New particle

[Student of Trinity]

CERN has just reported the discovery of a new particle, with a rest mass around 2 x 10^(-22) grams — that is, a rest energy mc^2 of about 125 giga-electron-volts. That's teeny, but if it's really a fundamental particle, it's one of the biggest known (second only to the top quark).

 

Most importantly, this particle may well be the long-sought Higgs boson. If it proves to have the Higgs boson's predicted properties, then that will confirm the thirty-year-old 'Standard Model' of particle physics that lays out the basic building blocks of matter, and of all force except gravity, as far as we understand them.

 

The Higgs is currently the last outstanding piece of the puzzle — the only component of the Model that has not yet been fully confirmed by experiment. From the success of the rest of the Model, however, it has already long been known that something must evidently be doing the job that the Model assigns to the Higgs particle. The only question has been whether the job gets done in the simplest way we've imagined, with a single Higgs boson, or whether this key role might be managed by Nature in some more complex manner — perhaps as part of a larger pattern we have not yet glimpsed.

 

If the plain, vanilla Higgs turns out to be it, then particle physics is over. There may be more particles out there, but we have too few clues as to what they may be like. Physicists won't get billions of dollars to build colossally larger accelerators just to search blindly and hope to get lucky. So a lot is riding on what the detailed properties of this newly discovered particle turn out to be.

[Cairo Jim]

That's quite interesting. I remember hearing something about that Higgs boson a few months back, but I can't remember any of the details.

 

Originally Posted By: Student of Trinity

If the plain, vanilla Higgs turns out to be it, then particle physics is over.

 

Well hopefully it won't mean the end. I mean, once you've found it, doesn't mean you can't find out what you can do with it.

[Randomizer]

No, we must get more funding to build even more powerful accelerators. We still didn't get quantum black holes generated.

[Student of Trinity]

Unfortunately there is nothing we can foreseeably do with the Higgs boson. The particle only exists for a very tiny fraction of a second before disintegrating into a swarm of more mundane particles. It takes a machine the size of a county, with a whole lot of superconducting magnets and a frighteningly large electricity bill, running for several months, just to create a handful of these elusive critters. And then almost as soon as they appear, they're gone. We need to be lucky to glimpse them before they go.

 

Nobody can say what technology might achieve in a couple of centuries, but it's a pretty safe bet that none of us will live to see any practical application of the Higgs boson.

[Cairo Jim]

I probably should of worded that better. I wasn't necessarily meaning how to use the Higgs boson directly. The more we find out about all these different little buggers and how they act, I reckon we'd be able to one day improve larger things like semi conductors and who knows what else.

[Dikiyoba]

Originally Posted By: Cairo Jim

I reckon we'd be able to one day improve larger things like semi conductors and who knows what else.

But throwing the billions of dollars currently going into particle physics into any other sub-field of physics would result in larger improvements sooner, is what Student of Trinity seems to be saying.

Dikiyoba.

[Tridon]

The greatest achievement in science so far in my lifetime. I watched the webcast directly today, and it was surely something to remember=)

 

This kind of intensity of applause is not something you see very often in a physics presentation And of course it was memorable to see a moved Higgs.

[Kelandon]

They're hopeful, I should add, because while it looks like this Higgs boson is pretty close to the simplest possible Standard Model Higgs, it's not quite, and those "not quite" aspects might lead to some good new advances.

 

Of course, they don't really know much for sure yet, other than that they saw something, and they hope to have enough data by the end of the year to be able to say something more when it's all analyzed.

[keira]

As an American, I demand it be renamed the Freedom Particle.

[VCH]

SoT (or anyone really) could you explain the importance of this discovery in away that those of us with minimal physics knowledge might understand; newspapers are doing a poor job of that.

 

Thanks

[JamesMighty]

But, what if this Higgs boson isn't the missing peice to the 'Standard Model' of life and all that. What happens then?

[adc.]

A new particle? Unsurprisingly, I didn't understand a thing. WOO! Is it magical? What does it do? I'd rather think of better particles like Science Magic. Anyway, I'll keep this in mind...

--------

-Nightwatcher

[Tridon]

Originally Posted By: VCH

SoT (or anyone really) could you explain the importance of this discovery in away that those of us with minimal physics knowledge might understand; newspapers are doing a poor job of that.

I have minimal knowledge of physics as well, but I'll give it a shot by putting together what I've read these last days:
In the extremely early stages of the big bang (some millionth of a second after it started) everything was pure energy. However, some of that energy started "condensing" into something that later became mass. Even though we have a theory of this happening, we have no explanation why this happened, or how it could happen.

Scientist have a hope that a deep understanding of this particle (if this proves to actually be the particle they hope it is) will help to understand how this energy could condense. In other words, help us one step further along the way to understand how atoms and the things inside the universe came into being.

[Dintiradan]

Originally Posted By: VCH

SoT (or anyone really) could you explain the importance of this discovery in away that those of us with minimal physics knowledge might understand; newspapers are doing a poor job of that.

Thanks

I found this animated interview to be a useful explanation that doesn't require too much physics knowledge, but as someone in the target audience of people who don't have too much physics knowledge, I can't say whether it's accurate or not.

EDIT: Should be noted that this is older, from before the discovery of the last few days.

[Goldengirl]

There's beauty in brevity. This explanation has satisfied my lack of comprehension, and even won an award for being the best explanation for us laypersons.

[Student of Trinity]

Actually that PhDcomics animation is pretty good.

 

The Higgs boson is important because it's a direct manifestation of the Higgs field, which plays a critical role in the Higgs mechanism. The Higgs mechanism is a sort of cunning work-around that enables particles to have mass. Without it, as far as we can tell, every kind of particle would have to be like a photon, massless and hence forever moving at the speed of light. Matter as we know it, let alone life as we know it, would be quite impossible. So, according to theory, the Higgs field is a sort of glue that makes everything slow down, and possibly stick together into the kind of matter we know. The Higgs boson is the smoking gun that says the Higgs field is really there, and not just a theoretical figment made up to excuse matter for existing.

 

The reason we need a cunning work-around to explain why things have mass is not obvious, but it seems to be a huge and solid issue. The only way we have figured out to put quantum mechanics and relativity together is to have forces that follow a certain very strict principle called gauge symmetry, which is a mathematical rule that has the effect of rigging the game of physics, but good: the fix is in right from the beginning, and there's just no chance that anything can go wrong. Nothing less seems to give workable results. An unfortunate side effect of this very effective fix, though, is that it forces all particles to be massless — apart from the one loophole offered by the Higgs mechanism.

 

So the Standard Model of particle physics is a somewhat funny combination of a brutally effective 'fix' for the apparent contradictions of forces subject to both quantum mechanics and relativity, plus a sneaky loophole exploit to let particles have mass despite the fix. If this gives the impression that it's all a bit of a kludge, rigged up ad-hoc in response to problems and paradoxes, then in my opinion that's an accurate impression. The Standard Model is a pretty crudely written document to serve as the constitution for reality itself.

 

Most physicists assume, however, that Nature is not really as awkward as the Standard Model makes it seem. The Standard Model is a human product, which developed in stages, as a series of solutions to the problems created by previous solutions. The individual parts of the scheme, now that they are all in place, each work very well, even elegantly. But the structure as a whole has a seeming arbitrariness that betrays its historical origin. We all suppose that something deeper and much more elegant lies behind our Standard Model, and explains why all the things that now seem to be there as improvisations are really all profound motifs of a grand symphony whose score we have not yet glimpsed.

 

If the particle just discovered turns out to be just exactly the Higgs boson of the Standard Model, then our chances for discovering that grand symphony will actually have gone down a lot. We may be stuck with our awkward kludge of a system, and never learn anything more than that — unless somebody finds a way to discover it other than by discovering more particles. Maybe some radical new theory that re-imagines a lot of other parts of physics, and happens to revise particle physics as a mere by-product. Otherwise, we'd better hope that the new particle turns out not to work quite as advertised, and instead provides clues to the deeper pattern.

[Randomizer]

The result of the discovery.

[Callie]

"The substructure of the universe regresses infinitely towards smaller and smaller components. Behind atoms we find electrons, and behind electrons, quarks. Each layer unraveled reveals new secrets, but also new mysteries."

[Dintiradan]

Originally Posted By: Excalibur

"The substructure of the universe regresses infinitely towards smaller and smaller components. Behind atoms we find electrons, and behind electrons, quarks. Each layer unraveled reveals new secrets, but also new mysteries."

Personally, I prefer this Zakharov quote, but mostly because I find the voice acting amusing.

[Prince of Kitties]

This is a Big Deal IMO. Physics has been operating way out in the theoretical boondocks for a while, and I think it's nice to finally have some results, as opposed to more talk of superstrings and eleven-dimensional space and other as-yet-unprovables.

 

Of course, I'm not a physicist, and know very little of string theories; for all I know we could be on the verge of a working unified theory, etc. But the public will respond better to hearing about a new particle than hearing about a new twist on some theory... And more to the point, I think, physics students will be more excited by the possibility of discovery than the possibility of endless theorizing.

 

The above being all IMHO of course... Suffice to say I'm very glad to hear about this.

[*i]

Theory is and will always be very important because it informs the experimentalists what to look for and where they should conduct measurements. Of course, theorists are nothing without experiments, because experiments are the way to differentiate which theories describe reality and those that are merely interesting sets of equations.

 

Even if what we find out the vanilla result (see SoT's post above) I suspect it still will not be the end of particle physics as a discipline. There are still nuclear physicists busy today even though we have a pretty good fundamental understanding of how the nucleus works. There's a lot of important little details yet unresolved. I suspect there will be a shift from building bigger and bigger machines and finding new particles to understanding the details about the ones we already know.

 

This is probably a good thing, because if we spend more efforts looking at the interactions of more "mundane" particles like pions, kaons, etc. we can better understand the showers from cosmic rays, which impact the functionality of electronic components in airplanes or satellites. This is particular interest to people I work with, since part of our project is to write software to simulate all those interactions with all sorts of physical models, and we can't really match measured data yet.

 

So yeah, I think there's plenty of work left to be done in particle physics.

[Drayk Armitage]

SoT, thanks for that last post; that was a very helpful elaboration.

[Student of Trinity]

You're welcome. I studied this stuff in grad school years ago, but have since moved into other fields of physics, so it's been interesting to me to look back at this field with the jaded eye of a more experienced physicist.

 

Particle physics probably won't utterly die out in any case, but if the Higgs is the end of the road, then it will shrink dramatically. When the current generation of particle physicists retires, departments will decide to replace them (if they are able to replace them at all) with condensed matter physicists, or atomic or optical physicists. ('Quantum optics' is the rather awkward name for the study of the quantum properties of light; it has nothing to do with eyes.) Already, in fact, particle physics has long lost the cachet it once had as the peak of the physics mountain. The experiments just slowed down too much, with decades going by between major discoveries, and teams of hundreds of authors on each paper; while the theoretical side of the game shot off into the stratosphere, and pretty much into oblivion, with multi-dimensional superstring theories that have never even made any definite predictions at all, after decades of work, let alone been supported by any evidence.

 

There was a while where it seemed that there might be a smaller particle inside every particle we knew, ad infinitum; but that was a long time ago, now. We're pretty sure there is nothing inside electrons. They are just electrons. There are quarks inside protons, but there really doesn't seem to be anything inside quarks. The Standard Model pins down quite a small set of basic particles, and there has been no evidence of anything beyond them. They may really be all that there are.

 

One example of how awkward the Standard Model seems, though, is that all of the particles we normally think of as composing matter actually come in three different versions. It's a bit like Microsoft products. The electron is really only Electron Home Basic. There's an Electron Professional Edition, otherwise known as a muon; and an Electron Ultimate, which is really called a tauon. The muon and the tauon are exactly like the electron in every respect, except that they have much larger masses. There are also two heavier clones each of the electron neutrino, and of the "up" and "down" quarks that make up protons and neutrons.

 

The heavier versions are all unstable, since they can break up spontaneously into several of their lighter siblings. As far as we can tell they serve no purpose; they are kind of the appendix of the universe. They are not needed to make up ordinary matter as we know it. As far as we can tell, there are exactly three sets of the matter particles, not four or more. Nobody knows why these extra, heavier clones are there. The Standard Model doesn't say; it would apparently work fine without them, but they do turn up in accelerators, so the Model includes them.

[Drayk Armitage]

Originally Posted By: Student of Trinity (my bold)

There was a while where it seemed that there might be a smaller particle inside every particle we knew, ad infinitum; but that was a long time ago, now. We're pretty sure there is nothing inside electrons. They are just electrons. There are quarks inside protons, but there really doesn't seem to be anything inside quarks. The Standard Model pins down quite a small set of basic particles, and there has been no evidence of anything beyond them.

I certainly buy that there's no evidence of further component particles, and that there's no reason to assume they are there. But if you look at the history of our understanding of matter, isn't it the case that we repeatedly thought "okay, this is it, it doesn't break down any further" only to discover later that it really did? Is this situation different? If so, what makes it different?

I ask this because I am curious, though I will admit my heart pulls me to Anaxagoras.

[Randomizer]

We are at the limit for detection of smaller particles. Without a theoretical idea of what to look for interest will shift to the next big thing. New researchers will go off to other sub fields of physics.

 

If you look back at publications when there is a triggering event, you see that a novel discovery generates a surge in research and papers. About 25 years ago when IBM found the first high temperature super conductor, there was a surge of 300 papers within a year as new variations were discovered, the temperature was pushed higher, and theory was advanced for why they worked. Same with the discovery of a way to make carbon fullerenes and nanotubes a few years later.

[Student of Trinity]

Well, I guess it's just that there's no reason to think anything is inside the electron, and we've looked pretty hard. We've smashed electrons and positrons together hard enough to shake all kinds of stuff into existence out of the vacuum, but found no evidence that the electrons and positrons went into the collisions as composite objects.

 

It's actually a much more subtle issue than it seems, because in one way an electron is just a mathematical point, but in another way it is something much more complex. The issue is that electrons have both energy and electrical charge, but the astonishing thing is that these two properties are quantum mechanically inconsistent with each other. So there are quantum states with a definite electric charge concentrated entirely on one definite point; but these quantum states do not have a definite amount of energy, but rather a Schrödinger's Cat-style quantum superposition of many different amounts of energy. Conversely there are quantum states with definite amounts of energy (and momentum), which in mechanical terms fit the behavior one expects from a particle; but these states do not have a definite electric charge distribution. Instead they are quantum superpositions of lots of different charge distributions, always with a net charge equal to what we know as the electron charge, but possibly including many balancing pairs of positive and negative charges. Indeed, the vacuum state, where there is no energy at all, is a superposition of lots of different charge states. The vacuum itself is a sort of dielectric, or even a semiconductor.

 

It's not clear what one should really mean by 'an electron'. Is it the point charge, that lacks a definite energy? Or is it the particle with mass and momentum and energy, but whose electric charge is a quantum cloud of many point charges, positive as well as negative? Will the real electron please stand up?

 

In a sense, whichever choice you make as to what counts as 'the electron', 'the electron' is then made up of many electrons and positrons (in the other sense). Plus, actually, a lot of photons. And a few muons and quarks and everything, in fact.

 

The issue of whether the electron contains sub-components is a further issue beyond all of the above. It would be about whether, in every possible alternative branch of the quantum superposition of states, each of those point charges turned out to be not just one point, but a tiny cluster of separate points.

 

Fortunately for most everyday applications of electrons, all those pairs of opposite point charges within the vacuum are quite tightly correlated. The two charges in a pair are essentially never further than about 10^(-12) meters apart from each other. So unless you have picometer resolution, it all averages out, and the vacuum just looks empty of charge, and an electron just looks like a single charge. So for most purposes you can just ignore all this complexity. But when you look really closely at things, you discover all this unexpected complexity.

 

Particle physicists have looked hard, though, and gotten down to resolutions of maybe 10^(-25) meters or less, without seeing any sign that electrons are actually composite. So it could be, but there's no reason to guess that it's so.

[JamesMighty]

Originally Posted By: Student of Trinity

If the particle just discovered turns out to be just exactly the Higgs boson of the Standard Model, then our chances for discovering that grand symphony will actually have gone down a lot. We may be stuck with our awkward kludge of a system, and never learn anything more than that — unless somebody finds a way to discover it other than by discovering more particles. Maybe some radical new theory that re-imagines a lot of other parts of physics, and happens to revise particle physics as a mere by-product. Otherwise, we'd better hope that the new particle turns out not to work quite as advertised, and instead provides clues to the deeper pattern.

So, we're hoping it's NOT the correct particle, or did I read that wrong?

[Student of Trinity]

Well, we're kind of hoping to learn that it was indeed Colonel Mustard in the Conservatory, but not with the lead pipe. We want to be wrong about something, but not so wrong that the conclusion is that this is just not the boson we were looking for.

[JamesMighty]

Oh, so you have more reason to keep searching, correct?

[Necris Omega]

Hm... even if this particle isn't the Holy Grail of particle physics, it's still a new particle no? Wouldn't that in and of itself carry weight and implication and affect the current theoretical models?

[JamesMighty]

perhaps, but we do know the Higgs Bosson could only effect one model, the Standard Model of Life.

[Student of Trinity]

Originally Posted By: Necris Omega

Hm... even if this particle isn't the Holy Grail of particle physics, it's still a new particle no? Wouldn't that in and of itself carry weight and implication and affect the current theoretical models?

Yes, unless it turns out to be some combination of previously known particles. But I guess that must actually be pretty much ruled out at this point. The main thing is that it doesn't just go away, having turned out to be a mistake. They're pretty confident in their statistics, but it's worth bearing in mind that what they're actually seeing is a tiny little bump on a big, broad curve. Some kind of re-calculation could still conceivably make everybody just say, Woops.

[The Mystic]

I, for one, am lost.

*rereads posts, clicks links to explanations, etc.*

Still lost. I think it might be better for me that way, so don't bother to re-re-re-re-explain it for my sake.

 

Cool, though.

[keira]

There was another kind of big deal thing too, I guess.

[Prince of Kitties]

The Boston Herald is basically a tabloid. This looks legit though.

 

I wonder exactly what kind of dark matter it is. If it's a cosmic string, that might give more credence to the big bang/cosmic inflation theory, no?

 

(Edit: this being in response to Sylae's post above.)

[keira]

It was linked to on /. so I am assuming this has been reported elsewhere too.

[Student of Trinity]

Here's the publicly accessible summary page from Nature, which is a bit more authoritative than the Herald. It's pretty much the world's top scientific journal at the moment, with a modest lead in impact factor over the AAAS flagship journal Science. (A few other journals do have higher impact factors still, but they are specialized medical journals where everybody apparently rabidly cites each other.)

 

This one is potentially significant but much less decisive than the Higgs. It's not new evidence for the existence of dark matter. That has already been pinned down pretty well. It's not a new kind of dark matter — not, for instance, a so-called cosmic string. (At least, the observation doesn't seem to identify it as such. Even if all that mass were concentrated in a super-thin topological defect line, the observations wouldn't have enough resolution to tell.)

 

It's been several years since I last heard a talk on this, but if I put together what I remember from my days in a theoretical astrophysics group with what I read in this summary from Nature, it seems to be this. Nobody has seen dark matter directly (which is why it's called dark), but we can track its gravitational effects on stars (bright matter), and we can try to model its motion with massive computer simulations.

 

When we tune the models to produce mass distributions compatible with what we see in reality, we usually see the dark matter collecting into big, flattish pancakes. (That's nothing special about dark matter, really; the competition between gravity and angular momentum makes most gravitating matter form into some kind of disk shape.)

 

I'm not sure whether pancake distributions of dark matter have actually been confirmed with observations, but the point of this recent observation seems to be that we would occasionally expect two pancakes to cross each other at an angle, making an extra-high concentration of dark matter where they cross. That's a dark matter filament. Then the theory has been that where two dark matter filaments cross, we have a little blob of even greater concentration of dark matter.

 

These 'little blobs' are actually really enormous, and their 'extra density' is really very slight. Their total mass adds up to an awful lot, but only because they're so vast. What they're expected to do is explain why galaxies seem to cluster together in the way they're observed to cluster. A dark matter filament is just a faint hazy strip of invisible foggy something, that may be much thicker across than a galaxy; it's not something a starship would have to dodge.

 

Anyway, something consistent with that picture has now been seen. Yay!

[Mod.]

Was that whole "new equation" that explains dark matter effects without there being dark matter ruled out then?

[Student of Trinity]

Haven't heard about that, actually. Could be there was something and I missed it, but probably you just heard some crackpot.

[Kelandon]

There have been several attempts to explain away dark matter by, among other things, modifying gravity not to need dark matter. Some have been more promising and some have been less. As we get more data, it's getting harder and harder to design a new theory of gravity that dispenses with the need for dark matter.

 

I last paid attention to this issue about three or four years ago, but where we stood at that point was as follows. Just as the Moon goes around the Earth and the Earth goes around the Sun, so too the Sun goes around the galactic center (a "supermassive" black hole). All the other stars in the galaxy do this, too, so the galaxy rotates. You can calculate fairly easily with Newtonian mechanics (specifically, equations of gravity) what this rotation should look like; with a little more trouble, you can calculate with general relativity (the most advanced theory of gravity we have at present) what this rotation should look like. Then, when you look up at the sky, it turns out that the rotation doesn't look like that at all. And basically it looks as though there's a whole lot of mass up in the sky exerting gravity that you can't see. Since you can't see it (it doesn't emit light, unlike regular stars, etc.), we call it "dark." Hence, dark matter.

 

Now, at first people just thought that there was something else going on. Maybe general relativity was wrong. Maybe there are more neutrinos (which don't interact electromagnetically, so they don't emit light) than we thought. Maybe we could fix this somehow. But gradually the evidence began to pile up.

 

One of the more interesting things that we had just recently found was two galaxies that were colliding (or galaxy clusters — I forget), such that their dark matter was overlapping. I think maybe the light-emitting matter hadn't collided yet, only the dark matter halos (dark matter around the outside of the galaxies), and yet we could see, because of the gravitational effects, that the dark matter was doing exactly what you would expect dark matter to do: it was pulling on everything nearby gravitationally. It just didn't look like screwy gravity; it looked like more mass. It was really hard to explain this one away without dark matter.

 

As I understand it, this particular find is that there are tendrils of dark matter connecting galaxies, even ones that aren't colliding but are just near each other. I'm not sure, but I think this is really hard to explain away with a new form of gravity.

[Student of Trinity]

The other point is just that our current understanding of gravity really makes excellent sense. It's a very simple picture, such that if it somehow weren't true, you'd really have to scratch your head about why not. If general relativity is wrong, then Nature has passed up an obvious opportunity.

 

And, on the other hand, dark matter also just makes sense. There's no obvious reason at all why all matter has to emit light. A priori one would just expect that a lot of matter wouldn't.

 

So the theory of dark matter isn't in any sense a fudge. It's only saying, Well, it looks as though the obvious default scenario is, in fact, the case. So modifying gravity, as an alternative to dark matter, seems like really bending over backwards and going out of your way.

[Mod.]

Originally Posted By: Student of Trinity

Haven't heard about that, actually. Could be there was something and I missed it, but probably you just heard some crackpot.

It was actually a big thing back then if I remember. Found it on wikipedia:
http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

Quote:

Consequently, the velocity of stars on a circular orbit far from the center is a constant, and does not depend on the distance r : the rotation curve is flat.

The proportion between the "flat" rotation velocity to the observed mass derived here is matching the observed relation between "flat" velocity to luminosity known as the Tully-Fisher relation.

At the same time, there is a clear relationship between the velocity and the constant a0. The equation v=(GMa0)1/4 allows one to calculate a0 from the observed v and M. Milgrom found a0=1.2×10−10 ms−2. As expected, this quantity is far smaller than any acceleration typically found in solar system-scale interactions.

To explain the meaning of this constant, Milgrom said : "... It is roughly the acceleration that will take an object from rest to the speed of light in the lifetime of the universe. It is also of the order of the recently discovered acceleration of the universe."

Even if it was proved wrong, its still good to keep an open mind to other possibilities. With all the work put into dark matter research, we really hope we aren't wrong though.

[Kelandon]

Here we go: http://en.wikipedia.org/wiki/Bullet_cluster

 

That was the colliding galaxy cluster that I vaguely remembered. Wikpedia probably does a better job than I did of explaining why it makes a modified gravity theory really hard to sustain.

[Erebus the Black]

Originally Posted By: Student of Trinity

There's no obvious reason at all why all matter has to emit light. A priori one would just expect that a lot of matter wouldn't.

Thing is it doesn't seem to reflect light (at least, not enough for us to be able to see it down here) either which is why some can't seem to be able to accept the term.

If I may take a step back here for a moment, it's been over 2 years now since I was in the academia and something has struck me as quite odd. When is a quantum experiment considered to have been measured or seen? (I have a follow up question depending on your answers)

[Student of Trinity]

I wish I knew the real answer to that one myself. So far the practical answer is, Once the result has been recorded irreversibly. The good thing about that answer is that it does work in practice, in that it has always been pretty clear so far which processes are irreversible in practice, and which are not. The bad thing about the answer is that we don't really know exactly what makes a process irreversible. So connecting quantum measurement with thermodynamic irreversibility is a bit like luring two monsters together in the hope that they will keep each other busy.

 

My own suspicion is that measurement is simply a particular class of events, which include irreversibility by definition. As soon as we start building small enough devices that it becomes unclear whether what they are doing is irreversible, we'll begin realizing that it's also unclear whether what they are doing is really measuring. So we'll end up concluding that measuring is like cooking. If it's cooked it's cooked, and can't be uncooked; and if it's raw, it's raw. But things can be half-cooked, or warmed up a bit and then cooled down again, and so on. So cooking is not necessarily always a yes-or-no issue, though often it is. Measurement is probably similar.

 

In a sense I may be a biased witness on this question, though. Two of my students and I have a paper under consideration by Nature at the moment, which has to do exactly with quantum mechanics and irreversibility. The referees have been considering the thing for nearly three weeks now, and I'm becoming a nervous wreck, thinking every e-mail I get is going to be the acceptance or the rejection. It would mean a lot to me to get it accepted, and I really think our paper is pretty cool; but it's a high bar to get over. Most scientists never get anything published in Nature in their entire careers, and purely theoretical papers, like ours, are particularly rarely accepted.

[Erebus the Black]

Then the Schrodinger cat experiment is a bad example?

If you're dead you're dead not much you can do about it once the brain matter dies, it's irreversible.

 

Also what does recorded irreversibly means? When does nature record anything?

[*i]

SoT -- Sounds like quite an accomplishment if you get it accepted. I know what you mean about the stress of waiting for acceptance/rejection from a journal. Best of luck!

 

Re: Measurement -- This has always been a sticking point since the beginning of quantum mechanics. Before something is measured (whatever that means) an object exists in a weird symphony of all possible states. The act of measurement causes this symphony to collapse into one note, and which note it collapse to is related to the amplitude of its wavefunction (or more precisely the wavefunction times its complex conjugate).

 

Now exactly what constitutes measurement is a difficult question, whose answer has more often lied more in the realm of philosophy than physical science, often requiring a living, intelligent observer to make an observation. The interpretation that I personally find most plausible is that interactions with other quantum states cause the wavefunction to collapse and therefore constitute a "measurement". Now, the question is whether a measurement is a binary event or more fuzzy I believe is still an open question. Is one interaction between two quantum states enough to cause a complete collapse of the wavefunction, or does it require a large number of them such that there exists some number of interactions that the quantum state intermediately "measured" and is only partially collapsed and resolved? In other words, macroscopic objects may be able to measure themselves, but what does it mean when we have a small collection of a few hundred atoms?

 

In terms of irreversiblity, the only way to know the entropy of a system has increased (or decreased -- yes, this is possible since the second law only speaks to averages) is to measure it. Prior to measurement, the state exists as a probability cloud where it is possible the entropy has increased, it may have decreased, or it may be the same. For processes involving macroscopic objects, the pull of statistical averages is so strong that upon measurement (by an outside observer or by itself) the system has almost always gained entropy. In terms of the small collection of atoms, things are a lot less certain as to what is exactly going on.

[Erebus the Black]

Has there been any further advancement on the confirmation (or otherwise) of the pseudo-new particle as the Higgs?