[HN Gopher] An ultra-precise clock shows how to link the quantum...
___________________________________________________________________
An ultra-precise clock shows how to link the quantum world with
gravity
Author : theafh
Score : 145 points
Date : 2021-10-25 14:42 UTC (8 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| sam_goody wrote:
| Can someone please explain, since I am missing the obvious:
|
| Time travels sliiiiiightly faster by my feet than by my head. So
| how does all of me stay put together? I would expect that even if
| the shift was really tiny, as soon as part of my body is not in
| the same time reference point as another part, it would go * poof
| *.
| Lambdanaut wrote:
| The chemical bonds between your molecules are stickier than the
| force differences created by the difference in time.
| myaccount80 wrote:
| I didn't read the article yet but yesterday I was wondering about
| something: does gravity (the bending of space) bends the
| electromagnetic field? I guess they (spacetime and
| electromagnetic fields) are two independent fields but maybe they
| influence each other ?
|
| Edit: maybe because these two forces have very different
| magnitude it is not possible to measure it
| tsimionescu wrote:
| A field is, by definition, a physical quantity in space and
| time. The key idea of GR is that gravity _is_ the curvature of
| space time. The electromagnetic field is not bent, light for
| example always travels in perfect "straight lines" in the
| curbed space time created by mass/energy (more specifically,
| light always follows the shortest possible length of space-time
| between two points, which, in un-curved space-time is a
| straight line, but is a curved line if space-time is curved).
|
| Do note that current quantum field theories do not work in
| curved space-time, so this may turn out to be wrong in certain
| crucial ways.
| raattgift wrote:
| > quantum field theories do not work in curved space-time
|
| In _general_ curved spacetimes. But that includes a lot of
| obvious unphysicality.
|
| Modelling our universe, QFT in CS (the subject of textbooks,
| after all, like Birrell and Davies) works just fine away from
| strong curvature, all of which as far as we can tell is
| shrouded behind an event horizon or not-practically-
| observable in the very early universe.
|
| You don't have to take my word for it. See
| https://en.wikipedia.org/wiki/Robert_Wald 's first three
| slides (after the title slide) at
| http://gravity.psu.edu/events/abhayfest/talks/Wald.pdf )
|
| tl;dr: it is a fine _effective_ theory, but not a good
| candidate for a fundamental theory.
|
| (Also in your first paragraph you are implicitly carving up
| spacetime in to space + time, and not taking that into
| account in what you write about "straight lines". However,
| you've got one part right namely (paraphrasing the start, up
| to the second comma, of your parenthetical) the spacetime
| interval of a _null_ geodesic).
| AnotherGoodName wrote:
| Everything obeys the curvature of spacetime. We'd be breaking
| the speed of light and thus breaking causality if certain
| fields didn't have to obey the curvature of spacetime that
| gravity causes.
|
| In fact gravity is even self-interacting with itself. ie.
| Gravitational fields themselves influence the propagation of
| gravitational fields. If this wasn't the case we'd observe
| gravitational waves from distance objects earlier than the
| speed of light. Which would be a problem for all our current
| models of physics if true.
| goatlover wrote:
| How does this work with whatever (dark energy?) caused
| Inflation?
| AnotherGoodName wrote:
| Again everything obeys it.
|
| If there was something that didn't obey fundamental changes
| to spacetime itself we'd observe things like gravitational
| waves in a completely different location and time to their
| visual counterparts. We do not see any evidence of this. So
| for any theory that states a change in the fabric of
| spacetime itself you can guarantee that everything must
| conform to that change.
| raattgift wrote:
| LIGO, Virgo, and other gravitational wave observatory
| collaborations forthcoming in our solar system are _expected_
| to see the gravitational wave component of a
| https://en.wikipedia.org/wiki/Multi-messenger_astronomy event
| precede that event's electromagnetic (gamma rays, light,
| radio waves) component. Why? Both the electromagnetic wave
| and the gravitational wave obey the massless wave equation,
| for which there is the free parameter "c". This parameter is
| the wave's propagation speed _in vacuum_. But
| electromagnetism couples much more strongly with interstellar
| and intergalactic gas and dust than gravitation does, so such
| intervening media slows the electromagnetic wave compared the
| gravitational one.
|
| This is a handy feature, since when a high-redshift candidate
| event is detected by LIGO or Virgo, various telescopes can
| search the inferred location on the sky, looking for a
| trailing component. A neutron star-black hole merger, for
| instance, will have a such a component. So will a star
| falling apart in proximity to a black hole (a "tidal
| disruption event"). The spread for closer events isn't so
| big: detection of the LIGO/VIRGO G298048 (sourced about 140
| million light years away, so very low redshift) event's gamma
| rays trailed by about about 1.7 seconds after the
| gravitational waves.
|
| We can draw a direct comparison with neutrinos. Although they
| are not massless, and thus obey a different wave equation,
| they are very very very light, so we in multi-messenger
| astronomy we can treat them as if they _effectively_ move at
| the speed of light. (In fact, supernova multi-messenger
| astronomy is a strong constraint on the difference between
| the speed of light and the speed of neutrinos).
|
| Neutrinos also couple with gas and dust very very weakly, and
| so a neutrino signal and a gravitational wave signal will
| arrive at nearly the same time, with the electromagnetic
| components arriving later.
|
| > ... curvature ... curvature of spacetime ... Gravitational
| fields themselves influence the propagation of gravitational
| fields
|
| While you're right that different solutions of the Einstein
| Field Equations of General Relativity do not superpose
| linearly (around a Schwarzschild black hole, a very low-mass
| particle behaves very differently from a one with enough mass
| to have a gravitational self-force:
| https://arxiv.org/abs/0902.0573 for gory details) it's
| probably easy to be misled by mixing a field view of General
| Relativity ("GR") with a geometry ("curvature") view.
|
| We can take an effective field theory view of GR and say that
| there is some chosen background (e.g. Minkowski spacetime)
| that is perturbed by a non-rotating point mass, the
| combination of the two (Minkowski + perturbation) generates
| the Schwarzschild spacetime. We can then add another mass, a
| second perturbation, and see what the combination of three
| (Minkowski + perturbation_1 + perturbation_2) does. This is
| the approach of https://en.wikipedia.org/wiki/Post-
| Newtonian_expansion and as can be seen in the diagram on that
| page, it is only valid when the two masses are fairly far
| apart. It is hard not to think of the perturbations as fields
| in the sense that you seem to be thinking about.
| Unfortunately this has its limits. As you bring the masses
| closer together (increasing compactness, moving downwards on
| the Y axis in the diagram), obviously wrong predictions tend
| to creep in, destroying one's confidence in the idea that in
| a system with multiple gravitating masses, each generates its
| own independent gravitational field which can somehow be
| combined (or which somehow propagate through some
| background).
|
| In the most popular General Relativity reference book,
| Misner, Thorne & Wheeler's _Gravitation_ , the authors
| discuss the expression "prior geometry", meaning some aspect
| of the curvature which is externally fixed or non-dynamical.
| General Relativity is a theory with "no prior geometry", and
| they make a brief argument about this. While some decades
| later we are much better with post-Newtonian expansion
| approaches (which _do_ fix a prior geometry, which is then
| studied using perturbation methods), and can ignore "no
| prior geometry" as much more than a slogan in many cases,
| unfortunately we cannot do so for all of them.
|
| For highly relativistic problems (objects moving near c;
| "escape velocities" near c), one must resort to the full
| theory of General Relativity, either solving the exact form,
| a good approximation (see https://pos.sissa.it/081/015/pdf),
| or a numerical solution where neither of the previous two
| forms are known or to "hide" divergences in analytical
| approaches.
|
| Additionally, for speculative modelling of highly
| relativistic systems we may wish to require that the model
| enjoy the manifest
| https://en.wikipedia.org/wiki/Background_independence of the
| full theory of General Relativity, which in a practical sense
| means that all possible observers will agree on the point-
| coincidences of the system independent of the choice of
| observer's system of coordinates or relative motion (object
| "A" and object "B" are in contact at the same point in
| spacetime for all observers; you don't have fast-moving
| observers calculate them never to have been in contact; you
| don't have rotating observers calculating them as never-in-
| contact; you don't have observers in deep space disagreeing
| with planet-bound observers about whether "A" and "B" come
| into contact, etc).
|
| Approximations instead fix some aspect(s) into a background,
| and in some strongly relativistic systems, one may have to
| introduce counter-terms ("ghosts") for families of observers
| that are not ideal Eulerian observers within that background.
|
| (Einstein has a good argument about this in Chapter XXXII
| ("The Structure of Space According to the Theory of General
| Relativity") in his 1934 book, https://www.ibiblio.org/ebooks
| /Einstein/Einstein_Relativity.... whose "not-even-quasi-
| Euclidean" argument is extended in Appendix 4 and accompanied
| by a further fourteen pages as Appendix Chapter 5
| ("Relativity and the Problem of Space") in the (not-as-
| freely-available) 2nd edition
| https://doi.org/10.4324/9780203518922 )
| tzs wrote:
| When do gravitational waves actually arrive from distant
| objects relative to light from those objects?
|
| Generally the space between us and distant objects isn't
| actually a perfect vacuum. It should have an index of
| refraction greater than 1, and it should vary by frequency.
| Light from a distant object should arrive here spread out in
| time by frequency, and the earliest should arrive a little
| later than something moving at the speed of light would
| arrive.
|
| Is there something like the index of refraction for gravity
| waves? If not then we should see gravity waves from an event
| before we see any light from the event. If there is, then it
| should be possible for gravity waves to arrive before, at the
| same time, or after light from the same event depending on
| the frequency of the gravity wave and the light.
| gizmo686 wrote:
| Depending on frequency, the vaccumum of space is close
| enough to a vaccum. If the light needs to travel through
| something opaque, you generally just don't see it (although
| it may illuminate the dust)
|
| We have measured the relative speed of gravity and light.
| The difference is constrained to be no more that about
| 10^-15 times the speed of light. This us based on a signal
| that travelled 130 million light years.
|
| https://en.m.wikipedia.org/wiki/GW170817
| AnotherGoodName wrote:
| Every experiment so far has detected gravitational waves a
| tiny bit before they detected light based evidence.
| Consistent with the light being slowed by the refraction in
| that very small amount of matter that exists in the
| interstellar medium and gravity passing through that dust
| more or less unaffected.
|
| Of course going faster than light which is being slowed by
| absorption and re-emission isn't the same as breaking the
| speed of light since light itself is going slower than the
| speed of light in this case.
|
| So yes you're right that it isn't exactly the same arrival
| time but we're not talking about curvature differences
| here, we're talking about physical interactions that the
| light undergoes that gravity doesn't.
| piyh wrote:
| We need to replace faster than light with faster than
| gravity
| spsoto wrote:
| I don't have a physics background but I've always seen
| "c" as the speed of causality. The light happens to go at
| that speed in the absence of gravitational disturbances.
| Gravity and others fields should also move at this
| maximum speed.
|
| That said, I'm still trying to come to terms with the
| fact that breaking this speed limit just means that
| causality would be potentially broken. Isn't that just
| something we axiomatically believed based on experience
| and we just haven't observed otherwise?
| Twisol wrote:
| My (mostly layperson's) understanding is that our laws of
| physics demand that causality would be broken; it's not
| taken as an axiom.
|
| Because of how the three dimensions of space and one
| dimension of time are put together, you can think of
| there being a balance or trade between motion in space
| and motion in time. If you aren't moving in space, you're
| moving through time at the maximum possible "rate". The
| more rapidly you move through space, the slower you move
| through time. This trade bottoms out at "c", at which
| point you're not moving through time at all. (Since
| motion is impossible without time passing, "c" itself is
| unachievable; you can only approach it asymptotically.
| Something about massless particles makes "motion" not a
| thing in the first place, I think, meaning they can
| actually propagate at exactly "c" as seen by an
| observer.)
|
| You can visualize this as a dial on an X-Y graph which
| starts out pointing in the Y direction, and as you speed
| up, it turns more toward the X direction. When you're
| pointing completely in the X direction, you're moving "at
| the speed of light", purely in space and not at all
| through time. If you turn the dial even further, you're
| trading some of that speed _back_ for motion in time...
| but in the opposite direction.
|
| Of course, this is all super-handwavey; most importantly,
| velocity has to be measured relative to an observer, so
| all of this about rates has to be anchored relative to an
| observer. (But this is also precisely why massless
| particles propagate at the same rate _regardless_ of
| observer -- insert timey-wimey Doctor Who reference.)
|
| Greg Egan has a lovely trilogy, Orthogonal, set in a
| universe where space and time don't have this trade
| (formally, the sign on the time variable in some critical
| equation is flipped to match the spatial dimensions). He
| has some great material on the exact physics of such a
| world. [0]
|
| [0] https://www.gregegan.net/ORTHOGONAL/00/PM.html
| tsegratis wrote:
| May I ask since gravity obeys curvature like light, why
| do we see gravity from black holes, when it is the
| curvature that stops us seeing light?
|
| Maybe the gravity emanates from outside the event
| horizon, but then why would it pull us inside?
|
| Thanks
| AnotherGoodName wrote:
| >We study static, spherically symmetric black hole
| solutions of the Einstein equations with a positive
| cosmological constant and a conformally coupled self
| interacting scalar field. Exact solutions for this model
| found by Mart'inez, Troncoso, and Zanelli, (MTZ)
|
| >The final conclusion of our analysis is that there
| appear to be no physically acceptable stable solutions of
| the MTZ system
|
| https://arxiv.org/pdf/0710.1735.pdf
|
| Basically it's a huge hole in black hole theory right
| now. It should be made clear though that both gravity is
| self interacting and black holes do exist. It's just when
| you get down to specifics it's a case of "we don't know
| how to make this work".
| wlesieutre wrote:
| https://arxiv.org/abs/1710.05833
|
| > _On 2017 August 17 a binary neutron star coalescence
| candidate (later designated GW170817) with merger time
| 12:41:04 UTC was observed through gravitational waves by
| the Advanced LIGO and Advanced Virgo detectors. The Fermi
| Gamma-ray Burst Monitor independently detected a gamma-ray
| burst (GRB 170817A) with a time delay of ~1.7 s with
| respect to the merger time._
|
| So there is a delay between arrival of gravitational waves
| and accompanying gamma ray burst, but I couldn't tell you
| if that's purely because light travels slower than it would
| in a perfect vacuum, because the gravitational waves are
| generated before the gamma ray burst, or a bit of both. The
| GRB being less than two seconds long, I would guess they
| both happened at close to the same time, and it does have a
| speed difference.
|
| Coming from an object 130 million light years away, 1.7
| seconds is a very small difference in speed.
| z3t4 wrote:
| What if we had an object massive enough that gravity could
| not escape? Would it become weightless?
| ninepoints wrote:
| Look up "gravitational lensing"
| sega_sai wrote:
| Yes it does. Because the light bending by the Sun (predicted
| and measured in early 20th century) is bending of
| electromagnetic waves.
| javajosh wrote:
| It does but in the same way it's true that Jupiter's gravity
| affects you, personally. For all practical purposes GR has no
| effect on our planet, fun observations of Mercury's
| perihelion and GPS signal-beaming satellites aside. GR
| matters _a tiny little bit_ for certain specialized
| engineering problems like doing precise inter-planetary
| transits. It matters a bit more for long-term position
| prediction of highly eccentric bodies, and really only starts
| to really matter at the cosmological scale.
|
| It's a matter of perspective. Our Solar System's mass is 98%
| in the Sun. Earth is tiny and small and, as a GR object, is
| moving very slowly, and that only according to how its
| particles were set in motion at the beginning of time.
|
| As others have said, gravitational lensing is a real thing,
| but that is a cosmological effect, and we are completely at
| the whim of the Initial Conditions for these opportunities.
|
| (If there are real engineering applications for GR,
| especially in optics, I would be delighted and grateful to
| learn more!)
| raattgift wrote:
| > If there are real engineering applications for GR,
| especially in optics
|
| Large-frame optical Sagnac gyroscopes for precision
| geodesy:
|
| https://www.frontiersin.org/articles/10.3389/fspas.2020.000
| 4...
|
| And some detail on the GINGER project, "Sagnac Effect, Ring
| Lasers, and Terrestrial Tests of [post-Newtonian] Gravity"
| (clarification mine),
| https://www.mdpi.com/2075-4434/3/2/84/htm
|
| I imagine there is some literature on higher order modes in
| dispersion compensating fibre spools placed over
| underground flows (magma, water) but don't really have time
| to think about what decade practical engineering problems
| might emerge.
|
| Of possible interest to you, quoting preface of following:
| "These few words should make it clear that quantum optics,
| experimental gravitation and measurement theory are not
| nearly as far apart as one might first have thought.
| However, there has traditionally been little contact
| between physicists working in these various fields." (which
| is a little less true now because of e.g. LIGO)
| https://link.springer.com/book/10.1007%2F978-1-4613-3712-6
|
| Next, I'm pretty sure that the emissions spectra of
| galactic magnetars
| (https://en.wikipedia.org/wiki/SGR_1935%2B2154 , one of
| Arecibo's last big detections, SS2.1 of
| https://arxiv.org/abs/2103.06052v1 ) are far from the
| cosmological scale (see https://arxiv.org/abs/1507.02924
| n.b. figure 24).
|
| > gravitational lensing ... is a cosmological effect
|
| Also pretty sure the Magellanic Clouds, other non-naked-eye
| Milky Way satellites, and some galactic targets aren't
| "cosmological", https://en.wikipedia.org/wiki/Gravitational
| _microlensing#Obs...
|
| Finally, it strikes me as unfair to to invoke Initial
| Conditions as a way to discount the relevance of
| gravitational observations. What, if not Intial Conditions,
| determines the frequency of your HeNe laser? Where did the
| neon in particular come from? (spoiler:
| https://en.wikipedia.org/wiki/Neon#Occurrence) And that
| helium is mostly a cosmological effect! ("The vast majority
| of helium was formed by Big Bang nucleosynthesis one to
| three minutes after the Big Bang. As such, measurements of
| its abundance contribute to cosmological models.")
| jpgvm wrote:
| Specifically this phenomena is called gravitational lensing
| and it's incredibly cool.
| herlitzj wrote:
| Yep. Predicted by Einstein in 1912
| https://en.wikipedia.org/wiki/Einstein_ring
| ck2 wrote:
| I thought they already measured this via satellites and time
| dilation, GPS in particular has to know exactly how much time is
| lost
|
| https://en.wikipedia.org/wiki/Frame-dragging
|
| https://en.wikipedia.org/wiki/Time_dilation
| pm_me_your_quan wrote:
| Yea, the core questions are around superposition states, at
| least from a rough reading. Centrally, if the _same_ clock is
| "at two altitudes" (by superposition), which time dilation will
| it experience?
| Arwill wrote:
| If particles behaved like software we could have a guess.
|
| It could be that time dilation is caused by some underlying
| physical system having a bottleneck, thus causing the
| slowdown. The underlying physical system has to evaluate all
| possible states for a particle in superposition, even if
| working in parallel. Then i would guess a particle in
| superposition should always experience the biggest possible
| slowdown.
| ISL wrote:
| Yes, people have already measured the gravitational redshift in
| many other contexts. That should not take away from this
| impressive achievement.
|
| The team here measured the gravitational redshift by comparing
| the clock-rates inferred from atoms at the top and bottom of a
| cloud of atoms that was itself smaller than a grain of rice.
|
| I've spent much of my career building precision gravity-sensing
| systems -- I'm happy to assure you that this news is very cool.
| Clocks are an _awesome_ way to learn a lot of new things about
| gravity, particularly because they measure differences in
| potential, rather than differences in acceleration. In the past
| two decades, we have seen atomic clocks begin to become
| sensitive to terrestrial gravity. In the next two decades, we
| are likely to see clocks begin to open up previously-impossible
| measurements.
|
| The measurement described here is a wonderful stepping-stone on
| that path.
| lordnacho wrote:
| How are such precise measurements possible? Have you got an
| intro reading? I just don't get how we could have found and
| cancelled all the sources of noise on something like this.
| omgwtfbyobbq wrote:
| >The specific way they measured the shift -- comparing two
| parts of the same cloud -- allowed them to cancel out a lot
| of noise that was common to both parts. It's like measuring
| a sailboat in rough seas. Even as it lurches up and down
| unpredictably, the distance between the keel and mast will
| always stay constant. While a clock made of a cloud of
| atoms can drift due to any number of things -- electric
| fields, magnetic fields, the laser light itself, heat from
| the environment -- the difference in frequencies between
| the top and bottom of the cloud remains the same. Measuring
| that difference revealed the effect of gravity. "That's not
| trivial to do," said Andrew Ludlow, an atomic clock expert
| at the National Institute of Standards and Technology, who
| was not involved with the research.
| lordnacho wrote:
| But if the difference you're measuring is absolutely
| tiny, you would imagine that the differential cooling of
| the mast when the wind is blowing/not blowing would have
| an effect on the height?
| raattgift wrote:
| > this news is very cool
|
| 800 nanokelvins down to 100 nK! Brrr!
| [https://arxiv.org/abs/2109.12238 "Atomic sample
| preparation", pdf p. 16, and top of p. 3]
|
| Maybe it's soon time to plan updates to PARCS/ACES/SAGAS.
| jcims wrote:
| I am a total layperson in this area but I think the GPS
| correction/frame dragging also incorporates relative motion
| where this (i think) is purely about gravitational field
| strength.
|
| To your point, maybe, I wonder if they had to factor in the
| relative velocity difference of the top and bottom of the cloud
| due to the rotation of the earth at their latitude (or just do
| the experiment at the south pole). At the equator, assuming a
| cloud of 1mm diameter, it would be 2p mm/day, which according
| to a sloppy google search is 2.425675e-16c.
| sgillen wrote:
| Yes but I think the idea of this new paper is to measure the
| effect of gravity on time dilation for a system that is also
| subject to quantum effects.
| wnoise wrote:
| I would love to see a system that isn't subject to quantum
| effects.
| mikewarot wrote:
| Does anyone here have 2x10km of fiber optics handy? We could put
| a monochromatic light in one end of each spool, and then look at
| the the interference pattern as we recombine the beams after
| their 10 km trip through the fibers... then change the height of
| one of the two spools by a few CM... while keeping everything
| carefully at constant temperature, to see if a phase shift
| occurs.
| dr_orpheus wrote:
| Maybe you can find a couple of fiber optic gyros lying around.
| The really precise ones can have a couple km of fiber optics in
| them. Bonus, the good ones will also have some temperature
| control.
| akomtu wrote:
| Has anybody explored GR equations with multiple, say 2, timelike
| dimensions? We're making some "obvious" assumptions about how
| physics should work and one of the central assumptions is the
| single straight line of time. I feel like this assumption is
| similar to the Ptolematic model with Earth in the center.
|
| GR works equally well with all topologies of spacetime and we're
| just assuming that it's 3+1. The Kaluza-Klein theory adds a small
| 4th dimension and derives most of the EM equations out of GR. So
| I wonder if we just need to make a better guess what the timelike
| dimensions look like.
| throwaway81523 wrote:
| > Has anybody explored GR equations with multiple, say 2,
| timelike dimensions?
|
| Entire topic is too big for my brain, but I think the cool kids
| these days consider classical theories of any sort to be
| uninteresting, that the universe is really an infinite
| dimensional quantum state, and that GR and everything else
| emerges from that. https://arxiv.org/abs/1801.08132
| throw1234651234 wrote:
| Honestly, we might as well not post any of these. No one without
| a Phd in physics can remotely understand what's being conveyed,
| let alone any nuance.
| Cthulhu_ wrote:
| I don't have a PhD in physics and found this article pretty
| easy to follow - it's an accessible pop sci translation of a
| paper [1] that DOES require a PhD or at least a serious
| interest in physics to follow.
|
| [1] https://arxiv.org/pdf/2109.12238.pdf
| matt123456789 wrote:
| I think that the following paragraph cuts to the heart of an
| important question linking GR to QM:
|
| "Take the case where a massive object is put into a
| superposition of two possible locations at the same time.
| General relativity says that any object with mass should bend
| the fabric of space-time. But what if that object is in a
| superposition? Is the geometry of space-time also in a
| superposition?"
|
| I don't have a PhD in anything let alone physics, but it seems
| to me that answering the above question could potentially lead
| to a reformulation of GR in a way that gracefully incorporates
| quantum mechanics, which is worth getting excited about.
|
| But yes, you're correct that the nuance is lost on me. Still,
| there seem to be plenty of people who find it interesting.
| sharikous wrote:
| First of all what you said is correct. Physics is looking for
| a quantum theory of gravitation and of course that would
| allow for superposition.
|
| The first problem is that a quantum theory of gravity is not
| straightforward. It's not "just" a spacetime with
| superposition, it probably has an even richer structure, the
| way QED has a richer structure than an EM field with
| superposition. And from the little hints we have it might
| have "spin 2" representation, and thereshould be some quantum
| very complicated field that acts like the metric GR field at
| low energies. But not only there are many possibilities for
| that, they are also very complicated, basically untreatable,
| or have infinite degrees of freedom in their definition
| itself.
|
| The second problem is that there is no current way of
| experimentally probing such behavior. You say put a massive
| object in superposition, but to have observable effects you
| need to have so massive an object that is tens of orders of
| magnitude above "impossible".
|
| This experiment is nice but it probes "just" the
| gravitational field of the Earth (a ~10^25 kg object). It is
| very nice for accurate time measures, and maybe for measuring
| gravitational fields, it comes nowhere near to probe quantum
| gravity effects, just quantum effects coupled to _classical_
| gravity (classic in the sense of GR but not quantum)
| nanomonkey wrote:
| They key part here that is not easy to set up in an
| experiment is "massive object in superposition".
| Superposition is generally something that we observe in mass-
| less objects (bosons) like photons on a macroscopic level.
| The spin state of an electron or any other massive objects
| (fermions) isn't going to effect it's mass distribution
| observably on a macroscopic level as far as I understand
| it...these things usually collapse upon interaction with
| other particles.
|
| The curious thing in my opinion is how this will effect
| quantum computing. If gravity causes decoherence, then it is
| likely difficult to maintain coherence in large qubit devices
| that aren't in microgravity.
| mrami wrote:
| A bit of a tangent, but boson vs. fermion is not a
| massless/massive split. There are massive bosons. Rather,
| it's a split on the qualities of the objects' spin, and
| thus whether they obey the Pauli exclusion principle.
|
| https://en.wikipedia.org/wiki/Boson
| nanomonkey wrote:
| Ah yes, you're correct. I see that I remembered that
| incorrectly.
| rytill wrote:
| I don't understand. Isn't that a pretty basic hypothesis?
| There's no way we've gone almost 100 years and no one tried
| that.
| AnimalMuppet wrote:
| Creating an object that is 1) massive enough to
| _observably_ bend spacetime, and 2) still can be put in a
| quantum superposition is... not easy.
| Enginerrrd wrote:
| True... but you might be able to put an object in
| superposition with another object and separate those a
| bit within an existing gravity well and observe the
| implications.
| matt123456789 wrote:
| https://www.google.com/search?q=can+spacetime+geometry+be+i
| n...
|
| Seems like there is a lot of theory discussions around the
| Internet but not seeing any experimental evidence one way
| or the other in the first few results.
| tom-thistime wrote:
| It's too difficult to actually carry out in practice. You
| increase the mass, it quickly becomes impossible to
| distinguish the states.
| pm_me_your_quan wrote:
| It is. We haven't had the necessary experimental equipment
| to be able to measure it until recently. You need an
| extremely precise clock that you can put into a quantum
| superposition- that's difficult to make.
| throw1234651234 wrote:
| I get what you are saying, but I have no way to know whether
| or not it's valid to any degree. Maybe the "constructive"
| part of what I am trying to say is that we either need better
| education on the fundamentals here, better examples, better
| explanations, or should just admit that it's far beyond the
| layman.
|
| For example, I get entanglement. I can get answers to
| questions like "Is FTL comm possible? No." or "What is a
| practical application for entanglement? Quantum radar -
| sorting your initial emission from jamming." etc.
|
| This though? Just too abstract (for me).
| [deleted]
| steve76 wrote:
| Interactions occur using internals as a medium without changing
| externals. This was shown long ago with Planck and supported by
| speed of light measurements on a cosmic and subatomic scale.
| This causes:
|
| 1. Things like electrons, the atom using it's own internals to
| interact, to be smeared out.
|
| 2. The Earth having a much slower frame rate than atoms. Attach
| a laser and detector on Earth, it shares the same frame rate. A
| cloud of atoms not attached to the Earth will appear to behave
| differently based on their distance from Earth.
|
| The laser is the same throughout it's path. It curves, but it's
| the same. If you have a really good laser, and a really good
| detector, you can find out how it works, perhaps create a new
| model beyond electrons and the nucleus. Especially with new
| data with black holes, you can define it in terms of particle
| pairs producing from nothing, and a dense internal homogeneous
| structure with no further substructure. You can take that model
| and apply real life applications regarding plasma, the sun, or
| matter waves, particle beams in orbit.
| otikik wrote:
| It's a matter of interest. I don't have a Phd in physics, but
| have watched enough videos in Youtube about quantum physics[1]
| now to understand almost everything the article says. Some
| details escape me, but I believe Quantum physics does that also
| to Phds.
|
| On the other hand, when they publish an economics article here,
| with the interests and the rates and the valuations of the
| options and whatnot, I understand almost nothing. I can't force
| myself to care about those things.
|
| [1] I recommend kurzgesagt, Sabine Hossenfelder and PBS Space
| Time
| Thorncorona wrote:
| What do you believe would be a good line for posts?
| lisper wrote:
| Note that they are not actually measuring the gravitational
| influence of the atoms themselves. They are measuring the
| difference in _earth 's_ gravitational field at two locations
| separated by a millimeter. That is a stupendous technological
| achievement, but it is not in and of itself progress towards a
| theory of quantum gravity.
| hanoz wrote:
| _> They are measuring the difference in earth 's gravitational
| field at two locations separated by a millimeter._
|
| Just in case there is anyone here who didn't read the article
| (surely not?), for whom this summary might not convey the full
| astoundingness of the procedure, what they are actually
| measuring is the difference in the passage of _time itself_ ,
| due to gravity, when a millimeter higher up than the other.
| trhway wrote:
| My favorite experiment in that vain is
| https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment
| - using loudspeakers to cause Doppler to compensate for
| redshift. 1959 - 22m, 2021 - 1 mm, a Moore's law for such
| measurements seems to be "doubling precision about each 4
| years".
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