- 4245 - SPACE GRAVITY - how
fast is? How can the universe expand
faster than light travels? It seems
like it should be illegal. The supreme
iron law of the universe is that nothing can go faster than the speed of
light. Nothing further needs to be said
about the issue, but here is more!
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--------------------------
4245 - SPACE GRAVITY - how
fast is?
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- Some galaxies are
moving away from us faster than the speed of light. What gives?
-
- We live in an
expanding universe. Every day the galaxies get farther apart from each other,
on average. There are slight motions on top of that general expansion, leading
to instances such as the Andromeda Galaxy heading on a collision course for the
Milky Way. But in general, in the biggest of pictures, the galaxies are getting
farther away from each other.
-
- The objects close to you would appear to move away with
some speed, but the farther objects would appear to move faster. The apparent stretch of space changes with
distance.
-
- Edwin Hubble was
the first to measure the expansion rate. The number he got was way wrong. The more modern value is 68 kilometers per
second per megaparsec. One megaparsec
is 1 million parsec, which is 3.26 million light-years.
-
- It means that if
you look at a galaxy 1 megaparsec away, it will appear to be receding away from
us at 68 km/s. If you look at a galaxy 2 megaparsec away, it recedes at 136
km/s. Three megaparsec away? 204 km/s.
And on and on: for every megaparsec, you can add 68 km/s to the velocity of the
far-away galaxy.
-
- So it's easy
enough to compute: At some point, at some obscene distance, the speed tips over
the scales and exceeds the speed of light, all from the natural, regular
expansion of space.
-
- The movement of
that galaxy can be interpreted as a "speed": you can measure the
distance to it, wait awhile and measure it again. Distance moved divided by
time equals speed. The speed you measure
can be faster than light.
-
- The notion of the
absolute speed limit comes from “special relativity”, but who ever said that
special relativity should apply to things on the other side of the universe?
That's the domain of a more general theory,
“general relativity”.
-
- In special
relativity, nothing can move faster than light. But special relativity is a
local law of physics. In other words, it's a law of local physics. That means
that you will never, ever watch a rocket ship blast faster than the speed of
light. Local motion, local laws.
-
- But a galaxy on the
far side of the universe? That's the domain of general relativity, and general
relativity says that galaxy can have any speed it wants, as long as it stays
way far away.
-
- Concepts like a
well-defined "velocity" make sense only in local regions of space.
You can only measure something's velocity and actually call it a
"velocity" when it's nearby and when the rules of special relativity
apply. Stuff far away, like the galaxies
doesn't count as a “velocity” in the way that special relativity cares about.
-
- Another misintrepretation is that antimatter falls down,
not up. 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 ubiquitous forces such as electrostatic attraction or
magnetism, separating it from other effects in the laboratory is a delicate
affair.
-
- Gravity is just so
weak, you really have to be careful to get a good measurement.
Experiments 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 physics. 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.
-
- 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.
-
- What would happen
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. 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.
-
- The results were
consistent with the antiatoms experiencing the same force of gravity as
hydrogen atoms would. The error margins are still large, but the experiment can
at least conclusively rule out the possibility that antihydrogen falls upwards.
-
- In 2010 scientists
succeed at trapping antihydrogen for an extended time, and starting in 2016
they were able to measure how the antiatoms absorb light. But the gravity
experiment required a new level of sophistication.
-
- No one would have
expected antimatter to fall up 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.
-
- The science goes
on to understand the world we live in.
How does it work?
-
-
November 27, 2023
4242
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Wednesday, November 29, 2023 ---------------------------------
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