
Researchers have observed the Lyman-alpha transition in the antihydrogen atom - sohkamyung
https://home.cern/about/updates/2018/08/alpha-experiment-takes-antimatter-new-level
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japanuspus
As an atomic physicist, it is great to see clever atomic physics experiments
opening up new fields in high-energy physics. The team has been working
incredibly hard for this: Jeff Hangst who leads the experiment was already
working on this when he taught my undergrad electrodynamics course back in
1998!

An important piece of background info for non-atomic-physicists is that the
Hydrogen 1S-2S transition frequency is probably the most accurately measured
physical property in the world, thanks to the pioneering work by T. Hansch.
Back in 2011 they reported an uncertainty of 4e-12,
[https://arxiv.org/abs/1107.3101](https://arxiv.org/abs/1107.3101).

~~~
jschwartzi
I thought the charge on an electron had been measured out to 10^-19? One of my
professors posted the journal article from the NIST team that did so.

Here's a reference: [https://physics.nist.gov/cgi-
bin/cuu/Value?e](https://physics.nist.gov/cgi-bin/cuu/Value?e)

~~~
reacweb
Your reference gives an uncertainty of 6.1 x 10-9

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acd
If there is anti matter. Is there an anti gravity force if there is gravity?

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philipov
Antimatter is a consequence of quantum field theory, but all signs indicate
gravity can not be successfully described as a quantum field. Gravity has
properties such as having its interaction strength grow with the amount of
charge (energy), which is very different from the forces described by quantum
fields that have constant interaction strength. To get antigravity, you need
to find a way to have negative energy without it making the vacuum unstable.

It's most typical to describe gravity as dynamical spacetime, not a field
existing in that spacetime. The closest people have come to a quantum theory
of gravity has been to show that in certain toy universes gravity in a bulk
space is the dual of strongly-coupled field interactions existing on the
boundary of that space-- that is holography. Information-theoretic approaches
have have also suggested that gravity emerges as the 'hydrodynamics of
entanglement.' All this suggests gravity is not a fundamental force, but is
far from a coherent real-world theory at this point.

The closest to an "antigravity" in general relativity is the cosmological
constant, thought to explain dark energy. Its tiny (10^-122) positive value
behaves like a constant negative energy attributed to the vacuum itself,
driving the exponential expansion of space.

~~~
pdonis
_> all signs indicate gravity can not be successfully described as a quantum
field_

That's not quite correct. The quantum field theory of a massless spin-2 field
was developed in the 1960s and 1970s and is perfectly well-defined. The
problem with it then was that it is not renormalizable; but today basically
all QFTs are viewed as effective field theories anyway, which means not being
renormalizable is not a showstopper.

The real issue is that we have no prospect of measuring any quantum aspects of
gravity now or in the foreseeable future, so there is no way to test the QFT
that has been developed against experiment.

 _> Gravity has properties such as having its interaction strength grow with
the amount of charge (energy), which is very different from the forces
described by quantum fields that have constant interaction strength._

This is not correct either. The coupling constant of the massless spin-2 field
is just Newton's gravitational constant (modulo some constant factors that
depend on your units). The fact that the total interaction in a specific case
varies with energy is no different from the fact that the total EM force
between two charged objects varies with the charge. Energy is the source for
gravity just as charge is the source for EM; the source is not the same as the
coupling constant.

 _> To get antigravity, you need to find a way to have negative energy without
it making the vacuum unstable._

Agreed. And as far as we know that is not possible.

 _> All this suggests gravity is not a fundamental force_

This is quite possible, yes, but even if it were true it would not necessarily
make the spin-2 massless QFT I referred to above invalid; it would just make
it an effective theory that emerges from whatever is the next level down.
(Which, as I noted above, is basically how we view all QFTs today anyway,
including the Standard Model of particle physics.)

 _> The closest to an "antigravity" in general relativity is the cosmological
constant, thought to explain dark energy. Its tiny (10^-122) positive value
behaves like a constant negative energy attributed to the vacuum itself,
driving the exponential expansion of space._

No, a positive CC does not have negative energy. It has positive energy and
negative pressure. The effective source in the Friedmann equation is the
energy density plus three times the pressure, which for a positive CC is
negative (because the negative pressure outweighs the positive energy
density).

~~~
raattgift
> massless spin-2 field

>> To get antigravity, you need to find a way to have negative energy without
it making the vacuum unstable.

> Agreed. And as far as we know that is not possible.

Which part are you agreeing to?

Spin-2 essentially means that like charges attract and unlike charges repel.
Matter with the opposite gravitational charge from all the matter (incl.
antimatter) in the universe we know about could have segregated in the early
universe. Opposite-gravitational-charge matter (incl. antimatter) is
unobserved, but not in-principle unobservable.

Vacuum instability doesn't arise since m_i stays the same as m_g changes sign
under gravitational charge changes.

~~~
philipov
1) They meant there's no way to get negative energy particles which could be
the source of antigravity. That would destabilize the vacuum because it would
allow you to keep lowering the energy without bound.

2) Intuitively if antimatter is regular matter travelling backwards in time,
then at the start of time, wouldn't it all go the other way? Kind of like one
huge quantum fluctuation.

~~~
pdonis
_> if antimatter is regular matter travelling backwards in time_

It isn't. More precisely, while the mathematical trick that this phrase refers
to (which also, btw, requires reversing all charges and flipping parity) works
as far as making calculations simpler while still getting the right answer, it
doesn't correspond to anything physical.

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ianai
I skimmed and didn’t find it. Is the frequency the same as hydrogen?

~~~
gus_massa
From the abstract of the paper:
[http://www.nature.com/articles/s41586-018-0435-1](http://www.nature.com/articles/s41586-018-0435-1)

> _The transition frequency at a field of 1.033 tesla was determined to be
> 2,466,051.7 ± 0.12 gigahertz (1σ uncertainty) and agrees with the prediction
> for hydrogen to a precision of 5E−8._

I.E. It's the same for all practical purposes, but there may be a extremely
tiny difference.

Edit: Comparing the numbers with the results for Hydrogen in an article
someone else linked here

    
    
      Anti-Hydrogen:      2,466,051.7 ± 0.12 GHz
      (Normal)-Hydrogen:  2,466,061.413187035(10) GHz 
    

So they measure a tiny difference, and looking at the error estimations the
difference is significative. I'm not sure if that is an expected difference or
a unexpected difference or just an error in the experiment.

~~~
Blackbeard_
That would be more than a 5 sigma difference which would definitely be
significant but that number you're quoting for normal hydrogren is the 1S-2S
line.

This measurement is the 1S-2P line which has an average frequency of
~2,466,051.625 GHz (there's actually two levels). Which agrees to within 1
sigma - i.e. good agreement.

~~~
gus_massa
Thanks for the correction. I just assumed that the other article was about the
same transition and I forget to check the details.

PS: If someone else wants to know why the 1S-2S and the 1S-2P transition have
almost the same energy, but no exactly the same energy, it's an interesting
weird story
[https://en.wikipedia.org/wiki/Lamb_shift](https://en.wikipedia.org/wiki/Lamb_shift)

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ArtWomb
A step closer to unlocking the secret origins of matter itself. Some deep
background on the anti-matter imbalance as well as famed Soviet dissident
Andrei Sakharov's conditions:

The Mystery Of The Matter Asymmetry

[https://www.vanderbilt.edu/AnS/physics/panvini/babar/sakharo...](https://www.vanderbilt.edu/AnS/physics/panvini/babar/sakharov.pdf)

~~~
sulam
I read this entire article with some interest, but made the (naive) assumption
that it would actually speak about the conditions under which Sakharov made
his contributions to matter/anti-matter imbalance. Instead I learned something
new, which is that there exist a set of conditions Sakharov identified that
must be accounted for in any theory that addresses this imbalance. Don't read
it expecting to learn about Sakharov's life. :)

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ajuc
I always wondered how they put atoms in magnetic traps, isn't hydrogen 1
positron + 1 antiproton? It should be neutral?

~~~
castratikron
It's electrically neutral, but it still has a magnetic moment.

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iamgopal
Because electron is moving and proton not ?

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castratikron
The magnetic moment of an atom comes from the orbital motion of the electrons
about the nucleus and also the rotational motion of each electron about their
axes (called "spin"). In the case of a hydrogen atom there's one electron that
contributes to the magnetic moment, which is how you're able to magnetically
capture an electrically neutral atom.

