3 ms·

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/wik

by 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.
- wnoise 5y ago
  I would love to see a system that isn't subject to quantum effects.
ISL 5y ago
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 5y ago
  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 5y ago
    >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 5y ago
      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?

by ck2 5y ago

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.

wnoise 5y ago
I would love to see a system that isn't subject to quantum effects.

ISL 5y ago

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 5y ago
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 5y ago
  >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 5y ago
    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?