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The dominant Lambda-CDM model is the standard model of physical cosmology, and it has proved reasonably successful. It does, however, have problems, such as dark matter, whose true nature remains elusive. Dutch physicist Erik Verlinde has, in a recent paper, proposed that gravity might not actually be a fundamental interaction at all, but rather an emergent property of spacetime itself, and as such, what current cosmological theory considers dark matter is really an emergent gravity phenomenon. Sabine Hossenfelder has an article about several recent tests of Verlinde's theory, which show that the idea might have promise.
Physicists today describe the gravitational interaction through Einstein's Theory of General Relativity, which dictates the effects of gravity are due to the curvature of space-time. But it's already been 20 years since Ted Jacobson demonstrated that General Relativity resembles thermodynamics, which is a framework to describe how very large numbers of individual, constituent particles behave. Since then, physicists have tried to figure out whether this similarity is a formal coincidence or hints at a deeper truth: that space-time is made of small elements whose collective motion gives rise to the force we call gravity. In this case, gravity would not be a truly fundamental phenomenon, but an emergent one.
[...] Verlinde pointed out that emergent gravity in a universe with a positive cosmological constant – like the one we live in – would only approximately reproduce General Relativity. The microscopic constituents of space-time, Verlinde claims, also react to the presence of matter in a way that General Relativity does not capture: they push inwards on matter. This creates an effect similar to that ascribed to particle dark matter, which pulls normal matter in by its gravitational attraction.
[...] So, it's a promising idea and it has recently been put to test in a number of papers.
[...] Another paper that appeared two weeks ago tested the predictions from Verlinde's model against the rotation curves of a sample of 152 galaxies. Emergent gravity gets away with being barely compatible with the data – it systematically results in too high an acceleration to explain the observations.
A trio of other papers show that Verlinde's model is broadly speaking compatible with the data, though it doesn't particularly excel at anything or explain anything novel.
[...] The real challenge for emergent gravity, I think, is not galactic rotation curves. That is the one domain where we already know that modified gravity – at last some variants thereof – work well. The real challenge is to also explain structure formation in the early universe, or any gravitational phenomena on larger (tens of millions of light years or more) scales.
Particle dark matter is essential to obtain the correct predictions for the temperature fluctuations in the cosmic microwave background. That's a remarkable achievement, and no alternative for dark matter can be taken seriously so long as it cannot do at least as well. Unfortunately, Verlinde's emergent gravity model does not allow the necessary analysis – at least not yet.
Previously:
Emergent Gravity and the Dark Universe
(Score: 0, Troll) by Anonymous Coward on Wednesday March 01 2017, @05:38PM (33 children)
"Gravity", as in "an inherent property of mass" has never, ever been proven.
Ever.-
Look it up, it has not. Nowhere, EVER, has it ever been demonstrated that mass attracts mass because it is mass and not because of some other factor messing it up (like electricity). NE - VER. Never! Not from Cavendish, nor from Newton, neither from those Italian dudes in 'Nature' 2015. Not once, nil, zero, nada. Please keep that in mind, and behave in a civilized fashion when engaged in relevant conversation.
(just a heads-up so you guys know your shit n avoid embarrassment if you ever find yourselves in the position of defending `Gravity')
(Score: 0) by Anonymous Coward on Wednesday March 01 2017, @05:46PM
I wish Le Sage gravity got more attention today, in attempts to address the shortcomings:
https://en.wikipedia.org/wiki/Le_Sage%27s_theory_of_gravitation [wikipedia.org]
(Score: 0) by Anonymous Coward on Wednesday March 01 2017, @05:55PM
Calm down, it's going to be alright :)
(Score: 2, Informative) by Anonymous Coward on Wednesday March 01 2017, @06:05PM
No physical theory is "proven", and you could take your own advice about civilized discussion.
(Score: 4, Informative) by FatPhil on Wednesday March 01 2017, @06:10PM (17 children)
Actual proof, as a philosophical/mathematical concept, is *unrelated* to the "extremely unlikely to be wrong given our current knowledge" use of the term that the physical sciences have to put up with.
If you are confusing the two, then that shows a weakness in your own mind. If you think that scientists are confusing the two, then you are either listening to woo-woo "scientists", which demonstrates a weakness in your ability to select who to believe on such matters, or you are misinterpreting what those scientists are saying, which is simply again a weakness in your own mind.
Now go read some Popper, you're nearly a century behind the educated world. Actually, read some Hume first, as we need to get you out of short trousers first.
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 1, Interesting) by Anonymous Coward on Wednesday March 01 2017, @06:24PM (4 children)
I think Popper is pretty much debunked and Lakatos is the current king of philosophy of science. Basically science is about making surprising a priori predictions (that turn out to be accurate) and coming up with the least byzantine set of ad hoc explanations for the deviations from these predictions.
(Score: 2) by HiThere on Thursday March 02 2017, @12:40AM (1 child)
Popper hasn't, and can't be "debunked". You can argue with how he uses words, or whether his domains are properly specified..and at times I would, but this is far from being debunked. That's like saying Newton was debunked. I'll admit I've never read Lakatos, but you can be pretty sure that he, also, did not say the last word on the subject. He may cover a wider field than does Popper (who really only properly covers experimental sciences that have readily replicated experiments).
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 0) by Anonymous Coward on Thursday March 02 2017, @03:20AM
Falsifiability doesn't work as a criterion. The only reason it seemed to work is because scientists before that knew what they were doing and predicted something precise. Now we have NHST where everyone tries to predict whether A is correlated with B (the vaguest thing possible). That aint science, because an insane number of explanations can be easily put forward for such vague observations. Such predictions, and "tests" of them, do not help distinguish between the theories.
As lakatos put it, every theory grows in a "sea of anomalies". They are born and live falsified. It is all about making one/few precise predictions that otherwise have low prior probability.
(Score: 1) by khallow on Thursday March 02 2017, @12:51PM (1 child)
I think Popper is pretty much debunked and Lakatos is the current king of philosophy of science.
First, this is an inappropriate use of the term, "debunk". Disagreement or elaboration is not "debunking". This seems particularly common in the debate over climate change where once someone disagrees with another, it becomes a "debunk" followed by the rhetorical dismissal of the prior opinion or claim. I disagree with you, therefore I have "debunked" you and can ignore whatever it was you said. I hope readers understand the problems with that approach.
Because that leads to the second observation. From what I've read, Lakatos isn't really disagreeing on Popper's approach except to say that it tends to be too aggressive in falsification, discarding models too quickly.
Basically science is about making surprising a priori predictions (that turn out to be accurate) and coming up with the least byzantine set of ad hoc explanations for the deviations from these predictions.
Notice what this sentence says. First, what is an a priori prediction which is both surprising and accurate? It's an observed falsification of the current model. That's definitely a standard Popper-style approach right there. And coming up with the "least byzantine set" of ad hoc explanations? Well, that's coming up with a new model using a vague and subjective criteria. I think there could be some room for improvement there. In actual science, there would be multiple creations of not necessarily ad hoc (models can often be reused, for example, in different fields and so may not be created for that purpose but rather a different one) models selected for a variety of subjective criteria followed by more observation and so on.
So we have a Popper-style model falsification approach combined with a perhaps peculiarly human approach to generating additional models with what I see as some modest room for improvement in the description of the process. That's not much of a deviation from Popper and thus, not much of a debunking.
(Score: 0) by Anonymous Coward on Thursday March 02 2017, @03:12PM
Well, there is Popper0, Popper1, and Popper2. Popper0 is the naive falsificationism that lay people (eg statisticians) think Popper described, Popper1 is Popper, and Popper2 is how Lakatos refers to his own ideas.
(Score: 0) by Anonymous Coward on Thursday March 02 2017, @12:17AM (11 children)
"Now go read some [..] as we need to get you out of short trousers first."
How thoughtful, I ll take that under consideration. Now, if you are done with your childish nonsense and laughable patronizing attempts, please direct your attention here as I got something special for you too: it is called a free-falling spring, and will help you understand how you have been meticulously deluding yourself into believing that 'expanding the mind' means 'quoting shit others said or wrote'.
The challenge is simple: a spring of length x and stiffness k is hanging at rest inside a gravity field g, and is released. Describe (math model) the motion of this free-falling spring.
Good Luck (you are going to need it)
(Score: 2) by HiThere on Thursday March 02 2017, @12:45AM (7 children)
That's a very simple problem. You just model the "free falling spring", and stop before it collides with anything. Of course, the shape of the spring, and how it's rotating will introduce a few complexities even before the first collision. ... When, among other things, you need to start considering the elasticity of to two surfaces (as well as the reaction of the rest of the spring). So stop first.
A fairer example would be considering how the air moves past the blades of a rotating fan. There are lots of places you can look things about that up without needing to rent time on a supercomputer.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 0) by Anonymous Coward on Thursday March 02 2017, @01:22AM (6 children)
That's a very simple problem.
Oh really? Solve it then, and report back with your solution.
Or don't, and just keep believing you know "how simple" it is.
(Score: 1) by khallow on Thursday March 02 2017, @12:57PM (5 children)
Oh really? Solve it then, and report back with your solution.
The other poster already did.
(Score: 0) by Anonymous Coward on Friday March 03 2017, @12:08PM (4 children)
The other poster already did.
Hi khallow, no, he did not, perhaps my fault for not explaining the problem better. I welcome you to give it a go: a spring of mass m and stiffness k is at rest hanging vertically inside a gravitational field g and is released. The question is to derive the equation of motion for the bottom tip of the spring as a function of the vertical distance from the ground against time, from the moment the spring is released to the moment it contacts the ground, and the answer is to be expressed this in terms of m, k and g, or any other parameter pleases you as long as you formulate it in math (differentials, a polynomial or a plot are all good) as long as you can justify your derivation.
(Score: 1) by khallow on Friday March 03 2017, @03:08PM (3 children)
Hi khallow, no, he did not, perhaps my fault for not explaining the problem better. I welcome you to give it a go: a spring of mass m and stiffness k is at rest hanging vertically inside a gravitational field g and is released. The question is to derive the equation of motion for the bottom tip of the spring as a function of the vertical distance from the ground against time, from the moment the spring is released to the moment it contacts the ground, and the answer is to be expressed this in terms of m, k and g, or any other parameter pleases you as long as you formulate it in math (differentials, a polynomial or a plot are all good) as long as you can justify your derivation.
You should have said that in your original demand. But even so, the process for deriving the solution was laid out. But having said that, I'd use the Lagrange approach:
L = K - V.
Here, K and V are the kinetic and potential energy of the spring. There are a variety of possibilities depending on how the mass and potential energy are distributed. In general, you can always find a solution computationally, even with complex three dimensional dynamics or if the spring can break or other sort of "memory" (where the current behavior of the spring depends on what happened to it in the past).
But let's suppose the simplest case where the mass of the frictionless spring is all concentrated in a single point with height x - larger means higher, the potential energy of the spring follows Hooke's law, and the gravitational field is constant. Then
K = 1/2 mv^2 (v = dx/dt, m is the constant mass of the spring concentrated at point x) and
V = 1/2 k(x_0 - x)^2 + mgx where k is a stiffness coefficient (not necessarily your stiffness coefficient, g is the acceleration due to the gravity field). The potential energy of the spring is just the sum of the potential energy of the spring and the potential energy of the mass in the gravitational field.
The key trick to using the Lagrangian L is that the partial derivative of L with respect to x is equal to the normal derivative with respect to time of the partial derivative of L with respect to v = dx/dt. Hmmm, let's call it
\dee L/ \dee x = d/dt( \dee L/ \dee v).
Then
k(x - x_0) + mg = m dv/dt.
Still looks pretty simple to me.
(Score: 0) by Anonymous Coward on Friday March 03 2017, @05:12PM (2 children)
The question is for the bottom of the spring, so the spring has to be considered at least as a one-dimensional 'physical' object; no point masses.
k(x - x_0) + mg = m dv/dt.
Again, it is the bottom of the spring that is concerned as a function of time: does x here represent the bottom of the spring?
(Score: 1) by khallow on Friday March 03 2017, @06:34PM (1 child)
The question is for the bottom of the spring, so the spring has to be considered at least as a one-dimensional 'physical' object; no point masses.
If you're assuming that the mass distribution is distributed along the spring, then we'll have to do something different. Let's suppose uniform distribution of mass. x(a) (actually x(a) := x(a;t)) is the position of the spring where fraction a mass is on one side (as parameterized by the coefficient a) at given time t. Overlapping of the spring is allowed. Kinetic energy and the potential energy due to gravity do not change much:
K = 1/2 m integral{a=0 to 1} dx(a)/dt da, and
P_{grav} = mg integral {a=0 to 1} x(a) da.
The potential energy due to the spring is now proportional along the length of the spring to the square of change in parameter a from a default rest state spring with no force acting on it. P_{spring} = 1/2 k integral{a=0 to 1} (dx(A)/da - b)^2 da. And fix the initial parameters of the spring (x(a; t=0). b happens to be the at rest length of the spring without tension (so b is your "x"). Now, we have a variational equation. We still can use the Lagrangian:
d/dt \delta L/ \delta (dx(a)/dt) = \delta L / \delta x(a). Solving the math becomes either a matter of a computer model, or getting all your integrals in terms of the variation of x(a), not various derivatives of the variation of x(a) (there's a second derivative of time t, which needs to be reexpressed in terms of a Kronecker delta function, and a first derivative of parameter a, which can be transformed via some integration by parts manipulation, as integrals of some function of x(a) and its derivatives collectively times a single counter-factor, the variation of x. The first factor thus has to be identically zero resulting in a differential equation. I feel there's little point to going that far though. Notice that while there is a lot of it, the math has come through via a variety of simple rules.
And as HiThere noted, once you have the model, you just use the model to compute the position of the spring. That remains simple as expected. Once we have the basic equation of motion, we can then add other effects such as your interaction with a solid surface, internal friction of the spring, etc. While that may look very hard from our very limited viewpoint it doesn't actually change the computational complexity of the problem that much.
(Score: 0) by Anonymous Coward on Friday March 03 2017, @07:37PM
If you're assuming that the mass distribution is distributed along the spring
I am,
And as HiThere noted, once you have the model, you just use the model to compute the position of the spring.
No dispute there: only I do not have such a model, and it is not trivial to produce. Your honest analysis may change that, so I guess I can treat it as homework, implement a model and see what happens.
(Score: 2) by FatPhil on Thursday March 02 2017, @10:27AM (2 children)
Why are you asking a pure mathematician an applied maths question? How will that help you grasp metaphysics?
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 1) by curril on Thursday March 02 2017, @11:20PM
Sorry, that's not correct. The instant the spring is released, the top the spring will start moving down, releasing tension and the bottom of the spring will start falling (albeit slower than the top). The center of mass of the spring will fall strictly by the acceleration of gravity g at that point, while the ends of the spring will oscillate about the center of mass according to Hook's Law and simple harmonic oscillation. Of course, if the length of the spring is such that g can't be assumed to the same at the ends of the spring, then spring will also experience tidal forces that change the frequency of oscillation over time.
(Score: 0) by Anonymous Coward on Friday March 03 2017, @12:38PM
was under gravity/tension equilibrium before it was dropped, and therefore remains stationary until the tension disappears [..] where the neutral extension is totally collapsed, the spring will simply collapse, base unmoving
Are you proclaiming the slingy a special case spring that exactly cancels out it stiffness with gravity so its bottom always stays put until the top collapses, even when its mass or stiffness is messed with? If so, please math it up (express it as a function of time)
Why are you asking a pure mathematician an applied maths question?
Sorry, you lost me: who is the applied mathematician?
How will that help you grasp metaphysics?
Where did that one come from? Would you mind if we settle the math part of the discussion first?
(Score: 1, Insightful) by Anonymous Coward on Wednesday March 01 2017, @06:44PM (9 children)
Is this more electric universe crap?
"Magnetism", as in "an inherent property of mass" has never, ever been proven.
Ever.-
Look it up, it has not. Nowhere, EVER, has it ever been demonstrated that charge attracts charge because it is electromagnetic and not because of some other factor messing it up (like gravity). NE - VER. Never! Not from Maxwell, nor from Faraday, neither from those Italian dudes in 'Nature' 2015. Not once, nil, zero, nada. Please keep that in mind, and behave in a civilized fashion when engaged in relevant conversation.
(just a heads-up so you guys know your shit n avoid embarrassment if you ever find yourselves in the position of defending `Magnetism')
Fuckin' magnets. How do they work?
(Score: 1) by khallow on Thursday March 02 2017, @01:16PM (8 children)
Is this more electric universe crap?
Or it could be our resident flat Earther rearing [soylentnews.org] to strike. A NASA/Freemason conspiracy to fake the Moon landing for Satan is more real than gravity.
(Score: 0) by Anonymous Coward on Friday March 03 2017, @01:16PM (7 children)
Or it could be our resident flat Earther
'flat Earther' in lack of a better description, yes- I like this forum a lot. I could open an account I guess and observe to what amount 'flamebait' and 'troll' is a reflex to my posts, whether I have ever spread enough hate, fear and division to actually deserve those designations, and what sorts of AC flak I ll be receiving, assuming I don't get straight out banned from the start and can get an account in the first place.
(Score: 1) by khallow on Friday March 03 2017, @03:23PM (6 children)
"Gravity", as in "an inherent property of mass" has never, ever been proven.
We have yet to observe mass that doesn't have gravity of the appropriate extent for the mass. So sure, it looks like an inherent property which is the point of empiricism, looking at things to determine their properties.
Nowhere, EVER, has it ever been demonstrated that mass attracts mass because it is mass and not because of some other factor messing it up (like electricity).
And there's a fair number of theories that propose that there's really just one force, it just decomposes nicely into four pieces in our situation.
whether I have ever spread enough hate, fear and division to actually deserve those designations
Don't forget ignorance which is a key attribute of your posts. Let's throw out three considerations:
1) If Earth is not a round ball-like object, then how is it shaped?
2) Why isn't the Earth uniformly lit or dark?
3) Why is the shadow of Earth when cast on the Moon, always round?
(Score: 0) by Anonymous Coward on Friday March 03 2017, @04:26PM (3 children)
Different AC here. These questions are actually key to calculating the size of the 33 C greenhouse effect:
1) A concave disk facing the sun.
2) It is uniformly lit, thus the concavity.
3) The earth is disc-shaped.
This is all climate science 101. http://www.atmos.washington.edu/~davidc/ATMS211/articles_optional/Hansen81_CO2_Impact.pdf [washington.edu]
(Score: 1) by khallow on Friday March 03 2017, @06:41PM (2 children)
These questions are actually key to calculating the size of the 33 C greenhouse effect:
Just like assuming for a physics problem that cows are spherical and frictionless makes them so? The paper you linked provides zero support for your assertion. Even worse for the Earth, it used a one-dimensional model not the conditions you asserted. That must have hurt.
(Score: 0) by Anonymous Coward on Friday March 03 2017, @07:16PM (1 child)
How does a one dimensional object have a radius?
(Score: 1) by khallow on Friday March 03 2017, @07:45PM
How does a one dimensional object have a radius?
I can think of a variety of ways it could and couldn't be done. Every one of them is irrelevant to this discussion.
(Score: 0) by Anonymous Coward on Friday March 03 2017, @08:09PM (1 child)
I guess I called it then, eh?
Yes,
We have yet to observe mass that doesn't have gravity of the appropriate extent for the mass.
You speak as if you already know for a fact that mass gravitates, and you are expecting to falsify that by observing mass that doesn't. I have never observed mass that gravitates, either within or outside of 'established parameters' (big G). Where have you observed mass to gravitate, and how?
1) If Earth is not a round ball-like object, then how is it shaped?
According to a testimony of the first man to see it from the stratosphere, Auguste Piccard, it looks like a dish with an upturned surface, horizon at eye level. He did not manage to see the whole Earth.
2) Why isn't the Earth uniformly lit or dark?
Water is transparent, why can't you see the bottom of the ocean?
3) Why is the shadow of Earth when cast on the Moon, always round?
I pay attention to such predicted events and have seen the moon get dark in different ways. If you want to claim that what darkens a globular moon is the shadow of a globular Earth cast on it I must tell you that I see inconsistencies with that idea. Eclipses differ a lot. So it is more efficient if you refer to eclipses per eclipse, as they are becoming more and more documented and are easy to refer to.
Having said that, at which eclipse did you observe the shadow of the Earth being round and cast on the moon?
(Score: 1) by khallow on Friday March 03 2017, @09:12PM
You speak as if you already know for a fact that mass gravitates, and you are expecting to falsify that by observing mass that doesn't. I have never observed mass that gravitates, either within or outside of 'established parameters' (big G). Where have you observed mass to gravitate, and how?
I have observed 5-10 kg masses gravitate. I used a similar setup to the Cavendish experiment [wikipedia.org] in a college physics lab. One pair of the heavy weights swung freely from a tether suspended from the ceiling. The other pair, which was placed very closed to the first could be swung on a pivot in the same axis as the tether so that second pair was just a little offset rotationally of the first pair in the circle of rotation, either clockwise or counterclockwise. That is, the weights of the free-swinging beam could be pulled clockwise or counterclockwise by the second beam which allowed one to pivot between the two modes. One significant deviation was to attach a mirror to the wire of the oscillating masses and project a laser beam down a fairly long hallway to get a very accurate measure of the deflection of the beam.
So for example, you could wait till all oscillation had ceased, then pivot the second pair of weights so that their pulled on the first pair in the opposite direction and measure over time the deflection of the tether's rotation at the other end of the hallway. This gave you both the net deflection and as in the original Cavendish experiment, the period of oscillation of the tether gave you an estimate of the stiffness of the tether as a spring. The combination allowed you to estimate the net force change acting on the free swinging weights. Then from estimates of the distance between the weights and how gravity acts between non-point masses, my estimate was within 20% of the measured force acting on the weights. That's pretty good for an undergrad lab.
According to a testimony of the first man to see it from the stratosphere, Auguste Piccard, it looks like a dish with an upturned surface, horizon at eye level. He did not manage to see the whole Earth.
If it is dish-shaped, then where is the edge of the dish? If there is no edge, then that both strongly restricts what sort of shape the Earth can be (I'll note here that the Earth is finite in extent, that plus no holes means the Earth would be topologically equivalent to a sphere, that still allows for some weird shapes, but maps could still be projected without loss or overlap onto a globe) and leads to the question of why would there be large dimples?
Water is transparent, why can't you see the bottom of the ocean?
Because water is opaque on the scale of kilometers. The Sun doesn't significant change in size as it moves across the sky and we see it move below the horizon from our point of view - so there isn't a lot of moving back and forth and hence, opportunity for the Sun's light to be obscured from my position). Further, anyone with a cell phone can verify, by calling people elsewhere in the world, that the Sun is in a different position in a consistent way. For example, until the end of my season this week, I worked at Yellowstone National Park. At one point, a coworker had called a significant other with video who was currently residing in the Philippines, I believe around 9pm Mountain time. The real time image clearly showed the significant other in daylight, even though Yellowstone had been in darkness for some time.
So the shape of Earth has to be something that allows the Sun to shine on parts of the Earth while not shining on other parts of the Earth in a very predictable and consistent way, day after day.
I pay attention to such predicted events and have seen the moon get dark in different ways. If you want to claim that what darkens a globular moon is the shadow of a globular Earth cast on it I must tell you that I see inconsistencies with that idea. Eclipses differ a lot. So it is more efficient if you refer to eclipses per eclipse, as they are becoming more and more documented and are easy to refer to.
I don't. I too make a fair bit of such observations and the umbral shadow is consistently very circular. For examples, the September 16, 2016 lunar eclipse which turned out quite nice from Yellowstone and a 1993 or 1994 lunar eclipse which I observed from western North Carolina. I recall it as being a partial eclipse only which would make it the May 25, 1994 one. It's also worth noting the predictability of these events which can be predicted decades out (well actually much further out than that, but who would wait for an eclipse millennia from now?).
(Score: 4, Funny) by maxwell demon on Wednesday March 01 2017, @08:14PM
Yeah, intelligent falling is the only reasonable explanation.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1, Informative) by Anonymous Coward on Wednesday March 01 2017, @08:27PM
Wording suggestion: "Gravity is an observed phenomenon that follows a known simple model on a smaller scale, but not on a larger, galactic scale."
This doesn't require explaining or giving the mechanism of gravity. It's described merely as an observed pattern in nature.
And, I use "known" because perhaps there is a simple model that works on ALL observable scales, but we simple haven't found/devised it yet.