Gravitons... wild goose chase?

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Gravitons... wild goose chase?

Postby Aelfyre » Sat Dec 17, 2011 10:05 pm UTC

It's been a while since I have pestered the brain collective... :) anyway I have a question. Why does mainstream physics assume the existence of the graviton is a foregone conclusion? As far as I know it had never been actually detected and as a force gravity is a real outlier. I mean the standard model may predict its existence but then again the standard model doesn't really do gravity right?

I mean gravity as a force seems fundamentally different from the other forces... They say or may ber because it propagates through more dimensions than just our visible four but if that were the case you would think it would not follow the inverse square rule right? Like the inverse cube or inverse fourth our fifth our something...

I am certainly not offering up an explanation as of I know something but it just seems that gravity is an emergent effect of spacetime and spacetime is distinct from the energy that inhabits it.

I heard an explanation for lights inability to escape the event horizon of black hole and out ready made sense... It said that inside a black hole spacetime itself was being drawn towards the singularity faster than light and that's why light could not escape... Like trying to swim up a waterfall... Light as always is moving at c but the space it its moving through is moving towards the singularity at faster than the speed of light and so it has no part but inwards.
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Re: Gravitons... wild goose chase?

Postby thoughtfully » Sat Dec 17, 2011 11:36 pm UTC

Aelfyre wrote:Why does mainstream physics assume the existence of the graviton is a foregone conclusion?

This is greatly overstating the case. However, if a theory of Quantum Gravity looks like existing quantum field theories, then a graviton would be present in such a theory.

As for detecting them experimentally, it seems doubtful that it could ever happen. They interact too weakly. But, according to some theories with higher dimensions, gravitons would have siblings (think muons and electrons) that might be detectable. That would be a pretty strong indicator, if it ever came to pass.
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Re: Gravitons... wild goose chase?

Postby gmalivuk » Sun Dec 18, 2011 1:25 am UTC

Aelfyre wrote:They say maybe because it propagates through more dimensions than just our visible four but if that were the case you would think it would not follow the inverse square rule right? Like the inverse cube or inverse fourth or fifth or something...
Not if the additional dimensions are quite small.
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Re: Gravitons... wild goose chase?

Postby Argency » Sun Dec 18, 2011 4:56 am UTC

I should preface this comment by saying that it's informed by metaphysics and epistemology, and that the dominant opinions in physics may differ. At the same time, though, metaphysics and epistemology underlie heaps of important current physics and as we get closer to the GUT they're going to be more and more important.

Anyway, epistemologically speaking there comes a point when the boundary between a given type of particle existing and not existing blurs, especially for the messenger particles like gravitons. The harder the particle is to detect, the less difference there is between a world in which gravitons exist and one in which they don't.

By way of illustration, imagine a world in which photons don't exist, and in which the electromagnetic force operates instead by "spooky action at a distance". Instead of the force being communicated by photons, electrons just coordinate themselves correctly without passing messages. So, where in our universe an electron might drop down a wavelength, releasing a photon which travels forty light-picoseconds to another electron, bumping it up a wavelength, in this fictional universe the first electron just drops it's energy by a wavelength, lowering the total energy of the universe slightly, and forty light-picometres later the second electron "spontaneously" jumps up a wavelength. The point of this thought experiment is that all other things being equal it's impossible to determine experimentally which of the two universes I've just described is the one in which we live. Every experimental test we could possibly use to determine the existence of photons relies on their interactions with matter, so a universe in which matter just behaves as though photons existed is identical to one in which they actually do exist.

Of course since there's no way of knowing which is which we don't need to worry - Occam's Razor just guides us to use the simplest theory that explains all of the data and that is the theory that photons exist. But it's still important (I think) to keep in mind that choosing the simplest theory was something that we had to do, and that the other theories existed, because it gives us insight into the nature of particle existence. The simpler a particle's interactions with matter are, less useful it is to think about that particle as a real object and the more useful it is to think of it as a theoretical device for describing how matter interacts. Photons don't behave quite exactly like objects - sometimes they act like a wave and sometimes they act like a particle - but that shouldn't bother us because we remember that the photon is actually just a label we apply to a certain type of interaction between two other particles. Most of the time it's a particularly object-like interaction, but sometimes it's a bit spookier. Really, you could say the same of any other object - electrons, neutrinos, quarks, etc - but in most of those cases the interaction is extremely object-like most of the time, it's only with extremely advanced measurement techniques that the wave-particle duality of matter becomes really noticeable. Point is, we're at the razor-sharp cutting edge with gravitons, and there's no reason to believe that they'll be extremely object-like. Maybe the difference in simplicity between a universe that actually contains gravitons and a universe that just behaves like it contains gravitons is negligible or near-negligible. In that case, is it even a meaningful question to ask whether or not gravitons exist?
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Re: Gravitons... wild goose chase?

Postby doogly » Sun Dec 18, 2011 5:01 am UTC

Why a graviton?
Because there should be a quantum theory of gravity. Whatever that theory is, we are going to use the name "graviton" for the quanta. This is mostly just a naming pattern. Until there is a specific theory of quantum gravity, this is really just a placeholder name. Don't read too much into it.

Why a quantum theory of gravity of at all?
We've never noticed a need for it, so why can't it just be classical all the way down? Imagine using classical gravitational waves to make a measurement. If they aren't limited by Heisenberg, you could use them to make measurements that violate Heisenberg for all other components in the Standard Model. You could do this by changing quantum mechanics in some way. But there's no way to just take "standard model quantum mechanics as we know it" and "classical GR as we know it" and say "well, good enough! I think we're done here." We know they have to be able to interact, and *one* of the theories has to bend when they bump. Fairly absolutely certain it's gravity. Though QM might have some bending left to do as well.
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Re: Gravitons... wild goose chase?

Postby Diadem » Sun Dec 18, 2011 5:45 am UTC

doogly wrote:Why a graviton?
Because there should be a quantum theory of gravity. Whatever that theory is, we are going to use the name "graviton" for the quanta. This is mostly just a naming pattern. Until there is a specific theory of quantum gravity, this is really just a placeholder name. Don't read too much into it.

Pretty much this.

We haven't found the graviton yet. We don't even know what it would look like. We just decided that any particle that governs gravity is called a graviton, regardless of its properties. So logically then the graviton must exist, unless gravity is not governed by quantum mechanics at all. Which would be deeply troublesome for a great many reasons.
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Re: Gravitons... wild goose chase?

Postby PM 2Ring » Sun Dec 18, 2011 3:36 pm UTC

Argency wrote:Photons don't behave quite exactly like objects - sometimes they act like a wave and sometimes they act like a particle - but that shouldn't bother us because we remember that the photon is actually just a label we apply to a certain type of interaction between two other particles. Most of the time it's a particularly object-like interaction, but sometimes it's a bit spookier. Really, you could say the same of any other object - electrons, neutrinos, quarks, etc - but in most of those cases the interaction is extremely object-like most of the time, it's only with extremely advanced measurement techniques that the wave-particle duality of matter becomes really noticeable. Point is, we're at the razor-sharp cutting edge with gravitons, and there's no reason to believe that they'll be extremely object-like.

It seems to me that the distinction you're talking about here isn't so much the difference between particle-like and wave-like things, it's really the difference between fermions and bosons. Bosons have whole integer quantum spin, and thus obey Bose-Einstein statistics; fermions have half-odd-integer spin (all known elementary fermions have a spin of 1/2), and thus obey Fermi-Dirac statistics, which means they are subject to Pauli exclusion. All the force carriers are bosons, and if the graviton exists, it's surely a boson (with a spin of 2).


As for actually detecting gravitons, I'm not holding my breath. I expect that it will be much harder to detect individual gravitons than to detect gravitational waves, and currently no attempts to detect gravitational waves have been successful.

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Re: Gravitons... wild goose chase?

Postby WarDaft » Sun Dec 18, 2011 3:49 pm UTC

doogly wrote:If they aren't limited by Heisenberg, you could use them to make measurements that violate Heisenberg for all other components in the Standard Model.

Just out of curiosity, how? It's such a weak force, shouldn't every other kind of measurement we make to measure the resulting gravitational waves vastly obscure any 'extra' details it might detect?
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Re: Gravitons... wild goose chase?

Postby idobox » Sun Dec 18, 2011 5:25 pm UTC

I think the OP and I have the same problem with the concept of graviton.
Gravity is extremely different from other forces because it deforms space-time.

For example, a charged particle will emit virtual photons that will affect charged particles movements. But a massive particle will influence every single particle, fermions as well as bosons, their interactions, their half-lives.
Also, it appears there is only one type of mass, unlike charge, flavour, etc.

The behaviour of gravity is so different from the other forces, I don't see why it could be explained by a mechanism similar to other forces. Is there a quantum object that describes gravitation? probably! Does this object have properties similar to particles? I don't see why.
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Re: Gravitons... wild goose chase?

Postby eSOANEM » Sun Dec 18, 2011 6:21 pm UTC

doogly wrote:We know they have to be able to interact, and *one* of the theories has to bend when they bump. Fairly absolutely certain it's gravity. Though QM might have some bending left to do as well.


I hear this a lot in discussions about unification of gravity, but it rarely seems to be more than an assertion so, I was wondering, why is it that people seem so keen to bend relativity into a quantum shape rather than vice versa? As I understand it, twistor theory this opposite approach for example.

WarDaft wrote:
doogly wrote:If they aren't limited by Heisenberg, you could use them to make measurements that violate Heisenberg for all other components in the Standard Model.

Just out of curiosity, how? It's such a weak force, shouldn't every other kind of measurement we make to measure the resulting gravitational waves vastly obscure any 'extra' details it might detect?


Because, even if such a measurement were swamped in the uncertainties of the other forces, it would still imply that the HUP was not an absolute truth and as such, it would merely be a problem of engineering rather than physics. This seems very much not to be the case.

idobox wrote:I think the OP and I have the same problem with the concept of graviton.
Gravity is extremely different from other forces because it deforms space-time.

For example, a charged particle will emit virtual photons that will affect charged particles movements. But a massive particle will influence every single particle, fermions as well as bosons, their interactions, their half-lives.
Also, it appears there is only one type of mass, unlike charge, flavour, etc.

The behaviour of gravity is so different from the other forces, I don't see why it could be explained by a mechanism similar to other forces. Is there a quantum object that describes gravitation? probably! Does this object have properties similar to particles? I don't see why.


The pre-relativistic theories of electrostatics and gravitation look very similar (except for the fact that there are two signs of charge) and, indeed, it is possible to model electrostatics in a similar way to gravity under kaluza-klein theory so to say that gravity behaves differently from the other forces appears very much to be a lack of knowledge on our part rather than an actual physical truth. As such, it would seem highly unlikely for there not to be a graviton.
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Re: Gravitons... wild goose chase?

Postby doogly » Sun Dec 18, 2011 7:24 pm UTC

eSOANEM wrote:
doogly wrote:We know they have to be able to interact, and *one* of the theories has to bend when they bump. Fairly absolutely certain it's gravity. Though QM might have some bending left to do as well.


I hear this a lot in discussions about unification of gravity, but it rarely seems to be more than an assertion so, I was wondering, why is it that people seem so keen to bend relativity into a quantum shape rather than vice versa? As I understand it, twistor theory this opposite approach for example.


Well, we have a lot of experimental evidence for quantum mechanics. And we have Bell's theorem. So, what kind of classical theory can produce those results? If you have something deterministic but nonlocal, this is still not really classical the way GR is a classical theory. You have a different flavor of problem than in quantum mechanics, but you are still doing something very strange. 'tHooft seems to want some things like this though; they aren't nonsense. Just not a thing I like at all.

Twistors are a quantum theory. You get Dirac equations and such. I spent like a grand total of three hours looking at twistors though, life total.
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Re: Gravitons... wild goose chase?

Postby JWalker » Sun Dec 18, 2011 8:09 pm UTC

doogly wrote:Why a graviton?
Because there should be a quantum theory of gravity.

Why should there be?

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Re: Gravitons... wild goose chase?

Postby doogly » Sun Dec 18, 2011 8:14 pm UTC

If there is no quantum theory of gravity, then our quantum theory of everything-but-gravity can't just be left as is; our understanding of something has to change, as I stated above. It could certainly be that quantum mechanics changes and GR is perfect. Most consider this unlikely but it isn't off the table. If quantum mechanics as we know it does have to change, it is likely that it does so in a way that changes GR also. I don't know of any proposals at all that modify quantum mechanics instead of GR. Perhaps some proposals do exist, or could exist, but none to my knowledge.
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Re: Gravitons... wild goose chase?

Postby mfb » Sun Dec 18, 2011 8:33 pm UTC

Argency wrote:Really, you could say the same of any other object - electrons, neutrinos, quarks, etc - but in most of those cases the interaction is extremely object-like most of the time, it's only with extremely advanced measurement techniques that the wave-particle duality of matter becomes really noticeable.

Put an arbitrary object on a scale. About 99% of its mass comes from quantum mechanics, ~1% from quark masses and ~0.03% from electrons.
You can create effective theories without quantum mechanics. They work well if you shoot with billard balls and look how they hit each other. But they will fail if you want to describe why the billard balls are solid, why they have mass and so on.


For a unification of the standard model (with quantum field theory) and relativity, neither can be used without problems. But quantum field theory is really nice theoretical approach to describe forces. You can just write down gravity in it (with a graviton with spin 2). And fail, because it is not well-defined.
Anyway, the common approach is to modify and extend QFT enough so you can include gravity.

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Re: Gravitons... wild goose chase?

Postby doogly » Sun Dec 18, 2011 8:36 pm UTC

And hey, the straightforward approach to quantizing gravity as a field theory might even work! Weinberg and friends have work suggesting it has asymptotic safety. It's still on the table.
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Re: Gravitons... wild goose chase?

Postby krogoth » Mon Dec 19, 2011 1:19 am UTC

A little off topic, but I think it sort of fits with this discussion- gravity as is a 'property of'/'force exerted' by any object with mass. Does this property\force use up energy from the object that emits this force?
Would this mean non radioactive atoms decay?
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Re: Gravitons... wild goose chase?

Postby doogly » Mon Dec 19, 2011 1:47 am UTC

The short answer is yes. But, static configurations don't lose energy. It is only when you have gravitational radiation that power is lost. See the Landau formula for fun details! So an atom would not decay gravitationally if it is just sitting around. If you have two objects orbiting each other, say, ooooooooh, a binary pulsar system, they will lose rotational energy and give off gravitational waves, and then even if we can't observe the gravity waves, we know they're happening because we can watch their periods decay. This has happened!
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Re: Gravitons... wild goose chase?

Postby Qaanol » Mon Dec 19, 2011 12:18 pm UTC

JWalker wrote:
doogly wrote:Why a graviton?
Because there should be a quantum theory of gravity.

Why should there be?

The short answer is, “Because gravity is created by and acts on matter, which is quantized.” However, that’s not the whole story, since although gravity is created by matter, it actually “acts on” the geometry of space-time, to which matter reacts.

doogly wrote:The short answer is yes. But, static configurations don't lose energy. It is only when you have gravitational radiation that power is lost. See the Landau formula for fun details! So an atom would not decay gravitationally if it is just sitting around. If you have two objects orbiting each other, say, ooooooooh, a binary pulsar system, they will lose rotational energy and give off gravitational waves, and then even if we can't observe the gravity waves, we know they're happening because we can watch their periods decay. This has happened!

Gravitational radiation happens in classical GR as well, just the same as an orbiting charge radiates in classical EM. In fact, it was the observation that electrons don’t continuously emit radiation that provided substantial motivation for QM.

There are definitely gravitational waves. The question is whether a gravitational wave, as it radiates spherically in all directions at speed c, will affect all particles as it bends space-time under them, or if it will only affect a finite number of particles by causing discrete changes to geometry of space-time in their neighborhoods.

Think about that. QM theory of light says, if a star burns for 10 billion years, all the light it emits during that time—all the electromagnetic radiation going off in all directions—amounts to some finite number of photons. A large number to be sure, but nonetheless finite. Each photon will either fly on forever, or else be absorbed by exactly one particle, most likely an electron. That is to say, the light exiting the star will affect only a finite number of particles. Those particles may then re-emit “scattered” photons of their own, which affect more particles, but those are new photons that did not originate with the star, and they represent waves propagating out from those new particles.

This is in stark contrast to the classical theory of electromagnetism, where the outgoing EM waves could “push around” an unlimited number of particles.

Now consider the gravitational picture. You have two stars orbiting each other for 10 billion years, radiating energy as gravity waves. Do those waves sweep over the whole of their future light-cone, affect all particles they come across by bending the geometry of space-time everywhere they go? Or do those waves only affect a finite number of particles? That’s really the distinction between classical GR and a theory of quantum gravity.

If gravity is quantized, then gravitational waves can exist in discrete amounts, and thus a particular chunk of gravitational radiation can only affect a finite number of particles.
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Re: Gravitons... wild goose chase?

Postby mfb » Tue Dec 20, 2011 2:57 pm UTC

Just playing around with numbers:

I think that finite number of particles can be really large. A quick search showed no result for the energy of gravitons, but I would expect that it is similar to elecromagnetic waves via E=hc/lambda. The earth/sun system radiates about ~200W gravitational waves with a wavelength of ~1 lightyear, and the energy per graviton would be 2*10^(-41)J. So the earth/sun-system emits about 10^39 gravitons per second (ignoring other planets).
The current orbital energy of the earth is about 3*10^33 J, enough to emit 10^74 gravitons. The total released energy of the orbital decay is much larger, but as the radius shrinks (and more potential energy is released) the energy per graviton goes up, so most gravitons are released at ~1AU.
So we are already near the number of particles in the observable universe with our small earth.
Another interesting number is the division of 10^39 gravitons per second by 10^52 leptons and baryons in the earth (sun is nearly at rest in the solar system). So if gravity would work like electromagnetism, each particle emits a graviton every ~10^13s = 300 000 years. However, I think that the interactions of gravitons with other gravitons is really important here to describe the system.

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Re: Gravitons... wild goose chase?

Postby SU3SU2U1 » Tue Dec 20, 2011 4:29 pm UTC

Remember, we can easily write down a theory that:
1. is quantum mechanical
2. goes to GR in the classical limit
3. goes to the standard model in the no gravity limit

this theory has gravitons (spin 2 bosons that couple to mass-energy). The only problem with it is that we can't use it to calculate to arbitrary accuracy- this could be a problem with our calculation techniques (we often use renormalization to cover the fact that perturbation theory is an imperfect tool), or it could be a fundamental problem with the theory (we don't yet know wether the theory is 'asymptotically safe').

But, our theory is 'close-enough' that we expect at minimum that it correctly approximates some correct theory.

And always remember- particle is a funny concept! Two different observers won't generally agree on what particles are hanging around (if you are falling in to a black hole, you won't see the Hawking radiation that a stationary observer watching the hole will see)

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Re: Gravitons... wild goose chase?

Postby eSOANEM » Tue Dec 20, 2011 5:40 pm UTC

mfb wrote: I would expect that it is similar to elecromagnetic waves via E=hc/lambda.


E=hf isn't just EM waves, it's also readily apparent from the time dependent schroedinger equation (although readily that implies more of an E=-ihf) p=h/lambda is also implied from the form of the momentum operator. Because of this, if gravity is quantised in any way even remotely resembling standard quantum mechanics, E=hf must hold (at least approximately) for gravitational waves.
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Re: Gravitons... wild goose chase?

Postby doogly » Tue Dec 20, 2011 6:39 pm UTC

You want to go to general relativity though. E doesn't quite mean what you want even in classical GR.
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Re: Gravitons... wild goose chase?

Postby Qwert » Tue Jan 03, 2012 8:00 am UTC

Per wiki:
Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector.[12] The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. It would be impossible to discriminate these events from the background of neutrinos, since the dimensions of the required neutrino shield would ensure collapse into a black hole.[12]


So we haven't detected actual individual gravitons(gravity) like we have photons(EM), W/Z Bosons(weak), and gluons(strong) simply because we can't. :)
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Re: Gravitons... wild goose chase?

Postby Thomas18 » Sun Feb 26, 2012 10:00 pm UTC

Diadem wrote:We haven't found the graviton yet. We don't even know what it would look like. We just decided that any particle that governs gravity is called a graviton, regardless of its properties. So logically then the graviton must exist, unless gravity is not governed by quantum mechanics at all. Which would be deeply troublesome for a great many reasons.


Also my point on this graviton matter :-)

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Re: Gravitons... wild goose chase?

Postby tomandlu » Thu Mar 01, 2012 7:54 am UTC

As a non-scientist, and therefore talking complete nonsense, I have to admit that I'm very sceptical about a lot of quantum physics. I often get the impression that layers of complexity, in the form of extra types of fundamental particles and interactions, are added to the model when perhaps a deeper truth is that things are probably simpler than that. The quantum world seems suspiciously obliging at showing us what we ask for.

As an analogy, it's as though we are sitting in a restaurant with no knowledge of food preparation and deciding, on the basis that whatever meal we ask for is served to us, that there are an infinite number of meals in the kitchen, rather than a finite number of ingredients. The meals are good and useful, and we have interesting and accurate things to say about them, but nevertheless a basic part of the process has escaped our attention...
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Re: Gravitons... wild goose chase?

Postby tooyoo » Thu Mar 01, 2012 12:57 pm UTC

tomandlu wrote:As a non-scientist, and therefore talking complete nonsense, I have to admit that I'm very sceptical about a lot of quantum physics. I often get the impression that layers of complexity, in the form of extra types of fundamental particles and interactions, are added to the model when perhaps a deeper truth is that things are probably simpler than that. [...]


It's true that when looking at science - especially modern physics - from the outside, things seem needlessly complicated. You should always remember however, that physics has been around for at least 300 years (if we date the beginning of theoretical physics with Newton, which kind of makes sense if you consider the importance of calculus). And during all that time, people have tried really hard to use simple methods. The only reason they came up with stuff like quantum mechanics and relativity is, that the simple things they tried didn't work. And they tried quite a lot.

Furthermore, that what humans consider as "sensible" or "simple", is usually owed to the fact that humans are well humans. That means that you're used to timescales between a few seconds and a few decades, length scales from mm to miles and so forth. If humans were the size of atoms and ruled by quantum effects, all those sensible & simple things that we are used to would appear highly strange. If humans lived a few seconds after the big bang (let's assume that there was a big bang), you'd never experience visible light since the early universe was opaque.

Now, coming full circle, you can actually measure and even see these "not-simple" and "not-sensible" things with the naked eye, if you make the right experiments. Take a look (google...) at Bose Einstein Condensates and superfluid helium.

So, one can actually write that among the greatest achievements in modern science (physics) is to teach us that the universe is not as we would expect, because we inhabit a very special place (and time) in it.

Sure, there might be something really easy out there, that people are missing, but literally all evidence points into the opposite direction.

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Re: Gravitons... wild goose chase?

Postby yurell » Thu Mar 01, 2012 1:15 pm UTC

Quantum mechanics is one of the most tested fields in physics precisely because many scientists throughout history felt as you, tomandlu — they were sceptical, and tried their best to prove it wrong, and the fact that it survived is testament to how incredibly well it works. This, of course, doesn't mean it's entirely correct (and at some points it may incomplete), but should it be wrong it can't be very wrong, since any new theory still has to predict the same results as QM in such situations that QM has been verified experimentally.
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Re: Gravitons... wild goose chase?

Postby mfb » Thu Mar 01, 2012 1:52 pm UTC

tomandlu wrote:As an analogy, it's as though we are sitting in a restaurant with no knowledge of food preparation and deciding, on the basis that whatever meal we ask for is served to us, that there are an infinite number of meals in the kitchen, rather than a finite number of ingredients. The meals are good and useful, and we have interesting and accurate things to say about them, but nevertheless a basic part of the process has escaped our attention...

That is a good description for the discovery of quarks. Particle accelerators produced more and more "particles" (mainly stuff we now call hadrons), and physicists wondered how many elementary particles they might find this way. And then, the quark model explained all these "particles" as product of a few quarks and anti-quarks (5 each, but that number is newer than the concept of quarks (there are 6, but the top does not hadronize before its decay)).

It is usually believed that there is a theory which is more fundamental than the Standard Model. However, "more fundamental" does not mean easier. It might be shorter when written down, but it has to contain the entire Standard Model (probably as limit for low energy) in it. It can explain at least all particles found so far - and maybe it has even more particles. It is possible that it can explain some of our current particles as composition of fewer more fundamental particles, but up to now there is no hint how.

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Re: Gravitons... wild goose chase?

Postby tomandlu » Thu Mar 01, 2012 2:07 pm UTC

yurell wrote: (and turoo and mfb)...


Thanks for the responses - I should add that I'm not for a moment suggesting that QP is 'wrong' (and rereading my post, it could be seen as saying that), more that there might be a simpler, fundamental truth underlying what's been found so far. To rephrase my restaurant analogy, an alien might look at a dictionary and not notice that a defined set of letters made up each word. They would then conclude that each word was a distinct, discrete object and marvel as to how we could ever learn each of them.

To phrase it more directly, there are 118 (?) elements, and even more isotopes, but we only need three particles to create all of them... so we've gone from the ancient four-elements of earth, air, fire and water, to then identifying the naturally occurring elements, to finally understanding that these elements were composed of the basic building blocks of protons, neutrons and electrons. In other words, the pattern was simplicity -> complexity -> simplicity. I just have a nagging suspicion that, where QP is concerned, we may still be on the 'complexity' stage...
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Re: Gravitons... wild goose chase?

Postby eSOANEM » Thu Mar 01, 2012 7:36 pm UTC

mfb wrote:(there are 6, but the top does not hadronize before its decay)


Huh? If the top doesn't form baryons before it decays, that would imply non-confinement of colour charge and that's a big deal. It seems to me that it's probably just that such baryons don't reach the detector before the top decays and the top-baryons become some other baryons.

tomandlu wrote:Thanks for the responses - I should add that I'm not for a moment suggesting that QP is 'wrong' (and rereading my post, it could be seen as saying that), more that there might be a simpler, fundamental truth underlying what's been found so far. To rephrase my restaurant analogy, an alien might look at a dictionary and not notice that a defined set of letters made up each word. They would then conclude that each word was a distinct, discrete object and marvel as to how we could ever learn each of them.


It's also worth noting that most people who can't read Chinese characters will not notice that, instead of each character being an indivisible whole, there are only something like half-a-dozen strokes.

Also, another example of how this sort of thing does happen (in astronomy this time):

Medieval astronomers liked to plot the orbits of planets. They started with circles around the earth, but that didn't work so well with Mars which went backwards for a bit, then they added loads and loads of "epicycles" (little loops around the circular orbit) to make them fit. Eventually it was realised that these epicycles disappeared if you centred your circles around the sun and put the earth on its own circle around the sun. Then Kepler showed that the circles were in fact ellipses and finally we got Newtonian gravitation coming out. (Timing of ellipse may not be correct wrt timing of heliocentrism).
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Re: Gravitons... wild goose chase?

Postby doogly » Thu Mar 01, 2012 8:12 pm UTC

If you put the orbits around the sun but still insisted on circles, you needed epicycles.
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Re: Gravitons... wild goose chase?

Postby andyisagod » Thu Mar 01, 2012 8:17 pm UTC

eSOANEM wrote:
mfb wrote:(there are 6, but the top does not hadronize before its decay)


Huh? If the top doesn't form baryons before it decays, that would imply non-confinement of colour charge and that's a big deal. It seems to me that it's probably just that such baryons don't reach the detector before the top decays and the top-baryons become some other baryons.



This doesn't imply non-confinement of colour charge, the top decay products carry the colour charge and they hadronize so the colour charge is confined. We detect most baryons and mesons through their resonances. There aren't any resonances that look like a confined top + some other quark.

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Re: Gravitons... wild goose chase?

Postby mfb » Thu Mar 01, 2012 9:41 pm UTC

eSOANEM wrote:
mfb wrote:(there are 6, but the top does not hadronize before its decay)


Huh? If the top doesn't form baryons before it decays, that would imply non-confinement of colour charge and that's a big deal. It seems to me that it's probably just that such baryons don't reach the detector before the top decays and the top-baryons become some other baryons.

Confinement is an effective low-energy thing (with the typical QCD scale of ~200MeV). The top quark has a decay width of ~2 GeV, which means it does not live long enough to take part in the hadronization. The bottom-quark as its decay product can hadronize then.
Another system without the usual confinement is the quark-gluon plasma in heavy-ion collisions.


There are few particles which reach the detector, mainly pions, kaons, electrons, muons, neutrinos, protons, neutrons and (in heavy ion collisions) other nuclei.

If a particle can decay only via the weak interaction (in addition to the particles mentioned above: Tauon, D0, D+, B0, B+, Bs, some baryons, all with their antiparticles) and is not a top-quark, they can live long enough to reconstruct their flight distance. It is even possible that they reach the innermost parts of detectors. But that is not the usual way they are studied: You combine 2 or more of the long-living particles mentioned above and reconstruct the invariant mass of a (possible) mother particle which decayed into these particles.

Most particles decay via the strong interaction and have a flight distance of nanometers or less. The only way to study them is via their decay products. And as andyisagod mentioned, no resonance with a top-quark has been found yet, and it is unlikely that there are any, due to the short top lifetime.

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Re: Gravitons... wild goose chase?

Postby eSOANEM » Thu Mar 01, 2012 10:30 pm UTC

doogly wrote:If you put the orbits around the sun but still insisted on circles, you needed epicycles.


Aren't the epicycles smaller and therefore harder to detect (and so more likely to have been discounted) though? Also, as I say, I'm not sure of the relative timings of heliocentricism and the realisation that orbits are elliptical.

mfb wrote:[
Confinement is an effective low-energy thing (with the typical QCD scale of ~200MeV). The top quark has a decay width of ~2 GeV, which means it does not live long enough to take part in the hadronization. The bottom-quark as its decay product can hadronize then.
Another system without the usual confinement is the quark-gluon plasma in heavy-ion collisions.


Hmmm... I guess that makes sense. I'd always wondered about how quark-gluon plasma worked with confinement and hadn't ever got a satisfactory answer so this also helps with that. As an aside, if at high energies, colour charge isn't confined (like other charges are at normal energies), would it not seem likely that at low energies, the other charges should be confined (and, as the strong force is the strongest and confined first, it seems to me that they should become confined in order of strength)? Obviously this couldn't work for gravity without negative gravitational mass though so I guess it's confinement energy would have to be 0.
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Re: Gravitons... wild goose chase?

Postby mfb » Fri Mar 02, 2012 4:46 pm UTC

Confinement is related to the self-interaction of gluons. This is not present in the electromagnetic interaction and too weak in the other interactions.

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Re: Gravitons... wild goose chase?

Postby eSOANEM » Fri Mar 02, 2012 5:38 pm UTC

I thought that gravity was highly self-interacting (with respect to its strength) though (I was under the impression that this the reason for the non-linearity of GR which was one of the big problems with quantising it) so should some similar process exist with gravity (or does the fact that there's no way for the mass-energy of the objects to sum to 0 overrule this)? The only gravitational phenomenon with even a passing resemblance to confinement that springs to mind are black holes in that they prevent some of the information about their constituents being observed (such as relative positions and momenta interally whereas with colour confinement its the individual particles' colour charges) but it also seems so passing that this is probably my brain seeing a pattern where none exists.
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Re: Gravitons... wild goose chase?

Postby starslayer » Fri Mar 02, 2012 7:11 pm UTC

eSOANEM wrote:Aren't the epicycles smaller and therefore harder to detect (and so more likely to have been discounted) though? Also, as I say, I'm not sure of the relative timings of heliocentricism and the realisation that orbits are elliptical.
No. The Copernican model itself required just as many, if not more, epicycles as the Ptolemaic model, which is one of the reasons it took so long for it to become generally accepted even among astronomers - there was no real reason to go for it over the geocentric model of the time. The data simply weren't accurate enough to tell the difference between the two. That changed with Brahe's observations, which were more accurate than any done before, and Kepler's analysis of those observations (specifically, Kepler published his analysis of the orbit of Mars in 1609). After Kepler, scientific objections to the heliocentric model largely began to disappear. By Newton's time, heliocentrism was the scientifically accepted view for the most part.

Copernicus himself died in 1543, and his work was published the same year, although details of it had been leaking out for years beforehand.

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Re: Gravitons... wild goose chase?

Postby eSOANEM » Fri Mar 02, 2012 10:07 pm UTC

Oh. Hadn't realised that, I'd always heard epicycles cited as an example of over complicating a bad model which went away with a better one. Thanks for putting me right.
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