[HN Gopher] The first nuclear clock will test if fundamental con...
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The first nuclear clock will test if fundamental constants change
Author : beefman
Score : 117 points
Date : 2024-09-04 16:23 UTC (6 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| 1970-01-01 wrote:
| Matter in other galaxies would behave differently from matter in
| the Milky Way if fundamental constants are not always true. I
| argue about this sometimes. Others keep stating that the
| wavelengths are equal, so everything else must be.
| gmueckl wrote:
| I think the better way to ask this question is: how much large
| scale spatial variation can there be in the laws of physics so
| that the observable behavior doesn't contradict existing
| observations? As far as I remember, this has been studied, but
| I can't find a reference right now.
| jepler wrote:
| wikipedia has a high level review of current constraints:
| https://en.wikipedia.org/wiki/Time-
| variation_of_fundamental_... fine-structure
| constant: less than 10^-17 per year gravitational
| constant: less than 10^-10 per year proton-electron
| mass ratio: less than 10^-16 per year
| rkagerer wrote:
| What's meant by "the wavelengths are equal"? (And have we
| measured comparable wavelengths in other galaxies?)
| fnordpiglet wrote:
| Presumably they mean propagating EM radiation we observe from
| earth appears to behave the same on earth as we observe from
| distant galaxies since the event that created them happened
| at a time much different than ours and a distant region of
| space.
| itishappy wrote:
| The wavelengths _of physical processes_ are equal. If
| fundamental constants changed, we 'd expect, say, the Lyman
| series to change too.
| mysecretaccount wrote:
| If the fundamental constants are not constant, why not expect
| them to change in this galaxy as well? The appeal to "other
| galaxies" seems suspect to me, a way to evade falsifiability.
| itishappy wrote:
| "A way to evade falsifiability" is the goal of the statement,
| given that we've been searching for evidence to the contrary
| for as long as we've been able. We haven't found any, and
| we've searched close-at-hand the most thoroughly.
| 1970-01-01 wrote:
| The idea is they're fixed/set by the overall size of the
| galaxy.
| mbrubeck wrote:
| If the constants change over very long time spans, we could
| observe this by looking at distant galaxies from billions of
| years ago. We don't have a way to make similar observations
| within our own galaxy.
| renewiltord wrote:
| One thing I have been arguing for a long time is that the
| fundamental constants are different until we observe them. i.e.
| if we don't observe it, it's possible for a tennis ball to
| travel through a wall. But in the universal program, if we will
| now or later observe the result, then it won't happen. But
| it'll happen so long as we will never observe the result. In
| fact, it's probably happened many times.
|
| No one has proven that this is impossible, AFAIK.
| ezrast wrote:
| What does "impossible" mean to you if not that a thing and
| it's consequences can never be observed?
| renewiltord wrote:
| Impossible means it does not happen, not that it does not
| happen only when we look. Just because we can't see it
| doesn't mean that it doesn't happen. After all, as the
| comment I replied to pointed out, other galaxies can have
| different constants. We have to be humble and admit we just
| don't know.
| jiggawatts wrote:
| The problem with these type of arguments is rigorously
| defining "we" and "look".
|
| Turns out that our gaze has no effect on anything and
| we're uninteresting squishy bags of mostly water as far
| as physical processes are concerned.
| renewiltord wrote:
| Yeah, but no one has proven that this is impossible so
| it's still possible. Just like OP comment.
| wizzwizz4 wrote:
| https://www.smbc-comics.com/comic/2014-03-25
| ezrast wrote:
| This seems like a distinction without a difference, since
| we can never positively categorize any unobserved
| phenomenon as impossible (vs merely unobservable). To me,
| it seems ontologically cleaner to treat existence and
| observability as the same thing. _shrug_
| renewiltord wrote:
| Okay, fine, I'll come clean, I was just making an
| unfalsifiability joke. The original god-of-the-gapsy
| comment was the one that got me. Always just out of reach
| of our verifiability is the magic. Why not _all_ the way
| out?
| ezrast wrote:
| Whelp, looks like I'm today's Poe's Law poster child. ;)
| Gooblebrai wrote:
| How can you even prove a negative?
| gitaarik wrote:
| Well, if you think about it, on a large scale of the universe,
| our laws are helped by our mathematical inventions of dark
| matter and dark energy. So is there really dark matter and dark
| energy, or is our understanding of the laws of the universe
| incomplete?
| thewarpaint wrote:
| > So is there really dark matter and dark energy, or is our
| understanding of the laws of the universe incomplete?
|
| These propositions are not mutually exclusive, the former
| implies the latter, right?
| foxyv wrote:
| As I understand it, dark matter and dark energy are just
| placeholders for discrepancies between our current physical
| model and observations made by telescopes like Hubble and
| Kepler. This could mean either that our measurements are
| inaccurate, or that the model is incomplete. Honestly, I
| think that both are extremely likely.
| AlexAndScripts wrote:
| Dark matter (matter that has mass but does not interact in
| any other way) _might_ be the literal solution. But there
| are also other suggestions (MOND is a big one).
|
| The https://en.m.wikipedia.org/wiki/Bullet_Cluster is
| pretty interesting.
| roywiggins wrote:
| Our understanding of the laws of the universe is incomplete
| either way. If dark matter exists, we still don't know what
| it's made of or exactly what properties it has.
| mseepgood wrote:
| They probably do change, but extremely slowly. It would feel
| strange if there were something fixed in the universe.
| bitmasher9 wrote:
| If they changed in a way to have meaningful impacts on how
| astronomical bodies operate we should be able to observe the
| change as some of the oldest light we observe is billions of
| years older than the newest light.
|
| In fact, based on this we can tell that the fundamental
| constant the speed of light has not changed which I agree is
| very strange.
| vl wrote:
| It comes down to what time is. I.e. what was before the Big
| Bang? If time didn't exist before big bang, then speed of
| light emerged after big bang, and as such "changed".
| psychoslave wrote:
| Either there is some unversal constants, or everything
| constantly change.
| hughesjj wrote:
| Could be both. Some things determined by some mathematical
| constraints will always be followed. Ex things like group
| theory and statistics will always be followed by any object
| subject to them, but how that manifests if the objects those
| rules act upon changes in form
| kimixa wrote:
| Why would it be "strange"? What reference can we possibly use
| to compare?
|
| This sort of thing tends to be so far from "common sense" it
| probably doesn't make sense to try to reason about it from that
| perspective.
| gus_massa wrote:
| It's possible to measure the ratios of the constants, like
| mass_of_proton/mass_of_electron . Another is the fine
| structure constant, that is related to the charge of the
| electron (divided by a lot of other constants to cancel the
| units). Both of them are related to the spectral lines of the
| light emitted and absorbed by atoms, so if they changed the
| "color" of the other galaxies should have changed a little. h
| ttps://en.wikipedia.org/wiki/Dimensionless_physical_constan..
| .
| jjeaff wrote:
| this is a bit tangential, but I once had a physics professor
| describe light waves as standing still and everything else is
| just moving around it.
| Vecr wrote:
| It's kind of silly to take the perspective of light, because
| it doesn't experience time (obviously, but you know what I
| mean). Maybe there will be new physics on that like there was
| with neutrinos, but it can't be too much of an effect.
| bluGill wrote:
| > it can't be too much of an effect.
|
| That is the problem with any argument for some new physics
| - it might exist, but it can't have much effect or we would
| detect it. Generally I only see people arguing for new
| physics because they really want faster than light travel
| (typically also without all the weird time effects, but a
| small minority would accept it with time effects)
| Vecr wrote:
| Also many people want to find libertarian free will
| somewhere in new physics.
| mystified5016 wrote:
| In case anyone else is curious about this fact: it has to
| do with time dilation. As your velocity through space
| approaches c, your velocity through _time_ approaches zero.
|
| Since photons move at c, they experience zero time between
| creation and destruction.
| ant6n wrote:
| Like the Planet Express ship? Sounds like professor
| Farnsworth.
| shagie wrote:
| The most recent Kurzgesagt video (on time travel)
| https://youtu.be/dBxxi5XAm3U had this passage:
|
| > To explain how this actually works without making a math
| video, we have to make a lot of physicists grumpy, so please
| keep in mind that we are simplifying and lying a bit.
|
| And that simplification / lie is that everything moves at the
| speed of light in spacetime. We are moving at basically 0 in
| the space coordinates and 1s/s in the time dimension (which
| is "light speed" in the time dimension). However... (1:45 in
| the video)
|
| > Photons, light particles, move at the speed of light
| through space. They don't experience any time passing because
| their speed in that time dimension is 0. In the time
| dimension they are frozen in place. If you see light on
| earth, from the photon's perspective it was just on the
| surface of the sun and then suddenly crashed into your eye
| with nothing happening in between.
|
| ... and this falls into the Lie-to-children domain.
| https://en.wikipedia.org/wiki/Lie-to-
| children#Examples_in_ed...
| thowawatp302 wrote:
| Yeah isn't it a simplification of the idea an object at
| rest has has a four-velocity where U^0 = c (so a velocity
| of c entirely the time direction) but a photon doesn't have
| a rest frame to do this calculation?
| User23 wrote:
| Makes sense really. If velocity is the derivative of position
| with respect to time and photons don't experience time how
| would they have velocity?
|
| It reminds me of my silly One Photon Conjecture. That is,
| there's only one photon that pops in an out of space as
| required by coupling events. Since it doesn't experience time
| saying it can't be in two or more places at the same time
| isn't meaningful!
| ck2 wrote:
| well no, photons move at the speed limit of causality in this
| universe
|
| they actually arrive slightly later than neutrinos to
| observers on earth because neutrinos just plow through
| virtually anything including stars and planets while photons
| have to travel the path affected by gravity
|
| photons aren't affected by gravity directly because massless
| but their path, their limit of causality, is affected
| AstralStorm wrote:
| Even if it had a rest frame, Schrodinger is a pain.
|
| An object at full rest is according to its wave/path
| equation literally everywhere at all times.
|
| However superconductivity has a bunch of truck sized holes
| for this. Specifically we don't quite understand Bose-
| Einstein condensate completely. Funky entities like time
| crystals appear in the mathematics, etc.
| shagie wrote:
| The fossil reactor at Oklo
| https://apod.nasa.gov/apod/ap100912.html and
| https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...
| can be used for that question.
|
| From Wikipedia: The natural reactor of Oklo
| has been used to check if the atomic fine-structure constant a
| might have changed over the past 2 billion years. That is
| because a influences the rate of various nuclear reactions. For
| example, 149Sm captures a neutron to become 150Sm, and since
| the rate of neutron capture depends on the value of a, the
| ratio of the two samarium isotopes in samples from Oklo can be
| used to calculate the value of a from 2 billion years ago.
| Several studies have analysed the relative concentrations of
| radioactive isotopes left behind at Oklo, and most have
| concluded that nuclear reactions then were much the same as
| they are today, which implies that a was the same too.
| User23 wrote:
| Is there a good explanation of how that isn't just measuring
| the expansion and contraction of a ruler with itself? Don't
| we know the reactor is 2 billion years old because of radio
| dating?
| Vecr wrote:
| It would be somewhat hard to tell if there's circularity
| somewhere, but you should be able to date it somewhat with
| the quantity of oxygen in the atmosphere at various times
| and general geological processes.
| AstralStorm wrote:
| Well, it's dated against pulsars and stars. But those
| sources of information have a bit of an error bar on time-
| space distance.
|
| Which is why a synthetic clock is needed here. That will
| have a known inception date and the changes if any can be
| compared.
|
| The problem with both is they're not exactly fully closed
| systems anyway so there will be some margin of error ever
| with the length of the operation.
|
| And during the test, we might just find out something
| completely unaccounted for in current physics... That isn't
| a universal constant related at all.
| thowawatp302 wrote:
| No, because you're comparing the various proportions, it's
| like comparing the contraction of various rulers made from
| different woods.
| adrian_b wrote:
| Most so called fundamental constants appear in the
| relationships between physical quantities only as a consequence
| of choosing arbitrary units.
|
| It is possible to eliminate almost all fundamental constants by
| choosing so-called natural units for the base physical
| quantities, for instance the elementary charge as the unit of
| electric charge.
|
| For all fundamental constants that can be eliminated by
| choosing natural units it makes no sense to discuss about
| changes of them.
|
| Nevertheless, even when a natural system of units is used,
| there remain 2 fundamental constants (plus a few other
| fundamental constants that are used only in certain parts of
| quantum field theory).
|
| The 2 important fundamental constants that cannot be eliminated
| are the Newtonian constant of gravitation, which is a measure
| of the intensity of the gravitational interaction, and a second
| fundamental constant that is a measure of the intensity of the
| electromagnetic interaction, which is frequently expressed as
| the so-called constant of the fine structure.
|
| The meaning of the constant of the fine structure is that it is
| the ratio between the speed of light in vacuum and the speed of
| a charged particle with unit charge, like an electron, that
| rotates around another charged particle with unit charge, which
| is much heavier, like a nucleus, in the state with the lowest
| possible energy, i.e. like the ground state of a hydrogen atom,
| but where the nucleus would have infinite mass. The speed of
| the rotating particle is a measure of the strength of the
| electromagnetic interaction between two elementary charges.
|
| So the only fundamental constants for which there could be a
| evolution in time are those that characterize the strengths of
| the electromagnetic interaction and of the gravitational
| interaction (and also the fundamental constants that
| characterize the strengths of the nuclear strong interactions
| and nuclear weak interactions).
|
| The values of these fundamental constants that characterize the
| strengths of the different kinds of interactions determine the
| structure of the Universe, where the quarks are bound into
| nucleons, the nucleons are bound into nuclei, the nuclei are
| bound into atoms, the atoms are bound into molecules, the
| molecules are bound into solid or fluid bodies, which are bound
| by gravitation into big celestial bodies, then into stellar
| systems, then into galaxies, then into groups of galaxies.
|
| Any changes in the strengths of the fundamental interactions
| would lead to dramatic changes in the structure of matter,
| which are not seen even in the distant galaxies.
|
| So any changes in time of the true fundamental constants are
| very unlikely, while changes in the constants that appear as a
| consequence of choosing arbitrary units are not possible
| (because such fundamental constants are fixed by conventions,
| e.g. by saying that the speed of light in vacuum is 299,792,458
| m/s).
| addaon wrote:
| What about the constants that describe the (relative) rest
| masses of elementary particles? Since we don't know the order
| of magnitude of neutrino masses, it seems improbable that
| even an order of magnitude change of those masses over time
| would lead to "dramatic changes in the structure of matter."
| adrian_b wrote:
| The masses of the particles and other specific properties,
| like magnetic moments, are not fundamental constants.
|
| They are the properties of those particles. There are such
| properties for leptons, for hadrons, for nuclei, for atoms,
| for molecules, for chemical substances, for humans and so
| on.
|
| Any object, either as small as an electron or as big as the
| Sun is characterized by various numeric properties, such as
| mass.
|
| The fundamental constants are not specific to any
| particular object. As I have said, after eliminating the
| fundamental constants that are determined by conventional
| choices of the system of units, the only fundamental
| constants that remain are those that characterize the
| strength of each fundamental interaction, as expressed in a
| natural system of units.
|
| Because most objects are composed of smaller subobjects, it
| should have been possible to compute their properties from
| the properties of their components. Starting from the
| properties of leptons and quarks, it should have been
| possible to compute the properties of hadrons, nuclei,
| atoms, molecules and so on.
|
| Unfortunately we do not have any theory that can compute
| the desired properties with enough precision and in most
| cases even approximate values are impossible to compute. So
| almost all properties of particles, nuclei, atoms or
| molecules must be measured experimentally.
|
| Besides the question whether the fundamental constants can
| change in time, one can put a separate question whether the
| properties of leptons and quarks can vary in time.
|
| Some of the properties of leptons and quarks are
| constrained by symmetry rules, but there remain a few that
| could vary, for instance the mass ratio between muon and
| electron. It is likely that a future theory might discover
| that this mass ratio is not an arbitrary parameter, but the
| muon is a kind of excited state of the electron, in which
| case this mass ratio could be computed as a function of the
| fundamental constants, so the question whether it can vary
| would be reduced to the question about the variation of the
| fundamental constants.
| jfengel wrote:
| In natural units, the Newton gravitational constant can be
| set to 1 as well.
|
| You do still need a term to characterize the strength of
| gravity. They sometimes use e, which can be defined in terms
| of G, c, Planck's constant, and a fundamental mass like the
| electron. The result is a truly fundamental unitless
| constant.
|
| The Standard Model has a dozen or so other fundamental
| constants, describing various mixing angles and fundamental
| masses (as ratios).
| adrian_b wrote:
| Nope. While the Newton gravitational constant can be set in
| theory as 1, it cannot be set in practice.
|
| The so-called Planck system of units where Newton's
| constant is set to 1 is an interesting mathematical
| curiosity, because in it all the physical quantities become
| dimensionless.
|
| Nevertheless, when Newton's constant is set to 1, the
| number of fundamental constants is not reduced, but another
| constant that was 1 in other systems of natural units
| becomes a fundamental constant that must be measured
| experimentally, for instance the elementary charge.
|
| Besides not having any advantage, because the number of
| fundamental constants in non-nuclear physics remains 2, the
| system where Newton's constant is set to 1 cannot be used
| in practice.
|
| The reason is that the experimental measurement of Newton's
| constant has huge uncertainties. If its value is forced to
| be the exact "1", then those uncertainties are transferred
| to the absolute values of all other physical quantities. In
| such a system of units the only values that would be known
| precisely would be the ratios of two quantities of the same
| kind, e.g. the ratios of 2 lengths or of 2 masses. Any
| absolute value, such as the value of a length or the value
| of a mass, would be affected by huge uncertainties.
|
| So the use of such a system of units is completely
| impossible, even if it is mentioned from time to time by
| naive people who know nothing about metrology. The choice
| of units for the physical quantities cannot be completely
| arbitrary, only units that ensure very low uncertainties
| for the experimental measurements are eligible.
|
| Currently and in the foreseeable future, that means that
| one of the units that are chosen must be a frequency. For
| now that is the frequency corresponding to a transition in
| the spectrum of the cesium atom, which is likely to be
| changed in a few years to a frequency in the visible range
| or perhaps in the ultraviolet range. In a more distant
| future it might be changed to a frequency in a nuclear
| spectrum, like this frequency that has just been measured
| for Th229, if it would become possible to make better
| nuclear clocks than the current optical atomic clocks,
| which use either trapped ions or lattices of neutral atoms.
|
| Some of the parameters of the "standard model" are
| fundamental constants associated to the strong and weak
| interactions. It is debatable whether it makes sense to
| call as fundamental constants the rest of the parameters,
| which are specific properties of certain objects, i.e.
| leptons and quarks.
| __MatrixMan__ wrote:
| If _nothing_ remains constant then there 's no identifying
| feature to point at and conclude that my experience yesterday
| and my experience today occurred in the same universe. Surely
| that feels even weirder than letting there be something that
| can be used as primary key for universe identification.
| Bluestein wrote:
| "When you absolutely, totally, _fundamentally_ , have to,
| fundamentally be sure" :)
| User23 wrote:
| It's still something of an open question whether or not G is
| actually constant.
|
| Not only that, but the results differ depending on whether atomic
| or dynamical time is used! In the latter case no change is
| measured using lunar reflectors.
| BurningFrog wrote:
| If the laws of physics can drift over time, might that explain
| the Big Bang?
| __MatrixMan__ wrote:
| I don't think so. There was no time before the Big Bang, so
| it's not like the laws of physics have anywhere to drift _from_
| such that they 're in a bang-causing configuration at t=0.
| mikewarot wrote:
| Let's assume they manage to make a nuclear clock out of this,
| with an Allan drift that's low enough to be useful. Once that's
| done, it'll take years of observation to measure any meaningful
| differences and gather enough data to notice something.
|
| Meanwhile, moving the height of anything a centimeter, the
| position of the moon, and a whole other host of noise sources
| have to be canceled out.
|
| I have no doubt this will be done... and it will be awe inspiring
| to hear it all told after the fact.
|
| While you're waiting... I found this really cool meeting
| documented on YouTube[1] that has the clearest explanation of how
| Chip Scale Atomic clocks work I've ever seen.
|
| I look forward to Chip Scale Optical Lattice clocks
|
| [1] https://www.youtube.com/watch?v=wHYvS7MtBok
| qsdf38100 wrote:
| If fundamental constants could change, this would violate energy
| conservation, and the second law of thermodynamics. Someone once
| said, if your pet theory violates the second law, there is no
| hope. Or am I missing something?
| elihu wrote:
| > Lots of nuclei have similar spin transitions, but only in
| thorium-229 is this cancellation so nearly perfect. > > "It's
| accidental," said Victor Flambaum(opens a new tab), a theoretical
| physicist at the University of New South Wales in Sydney. "A
| priori, there is no special reason for thorium. It's just
| experimental fact." But this accident of forces and energy has
| big consequences.
|
| ...
|
| > Physicists have developed equations to characterize the forces
| that bind the universe, and these equations are fitted with some
| 26 numbers called fundamental constants. These numbers, such as
| the speed of light or the gravitational constant, define how
| everything works in our universe. But lots of physicists think
| the numbers might not actually be constant.
|
| Putting these things together, if the physical constants do
| change over time, then perhaps there really isn't anything
| special about thorium-229, it's just that it's the one where the
| electrical repulsion and strong nuclear forces balance out right
| now. In a billion years maybe it would be some other element.
| Maybe we're just lucky to be alive at a time when one of the
| isotopes of an existing element just happens to line up like
| this.
|
| Perhaps too there's an optimal alignment that will happen or has
| already happened when those forces exactly balance out, and maybe
| that would be an ideal time (or place, if these constants vary by
| location) to make precise measurements in the changes to these
| constants, much like a solar eclipse was an ideal opportunity for
| verifying that light is bent by gravity.
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