
Simple Thought Experiment Shows Why We Need Quantum Gravity - ohaikbai
https://www.forbes.com/sites/startswithabang/2018/07/20/this-simple-thought-experiment-shows-why-we-need-quantum-gravity/
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abainbridge
I think the simple thought experiment is:

Imagine the famous two slit experiment. In this version we will fire electrons
at the slits, one at a time. Put a very accurate spring balance behind each
slit and measure the gravitational attraction of the electron as it goes past.

The author then claims, "The description that General Relativity puts forth
... needs to be augmented to include an uncertain position that has a
probability distribution to it."

Does that follow? Wouldn't, say, the Pilot Wave Theory avoid the need for GR
to change?

~~~
BlackFly
Yes, this is my thought as well. It simply does not follow that the two
alternatives are a probabilistic stress energy tensor or a violation of the
uncertainty principle.

Since quantum mechanics describes electrons as a field and not as a particle
with position and momentum, there is always the possibility that an extension
to the field or a function of the field itself can generate a classical stress
energy tensor.

~~~
teilo
Yes, but that doesn't solve the problem. That is just another way of
rephrasing the problem. A stress-energy tensor is still dependent upon
position and strictly speaking is not a thing in itself, but a mathematical
description of an thing/effect - in this case a graviton field or equivalent
which is associated with the higgs field of a particle in superposition. The
result will always be described via a stress-energy tensor, but this is a
description of a result, not a cause.

However this works out, we still have to deal with the "position problem" when
it comes to describing the gravitational field of a particle.

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justifier
> The description that General Relativity puts forth ... needs to be augmented
> to include an uncertain position that has a probability distribution to it.

i am sorry but 'needs' is too strong here

quantisation(o) was established before, necessarily so, probability was
introduced to explain the behaviour of electromagnetic phenomena

to claim that probability is necessary for quantisation seems to have it
reversed

the current understanding is both unknown whether gravity is quantised but
also whether probability is necessary to describe quantum behaviour

with current understanding the only thing one can say with certainty is that
requiring probability is a result of the math used to best model quantum
behaviour of electromagnetic phenomena, namely fourier analysis(i)

(o)
[https://en.wikipedia.org/wiki/Ultraviolet_catastrophe](https://en.wikipedia.org/wiki/Ultraviolet_catastrophe)

(i) [https://physics.stackexchange.com/questions/35746/is-
there-a...](https://physics.stackexchange.com/questions/35746/is-there-a-
relation-between-quantum-theory-and-fourier-analysis)

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axilmar
Sorry for asking this in here, but since the double slit experiment is
mentioned, I have a question.

Is the wave formed by light a 2d wave, like waves on the surface of the water?

And if so, then if we turn the slits sideways, why does the interference
pattern also turns sideways? it shouldn't turn sideways if light is a 2d wave.

My apologies for the offtopic question, but I couldn't find any answer about
this anywhere.

~~~
TheOtherHobbes
Light acts like a 2D wave when it's linearly polarised. But it can also have
circular polarisation - which rotates - or some mix of polarisations.

If you put a vertical polariser in front of one slit and a horizontal
polariser in front of the other slit, the usual pattern of fringes disappears.
So polarisation does matter for the experiment.

But I'd strongly suggest not thinking of light in wave and/or particle terms.
In Quantum Field Theory everything is made of fields, which are more like
carriers of probability which strongly hint at some weird non-local
properties, than a nice smooth lake surface waiting for some ripples.

If you try to apply physical intuition to QM it soon stops making any sense at
all. So IMO it's best not to start there.

~~~
axilmar
If we can't make the analogy between the surface of water and light, then how
come the double slit experiment shows us that the interference pattern is due
to the wavy nature of light?

~~~
spiralx
Because a wave is a more general phenomenon than what happens on the surface
of water, and waves of all kind result in interference patterns. Also, waves
in the ocean certainly aren't two dimensional either, the surface "wave" just
a visible manifestation of a larger system.

But the polarisation of light demonstrates more quantum effects. Put two
polarising filters in front of a light at 90 degrees to each other and no
light gets through them both; yet add a third filter in between at 45 degrees
angle to each, and now half of the light gets through all three filters!
Adding extra filters can make them let more light through, not less...

[http://alienryderflex.com/polarizer/](http://alienryderflex.com/polarizer/)

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sonnyblarney
"General Relativity describes gravity perfectly everywhere we've ever looked.
From the smallest-scale attractions we've ever measured in a laboratory to the
expansion and curvature of space due to Earth, the Sun, black holes, galaxies,
or the entire Universe, our observations and measurements have never deviated
from what we've observed. "

So correct me if I'm wrong, but the rotation of galaxies completely defies
even Newtonian principles, right?

So we invented concepts of Dark Matter / Dark Energy as a placeholder,
crossing our fingers that we're right on that?

~~~
PuffinBlue
Not exactly.

Imagine a china mug full of water. You can see the mug and the water being
held in by the mug.

Now imagine a glass of water. The purest most transparent glass ever, with an
identical refraction index as the water. You can't see the glass. You just see
the water being held together by something. You don't know the glass exists.

You 'invent' the glass to explain the behaviour of the water, seeing as you
know the how the mug affects the water.

~~~
oldandtired
The glass is testable though and would quickly be found. Dark matter and dark
energy are assumed to exist with no experimental observations occurring. Every
experiment has so far failed to show the existence of said entities.

One possibility is that we do not have any way to find said entities with any
means at our disposal. Another possibility is that they do not exist (just a
figment of imagination) and that the theories requiring them are wrong. In
that case, one should be looking for other theories that might be a more
capable explanation.

The question here is to determine when or whether we should stop looking for
these entities and maybe move onto another model/theory.

The basic assumption of all gravity based theories and models is that the
universe is electrically neutral everywhere and that magnetic and electric
fields have no possible influence at stellar, interstellar, galactic and
intergalactic distances. Whether this assumption is true or not is difficult
to gauge since we only have visual observations (at all sorts of frequencies)
and cannot actually get out there to make the required measurements.

So we have to fall back to what we think is reasonable and work from there.

~~~
SiempreViernes
> The basic assumption of all gravity based theories and models is that the
> universe is electrically neutral everywhere and that magnetic and electric
> fields have no possible influence at stellar, interstellar, galactic and
> intergalactic distances

This is _impressively_ inaccurate: plasmas (charged and magnetic fluids) are
basically the entirety of stellar evolution and the main part of everything up
to galaxy evolution, so we have pretty good limits on the magnetic fields out
in the universe.

There is also direct measurements of magnetic fields integrated along the line
of sight out to cosmological distances, and of course every spectra ever taken
has measured the ionization level of the luminous gas.

~~~
oldandtired
I agree. Yet the models assume that gravity is the only influence for the
motion of the galaxies, etc. We can see evidence of other phenomena but....

I have had this discussion with trained working physicists and the general
reaction has been that these phenomena are completely insignificant in
relation to stellar, interstellar, galactic and intergalactic motion. So where
do we stand? If the orders of magnitude difference in strength between
electric and magnetic fields and the strength of gravity is over 35. What
kinds of effects do we expect with these kinds of differences? Especially when
we are looking at plasmas and all sorts of ionisation.

I don't know, but it does lead to intriguing questions.

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qubex
When I try to picture the “smeared out” gravitational field of a single
“smeared out” quantum particle my head hurts. Try to nail down its position by
hitting it with a photon and you thereby increase its momentum and thus
relative mass and gravitational field... plus all those gravitons have
energies of their own and therefore associated graviton-mediated gravitational
fields... it’s a heroic mess.

~~~
abdullahkhalids
> you thereby increase its momentum

You don't increase its momentum. You increase the uncertainty in the momentum.
So if initially position was 5 +- 3 and momentum was 10 +- 2, then afterwards,
you might know that position is 6 +- 1, and momentum is 10 +- 4.

The rest of your train of thought doesn't follow after this refutation.

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konschubert
> You set up a photodetector around each slit, and measure when a particle
> passes through it.

[...No interference happens...]

> This is weird!

No, it's not. You made that particle's wave function collide with the huge
wave function of a photodetector, of course it's going mess up the wave of the
particle itself.

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konschubert
> Does the gravitational field always go primarily through one slit or the
> other? And does the act of observing (or not observing) change the
> gravitational field? And if so, how?

The Copenhagen interpretation is really throwing a wrench into our
understanding of physics.

