[HN Gopher] The S-matrix is the oracle physicists turn to in tim...
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The S-matrix is the oracle physicists turn to in times of crisis
Author : Brajeshwar
Score : 33 points
Date : 2024-05-23 16:35 UTC (6 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| CamperBob2 wrote:
| Interesting bit of trivia: the S-matrix concept is also where the
| so-called S-parameters came from, which are used by RF engineers
| to describe amplitude and phase relationships at input and output
| ports [1].
|
| There was some cross-pollination with the physics community in
| the 1960s that helped the RF people with circuit modeling
| problems they were facing due to the limitations of existing
| models. [2] Hewlett-Packard jumped on this with both feet [3],
| because S-parameters were a natural fit not only for modeling
| linear devices and circuits, but for measuring them using the
| vector network analyzers they were developing at the time.
|
| [1] https://cds.cern.ch/record/1415639/files/p67.pdf
|
| [2] https://typeset.io/papers/power-waves-and-the-scattering-
| mat...
|
| [3] http://www.hparchive.com/Application_Notes/HP-AN-95-1.pdf
| nico wrote:
| Maybe we don't need a new model, but rather just accepting that
| the universe can be two seemingly opposite things at the same
| time
|
| Like the particle/wave duality
|
| Maybe the whole universe is both quantum and not, at the same
| time. And we just perceive it differently at different scales.
| Like with a stroboscopic light, we might perceive things
| differently depending on how we are sampling (and not necessarily
| because the thing is different)
| nathan_compton wrote:
| Nice idea, but there are intermediate scales and sampling
| methods and the universe has to have rules that determine how
| things happen at those scales.
| NoMoreNicksLeft wrote:
| > and the universe has to have rules that
|
| Has anyone bothered to tell the universe this?
| MengerSponge wrote:
| I won't have any Last Thursdayists under my roof.
| tsimionescu wrote:
| The wave/particle duality has long-since been accepted, as have
| other dualities (e.g. the event horizon of a black hole is both
| a completely normal portion of spacetime, and a boundary region
| that emits Hawking radiation, depending on your trajectory),
| where it is impossible to make an experiment that could see
| both at the same time.
|
| But QM can't be reconciled with GR in the same way. One day
| we'll have precise enough instruments to measure the
| gravitational interactions of large particles. One day we'll
| have large enough particle accelerators to be able to measure
| quantum properties of huge molecules whose gravitational
| properties we already know. When those days come, we'll need a
| unified theory of how quantum fields cause gravitational
| effects.
|
| And even if it turns out there is some fundamental limit of
| some type of measurement that does cause a QM/GR duality to
| exist, a theory that includes that specific limit would itself
| be a new, unified theory, that supersedes both QM/QFT and GR.
| nico wrote:
| > But QM can't be reconciled with GR in the same way
|
| Would you care to explain?
|
| Why are the models so fundamentally incongruent?
|
| What exactly is the issue?
| tsimionescu wrote:
| I'm not a physicist, just a layman interested in this
| topic. So, if you're looking for very rigorous
| explanations, I can't help.
|
| That said, my understanding is this: if you try to
| calculate the trajectory of small particles using the
| equations of GR, and then compare that to measurements,
| you'll find that GR is dead wrong. Conversely, if you try
| to calculate the motion of stellar objects in strong
| gravitational fields using the equations of QM or QFT,
| you'll again get results that don't match observations at
| all.
|
| Further, for more complex mathematical reasons that I can't
| personally follow, if you just try to introduce the GR
| space-time deformation terms into QM equations, you can't
| compute anything because the equations diverge.
| hifromwork wrote:
| (Also not a physicist). GR works and has been tested
| experimentally on a large scale, but it doesn't always work
| on a small scale. QM works, has been tested experimentally
| on a small scale, but it doesn't always work on a large
| scale.
|
| They just give different predictions in some very small or
| very large cases. Obviously they can't both be right: if
| one theory predicts A and the other theory predicts B then
| we can just do the experiment and one of them will turn out
| to be wrong. Unfortunately, they only differ in extreme
| circumstances that are hard to recreate, and we were not
| able to perform an experiment that will tell us which is
| right (or how to tweak them to make them compatible with
| each other).
|
| Fun fact: There is a thing called a cosmological constant
| problem. It's the disagreement between the observed values
| of the cosmological constant (using GR) and a theoretical
| value suggested by QM. The observed value is... 10**120
| times smaller than QM predicts it should be [1]. Now this
| is what I call a fundamental incongruence :).
|
| [1] This is also called "probably the worst theoretical
| prediction in the history of physics".
| petsfed wrote:
| > _And we just perceive it differently at different scales.
| Like with a stroboscopic light, we might perceive things
| differently depending on how we are sampling_
|
| That's actually exactly what's going on with wave/particle
| duality. Everything's a wave, all the time, but in certain
| regimes, wave behavior very closely approximates particle
| behavior. The realization that particle behavior is therefore
| just a subset of wave behavior is part of the motivation for
| looking for an omnibus theory that accounts for everything.
|
| Unification is not unprecedented, consider e.g. the unification
| of the weak nuclear force and electromagnetic forces into the
| electroweak force.
| nico wrote:
| Where's the incongruence?
|
| At a macro scale, anything that happens in time can be
| considered as a sort of wave that we perceive as an object
| because of the way we are sampling it
|
| Even basic things, like when you measure your own weight,
| will vary throughout the day/time, but instead of considering
| them a wave, we usually just "collapse it" into some sort of
| average of observations
|
| Conceptually at least, both models seem to represent the same
|
| How are they not compatible? Is it a math issue?
| tahoeskibum wrote:
| The main incompatibility is that QM predicts that all
| fields have a zero point energy associated with each mode.
| However, GR predicts zero point energy will cause spacetime
| to curve. However there are infinite modes in QM, so now
| using this in GR leads to all spacetime instantly
| collapsing. However we exist so this collapse hasn't
| happened. The solution is the elusive Quantum Gravity,
| something that physicists from Einstein, Feynman haven't
| been able to solve.
| nico wrote:
| Thank you for the explanation
|
| What does it mean for "all spacetime instantly
| collapsing"?
|
| It would be amazing to see a video of a simulation of
| that
| barfbagginus wrote:
| I propose the sharticle. Whereas particles and waves have
| energy and phase, the sharticle is always just in different
| states of shitting its pants. This makes it much more relatable
| to a human, who is already a type of sharticle.
| ykonstant wrote:
| Nice exposition on the physics side; the S-matrix formalism has
| also played a major role in the mathematical theory of quantum
| chaos and aspects of representation theory.
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