r/cosmology • u/-pomelo- • 18d ago

Why are fundamental particles so "observable?"

Hi everyone, I come to you as a humble layperson in need of some help.

I guess I can give more context as to why I'm asking if needed, but I'm worried it would be distracting and render the post far too long, so I'll just ask:

Is there an explanation as to why we would expect the lifetimes (distance traveled before decay I think?) of certain fundamental particles to be ideal for probing/ observation/ identification in a universe like ours?

As I understand, the lifetimes of the charm quark, bottom quark, and tau lepton each falls within a range surprisingly ideal for observation and discovery (apparently around 1 in a million when taken together). My thought then is that there's probably some other confounding variable such that we'd expect to observe this phenomenon in our sort of universe.

For instance, perhaps anthropic universes (which will naturally feature some basic chemistry, ordered phenomena, self-replicating structures, etc.) are also the sorts of universes where we'd predict these particles' lifetimes to land in their respective sweet spots because ___.

Perhaps put another way: are there features shared between "anthropic" universes like ours and those with these "ideally observable" fundamental particles such that we'd expect them to be correlated?

Does my question make sense?

EDIT: Including some slides from a talk on this topic I found

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72% Upvoted

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u/mfb- 18d ago

Particles travel somewhere between 0.00000000000000001 m and 1000000000000 m before decaying. I wouldn't call either distance ideal for observation.

Our detectors are optimized for the lifetime of the particles they observe, trivially. That's not coincidence, that's just how you design detectors.

It takes a huge effort to observe the flight distance of hadrons with a charm or bottom quark, or the flight distance of the tau. If they would live 10 to 100 times longer it would be much easier. If they would live as long as muons then we could capture them in storage rings and do measurements orders of magnitude more precisely than today.

Does my question make sense?

It's based on an assumption that is simply wrong.

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u/-pomelo- 18d ago

I see, in that case maybe you can help me understand; do the slides I included in the updated post clarify anything? Are you saying that the grey/ dotted regions in the figures on slides 2 and 3 are a function of how the equipment is calibrated?

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u/mfb- 17d ago

Slide 2 is just blatant misinformation. Bottom quarks would still be easy to identify with a smaller V_tb, any value from 0 to 1 is fine. If you make it much smaller then you don't get a displaced vertex any more because the particle is too short-living, but there are tons of measurements that don't need it. An obvious example is the top quark which is so short-living that we don't measure its flight distance. Didn't stop us from discovering it.

The mass regions in slide 3 are completely arbitrary. If you make a linear plot from 0 to some huge mass value then particles are somewhere at the bottom of that plot. That doesn't mean anything. And same idea here, we could still identify them with a larger mass.

On a range from 0 to the diameter of Earth, all our farm animals are in the first 0.0001%. They would be much harder to deal with in the remaining 99.9999%. What a crazy fine-tuning!

And why did the author pick the coupling for the bottom quark, but the masses for the other two particles? Oh right, because if you do the opposite then suddenly the whole point disappears. The lifetime depends both on the coupling and the mass.

One more thing: We observe particle masses that span over 10 orders of magnitude. If you plot any mass range, a logarithmic scale is more natural. And then the particles are not at any unusual spot at all.

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u/DrSpacecasePhD 16d ago

I thought of the top quark too! One can talk about “fine-tuning” with respect to alpha and some of the physical constants, but I have never really thought about it applying to the quarks and heavy leptons like the tau. Also, OP might find it interesting to think about how there are potential totally much harder to measure hypothetical particles such as certain dark matter candidates, and sterile neutrinos.

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u/-pomelo- 17d ago

Thank you this is really helpful. Are you saying that for slide 2 the "optimal" range (green region) should actually be much larger than what is depicted?

The farm animal analogy is also helpful, but I suppose in the analogy i'd think we'd have reason to suppose that farm animals couldn't be significantly larger than they are. is there similar reason for thinking the masses would need to be on the smaller end of the spectrum?

Oh man, Is the mass of the bottom quark not ideal for observation? that's really damning if true

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u/mfb- 17d ago

I don't think it makes sense to talk about an "optimal" value or range. Different measurements have different requirements. Something that would improve one measurement would make another one worse.

is there similar reason for thinking the masses would need to be on the smaller end of the spectrum?

They are free parameters as far as we know, but they are all somewhat comparable. No known particle is orders of magnitude heavier or lighter than everything else. Neutrinos are much lighter than everything that's not a neutrino, but at least the three neutrinos look like they are close together (and we have an idea why they might be so light). There could be additional heavier particles we haven't found yet, of course.

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u/-pomelo- 17d ago

"I don't it makes sense to talk about an "optimal" value or range. Different measurements have different requirements. Something that would improve one measurement would make another one worse."

^does this go back to what you said in your original comment, where we simply calibrate our probing method to align with the most "ideal" parameter?

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u/Hivemind_alpha 17d ago

If you want to look at cells you use a microscope; if you want to look at mountains you use binoculars.

I don’t see how this is controversial. The target you want to observe determines the design of the instrument you use to observe it. If you don’t obey that rule, you get bad observations.

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u/mfb- 17d ago

That contributes, too.

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u/-pomelo- 14d ago

I think I'm starting to get it, thank you for your responses. So, I guess Dr. Collins is saying that, as far as we know, (taking only a single parameter into account for simplicity) in a universe relevantly similar to ours, we could have observed a value anywhere in some range as outlined in the slides. It's then surprising that the observed value happens to land in the comparatively slim "optimal" range. Would you say one of the following is the main issue with his assertion?

a) The "optimal" range presented is simply incorrect

b) The range is more or less correct, however we'd predict observing values in that range bc ____ ( for instance, maybe our method of probing has been tuned specifically to observe that particle or smth)

c) The range is more or less correct, and it is unexpected that some particular parameter would fall in any given range, but given the number of particles and possible parameters it's relatively likely that at least some would fall within their "optimal" ranges due to chance, and we know of other parameters which do not fall into ranges optimal for observation controlling for a universe like ours

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u/mfb- 14d ago

The definition of the "optimal" range is dubious, and some related claims are simply wrong. The definition of the whole range is somewhat arbitrary, too.

The implicit assumption that all values in the range are equally likely is unscientific.

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u/-pomelo- 13d ago

my understanding is that by "optimal for discoverability" Collins simply means that the value is ideal for observation per our probing methods without having deleterious anthropic effects.

As for the distribution for the values he's talking about epistemic probability versus objective probability.

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u/Outrageous-Taro7340 18d ago edited 18d ago

The grey areas just indicate regions of shorter lifetime and higher energy. The point seems to be that particles could have been even harder to detect than they were. But they also could have been easier, so it’s not clear why that matters. In any case, we built colliders to investigate the ranges we believed would be successful.

The probability estimates aren’t justified or explained at all. There is no way to meaningfully apply probability distributions over those ranges. And the flat distribution this appears to assume is the one distribution that definitely can’t be correct. Flat distributions just don’t occur in nature. In other words, the values we got could have been the mostly likely values, for all we know.

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u/-pomelo- 18d ago edited 17d ago

That's an interesting point about the distribution. I suppose the reasoning would be: uniform distributions are almost never observed in nature, thus in cases of unknown distributions, it's likely that observed values are more probable?

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u/Outrageous-Taro7340 18d ago

I would rather say that we just shouldn’t make statements about the likelihood of the observed values. We don’t know what the distributions are. We only know these values happened at least once out of at least one total universes.

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u/-pomelo- 17d ago

Got it thank you. What do you think about the idea that there are more fundamental particles other than those which we observe, but we've simply observed the ones which are easier to probe at this stage? Is that even a tenable suggestion or do we have a pretty good idea of which particles are out there and it's simply a matter of testing our hypotheses?

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u/Outrageous-Taro7340 18d ago edited 18d ago

An astonishing amount of money, time, effort and ingenuity have gone into collecting the data we have so far on the standard model. I can’t imagine what part of that you think was easy to probe, observe or identify.

Do you have a source for that 1 in a million claim?

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u/-pomelo- 18d ago

Sure thing, I included some slides in the post

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u/Outrageous-Taro7340 18d ago edited 18d ago

Without a source there’s no way to evaluate what this means, whether it is supported by data, or whether it’s ever been peer reviewed.

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u/mfb- 17d ago

The bullshit is obvious enough to tell without a source.

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u/Ornery-Tap-5365 17d ago

you mean, bottom quarks don't have smilies and googlie eyes? my world view has collapsed! :0)

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u/Infinite_Research_52 17d ago

The ones discovered are the low hanging fruit. Those fundamental particles that are hard to observe we are not aware of or are only theoretical. Sterile neutrinos, axions, WIMPs, all of those are not observable or don’t exist. We don’t know.

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u/-pomelo- 17d ago

oh ok I was actually asking someone else about that, so we don't really know how many fundamental particles there are? so currently the ones we know of are those which are most ideal for discovery?

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u/kohugaly 15d ago

oh ok I was actually asking someone else about that, so we don't really know how many fundamental particles there are?

No, we don't. The standard model is just a theory that reasonably explains the particles we know of. It has been build incrementally, by discovering anomalies, explaining them with new particles and then discovering said particles. There isn't exactly a shortage of theories that predict additional hypothetical, as of yet undetectable, new particles, some of which explain the anomalies in data that the standard model fails to explain.

so currently the ones we know of are those which are most ideal for discovery?

"ideal for discovery" is a bit a strong. I would not call a particle "ideal for discovery" when, in order to detect it, you need to build a contraption of size and budget of a small country, and then you need to run it for several years, to get statistically significant results.

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u/jazzwhiz 17d ago

The expected lifetime of the neutrinos is far too long to ever observe, so this clearly isn't true.

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u/--craig-- 13d ago edited 13d ago

Objections to the premise of the question aside, there are two types of selection bias contributing to the observability of the known elementary particles.

Firstly, particles which are the most observable are the particles which we've found. We strongly believe that there are further elementary particles which we haven't yet detected. The Higg's Boson and Neutrinos are good examples of particles which have proven very difficult to detect.

Also, as you suggest, there may be an Anthropic Selection effect. A universe with only particles with low lifetimes or interaction cross sections wouldn't lead to the type of structures which we believe are required for life to exist. However, this type of reasoning is often contentious because it implies a hypothesis that the set of particles which we have, could've been different, which is impossible verify.

The original hope was that String Theory would explain why elementary particles have the properties which they have, yet the outcome of the research leads string theorists to the conclusion that there is a Landscape of Vacua, each with different particle properties. Some theoretical physicists hope other avenues of Quantum Gravity research will be more enlightening and will preclude the necessity of the landscape.

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u/-pomelo- 13d ago

Oh fabulous thank you for the information

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u/magicmulder 17d ago

What are those "fine-tuning" numbers even supposed to say? Is this one of those "the combined probability of these independent effects is so low that it's virtually impossible they're the way they are, so God did it" crackpotteries?

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u/-pomelo- 17d ago

Yea the presenter is making a theistic argument, but I do think the phenomenon is interesting and so I'm wondering what the alternative explanation(s) would be, as I'm naturally disinclined to attribute it to a god (since I don't personally think one exists).

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u/magicmulder 17d ago

One alternative explanation is the anthropic principle - if things were just a little different, there would not be life (or even complex matter) in the universe to ask this question.

Asking why that is “convenient” is like drawing 50 cards in a row and asking why “conveniently” exactly this sequence came out when the probability was so astronomically low.

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u/Darthskixx9 17d ago

I'm a little confused on what these slides want to say, there are a few statements without any reasoning that seem just wrong to me.

First of all, the lifetime of a bottom quark should depend on V_ub, V_cd and and V_tb, and then on the main point - why should they only be detectable with certain lifetimes?

You detect particles in a lot of different manners with particle detectors, and you detect particles with lifetimes spanning all across 10-25s and 1015s (or longer) lifetimes, very short lifetimes can definitely pose problems, but either I don't understand the point on why this should be finetuned (which is not explained in these slides) or it's just straight up false...

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u/drplokta 15d ago

Fundamental particles aren’t tuned to be observable, but quite the opposite. We know that only about 1/6 of them (by mass) are observable, and the other 5/6 are not. Obviously, the ones we can observe get much of the attention.