7 ms·

Equivalence principle of general relativity holds even at gravitational extremes

121 points by okket 8y ago

acqq 8y ago
The new paper claims to add to the confirmation of "the strong equivalence principle of general relativity."(0) Reading Wikipedia to find which theories are (or were) the alternatives which assumed that principle not to hold: "The strong equivalence principle": "The first part is a version of the weak equivalence principle" "The second part is the Einstein equivalence principle (with the same definition of "local"), restated to allow gravitational experiments and self-gravitating bodies." "This is the only form of the equivalence principle that applies to self-gravitating objects (such as stars), which have substantial internal gravitational interactions. It requires that the gravitational constant be the same everywhere in the universe and is incompatible with a fifth force. It is much more restrictive than the Einstein equivalence principle." "Einstein's theory of general relativity (including the cosmological constant) is thought to be the only theory of gravity that satisfies the strong equivalence principle. A number of alternative theories, such as Brans–Dicke theory, satisfy only the Einstein equivalence principle." (1) So it seems that "Brans–Dicke theory" and similar are ruled out by this observation, that is that the following line from one Wikipedia page can't remain: "At present, both Brans–Dicke theory and general relativity are generally held to be in agreement with observation."(2) But note that the authors of (0) don't claim that the alternative is completely impossible, but that they have measured even bigger space where the "strong equivalence principle" holds, compared to all measured up to now: "our limit on the strong-field Nordtvedt parameter, which measures violation of the universality of free fall, is a factor of ten smaller than that obtained from (weak-field) Solar System tests"(0) That is, of course, a success, as every scientific and measured increase of "known" is. Note that other "Solar System" tests mentioned in (0) are also very new, from this year. Also: the parameter is "a factor of almost a thousand smaller than that obtained from other strong-field tests"! The article from Nature, of course more scientific that the OP on HN (currently: from discovermagazine.com), ends with: "Although the [scalar–tensor] theories are not completely quashed, their hopes for validity have been made that much fainter."(4) 0) https://www.nature.com/articles/s41586-018-0265-1 https://www.nature.com/articles/s41586-018-0265-1 1) https://en.wikipedia.org/wiki/Equivalence_principle#The_strong_equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle#The_stro... 2) https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory 3) https://journals.aps.org/prd/abstract/10.1103/PhysRevD.48.3436 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.48.34... 4) https://www.nature.com/articles/d41586-018-05549-4 https://www.nature.com/articles/d41586-018-05549-4
- hymen0ptera 8y ago
  A gravitational constant that is the same everywhere in the universe and is incompatible with a fifth force. The fallout of this ramification is that, gravity is some sort of ambient side effect of material presence, sort of like a shadow cast, more than an emission radiated. As a constant, that means that its invariance is significant, in the same way the speed of light is significant. There is some externality pegging the phenomena we notice, at the value we observe. Perhaps some sort of Planck-level absolute fact, which is irreducible in the same way that the concept of color doesn't exist very much beneath 400 nanometers (violet/near-ultraviolet), since color is only an abstraction of our eyeballs and the language we use to describe our sensations.
  - saagarjha 8y ago
    > The fallout of this ramification is that, gravity is some sort of ambient side effect of material presence, sort of like a shadow cast, more than an emission radiated. But then gravity changes would be felt instantaneously rather than propagating at the speed of light.
    - hymen0ptera 8y ago
      Since gravity is a symptom of the presence of matter, gravitational changes ride along with matter travelling as fast as the matter (responsible for this gravitational symptom) can possibly move. At best, only the destruction of matter can move at the speed of light, so a reduction of gravity may propagate, as fast as the dispersal of the dematerialization and subsequent radiation. Accumulation of mass as a knock-on pile-up of massive particles would need to be studied, to assess how quickly the shadow of newly created matter can induce its respective gravitational phenomenon. Figure star nurseries and supernovae events (and similar generative or cataclysmic reactions) would be the place to look. LIGO studies try to do exactly that, but the investigators seem to relish conflating an interpretation that favors signal transmission, and thus "particle/wave duality" because math. Gravity-as-a-wave, meanwhile would be sort of like an illusory sound-as-a-wave being a byproduct of kinetic energy in a material aggregate serving as a bystander medium of propagation. Sound is not it's own "force" although we can readily recognize the phenomenon as a principle that behaves with similar attributes, parameters and effects as observed elsewhere, in other fundamental systems. That LIGO instruments can detect gravitational influence in one part of the apparatus, before the other part of the apparatus registers the phenomenon, is really not unlike saying dominos demonstrate gravitational waves, because toppling the first domino does not topple the last domino instantaneously. That photons can be influenced by a side-effect of material accumulation, bending or altering their path of travel probably says something about the photon or the environment being travelled through, more than it does about space and time.
mar77i 8y ago
I've heard of the new atomic clocks underway to make this ridiculously precise measurement even more precise: https://www.nist.gov/news-events/news/2013/08/nist-ytterbium-atomic-clocks-set-record-stability https://www.nist.gov/news-events/news/2013/08/nist-ytterbium...
- mchahn 8y ago
  Although that news is actually five years old. I guess in some time-frames that could be considered new.
egjerlow 8y ago
*Equivalence principle
millstone 8y ago
This was a measurement of the Nordtvedt parameter which characterizes the difference between a gravitational field and an accelerating reference frame. If there is no difference, as GR predicts, then the parameter is 0. This experiment tests the idea that gravity itself gravitates: gravity imbues very massive bodies with gravitational binding energy, and therefore more inertia. If your inertia is increased due to gravity, do you nevertheless fall at the same rate? i.e. does gravity affect the energy that itself imbues? The conclusion is consistent with "yes." http://www.scholarpedia.org/article/Nordtvedt_effect http://www.scholarpedia.org/article/Nordtvedt_effect
- drjesusphd 8y ago
  I've never thought of the equivalence principle like this. I knew gravitational fields have their own mass-energy and can cause a runaway effect (black holes), but I never realized this could mean more inertia too. Neat! I'm curious about what this means as the event horizon is crossed.
  - raattgift 8y ago
    Sorry I'm a few days late to this party. There are lots of things I am tempted to comment about other comments, but even though the OP doesn't really have anything to do with black holes directly, I'll try an answer restricting myself to your good, honest question: > I'm curious about what this means as the event horizon is crossed tl;dr it means General Relativity is how we answer this; more specifically, we expect that if we do a Galileo-like experiment replacing the Leaning Tower and the ground with a giant scaffold surrounding a black hole, and we carefully drop in a feather, a planet, and a neutron star, neither object's centre of mass wins a race to the horizon (it's a tie, even though the neutron-star is strongly gravitationally self-bound, and the feather is not gravitationally self-bound at all). In practical terms, and strictly with respect to your question, the results of Archibald et al. [2018] (the work described in the original post) mostly (more below) vindicate the use of a post-Newtonian approximation (PNA) formalism as a short-cut to the results we would get from the full theory of General Relativity. At the horizon of anything but the smallest black hole[1] one expects "no drama" for a freely-falling infaller. This is not terribly surprising, as we could already make a model black hole's mass arbitrarily high, bringing the curvature at the horizon down well below the curvature we experience in laboratories here on Earth. Clifford Will, who has written many papers on post-Newtonian methods, in (deliberately accessible to non-specialists) Will [2011] [2] writes: The reason is a remarkable property of general relativity called the Strong Equivalence Principle (SEP). A consequence of this principle is that the internal structure of a body is “effaced,” so that the orbital motion and gravitational radiation emitted by a system of well separated bodies depend only on the total mass of each body and not on its internal structure, apart from standard tidal and spin-coupling effects. In other words, the motion of a normal star or a neutron star or a black hole depends on the body’s total mass and not on the strength of its internal gravitational fields. This behavior was already implicit in the work of Einstein, Infeld, and Hoffman, where only the exterior nearby field of each body was needed, and has been verified theoretically to at least second post-Newtonian order by more modern methods. (So, for example, SEP means a neutron star's strong internal curvature -- the strongest we can access observationally with current technology -- doesn't cause the neutron star to radiate energy-momentum away as dipole gravitational waves.) Archibald [2018] provides observational verification for SEP for the inner radio pulsar of the triple system to good sub-leading post-Newtonian order, improving on results from (among others) the Hulse-Taylor binary and (indirectly) LIGO. Will [2011] uses the "xPN" notation for subleading orders, where 1PN is leading-order, 2PN is next-to-leading order, 3PN is next-to-next-to-leading order, and so forth. A problem raised in Archibald [2018] is that the most popular formal system (the parameterized post-Newtonian (PPN) formalism) for comparing alternative theories of gravitation which may distinguish inertial mass from gravitational mass is only good to 1PN; any theory that does not match the results of Archibald [2018] at 1PN can be excluded, but anything that differs at subleading order needs a different comparison framework. Again, this is not tremendously surprising; PPN was explicitly constructed to fully contain weak-field results (mostly within our own solar system), and the region around the central binary in Archibald [2018] is clearly not weak-field. So while this spells trouble for several families of alternatives to General Relativity (GR), there are several others where the results of breaking GR's inertial mass == gravitational mass equality appear only in the strong field limit. An extension of PPN is needed to capture the results of Archibald [2018] into a formal comparison system that may distinguish between such theories and General Relativity, assuming the compared theories match in every other PPN parameter (if they don't we can exclude one on that basis). Or, in short, General Relativity looks really sound still, and post-Newtonian programmes are not wildly off-track. - -- [1] The region near (but outside) the horizon of a small black hole is going to be dramatic for other reasons, ranging from astrophysical ones like the virtual certainty of a hot accretion structure to theoretical ones like the increasing heat of Hawking radiation as one takes the black hole mass to zero. An object like a space capsule with a person inside would be vapourized by the hot matter outside a small black hole well before reaching the horizon. As we make the mass of a black hole larger, the matter just outside the horizon -- at least on average -- is a lot cooler and sparser, so a space capsule could easily freely fall through the horizon. [2] Proceedings of the National Academy of Sciences of the United States of America, April 12, 2011. 108 (15) 5938-5945; https://doi.org/10.1073/pnas.1103127108 https://doi.org/10.1073/pnas.1103127108 Thankfully PNAS makes it freely available at http://www.pnas.org/content/108/15/5938 http://www.pnas.org/content/108/15/5938
zerostar07 8y ago
why do they call it Principal?
- okket 8y ago
  Double spelling error (first two occurrences), from the third on the article got it right.
lawlessone 8y ago
>with a margin of error of only 30 meters, according to a Amazing they can measure it that accurately
- RedOrGreen 8y ago
  We can do even better some of the time - for example, we can get to ~few nanosecond timing precision for pulsars like J1909-3744 [1], which is equivalent to measuring (changes in) the path length to that pulsar to a few meters or better. [1] https://arxiv.org/abs/1406.4716 https://arxiv.org/abs/1406.4716
Santosh83 8y ago
Just a layman question: does this have any implications for theories of quantum gravity, like LQG or String theory?
- RedOrGreen 8y ago
  Yes, the authors discuss the implications for scalar-tensor theories, and it doesn't look great for many of them. The Nature News and Views article by Clifford Will [1] offers a decent overview. [1] https://www.nature.com/articles/d41586-018-05549-4 https://www.nature.com/articles/d41586-018-05549-4
phyzome 8y ago
> Galileo famously (and likely apocryphally) demonstrated the principal by dropping lead balls of different weights off the Leaning Tower of Pisa and observing them hit the ground at the same time. This never made sense to me as something you'd need to test, once the question had been considered. Imagine 1000 cannonballs all connected to each other with threads. The result is a single object with 1000x the mass of a cannonball. Would you expect it to fall faster?
- contact_fusion 8y ago
  It may seem obvious to our modern eyes, but the notion that objects of different mass fall at the same rate is not at all obvious outside of the framework of physics that Galileo helped build. Without proper notions of force, gravity, mass, density, and the form of the laws of gravity, surely you realize that the answer to your hypothetical is not trivial. This was kind of the whole point of Principia.
  - phyzome 8y ago
    I agree that it's well-established at this point. What I'm poking at is the question of "what is an object?" since that's also fundamental to the question of "do heavier objects fall faster?" And it's a question that Newton could easily have asked, as far as I can tell.
    - contact_fusion 8y ago
      But a more complete notion of "object" isn't what solved the problem. Much of what constituted physics prior to the advances by Galileo and Newton (and many others) was essentially what was developed by Aristotle. Within the Aristotelian framework, which is what most educated people knew at the time, the concept of a distinct "object" is perfectly well defined. My point is that the natural philosophers at the time were quite capable of asking the question, "do heavier objects fall faster?" without being led astray by an incomplete notion of "object." In other words, the problem is conceptual, not semantic. The Aristotelian - again, the dominant framework back then - was instead led astray by the notion that all objects had a "natural" tendency to move towards the center of the Earth, and that this tendency was stronger for heavier objects. This worldview had a very long history, and a fair amount of evidence (such as it was) to back it up. In this context, what matters is not the distinct objects but instead what properties influence their dynamics. It should be clear then why it was necessary to demonstrate the empirical truth. Many plausible theories had emerged, including the prevailing Aristotelian one, which accounted for the differences in, say, a lead weight falling versus a feather. Or, to get back to your original example, the Aristotelian would have answered, probably confidently, that yes, your chain of cannonballs would fall faster than separate ones. Your (correct Newtonian) intuition is that the mere act of chaining them together should not alter them, given that the threads would not exert forces in free fall, and therefore mere mass should not alter the falling rate. But, those notions depend entirely on the Newtonian framework. "Force" and "free fall" are the undefined concepts, not "object." A key insight was needed, namely that you could separate out different causes in the motion; in this case, gravity and air resistance. (Though, of course, "cause" is itself a thorny concept...) Galileo's experiments demonstrated the point elegantly. More formally, this resulted in the development of the notions of inertial reference frames and linearity, which underpin much of classical physics. I hope you don't think I'm belaboring the point. Newton's revolution was so total, so complete, that it changed everything about how we think about physical problems. Centuries of difficult philosophical and scientific work has been condensed down into a semester of freshman physics. To us, the problem is trivial, and it is easy to think that such experiments are so trivial as to be useless. To Galileo, this would not have been the case. A single thought experiment would not have been enough back then, even though it suffices now.

121 points by okket 8y ago

acqq 8y ago

The new paper claims to add to the confirmation of "the strong equivalence principle of general relativity."(0) Reading Wikipedia to find which theories are (or were) the alternatives which assumed that principle not to hold: "The strong equivalence principle": "The first part is a version of the weak equivalence principle" "The second part is the Einstein equivalence principle (with the same definition of "local"), restated to allow gravitational experiments and self-gravitating bodies." "This is the only form of the equivalence principle that applies to self-gravitating objects (such as stars), which have substantial internal gravitational interactions. It requires that the gravitational constant be the same everywhere in the universe and is incompatible with a fifth force. It is much more restrictive than the Einstein equivalence principle." "Einstein's theory of general relativity (including the cosmological constant) is thought to be the only theory of gravity that satisfies the strong equivalence principle. A number of alternative theories, such as Brans–Dicke theory, satisfy only the Einstein equivalence principle." (1) So it seems that "Brans–Dicke theory" and similar are ruled out by this observation, that is that the following line from one Wikipedia page can't remain: "At present, both Brans–Dicke theory and general relativity are generally held to be in agreement with observation."(2) But note that the authors of (0) don't claim that the alternative is completely impossible, but that they have measured even bigger space where the "strong equivalence principle" holds, compared to all measured up to now: "our limit on the strong-field Nordtvedt parameter, which measures violation of the universality of free fall, is a factor of ten smaller than that obtained from (weak-field) Solar System tests"(0) That is, of course, a success, as every scientific and measured increase of "known" is. Note that other "Solar System" tests mentioned in (0) are also very new, from this year. Also: the parameter is "a factor of almost a thousand smaller than that obtained from other strong-field tests"! The article from Nature, of course more scientific that the OP on HN (currently: from discovermagazine.com), ends with: "Although the [scalar–tensor] theories are not completely quashed, their hopes for validity have been made that much fainter."(4) 0) https://www.nature.com/articles/s41586-018-0265-1 https://www.nature.com/articles/s41586-018-0265-1 1) https://en.wikipedia.org/wiki/Equivalence_principle#The_strong_equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle#The_stro... 2) https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory 3) https://journals.aps.org/prd/abstract/10.1103/PhysRevD.48.3436 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.48.34... 4) https://www.nature.com/articles/d41586-018-05549-4 https://www.nature.com/articles/d41586-018-05549-4

hymen0ptera 8y ago
A gravitational constant that is the same everywhere in the universe and is incompatible with a fifth force. The fallout of this ramification is that, gravity is some sort of ambient side effect of material presence, sort of like a shadow cast, more than an emission radiated. As a constant, that means that its invariance is significant, in the same way the speed of light is significant. There is some externality pegging the phenomena we notice, at the value we observe. Perhaps some sort of Planck-level absolute fact, which is irreducible in the same way that the concept of color doesn't exist very much beneath 400 nanometers (violet/near-ultraviolet), since color is only an abstraction of our eyeballs and the language we use to describe our sensations.
- saagarjha 8y ago
  > The fallout of this ramification is that, gravity is some sort of ambient side effect of material presence, sort of like a shadow cast, more than an emission radiated. But then gravity changes would be felt instantaneously rather than propagating at the speed of light.
  - hymen0ptera 8y ago
    Since gravity is a symptom of the presence of matter, gravitational changes ride along with matter travelling as fast as the matter (responsible for this gravitational symptom) can possibly move. At best, only the destruction of matter can move at the speed of light, so a reduction of gravity may propagate, as fast as the dispersal of the dematerialization and subsequent radiation. Accumulation of mass as a knock-on pile-up of massive particles would need to be studied, to assess how quickly the shadow of newly created matter can induce its respective gravitational phenomenon. Figure star nurseries and supernovae events (and similar generative or cataclysmic reactions) would be the place to look. LIGO studies try to do exactly that, but the investigators seem to relish conflating an interpretation that favors signal transmission, and thus "particle/wave duality" because math. Gravity-as-a-wave, meanwhile would be sort of like an illusory sound-as-a-wave being a byproduct of kinetic energy in a material aggregate serving as a bystander medium of propagation. Sound is not it's own "force" although we can readily recognize the phenomenon as a principle that behaves with similar attributes, parameters and effects as observed elsewhere, in other fundamental systems. That LIGO instruments can detect gravitational influence in one part of the apparatus, before the other part of the apparatus registers the phenomenon, is really not unlike saying dominos demonstrate gravitational waves, because toppling the first domino does not topple the last domino instantaneously. That photons can be influenced by a side-effect of material accumulation, bending or altering their path of travel probably says something about the photon or the environment being travelled through, more than it does about space and time.

mar77i 8y ago

I've heard of the new atomic clocks underway to make this ridiculously precise measurement even more precise: https://www.nist.gov/news-events/news/2013/08/nist-ytterbium-atomic-clocks-set-record-stability https://www.nist.gov/news-events/news/2013/08/nist-ytterbium...

mchahn 8y ago
Although that news is actually five years old. I guess in some time-frames that could be considered new.

egjerlow 8y ago

*Equivalence principle

millstone 8y ago

This was a measurement of the Nordtvedt parameter which characterizes the difference between a gravitational field and an accelerating reference frame. If there is no difference, as GR predicts, then the parameter is 0. This experiment tests the idea that gravity itself gravitates: gravity imbues very massive bodies with gravitational binding energy, and therefore more inertia. If your inertia is increased due to gravity, do you nevertheless fall at the same rate? i.e. does gravity affect the energy that itself imbues? The conclusion is consistent with "yes." http://www.scholarpedia.org/article/Nordtvedt_effect http://www.scholarpedia.org/article/Nordtvedt_effect

drjesusphd 8y ago
I've never thought of the equivalence principle like this. I knew gravitational fields have their own mass-energy and can cause a runaway effect (black holes), but I never realized this could mean more inertia too. Neat! I'm curious about what this means as the event horizon is crossed.
- raattgift 8y ago
  Sorry I'm a few days late to this party. There are lots of things I am tempted to comment about other comments, but even though the OP doesn't really have anything to do with black holes directly, I'll try an answer restricting myself to your good, honest question: > I'm curious about what this means as the event horizon is crossed tl;dr it means General Relativity is how we answer this; more specifically, we expect that if we do a Galileo-like experiment replacing the Leaning Tower and the ground with a giant scaffold surrounding a black hole, and we carefully drop in a feather, a planet, and a neutron star, neither object's centre of mass wins a race to the horizon (it's a tie, even though the neutron-star is strongly gravitationally self-bound, and the feather is not gravitationally self-bound at all). In practical terms, and strictly with respect to your question, the results of Archibald et al. [2018] (the work described in the original post) mostly (more below) vindicate the use of a post-Newtonian approximation (PNA) formalism as a short-cut to the results we would get from the full theory of General Relativity. At the horizon of anything but the smallest black hole[1] one expects "no drama" for a freely-falling infaller. This is not terribly surprising, as we could already make a model black hole's mass arbitrarily high, bringing the curvature at the horizon down well below the curvature we experience in laboratories here on Earth. Clifford Will, who has written many papers on post-Newtonian methods, in (deliberately accessible to non-specialists) Will [2011] [2] writes: The reason is a remarkable property of general relativity called the Strong Equivalence Principle (SEP). A consequence of this principle is that the internal structure of a body is “effaced,” so that the orbital motion and gravitational radiation emitted by a system of well separated bodies depend only on the total mass of each body and not on its internal structure, apart from standard tidal and spin-coupling effects. In other words, the motion of a normal star or a neutron star or a black hole depends on the body’s total mass and not on the strength of its internal gravitational fields. This behavior was already implicit in the work of Einstein, Infeld, and Hoffman, where only the exterior nearby field of each body was needed, and has been verified theoretically to at least second post-Newtonian order by more modern methods. (So, for example, SEP means a neutron star's strong internal curvature -- the strongest we can access observationally with current technology -- doesn't cause the neutron star to radiate energy-momentum away as dipole gravitational waves.) Archibald [2018] provides observational verification for SEP for the inner radio pulsar of the triple system to good sub-leading post-Newtonian order, improving on results from (among others) the Hulse-Taylor binary and (indirectly) LIGO. Will [2011] uses the "xPN" notation for subleading orders, where 1PN is leading-order, 2PN is next-to-leading order, 3PN is next-to-next-to-leading order, and so forth. A problem raised in Archibald [2018] is that the most popular formal system (the parameterized post-Newtonian (PPN) formalism) for comparing alternative theories of gravitation which may distinguish inertial mass from gravitational mass is only good to 1PN; any theory that does not match the results of Archibald [2018] at 1PN can be excluded, but anything that differs at subleading order needs a different comparison framework. Again, this is not tremendously surprising; PPN was explicitly constructed to fully contain weak-field results (mostly within our own solar system), and the region around the central binary in Archibald [2018] is clearly not weak-field. So while this spells trouble for several families of alternatives to General Relativity (GR), there are several others where the results of breaking GR's inertial mass == gravitational mass equality appear only in the strong field limit. An extension of PPN is needed to capture the results of Archibald [2018] into a formal comparison system that may distinguish between such theories and General Relativity, assuming the compared theories match in every other PPN parameter (if they don't we can exclude one on that basis). Or, in short, General Relativity looks really sound still, and post-Newtonian programmes are not wildly off-track. - -- [1] The region near (but outside) the horizon of a small black hole is going to be dramatic for other reasons, ranging from astrophysical ones like the virtual certainty of a hot accretion structure to theoretical ones like the increasing heat of Hawking radiation as one takes the black hole mass to zero. An object like a space capsule with a person inside would be vapourized by the hot matter outside a small black hole well before reaching the horizon. As we make the mass of a black hole larger, the matter just outside the horizon -- at least on average -- is a lot cooler and sparser, so a space capsule could easily freely fall through the horizon. [2] Proceedings of the National Academy of Sciences of the United States of America, April 12, 2011. 108 (15) 5938-5945; https://doi.org/10.1073/pnas.1103127108 https://doi.org/10.1073/pnas.1103127108 Thankfully PNAS makes it freely available at http://www.pnas.org/content/108/15/5938 http://www.pnas.org/content/108/15/5938

zerostar07 8y ago

why do they call it Principal?

okket 8y ago
Double spelling error (first two occurrences), from the third on the article got it right.

lawlessone 8y ago

>with a margin of error of only 30 meters, according to a Amazing they can measure it that accurately

RedOrGreen 8y ago
We can do even better some of the time - for example, we can get to ~few nanosecond timing precision for pulsars like J1909-3744 [1], which is equivalent to measuring (changes in) the path length to that pulsar to a few meters or better. [1] https://arxiv.org/abs/1406.4716 https://arxiv.org/abs/1406.4716

Santosh83 8y ago

Just a layman question: does this have any implications for theories of quantum gravity, like LQG or String theory?

RedOrGreen 8y ago
Yes, the authors discuss the implications for scalar-tensor theories, and it doesn't look great for many of them. The Nature News and Views article by Clifford Will [1] offers a decent overview. [1] https://www.nature.com/articles/d41586-018-05549-4 https://www.nature.com/articles/d41586-018-05549-4

phyzome 8y ago

> Galileo famously (and likely apocryphally) demonstrated the principal by dropping lead balls of different weights off the Leaning Tower of Pisa and observing them hit the ground at the same time. This never made sense to me as something you'd need to test, once the question had been considered. Imagine 1000 cannonballs all connected to each other with threads. The result is a single object with 1000x the mass of a cannonball. Would you expect it to fall faster?

contact_fusion 8y ago
It may seem obvious to our modern eyes, but the notion that objects of different mass fall at the same rate is not at all obvious outside of the framework of physics that Galileo helped build. Without proper notions of force, gravity, mass, density, and the form of the laws of gravity, surely you realize that the answer to your hypothetical is not trivial. This was kind of the whole point of Principia.
- phyzome 8y ago
  I agree that it's well-established at this point. What I'm poking at is the question of "what is an object?" since that's also fundamental to the question of "do heavier objects fall faster?" And it's a question that Newton could easily have asked, as far as I can tell.
  - contact_fusion 8y ago
    But a more complete notion of "object" isn't what solved the problem. Much of what constituted physics prior to the advances by Galileo and Newton (and many others) was essentially what was developed by Aristotle. Within the Aristotelian framework, which is what most educated people knew at the time, the concept of a distinct "object" is perfectly well defined. My point is that the natural philosophers at the time were quite capable of asking the question, "do heavier objects fall faster?" without being led astray by an incomplete notion of "object." In other words, the problem is conceptual, not semantic. The Aristotelian - again, the dominant framework back then - was instead led astray by the notion that all objects had a "natural" tendency to move towards the center of the Earth, and that this tendency was stronger for heavier objects. This worldview had a very long history, and a fair amount of evidence (such as it was) to back it up. In this context, what matters is not the distinct objects but instead what properties influence their dynamics. It should be clear then why it was necessary to demonstrate the empirical truth. Many plausible theories had emerged, including the prevailing Aristotelian one, which accounted for the differences in, say, a lead weight falling versus a feather. Or, to get back to your original example, the Aristotelian would have answered, probably confidently, that yes, your chain of cannonballs would fall faster than separate ones. Your (correct Newtonian) intuition is that the mere act of chaining them together should not alter them, given that the threads would not exert forces in free fall, and therefore mere mass should not alter the falling rate. But, those notions depend entirely on the Newtonian framework. "Force" and "free fall" are the undefined concepts, not "object." A key insight was needed, namely that you could separate out different causes in the motion; in this case, gravity and air resistance. (Though, of course, "cause" is itself a thorny concept...) Galileo's experiments demonstrated the point elegantly. More formally, this resulted in the development of the notions of inertial reference frames and linearity, which underpin much of classical physics. I hope you don't think I'm belaboring the point. Newton's revolution was so total, so complete, that it changed everything about how we think about physical problems. Centuries of difficult philosophical and scientific work has been condensed down into a semester of freshman physics. To us, the problem is trivial, and it is easy to think that such experiments are so trivial as to be useless. To Galileo, this would not have been the case. A single thought experiment would not have been enough back then, even though it suffices now.