
The contest between gravity and quantum physics takes a new turn (2015) - dnetesn
http://nautil.us/issue/49/the-absurd/will-quantum-mechanics-swallow-relativity-rp
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H4CK3RM4N
> Just as a pixel is the smallest unit of an image on your screen and a photon
> is the smallest unit of light, he argues, so there might be an unbreakable
> smallest unit of distance: a quantum of space.

I thought that that role was fuffiled with the Planck Length. I've seen it
used in conjunction with the speed of light to calculate a theoretical
"refresh rate" of the universe, was that based on a flawed assumption?

~~~
gaze
The Planck length has no physical meaning. It's just a combination of physical
constants that gives a length. It's considered a scale at which quantum
gravitational effects become significant, but it's just that, a scale. It
might be a factor of 100 off from something significant, but what that thing
is we have no idea.

~~~
iaw
I have read alternate interpretations of the Planck length/time/volume/etc.

One says that they are a mathematical construct with no meaning, the other
says that they provide a bound for the limits of the measurement of reality as
we understand it (which many people take to mean the smallest unit).

Do you think these definitions disagree? Does one definitely supersede the
other? If so, could you point me at some literature?

~~~
gizmo686
Not a physisist, but I found this link [0] to be understandable. The
Heisenberg uncertainty principle says that the more accurately we know
position, the less accurately we know momentum. When you formalize this, the
result is that as the size size of a particle you are trying to measure
decreases, the energy necessary to measure it increases. Since energy causes
gravity, there is a length, _l_ , where measuring a particle would require so
much energy that the resulting gravity creates a black hole.

I think it is fair to say that _l_ is just a mathematical construct, since
nothing _really_ happens at this point. But it is a useful mathematical
construct that is, theoretically, relevant to our ability to observe the
universe.

Having said all of that, all of this is at a much smaller scale than we have
ever been able to observe. Seeing as we needed to invent new physics
(relativity) to explain scales as small Mercury's orbit [1], and as large as
atoms, and that these two theories are still incompatible, it is highly likely
that describing physics near the plank scale would require another reinvention
of physics.

[0] [http://backreaction.blogspot.com/2012/01/planck-length-as-
mi...](http://backreaction.blogspot.com/2012/01/planck-length-as-minimal-
length.html)

[1] Arguably smaller, since we have confirmed relativity without leaving Earth
orbit.

~~~
gaze
Yes I too can contrive a thought experiment that combines a bunch of physical
constants. It's hardly a construct. It's just a length. It might mean
something. It might mean something when multiplied by pi or a tenth or
whatever. It's the scale at which quantum gravity might maybe possibly could
be important. Nothing more nothing less.

~~~
gizmo686
I would not consider the explanation I describe to be a thought experiment,
but rather a mathematical derivation; although I might showing my roots as a
mathematician instead of a physicist there.

Additionally, all the argument, as I presented it, shows is that there is
_some_ length beyond which we cannot observe. It turns out that we can
calculate this length, and the result happens to be precisely the plank
length. As a mathematician, this seems highly unlikely to be a coincidence;
however I am not familiar enough with physics to know if this is a deep
result, or a trivial consequence of it's definition.

To clarify the disagreement: do you agree that our current theory predicts
that there is _some_ length beyond which we cannot measure?

~~~
gaze
Yeah, that would make sense, but I don't imagine it's a hard cutoff. It may be
that you get diminishing information per unit energy at a length scale related
to the Planck length or something.

------
platz
> Beneath its layers of conceptual complexity, the holometer is
> technologically little more than a laser beam, a half-reflective mirror to
> split the laser into two perpendicular beams, and two other mirrors to
> bounce those beams back along a pair of 40-meter-long tunnels.

> For the scale of chunkiness that Hogan hopes to find, he needs to measure
> distances to an accuracy of 10-18 meters, about 100 million times smaller
> than a hydrogen atom, and collect data at a rate of about 100 million
> readings per second.

How is this different from LIGO in any way?

~~~
6nf
It's the same thing but the Holometer is intended to be more sensitive than
LIGO.

------
vorg
> What if the universe were entirely empty except for two astronauts. One of
> them is spinning, the other is stationary. The spinning one feels dizzy,
> doing cartwheels in space. But which one of the two is spinning? From either
> astronaut’s perspective, the other is the one spinning. Without any external
> reference, Einstein argued, there is no way to say which one is correct

There is a 50% chance each is spinning, and we'll know which is correct when
an external reference "observes" them. The fact that there's no external
reference is what makes this situation be "quantum". If the conscious
astronauts are conceptually incapable of being 50% "feeling dizzy", then that
would mean conscious beings also require the external reference in order to
exist.

~~~
cobbzilla
This example totally confused me. An object spinning in space experiences
centripetal acceleration, relative to its own center of mass. For sake of
example, let's say that instead of spinning in cartwheels, the one spinning
astronaut is spinning like an ice skater. OK, which astronaut feels their arms
being pulled outwards due to centripetal force, and which one has their arms
hanging limply at their sides? Wouldn't that answer the question?

~~~
trhway
in the model where inertia is a result of interacting with all the other
gravitational fields empty Universe would mean no inertia and thus no
centripetal force. (of course there is another model - that of Higgs boson -
for which i haven't so far been able to find how it explains equivalence of
inertial and gravitational masses and thus it is so far non-starter for me, i
mean not the boson itself - that of course exists, i mean the boson's role in
inertial mass story until of course the boson happesn to be the main carrier
of the "gravitational charge" then all would fall nicely in place)

~~~
cobbzilla
Correct me if I'm wrong, but without the Higgs boson and its corresponding
scalar field, the concept of mass/inertia does not exist. Without mass, can
matter still exist? How would this "matter without mass" behave? My mind is
close to exploding.

~~~
trhway
imagine that there is only gravitational force in the Universe and the matter
has only gravitational mass("charge"). Accelerating any given piece of matter
with respect to all the other matter would cause propagation of change (ie.
wave) in the gravitational field of the whole system (Universe). Wave is
energy and to generate wave a work is required, ie. a force has to be applied
to the piece of matter being accelerated - the requirement for that force is
what we call inertia.

Now of course the question - what is the carrier of that "gravitational
charge". It would be great if Higgs boson was the carrier of that charge.

~~~
cobbzilla
I thought the idea that "the Higgs boson is the field responsible for
imparting mass" was the dominant theory, are there reasonable alternative
explanations?

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suprgeek
No it will not. Elements from both theories will be used to formulate the
structure/theory/paradigm that will then subsume both of them.

------
dogma1138
Can we stop ignoring the fact that Quantum gravity exists and works for the
most part.

The parts where it doesn't work are extremes like black holes where GR also
breaks apart.

~~~
vecter
Where's the evidence for your statement? This is what Wikipedia says:

> Currently, there is still no complete and consistent quantum theory of
> gravity, and the candidate models still need to overcome major formal and
> conceptual problems. They also face the common problem that, as yet, there
> is no way to put quantum gravity predictions to experimental tests, although
> there is hope for this to change as future data from cosmological
> observations and particle physics experiments becomes available.

[0]
[https://en.wikipedia.org/wiki/Quantum_gravity#Candidate_theo...](https://en.wikipedia.org/wiki/Quantum_gravity#Candidate_theories)

