13 ms·

An ultra-precise clock shows how to link the quantum world with gravity

189 points by theafh 5y ago

throw1234651234 5y ago
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_ 5y ago
  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 https://arxiv.org/pdf/2109.12238.pdf
- matt123456789 5y ago
  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.
  - rytill 5y ago
    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.
    - tom-thistime 5y ago
      It's too difficult to actually carry out in practice. You increase the mass, it quickly becomes impossible to distinguish the states.
    - matt123456789 5y ago
      https://www.google.com/search?q=can+spacetime+geometry+be+in+a+superposition https://www.google.com/search?q=can+spacetime+geometry+be+in... 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.
    - pm_me_your_quan 5y ago
      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.
    - AnimalMuppet 5y ago
      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 5y ago
        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.
  - deleted 5y ago
    [deleted]
  - nanomonkey 5y ago
    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 5y ago
      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 https://en.wikipedia.org/wiki/Boson
      - nanomonkey 5y ago
        Ah yes, you're correct. I see that I remembered that incorrectly.
  - sharikous 5y ago
    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)
  - throw1234651234 5y ago
    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).
- Thorncorona 5y ago
  What do you believe would be a good line for posts?
- steve76 5y ago
  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 5y ago
  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
- 4ad 5y ago
  As a physicist I find that the level of discourse around these topics among non-physicists ranges from hilarious to depressing. But I am still happy laymen are willing to have these discussions and that physics piques their interest. To be fair, non-experts have the same misunderstanding about physics as they have about any other subject -- economics, politics, medicine -- even music and arts. Hell, even programming on this very site. Should we ban people from discussing subjects outside their field of expertise? I don't think so. If anything, people should entertain more ideas, not less. There are no experts who didn't use to be noobs. That said, I do think that in general popular science articles are pretty bad, and laymen are mostly getting confusion out of them. And I do (probably?) agree with you that the people upvoting these submissions are pretty confused and I am not sure why are they upvoting them. Personally, I wouldn't post such articles here but as long as people seem to happily engage with one another I don't see any problem with such articles. To a first approximation there are the same proportion of people knowing physics here (arguably an offtopic subject) as there are people knowing real type theory (definitely an ontopic subject), so the niche aspect of it can't be a deciding factor on what to post.
myaccount80 5y ago
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
- AnotherGoodName 5y ago
  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.
  - tzs 5y ago
    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 5y ago
      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 https://en.m.wikipedia.org/wiki/GW170817
ck2 5y ago
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/Frame-dragging https://en.wikipedia.org/wiki/Time_dilation https://en.wikipedia.org/wiki/Time_dilation
- pm_me_your_quan 5y ago
  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 5y ago
    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.
- jcims 5y ago
  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 2π mm/day, which according to a sloppy google search is 2.425675e-16c.
- sgillen 5y ago
  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.
  - 5y ago
lisper 5y ago
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 5y ago
  > 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 5y ago
    My favorite experiment in that vain is https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment 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".
akomtu 5y ago
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 5y ago
  > 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 https://arxiv.org/abs/1801.08132
  - beecafe 5y ago
    You're right about the classical theory part. But the infinite dimensional is just one representation that is mathematically convenient - for example, you could consider a 2D image of 10x10 as a vector from a 100 dim space, each scalar being a pixel, or as one from a 2^100 space, with a vector of all zeros except at it's "index" in the list of all 10x10 images (this repr also allows you to express superposition within it by turning on multiple indices). Or as an element of a much lower dimensional subspace embedded in those spaces, like ML uses. In general infinite dimensional should just be taken to mean that using the inductive bias endowed by a specific representation isn't necessary for whatever we want to use the theory for. It's also how you (imo, crudely) represent functions as first-class objects in math.
- layer8 5y ago
  > Has anybody explored <idea> The answer to those questions is always “yes”, unless you’ve spent years of research to come up with <idea>. https://en.wikipedia.org/wiki/Multiple_time_dimensions https://en.wikipedia.org/wiki/Multiple_time_dimensions
mikewarot 5y ago
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 5y ago
  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.
sam_goody 5y ago
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 *.
- beecafe 5y ago
  You feel this difference as the pull of gravity. It's just that gravity is much, much weaker than electromagnetism which holds your atoms and molecules together.
- Lambdanaut 5y ago
  The chemical bonds between your molecules are stickier than the force differences created by the difference in time.
- 4ad 5y ago
  You seem to think that contiguous objects in space must live "in the same time" (whatever that means), so if time passed differently for different parts of the object then somehow this would break their topology because parts would be "left behind" (whatever that means). This is not how the physical reality works, in particular the first assumption doesn't hold. There is no global time. The mapping of 4-space is not only observer-dependent (special relativity) but it also is arbitrary, subject to mathematical constraints outside the level of this discussion (general relativity). Although perhaps it shouldn't be outside this discussion, because your definition of time is one that is not possible.
  - jacquesm 5y ago
    In a way this is a pretty accessible bit of evidence that there are no absolute reference frames.
    - 4ad 5y ago
      Yes, notice that even if you don't know what kind of coordinates are allowed in GR, from SR you know points of equal time coordinate must form surfaces, so any object with a non-zero 3-volume cannot lie on any such surface. As an aside, it really grinds my gears when I see popular science articles describing astronomical observations as being "in the past". Pretty much by definition everything that affects us must be in our causal past light cone, so in a strict sense that statement doesn't say anything at all. But it does try to say something, it tries to make astronomical observations seem different from everyday observations. In particular, people have this intuitive but non-physical idea on "now" happening around us, and these articles try to give the impression that "now" in those faraway astronomical objects is different, as if the notion of "now" (either here or there) would be a real physical thing, when only the observer's now is physically meaningful. They implicitely talk about THE faraway "now" when the whole point of GR is that there isn't a unique such thing. Articles get this backwards. The fact that light comes from the past is true both for astronomical phenomena and for everything else, that's not what's interesting, what's interesting is precisely that coordinate mappings can't be unique and by mixing ontologically different categories of "now", they obscure this idea further.
dd444fgdfg 5y ago
It's hard for me to have a useful mental model on this subject. As we know from the article, time passes differently for the top than the bottom, and this effect is in everything including you and me (obviously). What I find hard even less intuitive to grasp though, is whether or not this is cumulative. It should be right? 0.00000000000000001% slower on one side, well that adds up quickly. It can't be cumulative or my feet would be a lot older than my head, but I don't get how that's not true.
- leephillips 5y ago
  You’re feet are older than your head for relativistic reasons, as you say. But not “a lot” older.
  - dd444fgdfg 5y ago
    yeh it's the "lot" bit I'm confused about. like, aren't they always diverging, therefor the age difference should be a lot, right?
    - 4ad 5y ago
      How much is a lot? The parent comment came with 6x10^-7s over 100 years and for 2m tall people. I haven't checked the calculation, but we'll go with that. Is that a lot? If yes, why? Or if not, why not? Let's say it's not 6x10^-7s, but 10s over a lifetime. Is that a lot? 1 minute? 1 hour? So what if it's 1 hour? This difference causes tidal forces in your body. These tidal forces are minuscule compared to any other everyday interaction that you do. I haven't done the calculation, but I would wager that the tidal effect of even 1 hour over a lifetime is completely dwarfed by everyday interactions and effectively invisible in human everyday life.
    - akira2501 5y ago
      Even if it were, there's trillions of cells in your body. The time differences would still have to be incredibly pronounced for it to matter at this scale of entropy.
yummypaint 5y ago
The technique they use is similar in principle to how atoms are doppler cooled for BEC experiments (much of that early research was also at JILA), but it looks like the window has been narrowed significantly. What's special about this is that it's a step towards experiments involving gravity at non-macroscopic distances. If quantum gravity is possible, it would be quite exciting for the graviton to be discovered experimentally before theorists figure it out.
katabasis 5y ago
For anyone interested in this topic I highly recommend Carlo Rovelli's "The Order of Time", a beautifully written and up-to-date discussion of the physical nature of time. The book is written for a general audience (there is only one equation) and is as much a work of philosophy as of physics. https://bookshop.org/books/the-order-of-time/9780735216112 https://bookshop.org/books/the-order-of-time/9780735216112
RcouF1uZ4gsC 5y ago
> (The authors declined to be interviewed until the paper is published in a peer-reviewed journal.) Somehow I found this so encouraging. At a time when it seems even some scientists are just itching for a chance to get in front of the media with very preliminary studies, it is nice to see a team that seems to value actually getting the science right and getting others expert opinions before making a media blitz.

189 points by theafh 5y ago

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_ 5y ago
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 https://arxiv.org/pdf/2109.12238.pdf
matt123456789 5y ago
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.
- rytill 5y ago
  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.
  - tom-thistime 5y ago
    It's too difficult to actually carry out in practice. You increase the mass, it quickly becomes impossible to distinguish the states.
  - matt123456789 5y ago
    https://www.google.com/search?q=can+spacetime+geometry+be+in+a+superposition https://www.google.com/search?q=can+spacetime+geometry+be+in... 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.
  - pm_me_your_quan 5y ago
    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.
  - AnimalMuppet 5y ago
    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 5y ago
      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.
- deleted 5y ago
  [deleted]
- nanomonkey 5y ago
  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 5y ago
    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 https://en.wikipedia.org/wiki/Boson
    - nanomonkey 5y ago
      Ah yes, you're correct. I see that I remembered that incorrectly.
- sharikous 5y ago
  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)
- throw1234651234 5y ago
  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).
Thorncorona 5y ago
What do you believe would be a good line for posts?
steve76 5y ago
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 5y ago
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
4ad 5y ago
As a physicist I find that the level of discourse around these topics among non-physicists ranges from hilarious to depressing. But I am still happy laymen are willing to have these discussions and that physics piques their interest. To be fair, non-experts have the same misunderstanding about physics as they have about any other subject -- economics, politics, medicine -- even music and arts. Hell, even programming on this very site. Should we ban people from discussing subjects outside their field of expertise? I don't think so. If anything, people should entertain more ideas, not less. There are no experts who didn't use to be noobs. That said, I do think that in general popular science articles are pretty bad, and laymen are mostly getting confusion out of them. And I do (probably?) agree with you that the people upvoting these submissions are pretty confused and I am not sure why are they upvoting them. Personally, I wouldn't post such articles here but as long as people seem to happily engage with one another I don't see any problem with such articles. To a first approximation there are the same proportion of people knowing physics here (arguably an offtopic subject) as there are people knowing real type theory (definitely an ontopic subject), so the niche aspect of it can't be a deciding factor on what to post.

myaccount80 5y ago

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

AnotherGoodName 5y ago
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.
- tzs 5y ago
  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 5y ago
    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 https://en.m.wikipedia.org/wiki/GW170817

ck2 5y ago

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/Frame-dragging https://en.wikipedia.org/wiki/Time_dilation https://en.wikipedia.org/wiki/Time_dilation

pm_me_your_quan 5y ago
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 5y ago
  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.
jcims 5y ago
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 2π mm/day, which according to a sloppy google search is 2.425675e-16c.
sgillen 5y ago
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.
- 5y ago

lisper 5y ago

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 5y ago
> 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 5y ago
  My favorite experiment in that vain is https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment 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".

akomtu 5y ago

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 5y ago
> 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 https://arxiv.org/abs/1801.08132
- beecafe 5y ago
  You're right about the classical theory part. But the infinite dimensional is just one representation that is mathematically convenient - for example, you could consider a 2D image of 10x10 as a vector from a 100 dim space, each scalar being a pixel, or as one from a 2^100 space, with a vector of all zeros except at it's "index" in the list of all 10x10 images (this repr also allows you to express superposition within it by turning on multiple indices). Or as an element of a much lower dimensional subspace embedded in those spaces, like ML uses. In general infinite dimensional should just be taken to mean that using the inductive bias endowed by a specific representation isn't necessary for whatever we want to use the theory for. It's also how you (imo, crudely) represent functions as first-class objects in math.
layer8 5y ago
> Has anybody explored <idea> The answer to those questions is always “yes”, unless you’ve spent years of research to come up with <idea>. https://en.wikipedia.org/wiki/Multiple_time_dimensions https://en.wikipedia.org/wiki/Multiple_time_dimensions

mikewarot 5y ago

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 5y ago
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.

sam_goody 5y ago

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 *.

beecafe 5y ago
You feel this difference as the pull of gravity. It's just that gravity is much, much weaker than electromagnetism which holds your atoms and molecules together.
Lambdanaut 5y ago
The chemical bonds between your molecules are stickier than the force differences created by the difference in time.
4ad 5y ago
You seem to think that contiguous objects in space must live "in the same time" (whatever that means), so if time passed differently for different parts of the object then somehow this would break their topology because parts would be "left behind" (whatever that means). This is not how the physical reality works, in particular the first assumption doesn't hold. There is no global time. The mapping of 4-space is not only observer-dependent (special relativity) but it also is arbitrary, subject to mathematical constraints outside the level of this discussion (general relativity). Although perhaps it shouldn't be outside this discussion, because your definition of time is one that is not possible.
- jacquesm 5y ago
  In a way this is a pretty accessible bit of evidence that there are no absolute reference frames.
  - 4ad 5y ago
    Yes, notice that even if you don't know what kind of coordinates are allowed in GR, from SR you know points of equal time coordinate must form surfaces, so any object with a non-zero 3-volume cannot lie on any such surface. As an aside, it really grinds my gears when I see popular science articles describing astronomical observations as being "in the past". Pretty much by definition everything that affects us must be in our causal past light cone, so in a strict sense that statement doesn't say anything at all. But it does try to say something, it tries to make astronomical observations seem different from everyday observations. In particular, people have this intuitive but non-physical idea on "now" happening around us, and these articles try to give the impression that "now" in those faraway astronomical objects is different, as if the notion of "now" (either here or there) would be a real physical thing, when only the observer's now is physically meaningful. They implicitely talk about THE faraway "now" when the whole point of GR is that there isn't a unique such thing. Articles get this backwards. The fact that light comes from the past is true both for astronomical phenomena and for everything else, that's not what's interesting, what's interesting is precisely that coordinate mappings can't be unique and by mixing ontologically different categories of "now", they obscure this idea further.

dd444fgdfg 5y ago

It's hard for me to have a useful mental model on this subject. As we know from the article, time passes differently for the top than the bottom, and this effect is in everything including you and me (obviously). What I find hard even less intuitive to grasp though, is whether or not this is cumulative. It should be right? 0.00000000000000001% slower on one side, well that adds up quickly. It can't be cumulative or my feet would be a lot older than my head, but I don't get how that's not true.

leephillips 5y ago
You’re feet are older than your head for relativistic reasons, as you say. But not “a lot” older.
- dd444fgdfg 5y ago
  yeh it's the "lot" bit I'm confused about. like, aren't they always diverging, therefor the age difference should be a lot, right?
  - 4ad 5y ago
    How much is a lot? The parent comment came with 6x10^-7s over 100 years and for 2m tall people. I haven't checked the calculation, but we'll go with that. Is that a lot? If yes, why? Or if not, why not? Let's say it's not 6x10^-7s, but 10s over a lifetime. Is that a lot? 1 minute? 1 hour? So what if it's 1 hour? This difference causes tidal forces in your body. These tidal forces are minuscule compared to any other everyday interaction that you do. I haven't done the calculation, but I would wager that the tidal effect of even 1 hour over a lifetime is completely dwarfed by everyday interactions and effectively invisible in human everyday life.
  - akira2501 5y ago
    Even if it were, there's trillions of cells in your body. The time differences would still have to be incredibly pronounced for it to matter at this scale of entropy.

yummypaint 5y ago

The technique they use is similar in principle to how atoms are doppler cooled for BEC experiments (much of that early research was also at JILA), but it looks like the window has been narrowed significantly. What's special about this is that it's a step towards experiments involving gravity at non-macroscopic distances. If quantum gravity is possible, it would be quite exciting for the graviton to be discovered experimentally before theorists figure it out.

katabasis 5y ago

For anyone interested in this topic I highly recommend Carlo Rovelli's "The Order of Time", a beautifully written and up-to-date discussion of the physical nature of time. The book is written for a general audience (there is only one equation) and is as much a work of philosophy as of physics. https://bookshop.org/books/the-order-of-time/9780735216112 https://bookshop.org/books/the-order-of-time/9780735216112

RcouF1uZ4gsC 5y ago

> (The authors declined to be interviewed until the paper is published in a peer-reviewed journal.) Somehow I found this so encouraging. At a time when it seems even some scientists are just itching for a chance to get in front of the media with very preliminary studies, it is nice to see a team that seems to value actually getting the science right and getting others expert opinions before making a media blitz.