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Dandobi

Are the laws of physics really universal?

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I was told that on this forum there is no such thing as a stupid question so let me try one!

The law of gravity states that the further an object is away from a gravitational pull the slower it will orbit. Example, Venus takes longer to orbit the sun than Mercury, Jupiter longer than Mars etc.

Now I know that the arms of a galaxy have been shown to travel faster than an object closer to the centre, presumed to be due to 'Dark Matter'. What I want to know is have we 'invented' this to make it fit the laws as we know them or is this proof that laws as we know aren't universal? Do we need to rethink what we think we know?

Thanks, Marc (argh!)

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Dark matter is needed to make are current gravity theories (Newton, general realtivity) work. Without the extra gravity of dark matter, are current theories of gravity predict that the stars in galaxies and the galaxies in clusters of galaxies would "boil off". Dark matter makes explanation of other phenomena easier as well, for example, the formation of (the seeds for) galaxies in the early universe.

Because the as yet unseen dark matter plus current theory explains a list of phenomena, most physicists are betting on dark matter. It could be, however, that dark matter is a non-existent red herring, and that we need a completely new theory of gravity.

I think that we will have direct evidence for dark matter within a decade, either from the Large Hadron Collider, or from dedicated dark matter detectors. If direct evidence for dark matter is not found within two decades, more and more phyicists will start betting on the second option.

This uncertainy is what scientific research is all about. We live in exciting times!

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What I want to know is have we 'invented' this to make it fit the laws as we know them or is this proof that laws as we know aren't universal? Do we need to rethink what we think we know?

Science is a matter of our understanding of the universe (and its parts) so that it better fits the observations we are able to make. Nobody has got hold of a lump of "dark matter" and measured its physical properties; all we are able to estimate are its mass and its approximate location, based on its gravitational effects. Which boils down to trusting the best description of the universe we have, in this case Einstein's Theory of General Relativity.

We always need to rethink what we think we know ... better models may be needed ....

Now I know that the arms of a galaxy have been shown to travel faster than an object closer to the centre, presumed to be due to 'Dark Matter'.

That's not true. The speed falloff isn't as great as it would be if all the matter was concentrated in visible stars - but we can see locally that this isn't true - just look past your feet at what you're standing on; it's certainly not a star!

Fact of the matter is, the laws of physics have always been assumed to be universal, and that has given us a pretty good explanation of what we can observe of the physical universe. The fact that we can only observe dark matter indirectly is more or less irrelevant; we can't observe individual electrons directly, either, but it doesn't stop us having a very detailed grasp of their physics; and we can infer that the physics of electromagnetism are universal by the fact that we do receive electromagnetic radiation which began its journey long, long ago and far, far away, when the universe was a much smaller, hotter place.

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Yes, dark matter was "invented" to explain the motion of galaxy clusters. The alternative is to invent a different law of gravity - this is attempted by MOND (modified newtonian gravity). Problem with MOND is that you need to tweak the gravitational force to get exactly what you see, but for no other apparent reason, whereas particle theories abound with as-yet unobserved and unconfirmed particles which might make up the dark matter. So new particles tend to be favoured over new gravity laws at present.

Similar problem existed with the anomalous precession of Mercury: candidate explanations included something funny going on inside the Sun, or a modification to Newtonian gravity. Answer turned out to be general relativity.

Who knows, maybe the Pioneer Anomaly is evidence of new physics. Or maybe not.

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Thanks for your replies guys. Its good to get some feedback, its the only way I'll ever learn anything!

Just finished reading 'End of Science' by John Horgan who argues that maybe in the future people may look back and think of our understanding of physics as primitive as how we look back at Aristotle's. Or maybe we've figured it all out already!

Marc

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Also, I've never heard of MOND so that gives me something to look into next! Thanks again!

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To go back to the original question, How would we know?

Olly

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How would we know?

It's not given to us to be able to measure the physical constants on which our model are based anywhere except "here" and "now". The best we can do is to appeal, in the spirit of relativity, to the principle that neither "here" nor "now" are in any way special.

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This is getting deep, back to the books I think! Thanks for your input everyone!

Marc

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I'm reading a book at the moment that has a few chapters on this subject.

"13 things that don't make sense". Its currently in the top 20 in WH Smiths I think. Talks about Dark Matter and other stuff that science can't really explain yet.

Personally I think Dark Matter sounds a bit like the "Ether" concept that scientists were peddling ages ago because they didn't know any better. I think that our current theories just aren't good enough and we can see that they aren't, but haven't yet figured out how to improve them. I think that space travel may be the key to unlocking further leaps forward as this would enable testing of theories potentially outside the Earth's gravitational influence. But generally being able to do experiments deep in outer space would perhaps unlock a lot of mysteries.

The other thing that book sort of hints at is that scientists have a tendency to ignore results that don't fit the current theories. So they've kinda known that there is a problem for a long time and have only relatively recently invented Dark Matter so that the theories can continue. Sometimes they change things a bit, like introducing the Cosmological constant into formulae and removing it again, depending on whether they currently think the universe is expanding or contracting or standing still.

Not to knock what they're doing, but ignoring results that don't fit with the theory or can't be explained isn't the best thing in my opinion. But then I'm not the one that has to come up with the ideas to explain stuff...and I doubt I could do any better (or even anything remotely similar).

Dave

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I believe there's no such thing as dark matter. It's just another invention to make things fit.

Perhaps we should view the problem differently. If we dispense with the idea that gravity and motion are variables and time is fixed, we can then look at problems differently.

For example, gravity increases towards the centre of a galaxy, but motion decreases instead of increases. What if gravity and motion are constants and what we see is time running slower... This would explain the apparent low velocity of centre objects while getting rid of the need for dark matter.

At some stage we will have to free our minds of current theories in much the same way as people had to accept the world wasn't flat. I don't have the knowledge to have the answers, I suspect no-one does. Future scientists will look upon us and our theories, as we look upon neanderthals.

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Totally agree. In that book I mentioned, another chapter talks about the Viking probes, or maybe it was Pioneer...can't remember now. Anyway, as they were drifting out to the edges of our solar system, they suddenly started veering off course slightly so as the laws of gravity couldn't predict the course accurately.

For many years, they have studied this to try to work out why, but have come up with pretty much nothing. Except that some scientists use it as evidence that the laws don't function as we think they do on Earth once you start getting right out into deep space.

Of course, it could be some other simpler explanation for the course deviation, but no one has managed to find it in a large number of years.

Personally, I'm not sure it does mean the laws are subtly wrong, but it could do and its just another one of those things that don't add up.

David

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If we dispense with the idea that gravity and motion are variables and time is fixed

Eh?

We "dispensed with the idea that time is fixed" 105 years ago - Einstein's Theory of Special Relativity killed that off once and for all. Motion has to be variable for acceleration to be possible, and gravity is clearly variable depending on the local mass gradient.

Dark matter is at least an easily understandable concept. Dark energy isn't.

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The other thing I was thinking is that what if the missing 96% of the universe that they have invented Dark Matter to account for is actually not missing, but we just don't know where to look for it?

Maybe those black holes in galaxies have something to do with it? No one really understands how they work except as mathematical models. And also, the measurements made to assess the amount of matter in the universe, how do we even know if they're vaguely accurate. As far as I know, they keep getting revised as new techniques come along.

Maybe our universe is like a soap bubble and there are other universes outside ours expanding in the same sort of way, so that if you could see it from the outside, there'd be a whole pile of universe bubbles expanding away. Why should ours be the only universe, in the same way as there are clearly more than 1 solar system?

We have a lot to learn I feel....

David

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it could be some other simpler explanation for the course deviation, but no one has managed to find it in a large number of years.

Oh?

My understanding is that 90%-105% of the "Pioneer anomaly" can be explained by outgassing from the propulsion system on the spacecraft. That's close enough for me.

There is essentially zero anomaly reported in the trajectories of the Voyager probes, which have now overtaken the Pioneers & would certainly be affected equally by any unexpected change in the gravitational field.

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Maybe those black holes in galaxies have something to do with it? No one really understands how they work except as mathematical models.

Actually we have rather a lot of observational evidence, gathered from a safe distance. Special & General Relativity work very well up to the event horizon - and in fact can continue to do so within it, except very close to the singularity point, where a theory of quantum gravitation is required.

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Ok, sorry Brian. I probably read too many popular science books and watch too much stuff like Stargate to know enough about the real facts.

Still...I think Dark Matter/Energy does sound really dodgy and a theory of quantum gravitation would be really useful as you say.

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I think Dark Matter/Energy does sound really dodgy

OK, you tell us an alternative theory which explains what we can observe at least as well, and also an experiment (or set of observations) which can distinguish between your new theory and dark matter.

a theory of quantum gravitation would be really useful as you say.

There are several theories. The problem is a lack of experimental evidence ... quantum gravitation is only required to operate at very small scales and extremely high energy levels, many orders of magnitude higher than are accessible with our best tools e.g. the Large Hadron Collider. General Relativity is an excellent model of gravitation so far as the macroscopic structure of the universe is concerned, and I expect it to remain so. With the possible exception of the introduction of a "dark energy" correction which is significant only at huge scales. I'm not convinced about dark energy, the questionable assumption IMO is that the absolute luminosity of type Ia supernovae should have remained constant despite the long term variation in the density & composition of the interstellar medium ... and the difficulty of clearly distinguishing type Ia supernovae from other energetic events in galaxies with huge red shifts.

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Just as a spin-off, the theoretical physicist Lee Smolin has written about this and come up with the interesting hypothesis that the laws of physics might be subject to a kind of Darwinian system of natural selection. The book is The Life of the Cosmos and the underlying hypothesis seems to me to be outstanding. Highly recommended. It is hypothetical more than theoretical but Jocelyn Bell was saying recently that we should show more interest in the early hypothetical stages of theory-formation than we do at present. And why not, it is where the ideas come from.

Olly

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the theoretical physicist Lee Smolin has written about this and come up with the interesting hypothesis that the laws of physics might be subject to a kind of Darwinian system of natural selection. The book is The Life of the Cosmos and the underlying hypothesis seems to me to be outstanding.

The issue here is that the arguments are heavily based on the anthropomorphic principle ... a universe in which life can not exist (and in which we are therefore unable to discuss the matter) seems at least equally probable to me; and (adopting the principle of Occam's Razor) I'm inclined to the opinion that a universe in which life can exist will at least sometimes result in the existence of life - without requiring any external selection principle, which seems to me to be straying dangerously close to the concept of a creator divinity.

But I freely admit that there is a total lack of evidence to justify my prejudice.

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Brian, had I suspected the slightest divine intervention in the Smolin hypothesis it would have gone in the bin! I see it the other way round; the uncomfortable improbability of finding ourselves in a life-bearing universe are turned around and become probabilities. Nor do I see the anthropic principle in Smolin but that is not surprizing since I really can make nothing of this idea and have never understood it.

Just a word on using history of science to bash science; you need to be accurate. The flat earth is tossed around regularly in these conversations but to the best of my knowledge there has never been any kind of scientific flat earth theory in the history of humanity. (I exclude the loonies since peer review defines them as such.) The ancients knew the Earth was spherical. They saw constellations in the north set as they sailed south, and vice versa. They saw the Earth's shadow on the moon. This idea that educated people once believed in a flat earth needs to be put to bed. It never happened.

Now, 'peddling the luminferous ether.' Who peddled it? It was an hypothesis put forward to explain the propagation of light. It had its day but when Mickleson and Morely came up with a way to confirm or deny its existence they promptly did so and denied it. That seems a perfectly reasonable way to do science to me. I hardly think we should use this to bash scientists because, after all, M and M were scientists.

Olly

Edited by ollypenrice

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the uncomfortable improbability of finding ourselves in a life-bearing universe are turned around and become probabilities. Nor do I see the anthropic principle in Smolin

There you are. The probability of a life form finding itself in a life-bearing universe is, by definitition, precisely 1.

The idea that the universe (which is nothing more than the application of the laws of physics to a particular set of boundary conditions) can be subject to "natural selection" is dependent on there being an external influence to provide the selection ... as this is necessarily outside the observable universe, we're straight into metacosmology, or religion.

IMHO the idea that the universe is a sort of computer model makes more sense, it even starts to make sense of quantum physics that way. And there's no need to assume that any part of the "computer" exists outside the observable universe, because a finite state computer can emulate itself in a subset of its own hardware (thanks to Turing and von Neumann for providing the mathematical proof of this rather unexpected concept). Personally I'm not happy with this explanation but logically it does have considerable merit.

The question "why?" remains unanswerable. It may be a measure of our imperfection that we find it necessary to ask it.

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'There you are. The probability of a life form finding itself in a life-bearing universe is, by definitition, precisely 1. '

Indeed so, but what many cosmologists dislike about our present understanding of the universe is that it requires a very fine tuning of the numbers in order for it to happen in a manner that allows us to be here to see it. Now part of me says, 'Well, that's just how it is' but another side me would be glad to find some kind of improbablity- reducing theory sudh as Smolin's.

I don't know if you have read his book but he does make a case (not a strong one by his own admission) for it having some testable features and predictions. It cannot stand outside the present universe which, as you say, is a huge limitation, but it finds some internal characteristics which fit. The specific mechanism he proposes for universe natural selection is not, for me too important. What I like is the idea of universe evolution.

The universe as computer game has no appeal for me at all, I'm afraid. I think we are far too ready with computer anaolgies in our present obession with the things. I have no truck with artificial intelligence either but I'll start a new thread on that one!

Cheers,

Olly

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