The secret lives of long-lived particles | Hacker News Reader

The secret lives of long-lived particles

symmetrymagazine.org

58 pointsrch55 comments

I'm really fascinated by the idea that physics might be completed in my lifetime. I know it's a naive point of view - people thought the same around 1900, just before quantum mechanics and special relativity were discovered.

I also realise that CERN hasn't made the breakthrough researchers hoped for.

But what if the idea of long-lived particles, or another theory, comes good? It reconciles the standard model with relativity and explains dark matter, and then... it's all over. The rest is tidying up details.

Wouldn't that be the strangest time to be alive? I think some people would be disappointed, but I'd be thrilled.

>I'm really fascinated by the idea that physics might be completed in my lifetime.

? Unlikely, given the current situation where there isn't much real progress toward unification of QM and gravity..

I think it depends on how 'chaotic' new discoveries are.

For example, discovering relativity helped us to explain the world as it already was; e.g. it explained the precession of Mercury's orbit, but Mercury's orbit was already precessing.

On the other hand, since we discovered quantum mechanics, it's affected many aspects of the world. Many quantum effects only cause small deviations in measurements, similar to those of relativity, and naturally "average out" on the macro scale. Yet once we noticed them, we were able to amplify these effects, and now quantum devices like semiconductors are all over the place; even something mundane like the postal service now involves computers, and hence to be explained fully requires quantum mechanics.

I can imagine a future where all of our discoveries are of the relativity kind; i.e. making more precise predictions of some measurements, and having little effect outside of those measurements (although relativity has certainly changed our perspective and interpretation of the universe!)

I can also imagine a future where we discover some minor correction to a measurement, say a slight shift in the spectral lines of novae, which is explained via an effect we can harness and which will affect all manner of things in the way quantum physics has.

For example, maybe the discrepancy is explained as a weak gravitational repulsion; or due to matter making short jumps through time; or some other phenomenon which affects natural systems in subtle ways, but which we can amplify artificially to have far reaching effects.

http://www.nature.com/nature/journal/v436/n7047/full/436150a...

Nature publishes short sci-fi stories in each issue. This one is kind of amazing.

Well, it's a little amusing I suppose, but I don't see what makes it 'kind of amazing'.

I never understood claims like "if there's no free will, why do anything?"; it doesn't matter whether something's deterministic or not, only whether it's predictable or not.

The only non-BS form of "free will" I've come across is Wolfram's approach of computational irreducibility, where "I" am the computational process which calculates my behaviour. From that perspective, even if someone can predict my behaviour and choices with 100% accuracy, it's still "me" making those choices, since their model of me is just another instance of me.

Everything else is just geocentric fairytales.

If there's no free will, why do anything? Because I can't not do it, because I have no free will...

I like the hook of 1 second negative-time circuits being mass produced. So much so you can put them on keychains, buy them at a dollar shop, whatever.

That's the thing you really need to imagine: a world filled with gadgets more common then iPhones, that do a very trivial bit of future prediction that massively undermines our entire perception of reality.

There's a similar idea in Greg Egan's recent Orthogonal trilogy [1]. It's a fascinating series that explores a universe with different law of physics from our own, with the physical implications worked out in considerable detail.

One of the more intriguing (although hand-wavy) ideas is that in this universe, it's fairly straightforward to construct a device to send information backwards in time -- but if you actually do so, social and technological progress will immediately grind to a halt, on purely thermodynamic grounds. The idea is that any information that you send back seems to "appear from nowhere" as part of a stable causal loop, and so the entropically-favored outcomes are the ones where as much of the information as possible is uninteresting.

[1] http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html

The really amazing thing is that the "Predictor" device actually exists [1], and has since 1963 when it was invented by neurosurgeon W. Grey Walter. Here's a description of it from Dan Dennett's Book Consciousness Explained:

  Grey Walter performed his experiment with patients in whose motor cortex he
  had implanted electrodes. He wanted to test the hypothesis that certain
  bursts of recorded activity were the initiators of intentional actions. So
  he arranged for each patient to look at slides from a carousel projector.
  The patient could advance the carousel at will, by pressing the button on
  the controller. ... Unbeknownst to the patient, however, the controller
  button was a dummy, not attached to the slide projector at all! What
  actually advanced the slides was the amplified signal from the electrode
  implanted in the patient's motor cortex. ... One might suppose that the
  patients would notice nothing out of the ordinary, but in fact they were
  startled by the effect, because it seemed to them as if the slide projector
  was anticipating their decisions. They reported that just as they were
  "about to" push the button, but before they had actually decided to do so,
  the projector would advance the slide - and they would find themselves
  pressing the button with the worry that it was going to advance the slide
  twice!

Now that you know it's real, will you behave any differently?

  1. Not the negative time delay mechanism, though.

Reminds me of a short-story, "All the Myriad Ways" by Larry Niven, although that was almost the opposite, because it dealt with a different kind of "pointlessness".

Specifically, about how choices were robbed of meaning when -- in some universe or another -- you made every one of them, as opposed to losing meaning because choices were impossible or predetermined.

Whenever scifi invokes the "infinite number of parallel worlds" trope I become much more cynical about all of the subsequent events.

"Will he reach the airlock in time?" well there's an infinite number of him that do, and an infinite number which don't. And an infinite number where he's a banana. The only question is which outcome the writer has decided to show us.

we might also find out that there is no real way to travel faster than light, bounding us to colonizing few nearby star systems at best (which might be utterly inhabitable without massive terraforming), a rather depressing thought.

> we might also find out that there is no real way to travel faster than light

We don't need to find that out, we already know that

Well, there certainly seems to be no way for anything to move faster than light through spacetime. It's harder to rule out exotic matter or negative energy that could warp spacetime to (relatively) move an object faster than light.

Of course if computing power proceeds to its logical bounds, we'll have more then enough capacity to sustain thousands of personalities in substrate the same size as the human head. And once we're all immortal infomorphs, "time" - and thus distance, may end up having very different meanings to us.

EDIT: Basically just go read Accelerando for a pretty good take on this whole idea (it's free online).

Also read Permutation City, Diaspora, and Schild's Ladder by Greg Egan. If you only read one, read Schild's Ladder.

> I'm really fascinated by the idea that physics might be completed in my lifetime.

That's an unfortunate thought to be fascinated by. You should study some physics, instead of reading popular articles about it, or even what pre-eminent physicists say about their work.

I can simply illustrate the situation by saying if a question Q eventually provides answer A, the a new question Q' arises: why A?

You can tell yourself "it doesn't matter" but quite frankly, at the human scale, physics is already a done deal.

Which only further illustrates why your thought is a bit naïve.

I certainly could read more, but I do have a degree in physics and philosophy.

Certainly there will always be questions like: why something rather than nothing?

However, if you have a (relatively) simple set of rules, like the standard model, which work in every context and (in principle) predicts all observable phenomena, it does seem like you've crossed some kind of threshold. An intellectual threshold - as you say it has limited practical implications. It would be a watershed like observing alien bacteria. You're not going to do much with it, but it seems to put you in touch with the cosmic.

There might still be lots of emergent phenomena we don't fully understand, but we could still be confident that the underlying physics is totally understood. An example of this might be the Navier–Stokes equation - we don't fully understand it, but I don't think many people think that a complete understanding of why nature obeys this law will lead to any fundamentally new physics.

It's certainly possible it's just questions all the way down, perhaps smaller and smaller structures will be observable at higher energies, but it's an open question.

Another option is that we're close to bottoming out all of the complexity - and that's the possibility I find fascinating.

I'm not envisioning a time when every possible question has been answered, instead I'm thinking of a time when any questions about fundamental physics will either have been answered or we can be satisfied that no empirical evidence will answer them.

QM may already have hit this knowledge barrier. We know from Bell inequities there are no hidden variables, there is simply no empirical way to predict with certainty the state of a quantum system. We've hit the edge of the knowable.

"Whereof one cannot speak, thereof one must be silent" - as Wittgenstein might have said about the situation.

In defence of throwaway000002, when I talk with physicists about the idea that physics might be completed they mostly don't think it's likely. I don't think they subscribe to the questions all the way down model, instead they think we have a long way to go yet - though I think most researchers don't have clearly formed views on the subject.

I guess what I should have realized, and is my mistake, is that the idea of physics being completed is actually a philosophical issue, and given different philosophical bases you can arrive at different conclusions.

Ultimately, however, it's up to physicists to determine when their work is done, and as you say, they don't think it likely ever will be.

For me things like turbulence and critical phenomena are the more interesting questions out there, because they're still very much human-scale, and our only excuse is that we can't compute at the scale required to verify results, the calculations are just too fantastically large.

"An example of this might be the Navier–Stokes equation - we don't fully understand it"

We don't fully understand the flow of fluids, whether or not the NS equation is a good model for them. That the theory of elementary particles is more "fundamental" than the theory of turbulence is one point of view, not unreasonable. But another way to look at it is: the science of fluid flows has its own fundamentals, that are true whether or not a particular fluid is made of particles.

http://arstechnica.com/science/2014/08/the-never-ending-conu...

It's questions all the way down.

Even if we knew the complete theory of everything, the immediate question would become, "Why is the theory of everything the exact theory of everything?"

Because of the Anthropic principle !

Conceivably, you could have empirical evidence for multiple universes, at which point the anthropic principle becomes a normal explanation. Until then, obvious it remains a bit of a non-explanation.

I hate the Anthropic principle.. it appears to be able to justify anything

You should hate anything that isn't testable and falsifiable, that's good hatred.

Thanks!

Why? What fundamental reason is there to reject the untestable and the unfalsifiable? At worst, these claims fall into an epistemological netherworld where they can neither be ascribed truth nor falsity -- at best they may be useful tools.

So your idea of R&D is essentially random, but because you'll occasionally happen upon something useful, don't knock it?

Truly, the philosophy of science is dead.

That's not at all what I'm suggesting. I am not advocating for the arbitrary assignment of confidence intervals, expected values, or whatever your chosen epistemological metric is -- I am advocating for restraint in assigning falsity to what we merely cannot assign truth.

Logical positivism is not boundary of philosophy of science.

There's a difference between, "Can't assign truth" and "Not testable or falsifiable." A difference I hope not to have to explain.

Science can't tell us whether something will ever be testable or falsifiable; we just know what we know now.

So there's no scientific reason to hate any particular idea. Rather, the concepts direct the priority of effort--let's not spend time on things that are not currently testable or falsifiable.

The "hatred" aspect comes from somewhere else besides science, and honestly I think it's harmful more often than it is helpful.

Science can't tell us whether something will ever be testable or falsifiable; we just know what we know now.

That's... almost literally the opposite of reality.

I phrased it poorly.

What I meant is that, given a hypothesis that is not currently testable or falsifiable, we cannot use current scientific knowledge to prove that it will never be testable or falsifiable.

To your point, the way I wrote it above, it sounds like I believe nothing can ever be tested or falsified. Obviously we have tested and falsified many hypotheses.

I'm glad you clarified, because I was getting distressed at the idea of someone so eloquent thinking something so crazy.

To clarify a bit myself though, remember that we're talking about something for which the standards of "testable" and "falsifiable" often just mean, "even in some thought experiment." For example people predicting collisions with another universe have a pretty wild hypothesis, but they proposed some ways to explore that idea involving studying the CMB background.

When we're talking about the anthropic principle as a hypothetical foundation for a complete understanding of physics though... there's really no room even in theory for testing or falsifying that notion. That's what was being hated, and the hate of which I'm in support.

I can think of two hypotheses that can't currently be ruled out:

1) We're living in a computer simulation of a universe, which was built and operated for our benefit by some unknown intelligence(s).

2) Our universe is just one of an infinitely many universes, each of which has slightly different values for the physical constants. Our universe seems well-suited to our life because of selection bias--in the universes not suited to life, there is no life to observe that fact.

These both relate to various formulations of the anthropic principle. The principle itself can't be used to prove the correctness of one or the other. But it's possible that we will find some experiment in the future that will allow us to test one or the other.

I think that's why those concepts are entertained by a large number of people in the scientific community, over drinks at least. Of course, if someone tried to get serious funding to study it, right now, there might be a different reaction.

I think of it in terms of a network of beliefs. In the network of beliefs about physics, and complete theory of everything would make all of the constituent beliefs mesh together and support each other perfectly.

For sure, there would still be questions, but the network would have been tidied up.

Chemistry is already (I think?) in this position. There are no contradictions in the network of fundamental chemistry beliefs, even if there are some unknows in terms of practical issues.

Not only that, but empirical truths are always subject to measurement error -- we will never be completely certain that there is not a more accurate model of reality, and in all likelihood we will always continue to refine our models. Also consider the ever-increasing costs of this minute examination -- probing higher energy physics is only going to become more expensive. There will always be limits to what we know and what can be known about the Universe -- it's an inherent quality of empiricism.

We're not even remotely close, but if it makes you feel any better, people have been speculating about an end to physics for... well... you're in centuries of good company at least.

Condensed-matter physics and chemistry have been "complete" in this sense since forever. You can write down on a small piece of paper the Hamiltonian of electrons interacting via electromagnetic forces with themselves and the atom nuclei. And that's pretty much it.

As a complete layman trying to grok the physics of our reality's fundamentals, now and then during the occasional Wikipedia excursions, I get the feeling that there is no way there are only 3 physical dimensions, and that we as beings rooted in 3D may never be able to see the full picture first-hand.

I think having visual eyesight also makes us heavily biased towards picturing everything as ultimately made up of discrete "particles," and as far as I can tell, our instruments are also limited by trying to distill the results of our experiments into visual images, which results in things like the "wave-particle duality" conundrum.

This seems wrong to me. I know the LHC records total energy in and compares total energy out. Wouldn't any long lived particles would "appear" as areas of missing energy?

Yes, unless they appear as something you think you understand.

In the specific case of this article, they're interested in particles that look a lot like a muon. The signature of a muon in a particle detector is that it leaves a charged track in the inner tracker, passes through the substantial mass of the electromagnetic calorimeter without saying much, and then goes pow in the muon detector (usually alternating plates of steel and detector).

A long-lived particle could be quite similar; a charged track, a lot of nothing, and a decay in the muon tracker.

Any particle that might roughly replicate the muon's signature might have hidden from existing searches for new physics. The proposal in the article is to cover the muon detector with higher-resolution trackers. If the "muon"'s decay doesn't conform to the standard plan, higher resolution would make it apparent.

I'm not enough of an expert to know how sensitive this method is in comparison to other ways of hunting long-lived particles, but I have taken a class or two from one of the authors. If Lubatti's pitching the idea, it's worth thinking about.

As you can imagine, the long-lived particles can have "any" lifetime. Since the lifetime is unknown, it makes sense to probe many lifetimes. The experiment being suggested by Lubatti and his collaborators probes lifetimes on the order of 10^5 - 10^7 m. There aren't strong constraints on this range, which can appear as missing energy in CMS or ATLAS (or not). CMS, ATLAS, and LHCb are giving constraints at 10^-6 to 10^2 m. Interestingly, the different experiments are probing mostly different ranges.

To correct something in the grandparent (?) post, the LHC experiments do not know the incoming energy. Even though the beam energy is 6.5 TeV on 6.5 TeV, the collisions are only a fraction of that. We only know that the energy has to balance in the perpendicular direction. However, if a collision produces two invisible particles that balance each other, it would appear that there is no missing energy. In many of models, pair production of new particles is preferred, so if the particles are long-lived, they can be hard to find.

Well said but a few details are a bit off. The alternating plates of metal and detector is actually the hadronic calorimeter, which the muons also go straight through (but which stop almost everything else). The muon detectors are outside of that, and are just various types of spark chambers that get zapped as the muon passes through them. Generally the muons neither stop nor decay in the detector.

Ah, true story. That makes more sense. Thanks!

Supersymmetry and String Theories.

Two theories that are not even not even wrong.

Their proponents dance and shift in the prevailing intellectual currents such that there's no possibility they could EVER be proven wrong.

There's no null result, no complete experimental failure, that could ever possibly convince them they might, just might, have made an error.

I think you're trivializing the process through which these theories are created. This is incredibly difficult stuff. There is very, very difficult math involved and there are very difficult tests that have to be done with very expensive machines. These things aren't just made on a lark while have a pint or two one afternoon. And even while creating them, there is no guarantee that even if the math is correct that the theory is correct. Theorists know this. But they do it anyway because we need them to. Its hard work and they don't get enough recognition for it (aside from the few that make it on TV).

> But they do it anyway because we need them to. Its hard work and they don't get enough recognition for it (aside from the few that make it on TV).

They certainly don't do it because "we need them to". That's not how theoretical physicists and mathematicians think. It's more of a quest for the holy grail for some and more of an interesting puzzle to solve for the others. That's why some of them just can't stop, even if it's overwhelmingly clear that we have no way to create and probe the extreme conditions where something new and unexpected that would guide the theory could happen. Just because the math is hard does not mean they should get more recognition for it - one should look at the fruits of the work for that. It's not wrong to get occupied with solving famous puzzles, but everything in proportion.

I'd say string theory has had more dancing proponents than supersymmetry by a long shot. Supersymmetry can be tested--that's what the LHC was for--and as the evidence mounts against it, proponents are right to change their theories.

"no null result, no complete experimental failure"

Well, here, have a look at all the nice excluded theories.

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/Supersymmet...

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResult...

And here's Michael E Peskin (of the Supersymmetry Working Group) quoted on the late 2015 results:

http://www.nature.com/news/lhc-sees-hint-of-boson-heavier-th...

"To theoretical physicist Michael Peskin of the SLAC National Accelerator Laboratory in Menlo Park, California, the most relevant part of the talks concerned the failure to find a supersymmetric particle called a gluino in the range of masses up to 1,600 GeV (much farther than the 1,300-GeV limit of run 1). This pushes supersymmetry closer to the point at which many physicists might give up on it, Peskin says."

The "not even wrong" label does not actually apply to your impolite comment; it's just wrong.

> In the past, physicists assumed that new particles produced in particle collisions would decay immediately, almost precisely at their points of origin.

But wouldn't missing energy be evident?

The missing energy is evident indeed, but this is not the point. You usually use the missing energy as a proof of a neutral particle, not a fast decaying particle. And you can not say what that neutral particle was because you did not measure anything about it.

More importantly, the particle we are considering, the one that decays really fast, near the origin, will decay into other particles, and those can be detected.

If the new resultant particle(s) don't decay immediately near the collision why wouldn't their missing energy be evident?

Install a 10 foot thick lead shield at the end of the detector and glue a $10 motion detector light to it. Problem solved. You're welcome, CERN.

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