4 ms·

A couple of us from the Eot-Wash group are active on HN. Happy to answer questions. Hopefully the lead author (an HN-lurker, as far as I know) will jump in to

by ISL 7y ago

A couple of us from the Eot-Wash group are active on HN. Happy to answer questions. Hopefully the lead author (an HN-lurker, as far as I know) will jump in to bask in the limelight and answer your questions :).

abdullahkhalids 7y ago
Where will we be in ten years, in terms of distance scale for testing the gravitation law?
- ISL 7y ago
  We would be incredibly stoked if we could drop that number to ten microns; nobody knows yet if that can be done. I'd be surprised if either we or the group at Huazhong University don't reach at least 35 microns by 2030. We all aspire to ~20 microns. These all sound like small improvements, but Figure 5 tells the real story: reaching 10-microns requires a 3,000x improvement in sensitivity, in an environment where almost every experimental difficulty gets exponentially or power-law worse at short distances. That's why it is hard. The field is ripe for a totally-new technology, but whatever it is will be deeply clever and revolutionary. I've spent the last 16 years of my life trying to figure out how to do it with limited success.
  - pbhjpbhj 7y ago
    You say seismic vibrations increase the noise a lot but not, that I saw, anything about what measures you take against them? Presumably you do some sort of active noise cancelling for vibrations (like noise cancelling headphones)? Seismic noise must be a common problem in physics experiments, which other experiments have the best environmental vibration cancelling systems ... do people pay for use of them like they would for use of a space telescope?
    - ISL 7y ago
      We are moving from passive to active isolation now. The isolation that we need right now isn't too challenging. The hard part is ensuring that it doesn't disturb other properties of our instruments. We are sensitive to nanoradian rotations at millihertz frequencies, so we have to ensure that our isolation systems don't add any rotation noise. Regarding isolation-as-a-service: Most of these kinds of experiments run for many years in a university setting. The environmental requirements for each instrument vary widely, too. "Isolated-environment laboratories as a user facility" is an interesting idea to consider. I'm not sure if it would soar in the academic-research marketplace, but there might be a niche somewhere.
chaps 7y ago
Hi! Thanks for hopping on HN to answer questions. Are you expecting anything particularly interesting or exciting out of these tests?
- ISL 7y ago
  One day, nature will give up a secret. Gravity and the standard model are, as written, very different theories. Our short-range gravity experiments are simultaneously at the forefront of some classes of high-energy physics one might explore at the LHC and interesting astrophysical/cosmological questions. With our entire program of measurement, we are pushing as hard as we can on the theory of gravity, looking for a gap in the armor to see what's next. The draw of these short-range experiments is that it is totally-unexplored territory, and with every step forward, we continue to check whether or not our oldest-known force behaves as predicted. It's late, so I'm beginning to ramble: There are viable theories that predict that gravity should get stronger at these scales, and viable theories that gravity should get weaker at these scales. With this result, those theories just got squeezed into a smaller box. While this isn't my result (I helped with this one, but am not a prime-mover), for me, the big draw is to study gravity until we push it, like all the other interactions, into the realm of the microscopic.
  - chaps 7y ago
    Ah, I realized my comment sounded a little like I was trying to downplay the work! Wasn't my goal -- I truly find this work extremely interesting and at one point wanted to study physics so that I could do particle physics + cosmology. Parameters finding is an absolute must for us as a species if we want to push forward.
    - ISL 7y ago
      I didn't take it as downplaying at all. We are very grateful for the public support we receive, and are always happy to expound on why it matters.
      - chaps 7y ago
raverbashing 7y ago
Cool! Here's my question: Isn't the 1/r^2 just a consequence of Gauss' theorem or are you trying to look at possible extra dimensional effects here?
- ISL 7y ago
  1/r^2 and Gauss' law are two sides of one coin. We're looking for any deviation from that plan, in the form of new forces or a change in the behavior of gravity itself. Either one would be really cool to find, and the fact that we keep not finding anything places increasingly tight constraints on theories that attempt to extend the Standard Model or build a bridge from the SM to gravity.
  - raverbashing 7y ago
    Thanks for you answer and for the good work!
    - ISL 7y ago
      The biggest laurels go to John Lee and Ted Cook -- they were at the tip of the spear for a combined ~12+ years to make this happen.
bschwyn 7y ago
Cool to see you guys here - as a UW undergrad I visited the lab once and was amazed by all of the effects you had to take into account to reduce error. Wasn't it something like the experiment can pick up the sun shining on the bricks of the building and shifting it, and the changes in the level of groundwater after it rains? I never understood why you didn't have the apparatus in the middle of the desert. What sort of error reductions and changes allowed the new distance measurements?
- ISL 7y ago
  We contemplate moving elsewhere from time-to-time, but the proximity of our colleagues, machine shop, and an awesome lab keep us here. For some of our experiments, we need the hill we're built into, and we can handle the tilts. This short-range measurement was greatly benefited by a fortuitous snowstorm that brought Seattle's buses to a halt right as John was taking data with the fully-commissioned instrument. If we had foreseen with 6-8 weeks of foresight how much of a societal disruption Covid-19 would cause, we would have crushed it to get this short-range instrument back in operation in time. We're working on new seismic-isolation systems for our next upgrade. If that doesn't work out, we'll be looking for a remote hole in the ground to put a short-range instrument in. The big steps forward on this cycle were improved alignment systems, improved flatness of multiple critical parts, improved cleanliness, a new gluing arrangement that reduced Newtonian-gravity systematic uncertainty, even greater care to ensure non-magnetic fabrication, and another ~5 years of experience with the instrument. The next big jump for us is active seismic isolation. A small improvement there, with other tweaks, may buy us another ~5-10 microns.
  - magicalhippo 7y ago
    > We're working on new seismic-isolation systems for our next upgrade. If that doesn't work out, we'll be looking for a remote hole in the ground to put a short-range instrument in. Just put it on top of one of the LIGO mirrors? :P If you magically got an couple of order of magnitudes improvement (~0.5um), would you have to start to worry about van der Waals forces, or is that still weak enough? Or does it not matter at all for this kind of apparatus?
dvh 7y ago
On quora there is prolific nutjob who spreads misinformation about gravity claiming gravity as mass/distance² was never experimentally proved and instead claims gravity is volume/distance², his responses are often quite highly ranked, e.g. 2nd answer here: https://www.quora.com/Why-is-a-lunar-orbit-unstable https://www.quora.com/Why-is-a-lunar-orbit-unstable Can you please disprove his nonsense.
mirimir 7y ago
This seems like the key result: > We find that any gravitational-strength Yukawa interaction must have λ < 38.6 μm. This implies that the dilaton or heavy graviton mass and the radion unification mass must be greater than 5.1 meV and 7.1 TeV, respectively, and that the largest extra dimension must have a toroidal radius less than 30 μm. These are the tightest existing lab constraints on “string inspired” new gravitational phenomena. But the key words in it mean ~nothing to me :( So do they do anything more than constrain parameters?
- ISL 7y ago
  The first sentence above is, for a physicist, the key result. It says that, for the most common way one could modify gravity to incorporate a new force-mediator, the new interaction has to turn on at distances shorter than 39 microns. At distances longer than that, these measurements show that gravity acts, and in a way that roughly follows the inverse-square law. (at distances of ~100 microns it very-much follows the ISL, and at distances longer than that, it really follows the ISL). The easy way to think about this result: we are on the cusp of showing that gravity acts over distances shorter than the width of a human hair. Only the finest human hairs are smaller in diameter. For many years, my goal in life (see HN handle) was to push this limit below 10-microns. Turns out that is really hard to do :).
  - mirimir 7y ago
    Thanks. > ... and at distances longer than [100 microns], it really follows the ISL But at cosmic distances, there's also evidence that gravity doesn't follow ISL, right? Are there any implications of this work for hypothesized dark matter and dark energy?
    - fsh 7y ago
      You are probably thinking of galaxy rotation curves which do not agree with theoretical predictions that assume only visible matter exists. Gravity not following the ISL at large scales would indeed be a possible explanation for this. However, most alternative theories of gravity clash with other observations, such as the motion of binary pulsars or gravitational waves, whereas (unmodified) General Relativity is in excellent agreement. This is one of the reasons why the existence of dark matter is considered to be a much better explanation for the deviation of the rotation curves.

by ISL 7y ago

abdullahkhalids 7y ago

Where will we be in ten years, in terms of distance scale for testing the gravitation law?

ISL 7y ago
We would be incredibly stoked if we could drop that number to ten microns; nobody knows yet if that can be done. I'd be surprised if either we or the group at Huazhong University don't reach at least 35 microns by 2030. We all aspire to ~20 microns. These all sound like small improvements, but Figure 5 tells the real story: reaching 10-microns requires a 3,000x improvement in sensitivity, in an environment where almost every experimental difficulty gets exponentially or power-law worse at short distances. That's why it is hard. The field is ripe for a totally-new technology, but whatever it is will be deeply clever and revolutionary. I've spent the last 16 years of my life trying to figure out how to do it with limited success.
- pbhjpbhj 7y ago
  You say seismic vibrations increase the noise a lot but not, that I saw, anything about what measures you take against them? Presumably you do some sort of active noise cancelling for vibrations (like noise cancelling headphones)? Seismic noise must be a common problem in physics experiments, which other experiments have the best environmental vibration cancelling systems ... do people pay for use of them like they would for use of a space telescope?
  - ISL 7y ago
    We are moving from passive to active isolation now. The isolation that we need right now isn't too challenging. The hard part is ensuring that it doesn't disturb other properties of our instruments. We are sensitive to nanoradian rotations at millihertz frequencies, so we have to ensure that our isolation systems don't add any rotation noise. Regarding isolation-as-a-service: Most of these kinds of experiments run for many years in a university setting. The environmental requirements for each instrument vary widely, too. "Isolated-environment laboratories as a user facility" is an interesting idea to consider. I'm not sure if it would soar in the academic-research marketplace, but there might be a niche somewhere.

chaps 7y ago

Hi! Thanks for hopping on HN to answer questions. Are you expecting anything particularly interesting or exciting out of these tests?

ISL 7y ago
One day, nature will give up a secret. Gravity and the standard model are, as written, very different theories. Our short-range gravity experiments are simultaneously at the forefront of some classes of high-energy physics one might explore at the LHC and interesting astrophysical/cosmological questions. With our entire program of measurement, we are pushing as hard as we can on the theory of gravity, looking for a gap in the armor to see what's next. The draw of these short-range experiments is that it is totally-unexplored territory, and with every step forward, we continue to check whether or not our oldest-known force behaves as predicted. It's late, so I'm beginning to ramble: There are viable theories that predict that gravity should get stronger at these scales, and viable theories that gravity should get weaker at these scales. With this result, those theories just got squeezed into a smaller box. While this isn't my result (I helped with this one, but am not a prime-mover), for me, the big draw is to study gravity until we push it, like all the other interactions, into the realm of the microscopic.
- chaps 7y ago
  Ah, I realized my comment sounded a little like I was trying to downplay the work! Wasn't my goal -- I truly find this work extremely interesting and at one point wanted to study physics so that I could do particle physics + cosmology. Parameters finding is an absolute must for us as a species if we want to push forward.
  - ISL 7y ago
    I didn't take it as downplaying at all. We are very grateful for the public support we receive, and are always happy to expound on why it matters.
    - chaps 7y ago

raverbashing 7y ago

Cool! Here's my question: Isn't the 1/r^2 just a consequence of Gauss' theorem or are you trying to look at possible extra dimensional effects here?

ISL 7y ago
1/r^2 and Gauss' law are two sides of one coin. We're looking for any deviation from that plan, in the form of new forces or a change in the behavior of gravity itself. Either one would be really cool to find, and the fact that we keep not finding anything places increasingly tight constraints on theories that attempt to extend the Standard Model or build a bridge from the SM to gravity.
- raverbashing 7y ago
  Thanks for you answer and for the good work!
  - ISL 7y ago
    The biggest laurels go to John Lee and Ted Cook -- they were at the tip of the spear for a combined ~12+ years to make this happen.

bschwyn 7y ago

Cool to see you guys here - as a UW undergrad I visited the lab once and was amazed by all of the effects you had to take into account to reduce error. Wasn't it something like the experiment can pick up the sun shining on the bricks of the building and shifting it, and the changes in the level of groundwater after it rains? I never understood why you didn't have the apparatus in the middle of the desert. What sort of error reductions and changes allowed the new distance measurements?

ISL 7y ago
We contemplate moving elsewhere from time-to-time, but the proximity of our colleagues, machine shop, and an awesome lab keep us here. For some of our experiments, we need the hill we're built into, and we can handle the tilts. This short-range measurement was greatly benefited by a fortuitous snowstorm that brought Seattle's buses to a halt right as John was taking data with the fully-commissioned instrument. If we had foreseen with 6-8 weeks of foresight how much of a societal disruption Covid-19 would cause, we would have crushed it to get this short-range instrument back in operation in time. We're working on new seismic-isolation systems for our next upgrade. If that doesn't work out, we'll be looking for a remote hole in the ground to put a short-range instrument in. The big steps forward on this cycle were improved alignment systems, improved flatness of multiple critical parts, improved cleanliness, a new gluing arrangement that reduced Newtonian-gravity systematic uncertainty, even greater care to ensure non-magnetic fabrication, and another ~5 years of experience with the instrument. The next big jump for us is active seismic isolation. A small improvement there, with other tweaks, may buy us another ~5-10 microns.
- magicalhippo 7y ago
  > We're working on new seismic-isolation systems for our next upgrade. If that doesn't work out, we'll be looking for a remote hole in the ground to put a short-range instrument in. Just put it on top of one of the LIGO mirrors? :P If you magically got an couple of order of magnitudes improvement (~0.5um), would you have to start to worry about van der Waals forces, or is that still weak enough? Or does it not matter at all for this kind of apparatus?

dvh 7y ago

On quora there is prolific nutjob who spreads misinformation about gravity claiming gravity as mass/distance² was never experimentally proved and instead claims gravity is volume/distance², his responses are often quite highly ranked, e.g. 2nd answer here: https://www.quora.com/Why-is-a-lunar-orbit-unstable https://www.quora.com/Why-is-a-lunar-orbit-unstable Can you please disprove his nonsense.

mirimir 7y ago

This seems like the key result: > We find that any gravitational-strength Yukawa interaction must have λ < 38.6 μm. This implies that the dilaton or heavy graviton mass and the radion unification mass must be greater than 5.1 meV and 7.1 TeV, respectively, and that the largest extra dimension must have a toroidal radius less than 30 μm. These are the tightest existing lab constraints on “string inspired” new gravitational phenomena. But the key words in it mean ~nothing to me :( So do they do anything more than constrain parameters?

ISL 7y ago
The first sentence above is, for a physicist, the key result. It says that, for the most common way one could modify gravity to incorporate a new force-mediator, the new interaction has to turn on at distances shorter than 39 microns. At distances longer than that, these measurements show that gravity acts, and in a way that roughly follows the inverse-square law. (at distances of ~100 microns it very-much follows the ISL, and at distances longer than that, it really follows the ISL). The easy way to think about this result: we are on the cusp of showing that gravity acts over distances shorter than the width of a human hair. Only the finest human hairs are smaller in diameter. For many years, my goal in life (see HN handle) was to push this limit below 10-microns. Turns out that is really hard to do :).
- mirimir 7y ago
  Thanks. > ... and at distances longer than [100 microns], it really follows the ISL But at cosmic distances, there's also evidence that gravity doesn't follow ISL, right? Are there any implications of this work for hypothesized dark matter and dark energy?
  - fsh 7y ago
    You are probably thinking of galaxy rotation curves which do not agree with theoretical predictions that assume only visible matter exists. Gravity not following the ISL at large scales would indeed be a possible explanation for this. However, most alternative theories of gravity clash with other observations, such as the motion of binary pulsars or gravitational waves, whereas (unmodified) General Relativity is in excellent agreement. This is one of the reasons why the existence of dark matter is considered to be a much better explanation for the deviation of the rotation curves.