Will_Newsome comments on Why We Can't Take Expected Value Estimates Literally (Even When They're Unbiased) - Less Wrong

75 Post author: HoldenKarnofsky 18 August 2011 11:34PM

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Comment author: Will_Newsome 19 August 2011 04:25:12PM *  3 points [-]

Astronomy is pretty well understood, so it is pretty easy to estimate the cost of searching the sky for dangerous objects.

Sort of. The possibility of mirror matter objects makes this pretty difficult. There's even a reasonable-if-implausible paper arguing that a mirror object caused the Tunguska event, and many other allegedly anomalous impacts over the last century. There's a lot of astronomical reasons to take this idea seriously, e.g. IIRC three times too many moon craters. There are quite a few solid-looking academic papers on the subject, though a lot of them are by a single guy, Foot. My refined impression was p=.05 for mirror matter existing in a way that's decision theoretically significant (e.g. mirror meteors), lower than my original impression because mirror matter in general has weirdly little academic interest. But so do a lot of interesting things.

Comment author: timtyler 23 August 2011 07:40:15PM 1 point [-]

Astronomy is pretty well understood, so it is pretty easy to estimate the cost of searching the sky for dangerous objects.

Sort of. The possibility of mirror matter objects makes this pretty difficult. There's even a reasonable-if-implausible paper arguing that a mirror object caused the Tunguska event, and many other allegedly anomalous impacts over the last century. There's a lot of astronomical reasons to take this idea seriously, e.g. IIRC three times too many moon craters.

Reality check: mirror matter has a gravitational signature - so we know some 99% of non-stellar matter in the solar system is not mirror matter - or we would see its grav-sig. So: we can ignore it with only a minor error.

Comment author: Will_Newsome 23 August 2011 07:44:26PM 0 points [-]

Dark matter.

Comment author: timtyler 23 August 2011 09:49:53PM *  2 points [-]

There evidently aren't many "clumps" of that in the solar system - so we don't have to worry very much about hypothetical collisions with it.

Comment author: John_Baez 21 August 2011 06:31:48AM *  5 points [-]

I studied particle physics for a couple of decades, and I would not worry much about "mirror matter objects". Mirror matter is just of many possibilities that physicists have dreamt up: there's no good evidence that it exists. Yes, maybe every known particle has an unseen "mirror partner" that only interacts gravitationally with the stuff we see. Should we worry about this? If so, we should also worry about CERN creating black holes or strangelets - more theoretical possibilities not backed up by any good evidence. True, mirror matter is one of many speculative hypotheses that people have invoked to explain some peculiarities of the Tunguska event, but I'd say a comet was a lot more plausible.

Asteroid collisions, on the other hand, are known to have happened and to have caused devastating effects. NASA currently rates the chances of the asteroid Apophis colliding with the Earth in 2036 at 4.3 out of a million. They estimate that the energy of such a collision would be comparable with a 510-megatonne thermonuclear bomb. This is ten times larger than the largest bomb actually exploded, the Tsar Bomba. The Tsar Bomba, in turn, was ten times larger than all the explosives used in World War II.

On the bright side, even if it hits us, Apophis will probably just cause local damage. The asteroid that hit the Earth in Chicxulub and killed off the dinosaurs released an energy comparable to a 240,000-megatonne bomb. That's the kind of thing that really ruins everyone's day.

Comment author: Will_Newsome 21 August 2011 07:06:52AM *  2 points [-]

Mirror matter is indeed very speculative, but surely not less than 4.3 out of a million speculative, no? Mirror matter is significantly more worrisome than Apophis. I have no idea whether it's more or less worrisome than the entire set of normal-matter Apophis-like risks; does anyone have a link to a good (non-alarmist) analysis of impact risks for the next century? Snippets of Global Catastrophic Risks seem to indicate that they're not a big concern relatively speaking.

ETA: lgkglgjag anthropics messes up everything

Comment author: Douglas_Knight 19 August 2011 05:32:05PM 6 points [-]

Yes, you should compute the danger multiple ways, counting asteroids, craters, and extinction events. If there are 3x too many craters, then it may be that 2/3 of impacts are caused by objects that we can't detect. Giving up on solving the whole or even most of the problem may sound bad, but it just reduces the expected value by a factor of 3, which is pretty small in this context.

Comment author: wnoise 19 August 2011 04:50:31PM 2 points [-]

By "mirror matter", I assume you mean what is more commonly known as "anti-matter"?

Comment author: Will_Newsome 19 August 2011 05:07:39PM 5 points [-]

No, mirror matter, what you get if parity isn't actually broken: http://scholar.google.com/scholar?hl=en&q=mirror+matter&btnG=Search&as_sdt=0%2C5&as_ylo=&as_vis=0 http://en.wikipedia.org/wiki/Mirror_matter

Comment author: wnoise 19 August 2011 05:25:24PM 2 points [-]

Huh. Glad I asked.

My initial impression is that the low interaction rate with ordinary matter would make me think this would not be a good explanation for anomalous impacts. But I obviously haven't examined this in anywhere near enough detail.

Comment author: Will_Newsome 19 August 2011 09:49:48PM 3 points [-]

See elsewhere in the thread. E.g. http://arxiv.org/abs/hep-ph/0107132

Comment author: wnoise 20 August 2011 01:14:42AM 0 points [-]

I did see those replies. Thanks.

Comment author: gwern 19 August 2011 05:42:25PM 1 point [-]

Reading the Wikipedia article, I don't really see how mirror matter would be dangerous. It describes them as being about as dangerous as neutrinos or something:

Mirror matter, if it exists, would have to be very weakly interacting with ordinary matter. This is because the forces between mirror particles are mediated by mirror bosons. With the exception of the graviton, none of the known bosons can be identical to their mirror partners. The only way mirror matter can interact with ordinary matter via forces other than gravity is via so-called kinetic mixing of mirror bosons with ordinary bosons or via the exchange of Holdom particles.[10] These interactions can only be very weak. Mirror particles have therefore been suggested as candidates for the inferred dark matter in the universe

Comment author: Will_Newsome 19 August 2011 05:44:02PM *  1 point [-]

Read the papers, Wikipedia is Wikipedia. Kinetic mixing can be strong. The paper on Tunguska is really quite explanatory. (Sorry, I don't mean to be brusque, I'm just allergic to LW at the moment.) ETA: http://arxiv.org/abs/astro-ph/0309330 is the most cited one I think. ETA2 (after gwern replied): Most cited paper about mirror matter implications, not about Tunguska. See here for Tunguska: http://arxiv.org/abs/hep-ph/0107132

Comment author: gwern 19 August 2011 06:07:32PM 1 point [-]

The part on Tunguska doesn't really explain it though, but simply assumes a mirror matter object could do that and then spends more time on how the mirror matter explains the lack of observed fragments and how remaining mirror matter could be detected. The one relevant line seems to be

"If this impact event is due to a (pure) mirror matter body, it should not have slowed down as rapidly in the atmosphere as an ordinary matter body (for ǫ ∼ 10−8 − 10−9 as suggested by DAMA/NaI results, the air molecules typically pass through the body losing only a relatively small fraction of their momentum^36^)."

It must be explained elsewhere or the implications of 'ǫ ∼ 10−8 − 10−9' be obvious to a physicist. How annoying...

Comment author: Will_Newsome 19 August 2011 06:13:02PM 3 points [-]

Here you go: http://arxiv.org/abs/hep-ph/0107132

Abstract: Mirror matter is predicted to exist if parity (i.e. left-right symmetry) is a symmetry of nature. Remarkably mirror matter is capable of simply explaining a large number of contemporary puzzles in astrophysics and particle physics including: Explanation of the MACHO gravitational microlensing events, the existence of close-in extrasolar gas giant planets, apparently ‘isolated’ planets, the solar, atmospheric and LSND neutrino anomalies, the orthopositronium lifetime anomaly and perhaps even gamma ray bursts. One fascinating possibility is that our solar system contains small mirror matter space bodies (asteroid or comet sized objects), which are too small to be revealed from their gravitational effects but nevertheless have explosive implications when they collide with the Earth. We examine the possibility that the 1908 Tunguska explosion in Siberia was the result of the collision of a mirror matter space body with the Earth. We point out that if this catastrophic event and many other similar smaller events are manifestations of the mirror world then these impact sites should be a good place to start digging for mirror matter. Mirror matter could potentially be extracted & purified using a centrifuge and have many useful industrial applications.

Comment author: gwern 19 August 2011 07:11:25PM 1 point [-]

OK, I think that explains that - Wikipedia is making the first assumption identified below, rather than the other one that he prefers:

"If the only force connecting mirror matter with ordinary matter is gravity, then the consequences would be minimal. The mirror SB would simply pass through the Earth and nobody would know about it unless it was so heavy as to gravitationally affect the motion of the Earth.

However if there is photon - mirror photon kinetic mixing as suggested by the orthopositronium vacuum cavity experiment, then the mirror nuclei (with Z ′ mirror protons) will effectively have a small ordinary electric charge ǫZ ′ e. This means that the nuclei of the mirror atoms of the SB will undergo Rutherford scattering off the nuclei of the atmospheric nitrogen and oxygen atoms. In addition ionizing interactions can occur which can ionize both the mirror atoms of the space body and also the atmospheric atoms. The net effect is that the kinetic energy of the SB is transformed into light and heat (both ordinary an mirror varieties) and a component is also converted to the atmosphere in the form of a shockwave, as the forward momentum of the SB is transferred to the air which passes though or near the SB.

What happens to the mirror matter SB as it plummets towards the Earth’s surface depends on a number of factors such as its initial velocity, size, chemical composition and angle of trajectory. Of course all these uncertainties occur for an ordinary matter SB too. Interestingly it turns out that for the value of the kinetic mixing suggested by the Or- thopositronium experiment, ǫ ≈ 10−6, the air resistence of a mirror SB in the atmosphere is roughly the same as an ordinary SB assuming the same trajectory, velocity mass, size and shape (and that it remains intact). This occurs because the air molecules will lose their relative forward momentum (with respect to the SB) within the SB itself because of the Rutherford scattering of the ordinary and mirror nuclei as we will show in a moment. (Of course the atmospheric atoms still have random thermal motion). This will lead to a drag force of roughly the same size as that on an ordinary matter SB, implying an energy loss rate ....

The above calculation shows that the rate of energy loss of the SB in the atmosphere depends on its size and density. If we assume a density of ρSB ≃ 1 g/cm3 which is approximately valid for a mirror SB made of cometary material (such as mirror ices of water, methane and/or ammonia) then the body will lose most of its kinetic energy in the atmosphere provided that it is less than roughly 5 meters in diameter. Of course things are complicated because the the SB will undergo mass loss (ablation) and also potentially fragment into smaller pieces and of course potentially melt & vaporize. Thus even a very large body (e.g. R ∼ 100 meters as estimated for the Tunguska explosion) can lose its kinetic energy in the atmosphere if it fragments into small pieces.

...Returning to the most interesting case of large photon - mirror photon kinetic mixing, ǫ ≃ 10−6 which is indicated by the orthopositronium experiment, our earlier calculation suggests that most of the kinetic energy of a mirror matter SB is released in the atmosphere like an ordinary matter SB if it is not too big (∼ 5 meters) or fragments into small objects. It seems to be an interesting candidate to explain the 1908 Tunguska explosion (as well as smaller similar events as we will discuss in a moment).

Comment author: Will_Newsome 19 August 2011 09:50:10PM *  1 point [-]

OK, I think that explains that - Wikipedia is making the first assumption identified below

No, Wikipedia mentions kinetic mixing then says that if it exists it must be weak, Wikipeda doesn't say it wouldn't exist (the evidence suggests it would exist). The Wikipedia article is just wrong. (ETA: I mean, it is just wrong to assume that it's weak.) (Unless I'm misinterpreting what you mean by "the first assumption identified below"?)

Comment author: gwern 19 August 2011 10:48:50PM 0 points [-]

What I meant was that both the paper and Wikipedia regard kinetic mixing as weak and relatively unimportant; then they differ about the next effect, the one that would be strong and would matter to Tunguska.

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