5 ms·

Physicists have been searching for variations in the coupling of materials to gravity for more than a century, and by some measure, for more than a millenium.

by ISL 7y ago

Physicists have been searching for variations in the coupling of materials to gravity for more than a century, and by some measure, for more than a millenium. https://en.wikipedia.org/wiki/Equivalence_principle#Tests_of_the_weak_equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle#Tests_of... Every experiment anyone has ever tried has come up empty. The equivalence principle is a postulate (Einstein's "happiest idea") that underlies General Relativity, making it essential to test. (Source: This is literally what I do for a living.)

jeffdavis 7y ago
Assuming we find dark matter, do you think it will represent some departure from our current understanding of gravity and how it relates to mass? Or do you think it will just be some boring dust clouds?
- ISL 7y ago
  We have already tested the equivalence principle toward dark matter [1]. It appears to fall like everything else, at the 1% or better level. It is extremely difficult for modifications of gravity to simultaneously fit the cosmic microwave background, big-bang nucleosynthesis, galactic rotation curves, and the Bullet Cluster. The safe bet these days is on some sort of non-baryonic dark particle (which is why so many people are looking for them). Most other loopholes are closed or in the process of being closed. The coming decade or two should have something significant to say about dark matter. We will either discover its nature or severely constrain its properties through a plethora of varied experiments and searches. [1] https://arxiv.org/pdf/1207.2442.pdf https://arxiv.org/pdf/1207.2442.pdf
  - Brotkrumen 7y ago
    this is all fine and dandy, but when do you think will we have anti-grav drives?
    - ISL 7y ago
      Don't bet more than you can afford to lose.
      - karma_fountain 7y ago
        Make me a hover board, get to work!
        bluGill 7y ago
        He is working on it. Give him more money and he will hire more assistants, and in turn make progress faster. Of course you are risking that he eventually proves your hover board is impossible.
  - Zenst 7y ago
    From my understanding `dark matter` is an explanation for what we expected and what we see regarding the mass of the universe? It's a bit like exploding an apple and measuring all the bits that fly of and missing some bits. Whilst we do not know what the apple looked like initially, we are from our measurements, trying to reconstruct that apple. How convinced are we that their is dark matter and not a case of how we have measured things so far. Hence, differences in results based upon how we measure are evident, whilst many are close to the same, they just don't exactly match up - hence - dark matter theory. How likely is it that dark matter as we know it - does exist?
- poelzi 7y ago
  They will find dark-matter when they unify GR and QM, never :)
lisper 7y ago
How much money does society need to spend supporting the work you do before we can safely conclude that Einstein was actually correct and we don't need to test it any more?
- jeffdavis 7y ago
  Assuming Einstein is correct is not the end, because we still have so many questions. For instance, why is Einstein correct?
- ISL 7y ago
  That's a great and important question. (doubly so for me, as constrained resources may see me leave the field, to which I have devoted my entire professional life, in the coming months) We are perennially grateful for the support we receive to do the work that we do. To a high degree of approximation, Einstein's predictions are very, very correct. We are trying to look very carefully at Nature because we don't think we have the whole story. It is possible that Einstein is exactly right. If so, we would never be able to describe the four forces of Nature with one unified theory. Today, we need two theories to describe everything we see -- the quantum-mechanical/particle-physics description of the Standard Model and gravity. There is no quantum-mechanical fuzziness in the mathematics of gravity, and there is no hint of the differential geometry of gravity in the mathematics of the Standard Model. The aspiration of almost every fundamental-physics physicist is to find a way to either simplify the Standard Model or connect it with gravity. If the return on investment seems insufficent, know, too, that the technology we develop to push the boundaries of knowledge has important spin-offs. GPS is impossible without corrections from both special relativity and general relativity. The instrumentation we develop to make gravitational experiments possible on earth requires the development of new classes of seismometer [1] that may open new understanding of Earth's dynamics and allow better chip-fabrication instrumentation. The instrumentation developed to test gravity in space is also being used to measure the movement of mass (i.e. ice and water) on Earth's surface [2]. Even more important is the training we provide to students. Just as athletes train in the gym to get stronger, confrontation with the hardest known technical problems provides an efficient path for students and young faculty to become proficient at the entire range of modern measurement science. Alumni from our group have not only gone on to fancy academic positions, but also helped to redefine the kilogram, designed upgraded digital calipers used by tens of thousands of people worldwide, built key components of a major quantum-computing company's infrastructure, and more. The people who work on these kinds of experiments are making a fraction of what they could make in industry. We are doing it for love, not money, and the returns to society are myriad. [1] https://arxiv.org/abs/1707.03084 https://arxiv.org/abs/1707.03084 [2] https://gracefo.jpl.nasa.gov/ https://gracefo.jpl.nasa.gov/
zzzcarrot 7y ago
So we are litearaly at square 0 still here?
- chrisacky 7y ago
  No, sounds like we've eliminated 70 years worth of other possible squares.. if you want to take glass half full approach.
  - yourapostasy 7y ago
    Similar to the realization that saying "no" is one of life's important skills to internalize. Closing off possible avenues of explanation is just as much of an advancement as finding evidence to support an explanation.
- poelzi 7y ago
  Physics took 2 wrong roads and it got stuck in a local minima of the truth plane. Understanding that physics is the most ignorant field of science and how proud the field is of it's models, I don't think this will come anytime soon. https://www.pnas.org/content/112/24/7426 https://www.pnas.org/content/112/24/7426 PS: you need to understand how gravity is derived to manipulate it's propagation. Since the mechanisms are very stable due various reasons, only a very good understanding of many processes will allow you to design an experiment that works. I have adopted a very fringe physics model that is unified and explains more using less assumptions. From this perspective I can tell you that contemporary physics is so far from understanding, that most likely we will not see anything in this direction, before we fully trashed our planet.
  - conscion 7y ago
    What's your physics model?
    - poelzi 7y ago
      This one: https://www.amazon.de/Basic-Structures-Matter-Supergravitation-Unified/dp/1412083877 https://www.amazon.de/Basic-Structures-Matter-Supergravitati... Never could have come up with such a brilliant theory, took me a year to understand it. Now I understand 137 ^^

by ISL 7y ago

jeffdavis 7y ago

Assuming we find dark matter, do you think it will represent some departure from our current understanding of gravity and how it relates to mass? Or do you think it will just be some boring dust clouds?

ISL 7y ago
We have already tested the equivalence principle toward dark matter [1]. It appears to fall like everything else, at the 1% or better level. It is extremely difficult for modifications of gravity to simultaneously fit the cosmic microwave background, big-bang nucleosynthesis, galactic rotation curves, and the Bullet Cluster. The safe bet these days is on some sort of non-baryonic dark particle (which is why so many people are looking for them). Most other loopholes are closed or in the process of being closed. The coming decade or two should have something significant to say about dark matter. We will either discover its nature or severely constrain its properties through a plethora of varied experiments and searches. [1] https://arxiv.org/pdf/1207.2442.pdf https://arxiv.org/pdf/1207.2442.pdf
- Brotkrumen 7y ago
  this is all fine and dandy, but when do you think will we have anti-grav drives?
  - ISL 7y ago
    Don't bet more than you can afford to lose.
    - karma_fountain 7y ago
      Make me a hover board, get to work!
      - bluGill 7y ago
        He is working on it. Give him more money and he will hire more assistants, and in turn make progress faster. Of course you are risking that he eventually proves your hover board is impossible.
- Zenst 7y ago
  From my understanding `dark matter` is an explanation for what we expected and what we see regarding the mass of the universe? It's a bit like exploding an apple and measuring all the bits that fly of and missing some bits. Whilst we do not know what the apple looked like initially, we are from our measurements, trying to reconstruct that apple. How convinced are we that their is dark matter and not a case of how we have measured things so far. Hence, differences in results based upon how we measure are evident, whilst many are close to the same, they just don't exactly match up - hence - dark matter theory. How likely is it that dark matter as we know it - does exist?
poelzi 7y ago
They will find dark-matter when they unify GR and QM, never :)

lisper 7y ago

How much money does society need to spend supporting the work you do before we can safely conclude that Einstein was actually correct and we don't need to test it any more?

jeffdavis 7y ago
Assuming Einstein is correct is not the end, because we still have so many questions. For instance, why is Einstein correct?
ISL 7y ago
That's a great and important question. (doubly so for me, as constrained resources may see me leave the field, to which I have devoted my entire professional life, in the coming months) We are perennially grateful for the support we receive to do the work that we do. To a high degree of approximation, Einstein's predictions are very, very correct. We are trying to look very carefully at Nature because we don't think we have the whole story. It is possible that Einstein is exactly right. If so, we would never be able to describe the four forces of Nature with one unified theory. Today, we need two theories to describe everything we see -- the quantum-mechanical/particle-physics description of the Standard Model and gravity. There is no quantum-mechanical fuzziness in the mathematics of gravity, and there is no hint of the differential geometry of gravity in the mathematics of the Standard Model. The aspiration of almost every fundamental-physics physicist is to find a way to either simplify the Standard Model or connect it with gravity. If the return on investment seems insufficent, know, too, that the technology we develop to push the boundaries of knowledge has important spin-offs. GPS is impossible without corrections from both special relativity and general relativity. The instrumentation we develop to make gravitational experiments possible on earth requires the development of new classes of seismometer [1] that may open new understanding of Earth's dynamics and allow better chip-fabrication instrumentation. The instrumentation developed to test gravity in space is also being used to measure the movement of mass (i.e. ice and water) on Earth's surface [2]. Even more important is the training we provide to students. Just as athletes train in the gym to get stronger, confrontation with the hardest known technical problems provides an efficient path for students and young faculty to become proficient at the entire range of modern measurement science. Alumni from our group have not only gone on to fancy academic positions, but also helped to redefine the kilogram, designed upgraded digital calipers used by tens of thousands of people worldwide, built key components of a major quantum-computing company's infrastructure, and more. The people who work on these kinds of experiments are making a fraction of what they could make in industry. We are doing it for love, not money, and the returns to society are myriad. [1] https://arxiv.org/abs/1707.03084 https://arxiv.org/abs/1707.03084 [2] https://gracefo.jpl.nasa.gov/ https://gracefo.jpl.nasa.gov/

zzzcarrot 7y ago

So we are litearaly at square 0 still here?

chrisacky 7y ago
No, sounds like we've eliminated 70 years worth of other possible squares.. if you want to take glass half full approach.
- yourapostasy 7y ago
  Similar to the realization that saying "no" is one of life's important skills to internalize. Closing off possible avenues of explanation is just as much of an advancement as finding evidence to support an explanation.
poelzi 7y ago
Physics took 2 wrong roads and it got stuck in a local minima of the truth plane. Understanding that physics is the most ignorant field of science and how proud the field is of it's models, I don't think this will come anytime soon. https://www.pnas.org/content/112/24/7426 https://www.pnas.org/content/112/24/7426 PS: you need to understand how gravity is derived to manipulate it's propagation. Since the mechanisms are very stable due various reasons, only a very good understanding of many processes will allow you to design an experiment that works. I have adopted a very fringe physics model that is unified and explains more using less assumptions. From this perspective I can tell you that contemporary physics is so far from understanding, that most likely we will not see anything in this direction, before we fully trashed our planet.
- conscion 7y ago
  What's your physics model?
  - poelzi 7y ago
    This one: https://www.amazon.de/Basic-Structures-Matter-Supergravitation-Unified/dp/1412083877 https://www.amazon.de/Basic-Structures-Matter-Supergravitati... Never could have come up with such a brilliant theory, took me a year to understand it. Now I understand 137 ^^