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Posts Tagged ‘math’

I am going to argue that the Zimmerman verdict (for the shooting of Trayvon Martin) was the correct one.  You will either agree with me or you will not.  And then I will argue that either way, it doesn’t matter in the slightest to most people’s lives.

Let me just say, before you dismiss this post entirely because of some preconceived notion about my politics, then I am very liberal on social issues.  (As I’ve mentioned in the past, I literally don’t have an opinion on many complicated economic issues.)  I’m strongly supportive of privacy rights, voting rights, women’s rights, LGBT rights, and animal rights.  I think the idea of building a giant wall to keep out every illegal alien is absurd.  I am for the legalization of marijuana and for the decriminalization of other drug use in general.  I think man-made global warming is a self-evident fact.  I think big monolithic corporations, in the long term, have a negative effect on the happiness of the masses because they operate as entities without conscience, self-awareness, or humanity.

But when the Zimmerman verdict came back on July 13 as not guilty, I wasn’t surprised.  I wasn’t even outraged.  I just sort of shrugged and moved on.

Granted: there is still racism in this country.  I will even argue that there are often two disparate systems of justice in the U.S.: one for whites, and one for non-whites.  But still…what does that have to do with the Zimmerman verdict?

Scenario 1.  Let’s suppose the George Zimmerman is a total card-carrying KKK racist.  He may be, he may not be…I don’t have any evidence one way or another.  And most of you don’t, either.  But let’s just suppose he is.  Let’s say he follows Trayvon Martin looking for trouble; hoping for a confrontation; hoping to scare the boy.  A scuffle ensues and Martin is shot.

Is that murder?

I’m not a lawyer, but it doesn’t sound like murder.  Manslaughter seems a better fit.

Scenario 2.  Let’s be more realistic.  Let’s assume Zimmerman is a racist, but not the frothing-at-the-mouth kind.  He just feels uncomfortable having a black guy in his neighborhood.  However, if you asked him, he’d claim to not be a racist, claim to have black friends, and try to seem like a reasonable guy.

He follows Martin, hoping to scare him off, but not actively hoping for a fight; he genuinely wants to keep the peace.  If Martin gets scared, well that’s OK: he’s got no business being in this part of town.  A scuffle ensues and Martin is shot.

Is that murder, or even manslaughter?

Again, I don’t think so.  In this case, if Zimmerman is guilty of something, it’s…I don’t know…reckless endangerment?  Putting himself and another in a situation where only bad things could happen?

I didn’t follow the trial all that closely, but I will say that some people who followed the trial even less than I did were outraged at the verdict.  I can understand this, on some level; if a travesty occurs (the shooting of Trayvon Martin was certainly a travesty) then people want justice; they may even want revenge.  If Zimmerman wasn’t to blame, then who is?  Saying “the system” or “society” or “endemic racial profiling” are the root causes of Martin’s death isn’t satisfying, because you can’t put those nouns behind bars and throw away the key and feel good about yourself.  If no one gets blamed, then how does Trayvon Martin get justice?

Here are four ways Trayvon Martin could have gotten justice, or may still get justice:

  1. Florida’s inane stand-your-ground law gets repealed.  That would be justice.
  2. Community watch volunteers stop carrying guns and instead call trained police professionals if they see suspicious behavior.  That would be justice.
  3. Politicians stop listening to NRA lobbyists, and start listening to common sense: that would be justice.
  4. Zimmerman admits what he did was horribly bad judgment; pleads guilty to reckless endangerment; then performs 300 hours of community service as a sort of penance.  (In the long run this outcome would have been better for Zimmerman than the not guilty verdict, because I suspect Zimmerman may be a pariah for the rest of his life.  A little bit of remorse would have gone a long way.)

Anyway, all things considered, I think the jury did what 99% of juries would have done in this case, which was let Zimmerman go free.  The prosecution did not succeed in proving their case.  In retrospect, I think that going for a murder charge was ill-advised and entirely political; they should have aimed a little lower.  Going for manslaughter from the start had a much better chance of success.  Putting Zimmerman away for life on a murder rap isn’t justice; it’s revenge.

OK then.  Feel free to agree, or rabidly disagree.

It doesn’t matter.

The Zimmerman case was just one case.  One case, out of thousands of criminal cases in the U.S. every year.  That is, the Zimmerman trial was just one data point.

I’ve talked about this before.  You can’t really draw any conclusions about anything from one data point.  And yet, people do it all the time.  It’s a fallacy that probably has a name, but the name eludes me.  But to most people, it’s not a fallacy.  It has the weight of proof.

“I don’t believe in global warming.”  [Katrina devastates New Orleans]  “Wait, now I do!”

“I don’t think M. Night Shyamalan is a good director.”  [Watches The Sixth Sense] “Wait, now I do!”

“I don’t think racial profiling is a real thing.”  [Martin gets shot and his Skittles spill to the ground]  “Wait, now I do!”

I hope all three of these arguments is equally absurd to you.  If not, I think you lack the scientific mindset.  Now, don’t get me wrong: I think global warming is real, and I think racial profiling is real.  It’s just that you can’t make the case for those things with only one data point.  (Indeed, the case of M. Night Shyamalan shows that one data point can lead you horribly astray: after the wonderful The Sixth Sense Shyamalan has directed six turkeys in a row.)

I do think that racism still pervades the country.  I do think that whites get a different kind of justice than non-whites in our judicial system.  I do think that our country is obsessed…in an unhealthy way…with small metal devices whose sole purpose is to kill other human beings.  But I don’t believe any of these things solely because of a single data point.  You have to look at the big picture, look at the data in aggregate.  A preponderance of evidence is required to separate fact from fiction, truth from rumor, knowledge from urban legend.  As much as politicians love to bring up pithy examples, tell symbolic anecdotes, those examples and anecdotes are really rather meaningless.  Give me the data or go home.

And that is why the Zimmerman verdict is really rather meaningless.  Not to the family of Trayvon Martin, of course; I feel for them and am very sorry for their loss.  But as to what the trial says, in a larger context, about our society in general?  It says nothing.  A single data point says nothing.  It cannot say anything; that’s a simple mathematical fact.  It takes at least two points to make a line.

If you want to know how prevalent racism is, or how two “separate-but-equal” judicial systems pervade the U.S., or even whether putting guns in the hands of rent-a-cops endangers citizens, look at the data.  Data, plural.  Give Nate Silver a call.  Don’t argue by colorful anecdote.  And if you don’t have the hard data, at least have the courage to admit to yourself that what you believe is based on nothing.

That’s what I believe.  And yes, it’s really just based on nothing.  But I’m OK with that, because somewhere, hunched over a desk, Nate Silver is crunching all the numbers, and he’s still not a witch.

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When I was young, I once looked at a box of cereal and had an epiphany.  “Why is that cereal there?”  A universe of unfathomable complexity, with 100,000,000,000 galaxies, each with 100,000,000,000 stars, making 10,000,000,000,000,000,000,000 possible solar systems with planets around them—all that, and I’m sitting across from a box of Vanilly Crunch?

Vanilly

Since that existential crisis, I’ve always wondered why there was something instead of nothing.  Why isn’t the universe just one big empty set?  “Emptiness” and “nothingness” have always seemed so perfect to me, so symmetric, that our very existence seems at once both arbitrary and ugly.  And no theologian or philosopher ever gave me an answer I thought was satisfying.  For a while, I thought physicists were on the right track: Hawking and Mlodinow, for example, in The Grand Design, describe how universes can spontaneously appear (from nothing) according to the laws of quantum mechanics.

I have no problem with quantum mechanics: it is arguably the most successful theory devised by mankind.  And I agree that particles can spontaneously create themselves out of a vacuum.  But here’s where I think Hawking and Mlodinow are wrong: the rules of physics themselves do not constitute “nothing”.  The rules are something.  “Nothing” to me implies no space, no time, no Platonic forms, no rules, no physics, no quantum mechanics, no cereal at my breakfast table.  Why isn’t the universe like that?  And if the universe were like that, how could our current universe create itself without any rules for creation?

But wait—don’t look so smug, theologians.  Saying that an omnipotent God created the universe doesn’t help in any way.  That just passes the buck; shifts the stack by one.  For even if you could prove to me that a God existed, I would still feel a sense of existential befuddlement.  Why does God herself exist?  Nothingness still seems more plausible.

Heidegger called “why is there anything?” the fundamental question of philosophy.  Being a physicist, and consequently being full of confidence and hubris, I set out to answer the question myself.  I’d love to blog my conclusions, but the argument runs about 50,000 words…longer than The Great Gatsby.  Luckily for you, however, my book Why Is There Anything? is now available for the Kindle on Amazon.com:

rave book

You can download the book here.

You might wonder if my belief in the many-worlds interpretation (MWI) of quantum mechanics affected my thinking on this matter.  Well, the opposite is true.  In my journey to answer the question “why is there anything?” I became convinced of MWI, in part because of the ability of MWI to partially answer the ultimate question.  My book Why Is There Anything? is a sort of chronicle of my intellectual journey, one that I hope you will find entertaining, enlightening, and challenging.

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mcfly

“I am your probability density”

In an earlier post I discussed my philosophy of teaching special relativity.  My main idea was that physics professors should keep the “weird stuff” at bay, and start with non-controversial statements; once students are on board, you can push under the grass and show them the seething Lynchian bugs beneath.

Well, what about quantum mechanics?  Does the same philosophy apply?

My answer is yes, of course.  Don’t start with Schrödinger’s cat.  Don’t mention the Heisenberg uncertainty principle, or wave collapse, or the EPR experiment, or Bell’s theorem, or the double slit experiment, or quantum teleportation, or many worlds, or Einstein’s dice.  Start with the problems of physics, circa 1900, and how those problems were gradually solved.  In working out how physicists were gradually led to quantum mechanics, students will build up the same mental framework for understanding quantum mechanics.  At least, that’s how it works in theory.

Now, my perspective is from the point of view of a professor who teaches only undergraduates.  I only get to teach quantum mechanics once a year: in a course called Modern Physics, which is sort of a survey course of 20th century physics.  (If I were to teach quantum mechanics to graduate students, my approach would be different; I’d probably start with linear algebra and the eigenvalue problem, but that’s a post for another day.)  As it is, my approach is historical, and it seems to work just fine.  I talk about the evidence for quantized matter (i.e. atoms), such as Dalton’s law of multiple proportions, Faraday’s demonstration in 1833 that charge is quantized, Thomson’s experiment, Millikan’s experiment, and so on.  Then I explain the ultraviolet catastrophe, show how Planck was able to “fix” the problem by quantizing energy, and how Einstein “solved” the problematic photoelectric effect with a Planckian argument.  Next is the Compton effect, then the Bohr model and an explanation of the Balmer rule for hydrogen spectra…

We’re not doing quantum mechanics yet.  We’re just setting the stage; teaching the student all the physics that a physicist would know up until, say, 1925.  The big breakthrough from about 1825-1925 is that things are quantized.  Things come in lumps.  Matter is quantized.  Energy is quantized.

The big breakthrough of 1925-1935 is, strangely, the opposite: things are waves.  Matter is waves.  Energy is waves.  Everything is a wave.

So then, quantum mechanics.  You should explain what a wave is (something that is periodic in both space and time, simultaneously).  Here, you will need to teach a little math: partial derivatives, dispersion relations, etc.  And then comes the most important step of all: you will show what happens when two (classical!) wave functions are “averaged”:

ψ1 = cos(k1x – ω1t)

ψ2 = cos(k2x – ω2t)

Ψ(x,t) = (1/2) cos(k1x – ω1t)  + (1/2) cos(k2x – ω2t)

Ψ(x,t) = cos(Δk·x – Δω·t) · cos(k·x – ω·t)

where Δk ≡ (k1 – k2)/2, k ≡ (k1 + k2)/2, etc.

[Here I have skipped some simple algebra.]

This entirely classical result is crucial to understanding quantum mechanics. In words, I would say this: “Real-life waves are usually combinations of waves of different frequencies or wavelengths.  But such ‘combination waves’ can be written simply as the product of two wave functions: one which represents ‘large-scale’ or global oscillations (i.e. cos(Δk·x – Δω·t)) and one which represents ‘small-scale’ or local oscillations (i.e. cos(k·x – ω·t)).

This way of looking at wave functions (remember, we haven’t introduced Schrödinger’s equation yet, nor should we!) makes it much easier to introduce the concept of group velocity vs. phase velocity: group velocity is just the speed of the large-scale wave groups, whereas phase velocity is the speed of an individual wave peak.  They are not necessarily the same.

It is also easy at this point to show that if you combine more and more wave functions, you get something that looks more and more like a wave “packet”.  In the limit as the number of wave functions goes to infinity, the packet becomes localized in space.  And then it’s simple to introduce the classical uncertainty principle: Δk·Δx > ½.  It’s not simple to prove, but it’s simple to make plausible.  And that’s all we want at this point.

We’re still not doing quantum mechanics, but we’re almost there.  Instead, we’ve shown how waves behave, and how uncertainty is inherent in anything with a wave-like nature.  Of course now is the time to strike, while the iron is hot.

What if matter is really made from waves?  What would be the consequences of that?  [Enter de Broglie, stage right]  One immediately gets the Heisenberg relations (really, this is like one line of algebra at the most, starting from the de Broglie relations) and suddenly you’re doing quantum mechanics!  The advantage of this approach is that “uncertainty” seems completely natural, just a consequence of being wave-like.

And whence Schrödinger’s equation?  I make no attempt to “prove” it in any rigorous way in an undergraduate course.  Instead, I just make it imminently plausible, by performing the following trick.  First, introduce complex variables, and how to write wave functions in terms of them.  Next, make it clear that a partial derivative with respect to x or t can be “re-written” in terms of multiplication:

d ψ /dx  →  ik ψ

d ψ /dt  →  –iω ψ

Then “proving” Schrödinger’s equation in a non-rigorous way takes 4 lines of simple algebra:

E = p2/2m

E ψ = (p2/2m)ψ

Now use the de Broglie relations E = ħω and p = ħk…

ħw ψ = (ħ2k 2/2m) ψ

iħ(∂ψ/∂t) = (–ħ2/2m) ∂2ψ/∂x2

There’s time enough for weirdness later.  Right now, armed with the Schrödinger equation, the student will have their hands full doing infinite well problems, learning about superposition, arguing about probability densities.  As George McFly said, “I am your density.”  And as Schrodinger said, probably apocryphally, “Don’t mention my cat till you see the whites of their eyes.”

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As a physics professor, I have certain pet peeves.  For example, I cringe when someone says that “gravity” is 9.8 m/s2 when they mean the acceleration due to gravity.  I’m annoyed if someone says that an object “weighs” 7 kg.  And I stifle a laugh if someone says that a roller coaster is exciting because it goes so “fast”—humans can only detect acceleration, not speed, which is why we don’t notice that we’re traveling something like 67,000 mph right now in our orbit around the sun.

goose

“I feel the need for acceleration!”

But my biggest pet peeve may be students doing algebra with numbers.

Fellow physics professors will know exactly what I’m talking about, but for the uninitiated, here’s an example:

If you drop an object from a height of 20 m, how long will it take to hit the ground?

A student knows that a kinematics equation is needed, hits upon the correct one, Δyvi Δt + (1/2) a Δt2, and then correctly identifies Δy = –20 m, a = –9.8 m/s2, and vi = 0.  So far, so good.  They’ve studied their physics, right?  What happens next is sheer madness:

algebra_with_numbers

Sigh.

Over and over again I tell students, “don’t plug numbers in until the end.”  But students love plugging in numbers.  They feel they’re actually getting closer to the answer if they’re manipulating numbers.  On some level, they still feel uncomfortable with letters—as if manipulating letters isn’t really “math”.

How does this problem look in my answer key?  Like this:

algebra 2

You can now plug in values if you like…and get Δt = √[2(-20)/-9.8] = 2.02 s.

Which of these approaches is more beautiful, more powerful?  The approach you pick indicates whether you “get” algebra or not.  If you do algebra with numbers, the answer you get is very narrow and very specific, even if you do it correctly.  That hypothetical student could have gotten 2 seconds as an answer, and I would have given them full credit.  But their answer would have been ugly.

The second approach is beautiful, because it is completely general and applicable to multiple situations.  I try to tell students “Look!  You found the time to fall a certain distance.  You now know the answer no matter what the height is, and even no matter what planet you’re on, since g doesn’t have to be 9.8 m/s2.”  This is usually followed by a blank open-mouthed stare, much like Kristen Stewart in a Twilight movie.

There is a more practical reason to avoid doing algebra with numbers.  It’s simply that when you do algebra with numbers, other people cannot follow your work as easily.  And then, if you make a mistake, it’s harder for someone else to spot.  Quick: what algebra error did the student make above?  It takes a while to find the mistake.

My ultimate point is that students need experience seeing the power of algebra.  It’s all well and good that algebra classes stress real-world applications—else, why teach algebra in the first place?  But real-world doesn’t only mean with numbersE=mc2 is certainly a real-world application of algebra, and it’s a lot more elegant than saying that 378,000,000,000,000 Joules is released when a teaspoon of sugar with mass 4.2 grams  is converted to pure energy, given that the speed of light is 300,000,000 m/s.  The hard part, for us physics professors, is to help this spoonful of algebra go down.

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Poor Einstein.  Is there anyone else who is misquoted more often?  Is there anyone else to whom more nonsense is attributed?

I have no desire to rehash things that Einstein said about “God”.  Einstein was by all accounts an atheist, an agnostic, or a pantheist—depending upon your definitions—and various religious apologists have been trying to co-opt the man for years by misquoting him.  Others have already discussed this at length.

My goal today is to tackle that old chestnut, “Imagination is more important than knowledge,” as seen on T-shirts, bumper stickers, and even on the packaging of the Albert Einstein action figure.  Did Einstein really say this, and if so, what did he mean?

Here’s the quote in context:

“At times I feel certain I am right while not knowing the reason.  When the [solar] eclipse of 1919 confirmed my intuition, I was not in the least surprised.  In fact I would have been astonished had it turned out otherwise.  Imagination is more important than knowledge.  For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. It is, strictly speaking, a real factor in scientific research.”  [From A. Einstein, Cosmic Religion: With Other Opinions and Aphorisms, p. 97 (1931).]

So Einstein did say this.  However, I maintain that the full quote in context has a different feel to it than the quote in isolation.

When I see “Imagination is more important than knowledge” on a bumper sticker, I think this: “Flights of fancy and imagination are more important than learning stuff.  So why should I study?  Einstein didn’t study.  He just sat around and daydreamed and came up with the most remarkable breakthroughs about the workings of our universe.  Imagination is more important than learning all the proofs and figures ranged in columns before me.  So I am going to follow good ol’ Einstein and daydream about being Batman.”

The New Age meaning of the quote is this: “I’d rather daydream than study.”  It’s Walt Whitman’s “learn’d astronomer” nonsense all over again.

In context, it’s clear that Einstein was talking about doing science.  Imagination is more important in making scientific breakthroughs than knowledge, but that doesn’t mean that knowledge is not important.  Einstein worked very, very hard to learn an awful lot of physics.  By all accounts, it took him almost 10 years to flesh out general relativity, during which time he had to acquire a lot of mathematical knowledge about Riemannian geometry and tensor analysis.  The “intuition” that Einstein developed during this time frame is what allowed him to be so confident of the results of Eddington’s expedition.  What Einstein calls “intuition” is just knowledge that has become so ingrained that you are no longer cognizant of it.

Einstein may have been more famous than most of his contemporaries, and it was probably due to his superior imagination.  But take Einstein’s imagination today and give it to a twenty-five year old high school dropout, and he’d be lost in obscurity, stocking shelves at Wal-Mart.  Imagination is more important than knowledge.  But only slightly more.

[Note: my book Why Is There Anything? is now available for download on the Kindle!]

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As I was driving to school today, there was a story on NPR about an upcoming ballot initiative in Missouri.  Proponents want to raise the cigarette tax from $0.17 per pack (the lowest rate in the nation) to $0.90 per pack.  The idea is to generate some revenue and at the same time discourage smoking.

Missourians love their cancer sticks.

Now, I’m not going to go into the pros and “cons” of the initiative.  I would be for it, based on some pretty common-sense data, but I don’t live in Missouri so I guess my opinion doesn’t matter.  However, I’d like to comment on some squishy reasoning put forth on the radio by an opponent of the measure.

In a nutshell: “The measure,” the opponent says, “wouldn’t raise money at all.  It would actually lower revenue.  That’s because any gains made by raising the tax would lower the amount of regular sales tax accrued.” [Note: this isn’t a verbatim quote but a recreation based on my imperfect memory]

You see, Missouri has a 4.225% sales tax which also applies to cigarettes.  And, the “logic” goes, if less people are buying cigarettes, the less sales tax revenue would be generated.

Really?  Is this person completely innumerate?  Let’s say that a pack of cigarettes costs $6.00.  Without the ballot initiative, a Missourian would pay $6.00, plus $0.25 in sales tax, plus $0.17 in cigarette tax, for a total of $6.42.  The state gets $0.42 cents a pack.

Under the new plan, a smoker pays $6.00, plus $0.25 in sales tax, plus $0.90 in cigarette tax, for a total of $7.15.  The state gets $1.15 cents a pack, obviously an increase.

To be fair, the state does actually lose money if the gain in revenue per pack is offset by the loss in the number of sales.  But when would this occur?  Let’s use algebra to find out.  Suppose that N1 is the number of packs of cigarettes bought in Missouri in a given year.  Under the current system, and for a $6.00 average price per pack, the yearly intake for the state is just N1 x 0.42.  Now let N2 be the number of packs bought under the new plan.  It’s easy to see that if the proposal passes, the yearly intake for the state is N2 x 1.15.

The problem is that N2 < N1 (presumably).  So the proposal loses money if N1 x 0.42 > N2 x 1.15, which, after one line of 8th-grade algebra, is the same as N1/ N2 > 2.74.  That is, translated into English, the new proposal loses money compared to the current system if state cigarette purchasing goes down by a factor of almost 3.

I don’t know about you, but I seriously doubt this initiative will cut smoking in Missouri by that much.

The numerically savvy will notice that there is a subtlety.  This result is for a $6.00 pack of cigarettes.  What if the price is much different?  This matters, because the cigarette tax is a flat number added to a pack, whereas the sales tax is a percentage.  If cigarettes cost the same as houses the ballot initiative would be ridiculous, since 4.225% of a huge number is much, much greater than a measly $0.90.  No one would buy cigarettes at all, and the state would lose a lot of revenue.

Is there a tipping point?  That is, is there a price for a pack of cigarettes for which the proposal loses money for any decrease in purchasing? Surprisingly, the answer is no.  Mathematically, we would say that N1/ N2 = 1 only when the price of a pack is infinite.

But setting the bar at N1/ N2 = 1 is unrealistic.  Any tax hike will cause some smokers to buy less.  So let’s make an educated guess:  looking at the first graph at this site, I can see that a price hike of $0.73 produced a corresponding decrease in consumption of about 25%.  (The actual drop would probably be less than this, since in the 1980’s and 1990’s $0.73 represented a higher percentage of the cost of one pack).  So we let N1/ N2 = 1/0.75 = 1.33, and solve for the price of cigarettes.

You get $48.33.  For one pack of cigarettes.

Conclusion: the proposed tax increase would increase revenue even with a plausible decrease in sales, unless cigarettes cost around $48 or more per pack.

Yay algebra!

Some close analogue to Mark Twain or E. B. White (with so many misquotes on the web, you can’t be very sure these days) said that analyzing humor is like dissecting a frog: you don’t learn much, and the frog dies.  So why did I dissect this frog, and show all the gory math details?

I wanted to point out that there is a lot more mathematical detail in most stories you hear, most issues you examine, than you suspect.  And I want to emphasize: if you don’t know math, if you are not mathematically literate, then you don’t know much of anything.

There.  I said it.

Go study your algebra.

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