- 4191 -
ANTI-MATTER - and gravity? Matter fall down under the pull of
gravity. So, does anti-matter fall up
? Antimatter falls down too. How do we explain that?
--------------------- 4191 - ANTI-MATTER - and gravity?
- Physicists have
shown that, like everything else experiencing gravity, antimatter falls
downwards when dropped. This outcome is
not surprising. A difference in the
gravitational behavior of matter and antimatter would have huge implications
for physics.
-
- Because gravity is
much weaker than other forces such as electrostatic attraction or magnetism,
separating it from other effects in the laboratory is a delicate. Gravity is just so weak, you really have to
be careful.
-
- Experiments will
soon aim to test whether gravity acts with the “same strength” on antimatter as
it does on matter. Any tiny discrepancies could help to solve one of the
biggest problems in physic. How the
Universe came to be made almost exclusively of matter, even though equal
amounts of matter and antimatter should have arisen from the Big Bang.
-
- In the world of
antimatter, atomic nuclei are made of negatively charged antiprotons, orbited
by positively charged antielectrons, or positrons. According to the standard
model of particle physics, however, the opposite charges should be pretty much
the only difference: particles and antiparticles should have nearly all the
same properties.
-
- In particular,
experiments have confirmed that positrons and antiprotons have the same masses
as their matter counterparts, within the limits of experimental errors.
-
- According to
Einstein’s general theory of relativity, all objects of the same mass should
weigh the same. They should experience
exactly the same gravitational acceleration.
-
- Design an
experiment that would show what happened when the neutral atom antihydrogen was
dropped. It’s almost impossible to do
an experiment with a charged particle, so antihydrogen is the perfect
candidate.
-
- Antimatter
particles are routinely created in laboratories. For example, most particles
produced by high-energy particle collisions are made in pairs, one particle of
matter and its antiparticle. But it is hard to get antiparticles to combine
into antiatoms because antimatter particles are typically very short-lived.
-
- When an
antiparticle meets a particle, they both cease to exist and turn back into
energy, in a process called annihilation. In a world made primarily of matter,
this makes it hard for antimatter particles to find each other.
-
- CERN is currently
the only place in the world where antihydrogen can be made. It has an
accelerator that makes antiprotons from high-speed proton collisions, and a
‘decelerator’ called “ELENA” that slows them down enough to be held for further
manipulation.
-
- Several different
experiments feed off ELENA in CERN’s antimatter research hall. ALPHA-g is one
of them, and it combines antiprotons with positrons it collects from a
radioactive source.
-
- After making a
thin gas of thousands of antihydrogen atoms, researchers pushed it up a
3-meter-tall vertical shaft surrounded by superconducting electromagnetic
coils. These can create a magnetic ‘tin can’ to keep the antimatter from coming
into contact with matter and annihilating.
-
- Next, the
researchers let some of the hotter antiatoms escape, so that the gas in the can
got colder, down to just 0.5 °C above absolute zero and the remaining antiatoms
were moving slowly.
-
- The researchers
then gradually weakened the magnetic fields at the top and bottom of their trap
and detected the antiatoms using two sensors as they escaped and annihilated.
When opening any gas container, the contents tend to expand in all directions,
but in this case the antiatoms’ low velocities meant that gravity had an
observable effect: most of them came out of the bottom opening, and only
one-quarter out of the top.
-
- To make sure that
this asymmetry was due to gravity, the researchers had to control the strength
of the magnetic fields to a precision of at least one part in 10,000. This was
perhaps their most remarkable feat.
-
- The results were
consistent with the antiatoms experiencing the same force of gravity as
hydrogen atoms would. The error margins are still rather large, but the
experiment can at least conclusively rule out the possibility that antihydrogen
falls upwards.
-
- In 2010, the team
was the first one to succeed at trapping antihydrogen for an extended time, and
starting in 2016 they were able to measure how the antiatoms absorb light.
-
- No one would have
expected antimatter to fall up, if nothing else, because antiprotons are made
of antiquarks, but these only constitute less than 1% of an antiproton’s mass:
the rest is the energy that keeps them together. Any violation, if it exists, cannot be over
1% . Going beyond that would subvert not
only the theory of gravitation, but also the standard model of particle
physics.
-
- A third CERN
experiment, called “AEgIS” will attempt to measure the gravitational force on a
beam of antihydrogen atoms in the absence of any magnetic fields. They will aim
to reach 1% precision by first making positive antihydrogen ions (antihydrogen
with an extra positron), which will help to cool the gas down to a fraction of
a degree above absolute zero.
-
- Other efforts aim
to measure gravity acting on “positronium”, a short-lived particle made of one
electron and one positron orbiting each other. ALPHA-g itself plans to aim for
1% precision by letting antihydrogen atoms bump up and down and form a quantum
superposition with themselves.
-
-
October 19, 2023 ANTI-MATTER - and
gravity 4191
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