What Is Antimatter?

Antimatter is made of antiparticles.

For many known particles, there is a corresponding antiparticle with:

• the same mass
• the same spin
• opposite electric charge, if the particle is charged
• opposite values of certain quantum numbers

For example:

electron → positron

proton → antiproton

neutron → antineutron

The positron is the antimatter counterpart of the electron.

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Matter and Antimatter

Ordinary matter is made from particles such as:

• electrons
• protons
• neutrons

Antimatter is made from the corresponding antiparticles.

For example:

The bar above a symbol often indicates an antiparticle.

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The Positron

The antiparticle of the electron is the positron.

The electron has:

charge = −1

The positron has:

charge = +1

Both have exactly the same mass:

0.511 MeV/c²

Both are leptons.

However, their lepton numbers are opposite:

electron: L = +1

positron: L = −1

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The Antiproton

The proton contains three valence quarks:

uud

The antiproton contains the corresponding antiquarks:

ūūd̄

The proton has:

charge = +1

baryon number = +1

The antiproton has:

charge = −1

baryon number = −1

Their masses are the same.

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The Antineutron

The neutron has:

charge = 0

The antineutron also has:

charge = 0

So how are they different?

Their internal quark structures are different.

Neutron:

udd

Antineutron:

ūd̄d̄

Their baryon numbers are also opposite:

neutron: B = +1

antineutron: B = −1

This shows that an antiparticle is not always distinguished by electric charge alone.

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Antiparticles and Quantum Numbers

Antiparticles have opposite values of several additive quantum numbers.

For example:

This is important because conservation laws must still be obeyed when matter and antimatter interact.

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Matter and Antimatter Are Not "Opposite Mass"

A common misconception is that antimatter has negative mass.

It does not.

An antiparticle has the same positive mass as its corresponding particle.

For example:

electron mass = positron mass

proton mass = antiproton mass

The word "anti" refers to opposite quantum properties, not negative mass.

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Annihilation

When a particle meets its antiparticle, they can undergo annihilation.

During annihilation, the original particle-antiparticle pair disappears and new particles are produced.

For example:

e⁻ + e⁺ → γ + γ

An electron and positron can annihilate to produce two gamma-ray photons.

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Why Are Two Photons Produced?

Suppose an electron and positron annihilate while their total momentum is zero.

Producing only one photon would not work because a photon always carries momentum.

Two photons travelling in opposite directions can satisfy:

total momentum before = total momentum after

Therefore:

e⁻ + e⁺ → γ + γ

allows both energy and momentum to be conserved.

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Energy Released During Annihilation

The electron and positron each have a rest energy of:

0.511 MeV

Therefore, if they annihilate while essentially at rest:

total rest energy = 0.511 + 0.511

= 1.022 MeV

If two photons are produced equally:

energy per photon = 0.511 MeV

So:

e⁻ + e⁺ → 2γ

with each photon carrying approximately:

511 keV

in the centre-of-momentum frame when the pair begins essentially at rest.

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Conservation Laws in Annihilation

Consider:

e⁻ + e⁺ → γ + γ

Charge

Before:

−1 + 1 = 0

After:

0 + 0 = 0

Charge is conserved.

Baryon Number

Before:

0

After:

0

Lepton Number

Before:

+1 − 1 = 0

After:

0

All three are conserved.

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Proton-Antiproton Annihilation

A proton and antiproton can also annihilate.

A simplified representation is:

p + p̄ → other particles

Because protons contain quarks and antiprotons contain antiquarks, annihilation often produces several particles rather than just two photons.

The products commonly include hadrons such as pions.

The important principle is still:

matter + antimatter → other particles and energy

while all relevant conservation laws remain satisfied.

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Pair Production

Annihilation can be considered the reverse of pair production.

During pair production, energy creates a particle-antiparticle pair.

For example:

energy → e⁻ + e⁺

A high-energy photon can produce an electron-positron pair in the presence of another object, such as a nucleus:

γ + nucleus → e⁻ + e⁺ + nucleus

The nearby nucleus helps conserve momentum.

At least:

1.022 MeV

of photon energy is needed to provide the rest mass-energy of the electron and positron pair, with additional energy needed for recoil and kinetic energy.

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Matter-Antimatter Symmetry

The laws of physics treat particles and antiparticles in closely related ways.

For example:

electron and positron have equal masses

proton and antiproton have equal masses

Experiments have tested many particle-antiparticle properties to extremely high precision.

However, matter and antimatter are not perfectly interchangeable in every process.

Certain weak-interaction processes show small differences between the behaviour of matter and antimatter, known as CP violation.

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Why Is the Universe Mostly Matter?

One of the biggest unanswered questions in physics is why the observable universe contains much more matter than antimatter.

According to our current understanding, the early universe should have produced matter and antimatter in nearly equal amounts.

If exactly equal amounts had survived and annihilated, very little ordinary matter would remain.

Yet stars, planets, and living organisms are made overwhelmingly of matter.

Scientists are still investigating why this imbalance developed.

This problem is known as the matter-antimatter asymmetry.

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Is There an "Antimatter Periodic Table"?

In principle, antimatter could form structures similar to ordinary matter.

An antihydrogen atom, for example, consists of:

• one antiproton
• one positron

This mirrors ordinary hydrogen:

• one proton
• one electron

Scientists have successfully produced and trapped antihydrogen atoms for research.

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Antimatter and Gravity

Because antimatter has positive mass-energy, it is expected to respond to gravity.

Experiments with antihydrogen are used to test this directly.

Current measurements are consistent with antimatter falling downward in Earth's gravitational field rather than behaving as if it had negative mass.

This is an active area of precision experimental physics.

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Practical Application: PET Scanning

One of the most important practical uses of antimatter is positron emission tomography, or PET.

PET is a medical imaging technique.

A radioactive tracer containing a positron-emitting isotope is introduced into the body.

The isotope undergoes beta-plus decay and emits a positron.

The positron travels a short distance before meeting an electron.

Then:

e⁺ + e⁻ → γ + γ

Two gamma photons are produced.

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How PET Detects Annihilation

In PET imaging, the two annihilation photons typically travel in nearly opposite directions.

Detectors around the patient detect the photons.

If two detectors register photons at nearly the same time, the system can infer that the annihilation occurred somewhere along the line between them.

Many such detections are combined by computers to construct an image.

PET can help reveal patterns of biological activity in the body.

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Why PET Uses 511 keV Photons

When an electron and positron annihilate approximately at rest:

electron rest energy = 511 keV

positron rest energy = 511 keV

Therefore, the two photons each have approximately:

511 keV

This characteristic energy is fundamental to PET imaging.

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Antimatter in Scientific Research

Antimatter is also used in fundamental physics research.

Scientists study antimatter to investigate:

• matter-antimatter symmetry
• properties of antiparticles
• CP violation
• the behaviour of antimatter in gravity
• the origin of the matter-antimatter imbalance
• tests of the Standard Model

Antiprotons and antihydrogen are especially useful in these experiments.

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Antimatter Is Difficult to Store

Antimatter cannot simply be placed in an ordinary container.

If it touches ordinary matter, annihilation occurs.

Scientists therefore use combinations of:

• electric fields
• magnetic fields
• extremely high vacuum

to keep charged antiparticles away from the walls of a container.

Neutral antimatter such as antihydrogen requires specially designed magnetic traps.

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Why Antimatter Is Not a Practical Energy Source

Matter-antimatter annihilation converts a very large fraction of the initial rest mass-energy into other forms of energy.

That may make antimatter sound like an ideal fuel.

However, there is a major problem:

We must first produce the antimatter.

Producing antimatter requires much more usable energy than can presently be recovered from it.

Antimatter is also extremely difficult to manufacture in significant quantities and difficult to store.

Therefore, antimatter is currently a research tool and medical technology component, not a practical energy source.

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Comparing Matter and Antimatter

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Example 1: Identify the Antiparticle

What is the antiparticle of an electron?

Electron:

e⁻

The antiparticle must have:

• same mass
• opposite charge
• opposite lepton number

Therefore:

e⁺

The antiparticle is the positron.

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Example 2: Identify the Antiparticle

What is the antiparticle of a proton?

The proton has:

Q = +1

B = +1

The antiparticle has:

Q = −1

B = −1

Therefore:

p̄

The particle is the antiproton.

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Example 3: Check an Annihilation Reaction

Consider:

p + p̄ → γ + γ

Check charge:

Before:

+1 − 1 = 0

After:

0

Check baryon number:

Before:

+1 − 1 = 0

After:

0

So charge and baryon number are conserved.

However, actual proton-antiproton annihilation commonly produces multiple hadrons because protons and antiprotons are composite particles.

The simplified equation still illustrates the conservation principles.

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Example 4: Energy from Electron-Positron Annihilation

An electron and positron annihilate at rest.

Each has rest energy:

0.511 MeV

Total:

0.511 + 0.511 = 1.022 MeV

If two equal-energy photons are produced:

1.022 ÷ 2 = 0.511 MeV

Each photon has:

0.511 MeV = 511 keV

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Did You Know?

Antimatter occurs naturally.

Positrons can be produced during some radioactive decays, cosmic-ray interactions, and other high-energy processes.

Antimatter is therefore not purely artificial.

However, large concentrations of antimatter are not normally found around us because contact with ordinary matter leads to annihilation.

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Connecting the Ideas

Antimatter connects many topics in particle physics:

Fundamental particles

↓

Each particle may have an antiparticle

↓

Opposite charges and quantum numbers

↓

Matter-antimatter interaction

↓

Annihilation

↓

Mass-energy converted into other particles and radiation

↓

Conservation laws remain satisfied

This connects antimatter directly to:

• mass-energy equivalence
• particle collisions
• particle decays
• conservation laws
• medical imaging
• the Standard Model

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Key Terms

Antimatter – Matter composed of antiparticles.

Antiparticle – A particle with the same mass as its corresponding particle but opposite values of certain quantum properties.

Positron – The antiparticle of the electron.

Antiproton – The antiparticle of the proton.

Antineutron – The antiparticle of the neutron.

Antiquark – The antiparticle corresponding to a quark.

Annihilation – A process in which a particle and its antiparticle transform into other particles.

Pair production – The creation of a particle-antiparticle pair from energy.

Antihydrogen – An antimatter atom consisting of an antiproton and a positron.

PET – Positron emission tomography, a medical imaging technique that uses positron-emitting radioactive tracers.

Matter-antimatter asymmetry – The observed dominance of matter over antimatter in the universe.

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Key Takeaways

• Antimatter is made of antiparticles.
• Most particles have corresponding antiparticles with the same mass but opposite values of certain quantum numbers.
• The antiparticle of the electron is the positron.
• The antiparticle of the proton is the antiproton.
• Antimatter does not have negative mass.
• When a particle meets its antiparticle, they can undergo annihilation.
• Electron-positron annihilation can produce two gamma-ray photons.
• Mass-energy, momentum, charge, baryon number, and lepton number must remain conserved during antimatter interactions.
• The reverse of annihilation is pair production, in which energy produces a particle-antiparticle pair.
• Antimatter can form structures such as antihydrogen.
• Antimatter has important practical use in PET medical imaging.
• Antimatter is difficult and energy-intensive to produce and store, so it is not currently a practical energy source.
• The reason the observable universe contains much more matter than antimatter remains one of the major open questions in modern physics.