon.  Normally, as in diagram (a) above, the produced meson carries most of the momentum of the photon; the momentum transfer from the proton target is small.  But, sometimes, the proton acquires most of the photon momentum, leaving the meson nearly at rest.  This can happen if there is a large momentum transfer from the proton (diagram (c)) or if there is a small momentum transfer, but transfer also exchanges baryon number (diagram (b)).   Either way, the photon is rather directly probing the baryon number in the target.  

We use this reaction to 'image' the proton by looking at the momentum transfer in these backward production reactions.   The higher the momentum transfer, the smaller the object that is struck.  In more technical terms,  a two-dimensional Fourier transform of the transverse momentum of the outgoing meson gives us a density distribution of  baryon number within the proton.  In a preprint (arXiv:2603.03730) we (me, plus colleagues from U Washington, Livermore and UC Davis) did this for a number of different backward production reactions, along with, as a benchmark, some otherwise-matching conventional forward photoproduction reactions.  The plot below shows our results:

And there you have it - pictures of the essence of the proton - the seat of its baryon number. Here, the x axis is the radius (distance from the center of mass of the proton), in fermi; 1 fermi is 10^-15 m.   The y axis is the baryon number density.   For simplicity, we characterized these distributions by their average (rms) radius.  We found that the average transverse (after squashing the proton down to two dimensions) radius of the baryon number was 0.33 to 0.53 fm.  In comparison, a number of other measures of the proton size, based on its charge density, or parton densities, all found larger radii, at least 0.67 fm.  So, a protons protonness is concentrated near the center of the proton.  

There are caveats on this result.  If you are interested, please read the paper (click on the PDF link on the web page above), but overall it seems fairly (pardon the pun) solid.

 

 

 

 

3I/alpha re-emerges from behind the Sun. But, is it an alien artifact?

 The interstellar object (visitor from another sun) 3I/ATLAS has reemerged into view, after being hidden behind the sun.  It continues to attract considerable attention, from both telescopes and humans.  

 The images above, taken by the James Webb telescope are typical of recent images.  In this and other images, it appears to be a comet, with a small central nucleus, with a large coma (ice/dust cloud) on one side.  This cloud is expected; as a comet approaches the sun, surface ice is melted and ejected, taking some of the comet's rock/dust with it.  The coma is quite visible since the dust and ice is effective at reflecting sunlight.  Most of the other pictures are less detailed; there just wasn't much light to work with, so the images are fuzzy.  

3I/ATLAS has attracted much attention on two fronts.

First, it is only the third extraterrestrial object seen.  It appears different from both  1I/ʻOumuamua, which is a bit of an oddball by both accounts, and 2I/Borisov, which also looks a lot like a comet, but is  considerably smaller than 3I/ATLAS.   But, it looks enough like a comet so that NASA has officially called it one (rather than an asteroid).  In fact, if anything, the surprise is that it looks so much like the garden variety comes from our solar system. 

Second, there has been considerable speculation that it be an artifact produced by an alien civilization.  The reflected light has been postulated to be light emitted by the object.  There were also suggestions that it would alter course and head for Earth, or make other dramatic maneuvers.  However, the acceleration that we see is all consistent with a combination of gravity, plus the expected velocity changes due to the out-gassing mentioned above.

The alien-object hypothesis has some attraction.  It would be very very cool, and would radically change our view of the galaxy.  But, extraordinary claims require extraordinary proof.  That is lacking here.  There is no evidence of active maneuvering.  The observed light looks much like we expect from a comet.  

So, scientists have, very reasonably, fallen back on more mundane studies.  An interstellar visitor is plenty cool, even if it 'just' a comet from another solar system.

Antarctica - redux

The  Lawrence Berkeley National Laboratory Engineering Division communications people have put out a nice story about  Thorsten Stezelberger and my work in Antarctica - both ARIANNA and IceCube.  It is available at https://physicalsciences.lbl.gov/2025/10/14/visiting-antarctica-in-service-of-science/

This is somewhat timely, as the 2025/2026 Antarctic season ramps up.  For neutrino astronomy enthusiasts, this is a big one, featuring the deployment of the IceCube Upgrade.   The upgrade will consist of seven additional strings.  The strings will be densely packed (close together), with an energy threshold of a few GeV, giving IceCube a huge boost at low energies.  This should enable us to make a definitive measurement of tau-neutrino appearance through oscillations, and help us pin down the parameters for neutrino oscillation more precisely than previously possible.  The upgrade will also be filled with calibration devices that should allow us to measure the optical characteristics (scattering and absorption)  of the ice far more precisely.  this ice-property data should improve the accuracy of all IceCube measurements, by reducing our systematic uncertainties from ice property uncertainty.  

Alien visitors, redux: Why are we seeing them now?

 After my last post on 3I/ATLAS, an interested reader (full disclosure - my wife, Ruth Ehrenkrantz) asked an interesting question: Why are we only seeing the interstellar visitors now?  We have now seen three in eight years, after centuries of nothing?  

 The answer to this question is: technology.  Finding small (asteroids and comets both qualify) objects in our solar system is a matter of taking multiple photographs of the sky, spaced days or weeks apart, and looking for points of light that are moving against the fixed background of stars.    One needs to look at a lot of images to find a few moving objects.   Finding the even rarer (cf. three in eight years) objects that are moving too fast to originate in the solar system is even harder, and requires fast searches of large regions of the sky.

High-speed searching requires automation, to methodically take photos at appropriate intervals and automatically compare the images.  Correcting for different seeing conditions is not trivial - if the atmosphere is a bit more turbulent or hazy, images will look different.      This is technology that has been developed over the past few decade, driven by "Planetary Defense"which even has its own office at NASA.  Despite a name evocative of "Space 1999," this office is focused on a more prosaic concern, the possibility that an errant asteroid or comet might hit the Earth, ala the Tunguska meteorite or, worse, the asteroid that wiped out the dinosaurs (as shown in the visualization below, from New Scientist magazine). 

One well known search program is Asteroid Terrestrial-impact Last Alert System (ATLAS), which found 3I/ATLAS.  It uses five robotic telescopes, like the one shown above to search around the ecliptic plane (the plane containing the orbits of the planets around the Sun), looking for objects that might come close enough to Earth to pose a danger.  When it finds a moving object, it will follow it to determine its trajectory, and project its movement into the future, to see if it will come close to Earth, or even might hit it.  Of course, the further one projects into the future, the larger the uncertainty.  

Sometime - three times so far - these automated searches will find objects that are moving too fast to be gravitationally bound to the Sun.  It is not at all surprising that our observations of these extra-solar objects has started in the last decade, as searches like ATLAS have ramped up.   

Incidentally, although 3I/ATLAS looks like a typical comet, that has not stopped some astronomers - notably Harvard's Avi Loeb - from speculating that it may be an extra-terrestrial probe, on the basis that it is coming surprisingly close to several planets (not Earth, though).   Other astronomers continue to study it, and nothing too surprising has been observed.  Recently, it has been found to contain water, for example. 

Another alien visitor enters the Solar System

 The Solar System recently welcomed another visitor from another star:  3I/ATLAS.  We don't know much about it yet, but it seems considerably larger than ‘Oumuamua (which I wrote about previously) and 2I/Borisov - maybe 5 kilometers 3 miles) across, although considerably larger diameters - up to 12 km - have also been suggested, based on the hypothesis that it reflects only a small fraction of the incident light.  It is thought to be comet-like, with the outside made up of grains of water ice and silicates.  As the surface is heated by the sun, it is likely to form a coma - a long visible tail of gas and dust released as the ice melts.  Watching this evolution will tell us much about the nature of 3I/ATLAS. 

When it was spotted, it was roughly as far away as Jupiter. It is approaching rapidly and should reach 1.3-1.4 astronomical units (AU) from the Sun (1.3-1.4  times as far away from the Sun as the Earth is) in late November.  Unfortunately its closest approach to the Earth will only be at 1.8 AU.  Worse, when that happens, it will be on the far side of the Sun from us.   Depending on how much it brightens, it should be visible with relatively small telescopes, though.  So far, it appears to have a reddish color, perhaps a sign that it contains methane.   Interestingly, its trajectory is near the ecliptic plane of the Solar System - the plane that contains all of the planets orbits. 

It is also moving fast - faster than ‘Oumuamua -  and will reach 150,000 miles/hour during its closest approach to the Sun.  This simplified the detection, as can be seen from the gif below, combining images taken with the Asteroid Terrestrial-Impact Last Alert System:

 The moving dot shows how 3I/ATLAS stands out from the stationary background stars.  It is coming from the general direction of the center of our galaxy, which is also the direction of the highest density of nearby stars.   It was likely ejected from some other solar system by a close encounter with a planet which left it with a velocity high enough to escape from that sun.  

The three alien visitors that we have discovered are all quite different.  ‘Oumuamua is an oddball, while 2I/Borisov and 3I/ATLAS seem, except for their much higher velocities, comet-like, not too different from  the other comets that we see in the solar system.  More data is needed to draw any conclusions, but it does seem that these interstellar objects are relatively common, with three discovered in 8 years.  

Because of their large velocities and short notice of the arrival times, we probably won't be able to get a sample from one of these objects any time soon.  But, if we are patient, it may be possible to do a fly-by and get some high-resolution photos.