You and Carol had a busy day. You saw Uncle
John at Friendship Village in Dublin, had lunch at Max and Erma’s at Polaris
Shopping and picked up vases at Kim and Paul’s before heading home. You had a
nap, Carol took her walk in the park. You had Carol’s chili for supper, watched
NBC News and Madam Secretary. Carol is watching one of her shows presently.
2030 hours. Doug sent me an article on
quantized gravity he recently received from his son, Greg. I don’t understand
all this of course, maybe next to none of it actually, but I find it
interesting. The article is from ‘backreactionDOTblogspotDOTcom’. I added the
underlining to the interesting-to-me parts.
** **
Monday,
October 12, 2015
A newly proposed table-top experiment might be able to
demonstrate that gravity is quantized
Tl;dr:
Experimentalists are bringing increasingly massive systems into quantum states.
They are now close to masses where they might be able to just measure what
happens to the gravitational field.
Quantum effects of gravity are weak, so weak they are widely believed
to not be measurable at all. Freeman Dyson indeed is fond of saying that a
theory of quantum gravity is entirely unnecessary, arguing that we could never
observe its effects anyway. Theorists of course disagree, and not just because
they’re being paid to figure out the very theory Dyson deems unnecessary. Measurable
or not, they search for a quantized version of gravity because the existing
description of nature is not merely incomplete – it is far worse, it contains
internal contradictions, meaning we know it is wrong.
Take the century-old double-slit experiment, the prime example
for quantum behavior. A single electron that goes through the double-slit is
able to interact with itself, as if it went through both slits at once. Its
behavior is like that of a wave, which overlaps with itself after passing an
obstacle. And yet, when you measure the electron after it went through the slit
it makes a dot on a screen, like a particle would. The wave-like behavior
again shows up if one measures the distribution of many electrons that passed
the slit. This and many other experiments demonstrate that the electron is
neither a particle nor a wave – it is described by a wave-function from
which we obtain a probability distribution, a formulation that is the core of
quantum mechanics.
Well understood as this is, it leads to a so-far unsolved
conundrum.
The most relevant property of the electron’s quantum behavior is
that it can go through both slits at once. It’s not that half of the electron
goes one way and half the other. Neither does the electron sometimes take this
slit and sometimes the other. Impossible as it sounds, the electron goes
fully through both slits at the same time, in a state referred to as quantum
superposition.
Electrons carry a charge and so they have an electric field.
This electric field also has quantum properties and moves along with the
electron in its own quantum superposition. The electron also has a mass. Mass
generates a gravitational field, so what happens to the gravitational field? You
would expect it to also move along with the electron, and go through both slits
in a quantum superposition. But that can only work if gravity is quantized too.
According to Einstein’s theory of General Relativity though, it’s not. So we
simply don’t know what happens to the gravitational field unless we find a
theory of quantum gravity.
It’s been 80 years since the question was first raised, but we
still don’t know what’s the right theory. The main problem is that gravity
is an exceedingly weak force. We notice it so prominently in our daily life
only because, in contrast to the other interactions, it cannot be neutralized.
But the very reason that planet Earth doesn’t collapse to a black hole is that
much stronger forces than gravity prevent this from happening. The
electromagnetic force, the strong nuclear force, and even the supposedly weak
nuclear force, are all much more powerful than gravity.
For the experimentalist this means they either have an object
heavy enough so its gravitational field can be measured. Or they have an object
light enough so its quantum properties can be measured. But not both at once.
At least that was the case so far. But the last decade has seen
an enormous progress in experimental techniques to bring heavier and heavier
objects into quantum states and measure their behavior. And in a recent
paper a group of researchers from Italy and the UK propose an experiment that
might just be the first feasible measurement of the gravitational field of a
quantum object.
Almost all researchers who work on the theory of quantum gravity
expect that the gravitational field of the electron behaves like its electric
field, that is, it has quantum properties. They are convinced of this because
we have a well-working theory to describe this situation. Yes, I know, they
told you nobody has quantized gravity, but that isn’t true. Gravity has been
quantized in the 1960s by DeWitt, Feynman, and others using a method known as
perturbative quantization. However, the result one gets with this method
only works when the gravitational field is weak, and it breaks down when
gravity becomes strong, such as at the Big Bang or inside black holes. In other
words, this approach, while well understood, fails us exactly in the situations
we are interested in the most.
Because of this failure in strong gravitational fields,
perturbatively quantized gravity cannot be a fundamentally valid theory; it
requires completion. It is this completion that is normally referred to as
“quantum gravity.” However, when gravitational fields are weak, which is
definitely the case for the little electron, the method works perfectly fine.
Whether it is realized in nature though, nobody knows.
If the gravitational field is not quantized, one has instead a
theory known as “semi-classical gravity,” in which the matter is quantized but
gravity isn’t. Though nobody can make much sense of this theory conceptually,
it’s infuriatingly hard to disprove. If the gravitational field of the electron
remained classical, its distribution would follow the probability of the
electron taking either slit rather than itself going through the slits with
this probability.
To see the
difference, consider you put a (preferably uncharged) test particle in the
middle between the slits to see where the gravitational pull goes. If the
gravitational field is quantized, then in half of the cases when the electron
goes through the slit, the test particle will move left, in the other half of
cases it would move right (it would also destroy the interference pattern). If
the gravitational field is classical however, the test particle won’t move
because it’s pulled equally to both sides.
So the difference between quantized and semi-classical gravity
is observable. Unfortunately,
even for the most massive objects that can be pushed through double slits, like
large molecules, the gravitational field is far too weak to be measurable.
In the new paper
now, the researchers propose a different method. They consider a tiny charged
disk of osmium with a mass of about a nano-gram, held by electromagnetic fields
in a trap. The particle is cooled down to some hundred mK which brings it into
the lowest possible energy state. Above this ground-level there are now
discrete energy levels for the disk, much like the electron orbits around the
atomic nucleus, except that the level spacing is tiny. The important point
is that the exact energy values of these levels depend on the gravitational
self-interaction of the whole object. Measure the spacing of the energy
levels precisely enough, and you can figure out whether the gravitational field
was quantized or not.
For this calculation they use the Schrödinger-Newton equation,
which is the non-relativistic limit of semi-classical gravity incorporated in
quantum mechanics. In an accompanying paper they have worked out the
description of multi-particle systems in this framework, and demonstrated how
the system approximately decomposes into a center-of-mass variable and the
motions relative to the center of mass. They then calculate how the density
distribution is affected by the gravitational field caused by its own
probability distribution, and finally the energy levels of the system.
I haven’t checked this calculation in detail, but it seems both
plausible that the effect should be present, and that it is large enough to
potentially be measurable. I don’t
know much about these types of experiments, but two of the authors of the
paper, Hendrik Ulbricht and James Bateman, are experimentalists and I trust
they know what current technology allows to measure.
Suppose they make
this measurement and they do, as expected, not find the additional shift of
energy levels that should exist if gravity was unquantized. This would not,
strictly speaking, demonstrate that perturbatively quantized gravity is
correct, but merely that the Schrödinger-Newton equation is incorrect. However, since these are the only two alternatives I
am aware of, it would in practice be the first experimental confirmation that
gravity is indeed quantized.
Selected and edited
from -- http://backreaction.blogspotDOTcom/2015/10/a-newly-proposed-table-top experiment.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+Backreaction+%28Backreaction%29
** **
2058
hours. This article has many ‘reactions’ to it, which are as interesting as the
article. I love to see ‘reason’ in the arguments. The actual science is far
beyond me. For instance, Doug writes (to me), “Maybe this can explain black
hole behavior when developed.” – I have no reference for what he is saying
about black hole behavior here.
What you are showing is that you are quite
limited, that you look for the logic, the reasoning used and if it appears to
be put together well, that it will stand, at least as an argument but with
little background on the content you are at a loss to give even an opinion on
the matter at hand. – Amorella
2108 hours. This is so. Still, I
enjoyed the article because it stretches my sense that reason will eventually
rule the day as far as the science goes.
Are you saying you have faith in reason? –
Amorella
2110 hours. This was unexpected. – It does
sound like it. Reason reinforces what is real, that is, it reinforces what I
feel is real. If it doesn’t, then what I thought/felt was real was not or it is
unknown whether it is real or not, like; ‘Are UFO’s alien driven crafts?’ No
proof, then not real. – I think I am off base here. I have changed the subject
and or its intent.
This kind of experience leads you to believe
you are indeed weak-minded. – Amorella
Surprising, no it does not. Today’s
post shows that I am tired.– rho
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