the most energetic particles — starts with a bang! — medium

Throwback Thursday: TheMost Energetic Particles

These cosmic monsters make the LHC look like childs play, and yet even

they have their limits.

Image credit: Pierre Auger Observatory, via http://apcauger.in2p3.fr/Public/Presentation/.

Energy is liberated matter, matter is energy waiting to happen. -Bill

Bryson

You might think of the largest and most powerful particle accelerators in

the worldplaces like SLAC, Fermilab and the Large Hadron Collideras

the source of the highest energies well ever see. But everything weve ever

done here on Earth has absolutely nothing on the natural Universe itself!

https://medium.com/starts-with-a-bang/throwback-thursday-the-most-energetic-particles-6f1b6811abea

In fact, if you were interested in the most energetic particles on Earth,

looking at the Large Hadron Colliderat the 13 TeV collisions occurring

insideyou wouldnt even be close to the highest energies. Sure, those are

the highest human-made energies for particles, but were constantly

bombarded all the time by particles far, far greater in energy.

If youve never heard of them before, let me introduce you to a term I hope

youll never forget after learning about them now: cosmic rays, famous the

world over for their (ctional) eects, turning the four scientists aboard

Reed Richards spacecraft into the Fantastic Four.

Image credit: CERN, via http://people.physics.tamu.edu/kamon/research/refColliders/LHC/LHC_is_back.html.


You didnt need to be in space, or even to have any type of ight, to know

that these particles existed. Even before the rst human beings ever left the

surface of the Earth, it was widely known that up there, above the

protection of the Earths atmosphere, outer space was lled with

high-energy radiation. How did we know?

The rst clues came from looking at one of the simplest electricity

experiments you can do on Earth, involving an electroscope. If youve never

heard of an electroscope, its a simple device: take two thin pieces of

conducting, metal foil, place them in an airless vacuum, and connect them

Image credit: Stan Lee / Marvel Comics.


to a conductor on the outside that you can control the electric charge of.

If you place an electric charge on one of these deviceswhere two

conducting metal leaves are connected to another conductorboth leaves

will gain the same electric charge, and repel one another as a result. Youd

expect, over time, for the charge to dissipate into the surrounding air,

which it does. So you might have the bright idea to isolate it as completely

as possible, perhaps creating a vacuum around the electroscope once you

charge it up.

But even if you do, the electroscope still slowly discharges! In fact, even if

you placed lead shielding around the vacuum, it would still discharge, and

experiments in the early 20th century gave us a clue as to why: if you went

to higher and higher altitudes, the discharge happened more quickly. A few

scientists put forth the hypothesis that the discharge was happening

because high-energy radiationradiation with both extremely large

penetrating power and an extraterrestrial originwas responsible for this.

Image credit: Boomerias Honors Physics page, via http://boomeria.org/.


Well, you know the deal when it comes to science: if you want to conrm or

refute your new idea, you test it! So in 1912, Victor Hess conducted

balloon-borne experiments to search for these high-energy cosmic

particles, discovering them immediately in great abundance and henceforth

becoming the father of cosmic rays.

The early detectors were remarkable in their simplicity: you set up some

sort of emulsion (or later, a cloud chamber) thats sensitive to charged

particles passing through it and place a magnetic eld around it. When a

charged particle comes in, you can learn two extremely important things:

The particles charge-to-mass ratio and

its velocity,

simply dependent on how the particles track curves, something thats a

Image credit: American Physical Society.


dead giveaway so long as you know the strength of the magnetic eld you

applied.

In the 1930s, a number of experimentsboth in early terrestrial particle

accelerators and via more sophisticated cosmic ray detectorsturned up

some interesting information. For starters, the vast majority of cosmic ray

particles (around 90%) were protons, which came in a wide range of

energies, from a few mega-electron-Volts (MeV) all the way up to as high as

they could be measured by any known equipment! The vast majority of the

rest of them were alpha-particles, or helium nuclei with two protons and

two neutrons, with comparable energies to the protons.

Image credit: Paul Kunze, in Z. Phys. 83 (1933), of the rst muon event ever in 1932.


When these cosmic rays hit the top of the Earths atmosphere, they

interacted with it, producing cascading reactions where the products of

each new interaction led to subsequent interactions with new atmospheric

particles. The end result was the creation of whats called a shower of

high-energy particles, including two new ones: the positronhypothesized

in 1930 by Dirac, the antimatter counterpart of the electron with the same

mass but a positive chargeand the muon, an unstable particle with the

same charge as the electron but some 206 times heavier! The positron was

discovered by Carl Anderson in 1932 and the muon by him and his student

Seth Neddermeyer in 1936, but the rst muon event was discovered by Paul

Kunze a few years earlier, which history seems to have forgotten!

One of the most amazing things is that even here on the surface of the

Earth, if you hold out your hand so that its parallel to the ground, about

one muon passes through it every second.

Image credit: Simon Swordy (U. Chicago), NASA.


Every muon that passes through your hand originates from a cosmic ray

shower, and every single one that does so is a vindication of the theory of

special relativity! You see, these muons are created at a typical altitude of

about 100 km, but a muons mean lifetime is only about 2.2 microseconds!

Even moving at the speed of light (299,792.458 km/sec), a muon would only

travel about 660 meters before it decays. Yet because of time dilationor

the fact that particles moving close to the speed of light experience time

passing at a slower rate from the point-of-view of a stationary outside

observerthese fast-moving muons can travel all the way to the surface of

the Earth before they decay, and thats where muons on Earth originate!

Fast-forward to the present day, and it turns out that weve accurately

measured both the abundance and energy spectrum of these cosmic

particles!

Image credit: Konrad Bernlhr of the Max Planck Institute for Nuclear Physics.


Particles with about 100 GeV worth of energy and under are by far the most

common, with about one 100 GeV particle (thats 10^11 eV) hitting every

square-meter cross-section of our local region of space every second.

Although higher-energy particles are still there, theyre far less frequent as

we look to higher and higher energies.

For example, by time you reach 10,000,000 GeV (or 10^16 eV), youre only

getting one-per-square-meter each year, and for the highest energy ones, the

ones at 5 10^10 GeV (or 5 10^19 eV), youd need to build a square

detector that measured about 10 kilometers on a side just to detect one

particle of that energy per year!

Image credit: Hillas 2006, preprint arXiv:astro-ph/0607109 v2, via University ofHamburg.


Seems like a crazy idea, doesnt it? Its asking for a huge investment of

resources to detect these incredibly rare particles. And yet theres an

extraordinarily compelling reason that wed want to do so: there should be

a cuto in the energies of cosmic rays, and a speed limit for protons in the

Universe! You see, there might not be a limit to the energies we can give to

protons in the Universe: you can accelerate charged particles using

magnetic elds, and the largest, most active black holes in the Universe

could give rise to protons with energies even greater than the ones weve

observed!

But they have to travel through the Universe to reach us, and the Universe

even in the emptiness of deep spaceisnt completely empty. Instead,

its lled with large amounts of cold, low-energy radiation: the cosmic

microwave background!

Image credit: ASPERA / G.Toma / A.Saftoiu.


The only places where the highest energy particles are created are around

the most massive, active black holes in the Universe, all of which are far

beyond our own galaxy. And if particles with energies in excess of 5 10^10

GeV are created, they can only travel a few million light yearsmax

before one of these photons, left over from the Big Bang, interacts with it

and causes it to produce a pion, radiating away the excess energy and

falling down to this theoretical cosmic energy limit, known as the GZK

cuto. (More details here.)

So we did the only reasonable thing for physicists to do: we built a detector

that ridiculously large and looked, and saw if this cuto existed!

Image credits: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB:NASA/WMAP.


The Pierre Auger Observatory has done exactly this, verifying that cosmic

rays exist up to but not over this incredibly high-energy threshold, a literal

factor of about 10,000,000 larger than the energies reached at the LHC!

This means the fastest protons weve ever seen evidence for in the Universe

are moving almost at the speed-of-light, which is exactly 299,792,458 m/s,

but just a tiny bit slower. How much slower?

The fastest protonsthe ones just at the GZK cutomove at

299,792,457.999999999999918 meters-per-second, or if you raced a

photon and one of these protons to the Andromeda galaxy and back, the

photon would arrive a measly six seconds sooner than the proton would

after a journey of more than ve million years! But these ultra-high-energy

cosmic rays dont come from Andromeda; they come from active galaxies

with supermassive black holes like NGC 1275, which tend to be hundreds of

millions or even billions of light years away.

Image credit: Pierre Auger Observatory in Malarge, Argentina / Case WesternReserve U.


We even knowthanks to NASAs Interstellar Boundary Explorer (IBEX)

that there are about 10 times as many cosmic rays out there in deep

space as we detect here on-and-around Earth, as the Suns heliosheath

protects us from the vast majority of them!

In theory, there are collision occurring everywhere in space between these

cosmic rays, and so in a very real sense of the word, the Universe itself is

our ultimate Large Hadron Collider: up to ten million times more energetic

Image credit: NASA, ESA, Hubble Heritage (STScI/AURA).

Image credit: Adler Planetarium / Chicago.


than what we can perform here on Earth.

And thats the fantastic story of the highest energy particles in the Universe

from cosmic raysand the cosmic energy limit!

Leave your comments at the Starts With A Bang forum on Scienceblogs!

Starts With ABang!The Universe is outthere, waiting for youto discover it.

Follow Ethan SiegelThe Universe is:Expanding, cooling,and dark. It startswith a bang!#Cosmology Sciencewriter, astrophysicist,science communicator& NASA columnist.

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Published on May 7. All rights reserved by the author.


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