Scientists led by the University of Oxford have created extraordinarily dense beams of matter and antimatter in the laboratory. Containing around 10 trillion electron-positron pairs, these artificial “fireballs” could allow researchers to experimentally investigate some of the extreme plasma physics normally found around black holes and neutron stars.

Black holes are among the most extreme objects in the Universe.

Their immense gravity can pull surrounding matter inward while powerful jets of plasma are launched into space at velocities approaching the speed of light.

Some of those jets can extend over enormous cosmic distances.

Astronomers can observe them with telescopes. Physicists can simulate them with computers.

But reproducing their underlying physics experimentally on Earth has been extremely difficult.

Researchers led by the University of Oxford’s Department of Physics have now made a major step towards changing that.

Matter meets antimatter

Ordinary matter contains electrons, which carry a negative electric charge.

Electrons also have an antimatter counterpart: the positron, which has the same mass but a positive charge.

Around extreme astrophysical objects, electrons and positrons can exist together in enormous numbers, producing what physicists call an electron-positron pair plasma.

Such plasmas are thought to be important in environments associated with black holes, neutron stars, active galactic nuclei and gamma-ray bursts.

Creating a sufficiently dense version in a laboratory has been the problem.

Previous experiments could produce electron-positron pairs, but not necessarily in numbers large enough for the beam to behave collectively like a true astrophysical plasma.

The Oxford-led experiment changed that.

Ten trillion pairs

The international team successfully generated high-density relativistic plasma beams containing around 10 trillion electron-positron pairs.

That is important because at such densities the particles begin displaying collective, wave-like plasma behaviour rather than behaving merely as a collection of independent particles.

The researchers reported an electron-positron yield two to three orders of magnitude greater than had previously been achieved in comparable experiments.

For laboratory astrophysics, that represents a significant step.

CERN becomes an astrophysics laboratory

The experiment was conducted using the HiRadMat facility at CERN in Geneva.

Researchers harnessed the enormous energy of CERN’s Super Proton Synchrotron (SPS).

Approximately 300 billion protons were used to produce the electron-positron beams. Each proton carried kinetic energy around 440 times greater than its rest-mass energy.

When these extraordinarily energetic protons collide with atoms, they can break apart their internal constituents.

Quarks and gluons are released and rapidly recombine, producing cascades of particles.

Those cascades eventually generate large numbers of electrons and positrons.

The result is an artificial beam with properties sufficiently extreme to begin resembling the plasma found in some astrophysical environments.

Bringing the Universe into the laboratory

The achievement belongs to a growing field known as laboratory astrophysics.

Scientists obviously cannot travel to a black hole, place instruments beside it and manipulate its plasma jets.

Nor can they experimentally control a gamma-ray burst.

Instead, researchers attempt to reproduce selected physical conditions at a vastly smaller scale inside laboratories.

If the scaling is understood correctly, experiments on Earth can then be used to test theories about processes taking place billions of kilometres — or even billions of light-years — away.

Lead author Charles Arrowsmith, from Oxford’s Department of Physics, said the work opens an entirely new frontier by allowing scientists to experimentally investigate the microphysics of gamma-ray bursts and active-galactic-nuclei jets.

What telescopes cannot see

Modern astronomy possesses extraordinarily powerful telescopes.

But distance imposes a fundamental limitation.

Even the best satellite and ground-based observatories cannot resolve every microscopic process taking place inside distant cosmic plasma jets.

Scientists therefore rely heavily on theoretical models and computer simulations.

Professor Gianluca Gregori, the lead investigator at Oxford, explained that laboratory experiments can now provide another way of testing those calculations.

Researchers could, for example, study how these artificial plasma fireballs interact with other plasmas.

Those results could then be compared with theoretical predictions about what happens when cosmic jets encounter the thin plasma between stars.

Gamma-ray bursts

One potential application concerns gamma-ray bursts, among the most energetic events observed in the Universe.

They can be associated with catastrophic events such as the collapse of massive stars or mergers involving compact stellar remnants.

For a brief period, a gamma-ray burst can release extraordinary quantities of energy.

Understanding the behaviour of relativistic electron-positron plasmas is therefore important for understanding how such energy is generated, transported and dissipated.

The new laboratory method provides scientists with a controllable experimental system in which aspects of that physics can be investigated.

A collaboration across physics

The project brought together different branches of physics.

Oxford researchers involved specialists in atomic and laser physics, theoretical physics and particle physics.

The collaboration also included CERN, the Rutherford Appleton Laboratory, the University of Rochester’s Laboratory for Laser Energetics, Lawrence Livermore National Laboratory, the Max Planck Institute for Nuclear Physics, the University of Iceland and Instituto Superior Técnico in Portugal, among others.

That combination is itself significant.

A particle accelerator originally built to investigate fundamental particles can also become an experimental tool for studying some of the largest and most violent structures in the Universe.

A black hole has not been created

The term “black-hole plasma fireball” can easily create the wrong impression.

The researchers did not create a black hole.

Nor did they reproduce an entire black-hole environment.

What they created was a high-density electron-positron plasma beam with properties relevant to the relativistic plasma found around extreme astrophysical objects.

That distinction is important.

The breakthrough lies in reproducing a piece of the physics under controlled laboratory conditions.

From CERN to the cosmos

Modern science often advances by building machines that allow humans to reach scales nature normally keeps inaccessible.

Microscopes allow us to investigate the extremely small.

Telescopes reveal the extremely distant.

Particle accelerators recreate conditions that existed fractions of a second after the Big Bang.

This experiment combines those traditions in an unusual way.

A beam created inside a laboratory at CERN can now serve as a scaled experimental analogue for matter behaving around some of the most violent objects in the cosmos.

For astrophysicists, the Universe has effectively become a little more accessible.

Instead of only watching cosmic fireballs from afar, scientists can now begin experimenting with their physics here on Earth.

Source: Based on “Researchers generate black hole ‘plasma fireballs’ on Earth,” published by the University of Oxford on 14 June 2024. The research was led by Oxford’s Department of Physics in collaboration with CERN, Rutherford Appleton Laboratory and international partners. The underlying study, “Laboratory realization of relativistic pair-plasma beams,” by Charles Arrowsmith and colleagues was published in Nature Communications on 12 June 2024.