Einstein's Theory of Gravity Passes Toughest Test to Date: Equivalence principle in extreme conditions

[phands]

Einstein just keeps getting proved right! And this one was a challenge....

The new study could take some of the air out of their optimism. An international team of astronomers tested the equivalence principle under extreme conditions: a system composed of two superdense stellar corpses known as white dwarfs and an even denser neutron star.

 

[Peez]

Gravity is only a theory.

I'm just sayin'.

Peez

 

[repoman]

How many decimal places of precision compared to QED is this?

 

[bilby]

How many decimal places of precision compared to QED is this?

QFT.

 

[steve_bank]

That is a relief, for a while I was worried gravity might stop working.

 

[lpetrich]

Einstein's Theory of Gravity Passes Toughest Test to Date noting Universality of free fall from the orbital motion of a pulsar in a stellar triple system

Pulsar PSR J0337+1715 has two white dwarfs orbiting it. [1401.0535] A millisecond pulsar in a stellar triple system has more about it. The pulsar has a mass of 1.44 solar masses, rather typical of pulsars with known masses. It has a rotation period of 2.73 milliseconds, and its spindown timescale is about 2.5 billion years.

The inner white dwarf has a mass of 0.20 solar masses, and it orbits at a distance of 15.9 lts = 0.0319 AU, with a period of 1.63 days.

The outer white dwarf has a mass of 0.41 solar masses, and it orbits at a distance of 589 lts = 1.18 AU, with a period of 327 days.

Both orbits are nearly circular. lts = light-seconds. The Moon is 1.3 lts away, and the Sun 499 lts away.

 

[lpetrich]

Universality of free fall is also called equivalence principle. There are three kinds.

• Weak Equivalence Principle: Gravity is independent of nongravitational features like composition. Very well tested.
• Einstein Equivalence Principle: The WEP with local independence of position and velocity (Lorentz invariance).
• Strong Equivalence Principle: The EEP with gravity acting on an object's gravitational binding energy the same as on its other sorts of mass.

The WEP has been very well-tested, with strong upper limits on composition dependence.

The EEP has also been well-tested, with successors of the Michelson-Morley experiment reaching very high precision.

The SEP has been very difficult to test, because most objects' gravitational binding energy is very small compared to their masses. Here is a table:

• Moon: 2*10^(-11)
• Earth: 4.6*10^(-10)
• Jupiter: 10^(-8)
• Sun: 3.6*10^(-6)
• White dwarf (low mass): 3*10^(-4)
• Neutron star: 0.1

So pulsar orbits ought to be a good place to look for SEP violations. That is indeed the case for this triple system. In that recent Nature paper, it was reported that possible SEP violations are at most 2.6*10^(-6) (relative size). The best of similar tests was on pulsar PSR J1713+0747, which is orbited by a single white dwarf. SEP violation was at most 2*10^(-3) for it.

 

[lpetrich]

I had earlier discussed tests of general relativity in Trouble for Alternate Theories of Gravity

From  Test theories of special relativity, Lorentz-invariance tests have these upper limits on violations:

• Michelson-Morley: c dependence on direction: <10^(-11)
• Kennedy-Thorndike: c dependence on motion relative to some preferred frame: <10^(-7)
• Ives-Stilwell: relativistic Doppler effect and time dilation: <10^(-7)

Here is the arxiv version of that recent paper: [1807.02059] Testing the universality of free fall by tracking a pulsar in a stellar triple system They find good limits on (gravitational mass) / (inertial mass), though not on post-Newtonian parameters (beta) and (gamma) -- their limits are no better than Solar-System limits.

Here is what those parameters are:
(Newtonian gravity) = (Lowest-order time distortion)
(gamma) = (space distortion) / (time distortion)
(beta) = (nonlinearity in time distortion)

 

[Beer With Straw]

Saw this in news July 4: http://www.cbc.ca/news/technology/einstein-general-relativity-1.4733562

heh - zombie star

 

[Underseer]

Thanks for posting this news.

Here's something I wrote in a apologetics discussion group about this recent finding when I posted the above link:

In 1979, William Lane Craig produced and started to popularize a modified version of the Kalam Cosmological Argument. Craig's version vaguely used the language of modern physics to make it sound like science itself supported this version of the cosmological argument.

While the philosophers were able to poke holes in Craig's version of Kalam right away (the cosmological argument is very old), little by little more and more physicists have been getting involved showing with devastating arguments how badly Craig understands the actual physics he keeps using in his arguments.

Here's the thing: because of modern physics, we can longer say with certainty that there is such a thing as "before" before the universe since we now understand that spacetime as we know it came into existence with the universe. If there is no such thing as "before the universe," then the universe cannot have a cause, at least not in as much as we can understand the law of cause and effect.

At first Craig responded to this by claiming that the cause doesn't have to go before the effect. This was quickly shot down by philosophers and, well, anyone with a pulse. So now, Craig has furiously backpadaled and has been trying to use a syllogism to disprove Einstein's Relativity. (We've gone full circle: this whole thing started out trying to use Einstein's work to add credibility to the cosmological argument, remember?)

This is the most moonbat-stupid thing I've ever heard from an apologist.

Even Ray "banana man" Comfort understands enough about why things are true in science to lie about the evidence, but only William Lane Craig is stupid enough to believe that the evidence doesn't matter.

If you want to claim that relativity is false, step one has to involve the evidence in some way.

The most thorough and simplest way to disprove relativity would be to produce a new scientific theory that makes more accurate predictions about the relevant evidence than relativity does.

Of course, doing that would require coming up with a scientific theory and testing said scientific theory, not to mention requiring Craig to actually understand the evidence for relativity, all of which is clearly beyond him.

Which brings us to today's news article.

Now there is one more piece of evidence that Craig has to provide a different explanation for. Not that he will or can.

 

[Underseer]

That is a relief, for a while I was worried gravity might stop working.

Jokes aside, this wasn't about testing whether or not gravity exists.

Right now, Relativity is the only explanation for gravity that we have. Everyone knew about the fact of gravity long before science was a thing. Newton introduced the law of gravity in the 1600s. It wasn't until Einstein cooked up Relativity that we had a theory of gravity to go with the law of gravity. Relativity doesn't play nice with quantum anything, so either there is a problem with relativity, a problem with quantum, or a third thing no one has discovered yet.

 

[phands]

Relativity doesn't play nice with quantum anything, so either there is a problem with relativity, a problem with quantum, or a third thing no one has discovered yet.

Most physicist agree that Relativity is a special case approximation of QM, and most things QM can coexist with Special Relativity, and even with more strenuous maths, General Relativity. Even then, no-one has yet succeeded in making QM successfully describe a gravitational filed. This is why we keep looking, and have tried many GUTs, but so far nothing has been shown to work. I think a GUT and eventually a TOE is that third (and maybe 4th) thing.

 

[Underseer]

Relativity doesn't play nice with quantum anything, so either there is a problem with relativity, a problem with quantum, or a third thing no one has discovered yet.

Most physicist agree that Relativity is a special case approximation of QM, and most things QM can coexist with Special Relativity, and even with more strenuous maths, General Relativity. Even then, no-one has yet succeeded in making QM successfully describe a gravitational filed. This is why we keep looking, and have tried many GUTs, but so far nothing has been shown to work. I think a GUT and eventually a TOE is that third (and maybe 4th) thing.

I dropped out of physics before getting my 4 year degree, so I was just barely starting to learn quantum, but from what I've read (and bear in mind I am far from an expert), if you combine QM with anything that includes gravity, the resulting mess starts spitting out infinities where finite values are expected.

And that kind of thing tends to make physicists very cranky.

 

[skepticalbip]

That is a relief, for a while I was worried gravity might stop working.

Jokes aside, this wasn't about testing whether or not gravity exists.

Right now, Relativity is the only explanation for gravity that we have. Everyone knew about the fact of gravity long before science was a thing. Newton introduced the law of gravity in the 1600s. It wasn't until Einstein cooked up Relativity that we had a theory of gravity to go with the law of gravity. Relativity doesn't play nice with quantum anything, so either there is a problem with relativity, a problem with quantum, or a third thing no one has discovered yet.

My vote if for that third thing. Physicists working with Relativity and those working with QM both are aware that neither is a full description of the universe. Even with all the decades of attempts to unify the two theories, there has been no success. I am beginning to lean toward the idea that we may need someone (like another Newton or Einstein) capable of an entirely fresh look, able to ignore the two models and create an all encompassing model from scratch. Newton was able to see past the accepted thinking of his time and Einstein was able to see past Newton's assumptions.

 

[repoman]

What if gravity and QM don't inherently play nice and nothing can change that. Is that possible??

 

[bilby]

What if gravity and QM don't inherently play nice and nothing can change that. Is that possible??

Sure it is. Both could be approximations of a very different underlying reality - just as universal gravitation is an approximation of relativity, and chemistry is an approximation of quantum mechanics. These approximations work perfectly well as long as you don't need very high precision, or to predict the behaviour of objects in extreme conditions.

And that underlying reality is under no obligation to make sense to us, nor to be within our ability to discover or comprehend. But history suggests that sooner or later some truly impressive intellect will find the answer, and that once he explains it it may well seem obvious to those who learn it - just as happened with universal gravitation.

It's a LOT easier to understand an idea than it is to originate it. And once a new idea is out there, it is easy for those who understand it to test it up to the limits of our ability to measure. As those limits expand, there is always the chance that we will find that what seemed to be a universal law is, in fact, an approximation of a different and more accurate way of viewing things, just as our ability to precisely measure the orbit of Mercury showed that universal gravitation was inadequate in extreme conditions, such as close proximity to massive objects such as the sun.

Everything in science is provisional. But QM and Relativity both give extremely accurate predictions in the vast majority of conditions, so there is no chance that they will actually go away - People still use Newtonian models of gravitation for engineering, because the results are just as good in many applications - that is, the errors are too small to measure, much less to worry about. The important thing is to be aware when you are 'pushing the envelope' and might need to use the more accurate (but more arduous) mathematics of relativity.

 

[Underseer]

That is a relief, for a while I was worried gravity might stop working.

Jokes aside, this wasn't about testing whether or not gravity exists.

Right now, Relativity is the only explanation for gravity that we have. Everyone knew about the fact of gravity long before science was a thing. Newton introduced the law of gravity in the 1600s. It wasn't until Einstein cooked up Relativity that we had a theory of gravity to go with the law of gravity. Relativity doesn't play nice with quantum anything, so either there is a problem with relativity, a problem with quantum, or a third thing no one has discovered yet.

My vote if for that third thing. Physicists working with Relativity and those working with QM both are aware that neither is a full description of the universe. Even with all the decades of attempts to unify the two theories, there has been no success. I am beginning to lean toward the idea that we may need someone (like another Newton or Einstein) capable of an entirely fresh look, able to ignore the two models and create an all encompassing model from scratch. Newton was able to see past the accepted thinking of his time and Einstein was able to see past Newton's assumptions.

Votes aren't how opinions become accepted in science.

 

[Underseer]

What if gravity and QM don't inherently play nice and nothing can change that. Is that possible??

Sure it is. Both could be approximations of a very different underlying reality - just as universal gravitation is an approximation of relativity, and chemistry is an approximation of quantum mechanics. These approximations work perfectly well as long as you don't need very high precision, or to predict the behaviour of objects in extreme conditions.

And that underlying reality is under no obligation to make sense to us, nor to be within our ability to discover or comprehend. But history suggests that sooner or later some truly impressive intellect will find the answer, and that once he explains it it may well seem obvious to those who learn it - just as happened with universal gravitation.

It's a LOT easier to understand an idea than it is to originate it. And once a new idea is out there, it is easy for those who understand it to test it up to the limits of our ability to measure. As those limits expand, there is always the chance that we will find that what seemed to be a universal law is, in fact, an approximation of a different and more accurate way of viewing things, just as our ability to precisely measure the orbit of Mercury showed that universal gravitation was inadequate in extreme conditions, such as close proximity to massive objects such as the sun.

Everything in science is provisional. But QM and Relativity both give extremely accurate predictions in the vast majority of conditions, so there is no chance that they will actually go away - People still use Newtonian models of gravitation for engineering, because the results are just as good in many applications - that is, the errors are too small to measure, much less to worry about. The important thing is to be aware when you are 'pushing the envelope' and might need to use the more accurate (but more arduous) mathematics of relativity.

Yup.

I don't know why people think laws and theories are set in stone engraved in the pedestals of the statues of their creators. As you say, everything is provisional, a necessary consequence of the fact that evidence-based epistemology can never produce absolute truth (not that I think absolute truth is a thing).

Laws and theories are not things into themselves. They are models. They are little boxes in our heads that represent our best attempt at understanding reality.

Reality is under no obligation to fit into the little boxes we create inside our heads. Whenever we find a piece of evidence that doesn't fit into an existing box, we modify one of the existing boxes to fit the new evidence. If we can't modify the box, we replace it. If we can't replace it, we throw it away and hope someone in the future can make a better box.

 

[lpetrich]

I dropped out of physics before getting my 4 year degree, so I was just barely starting to learn quantum, but from what I've read (and bear in mind I am far from an expert), if you combine QM with anything that includes gravity, the resulting mess starts spitting out infinities where finite values are expected.

And that kind of thing tends to make physicists very cranky.

Pretty much it.

Actually, such infinities emerge more generally, but in the Standard Model, at least, they can be subtracted out, a process called renormalization. Troublesome infinities started to appear as physicists started to do quantum mechanics on elementary-particle fields, starting with photon and electron fields. But they eventually learned how to redefine masses, charges, and field strengths so as to absorb these infinities. The first successes were in the 1950's, with photons and electrons, giving quantum electrodynamics. Richard Feynman, Shinichiro Tomonaga, and Julian Schwinger won a Nobel Prize for that work in 1965. This work was easily extended to other particles' electromagnetic interactions, though strong and weak ones were more difficult.

In the late 1960's and early 1970's, physicists showed that a kind of theory called a "nonabelian gauge theory" is also renormalizable. Gerardus 't Hooft and Martinus Veltman got a Nobel Prize for that work in 1999. A nonabelian gauge theory is a theory much like electromagnetism, but with more than one "charge", and with the "photons" having some charge-anticharge combinations. This makes the "photons" interact with each other, thus producing the "nonabelian" feature.

This made it possible to construct well-behaved theories of the strong and electroweak interactions, with the electroweak interaction being a combination of the electromagnetic and weak ones. Thus giving the Standard Model of particle physics.


But if one tries to renormalize a quantized version of general relativity, one gets into trouble. One might try fixing it by adding more particles in suitable ways, but one is unable to stop. The closest that we have gotten to a well-behaved quantum theory of gravity has been string theory, and that works by having an infinite number of string modes. But string theory has plenty of problems of its own. One can get an approximation of the Standard Model out of it, but there are oodles of other possible solutions, and it is not very clear why our Universe would have one of them and not some other. Is it a sort of anthropic natural selection? That's because we'd see only a kind of Universe that allows us to come into existence. So there could be a string-solution multiverse with oodles of sterile string-solution universes.

 

[Underseer]

I dropped out of physics before getting my 4 year degree, so I was just barely starting to learn quantum, but from what I've read (and bear in mind I am far from an expert), if you combine QM with anything that includes gravity, the resulting mess starts spitting out infinities where finite values are expected.

And that kind of thing tends to make physicists very cranky.

Pretty much it.

Actually, such infinities emerge more generally, but in the Standard Model, at least, they can be subtracted out, a process called renormalization. Troublesome infinities started to appear as physicists started to do quantum mechanics on elementary-particle fields, starting with photon and electron fields. But they eventually learned how to redefine masses, charges, and field strengths so as to absorb these infinities. The first successes were in the 1950's, with photons and electrons, giving quantum electrodynamics. Richard Feynman, Shinichiro Tomonaga, and Julian Schwinger won a Nobel Prize for that work in 1965. This work was easily extended to other particles' electromagnetic interactions, though strong and weak ones were more difficult.

In the late 1960's and early 1970's, physicists showed that a kind of theory called a "nonabelian gauge theory" is also renormalizable. Gerardus 't Hooft and Martinus Veltman got a Nobel Prize for that work in 1999. A nonabelian gauge theory is a theory much like electromagnetism, but with more than one "charge", and with the "photons" having some charge-anticharge combinations. This makes the "photons" interact with each other, thus producing the "nonabelian" feature.

This made it possible to construct well-behaved theories of the strong and electroweak interactions, with the electroweak interaction being a combination of the electromagnetic and weak ones. Thus giving the Standard Model of particle physics.


But if one tries to renormalize a quantized version of general relativity, one gets into trouble. One might try fixing it by adding more particles in suitable ways, but one is unable to stop. The closest that we have gotten to a well-behaved quantum theory of gravity has been string theory, and that works by having an infinite number of string modes. But string theory has plenty of problems of its own. One can get an approximation of the Standard Model out of it, but there are oodles of other possible solutions, and it is not very clear why our Universe would have one of them and not some other. Is it a sort of anthropic natural selection? That's because we'd see only a kind of Universe that allows us to come into existence. So there could be a string-solution multiverse with oodles of sterile string-solution universes.

For any layman interested in this topic, I can't recommend the book The Infinity Puzzle enough.

That renormalization is basically spit and bailing wire, and yes, it deserved a Nobel, but it sure feels like something pointing at an underlying phenomenon that everyone is missing.