Friday, March 19, 2021

The Folly of Brain Copying: Conscious Identity vs. Physical Identity

The notion of “identity” is a recurring problem both in physics and in the nature of consciousness.  Philosophers love to discuss consciousness with brain-in-a-vat type thought experiments involving brain copying.  The typical argument goes something like this:

i)          The brain creates consciousness.

ii)         It is physically possible to copy the brain and thereby create two people having the same conscious states.

iii)        Two people having the same conscious states each identifies as the “actual” one, but at least one is incorrect.

iv)        Therefore, conscious identity (aka personal identity) is an illusion.

I spent a long time in Section II of this paper explaining why questioning the existence of conscious identity is futile and why the above logic is either invalid or inapplicable.  Yes, we have a persistent (or “transtemporal”) conscious identity; doubting that notion would unravel the very nature of scientific inquiry.  Of course, you might ask why anyone would actually doubt if conscious identity exists.  Suffice it to say that this wacky viewpoint tends to be held by those who subscribe to the equally wacky Many Worlds Interpretation (“MWI”) of quantum mechanics, which is logically inconsistent with a transtemporal conscious identity.

I showed in Section III of the above paper why special relativity prevents the existence of more than one instantiation of a physical state creating a particular conscious state.  In other words, at least one of assumptions i) and ii) above is false.  For whatever reason, the universe prohibits the duplication or repeating of consciousness-producing physical states.  In Section IV(A) of the same paper, I suggested some possible explanatory hypotheses for the mechanism(s) by which such duplications may be physically prevented, such as quantum no-cloning. 

Nevertheless, the philosopher’s argument seems irresistible... after all, why can’t we make a “perfect” copy of a brain?  If multiple instances of the same conscious state are physically impossible then what is the physical explanation for why two consciousness-producing physical states cannot be identical?  I finally realized that conscious identity implies physical identity.  In other words, if conscious identity is preserved over time, then physical identity must also be preserved over time, and this may help explain why the philosopher’s brain-copying scheme is a nonstarter.

I’d been struggling for some time with the notion of physical identity, such as in this blog post and this preprint.  The problem can be presented a couple ways:

·         According to the Standard Model of physics, the universe seems to be made up of only a handful of fundamental particles, and each of these particles is “identical” to another.  For example, any two electrons are identical, as are any two protons, or any two muons, etc.  The word “identical” is a derivative of “identity,” so it’s easy to confuse two “identical” electrons as being indistinguishable and thus having the same (or indistinct) identities.  So if all matter is made up of atoms comprising electrons, protons, and neutrons, then how can any particular clump of atoms have a different identity than another clump made of the same type of atoms?

·         Let’s assume that consciousness is created by physical matter and that physical matter is nothing but a collection of otherwise identical electrons, protons, and neutrons.  In the above paper I showed that if conscious identity exists, then conscious states cannot be copied or repeated.  And that means there is something fundamentally un-copiable about the physical state that creates a particular conscious state, which would seem odd if all matter is fundamentally identical. 

·         Consciousness includes transtemporal identity.  Assuming physicalism is true, then conscious states are created by underlying physical states, which means those physical states must have identity.  But physics tells us that physical matter comprises otherwise identical particles.

I finally realized that this problem can be solved if particles, atoms, etc., can themselves have identity.  (I do not mean conscious identity... simply that it makes sense to discuss Electron “Alice” and Electron “Bob” and keep track of them separately... that they are physically distinguishable.)  An object’s identity can be determined by several factors (e.g., position, entanglements and history of interactions, etc.) and therefore can be distinguished from another object that happens to comprise the same kind of particles.  Two physically “identical” objects can still maintain separate “identities” to the extent that they are distinguishable.  And we can distinguish (or separately identify) two objects, no matter how physically similar they may otherwise be, by their respective histories and entanglements and how those histories and entanglements affect their future states. 

Where does physical identity come from?  It is a necessary consequence of the laws of physics.  For instance, imagine we have an electron source in the center of a sphere, where the sphere’s entire surface is a detector (assume 100% efficiency) that is separated into hemispheres A and B.  The detector is designed so that if an electron is detected in hemisphere A, an alarm immediately sounds, but if it is detected in hemisphere B, a delayed alarm sounds one minute later.  The source then emits an electron, but we do not immediately hear the alarm.  What do we now know?  We know that an electron has been detected in hemisphere B and that we will hear an alarm in one minute.  Because we know this for certain, we conclude that the detected electron is the same as the emitted electron.  It has the same identity.  The following logical statement is true:

(electron emitted) ∩ (no detection in hemisphere A) à (detection in hemisphere B)

But more importantly, the fact that the above statement is true itself implies that the electron has identity.  In other words:

[(electron emitted) ∩ (no detection in hemisphere A) à (detection in hemisphere B)]

à (the electron emitted is the electron detected in hemisphere B)

(On retrospect, I feel like this is obvious.  Of course physical identity is inherent in the laws of physics.  How could Newton measure the acceleration of a falling apple if it’s not the same apple at different moments in time?)

So if electrons can have identity, then in what sense are they identical?  Can they lose their identity?  Yes.  Imagine Electron Alice and Electron Bob, each newly created by an electron source and having different positions (i.e., their distinct wave packets providing their separate identities).  The fact that they are distinguishable maintains their identity.  For example, if we measure an electron where Electron Bob cannot be found, then we know it was Electron Alice.  However, electrons, like all matter, disperse via quantum uncertainty.  So what happens if their wave functions overlap so that an electron detection can no longer distinguish them?  That’s when Bob and Alice lose their identity.  That’s when there is no fact about which electron is which.  (As a side note, Electron Bob could not have a conscious identity given that when he becomes indistinguishable with Electron Alice, even he cannot distinguish Bob from Alice.  This suggests that conscious identity cannot even arise until physical identity is transtemporally secured.)

This realization clarified my understanding of conscious identity.  My body clearly has an identity right now in at least the same sense that Electron Bob does.  What would it take to lose that physical identity?  Well, it wouldn’t be enough to make an atom-by-atom copy of the atoms in my body (call it “Andrew-copy”), because Andrew-copy would still be distinguishable from me by nature, for example, of its different location.  Rather, the wave functions of every single particle making up my body and the body of Andrew-copy would have to overlap so that we are actually indistinguishable.  But, as I showed in this paper, that kind of thing simply can’t happen with macroscopic objects in the physical universe because of the combination of slow quantum dispersion with fast decoherence.

What would it take for me to lose my conscious identity (or copy it, or get it confused with another identity, etc.)?  Given that conscious states cannot be physically copied or repeated, if conscious identity depends only the particular arrangement of otherwise identical particles that make up matter, then we need a physical explanation for what prevents the copying of that particular arrangement.  But if conscious identity depends on not just the arrangement of those (otherwise identical) particles but also on their physical distinguishability, then the problem is solved.  Here’s why.  Two macroscopic objects, like bowling balls, will always be physically distinguishable in this universe.  Bowling Ball A will always be identifiably distinct from Bowling Ball B, whether or not any particular person can distinguish them.  So if my conscious identity depends at least in part on the physical distinguishability of the particles/atoms/objects that create my consciousness, then that fact alone would explain why conscious states (and their corresponding transtemporal identity) cannot be copied.

Let me put this another way.  Identity is about distinguishability.  It is possible for two electrons to be physically indistinguishable, such as when the wave states of two previously distinguishable electrons overlap.  However, it is not possible, in the actual universe, for a cat (or any macroscopic object) and another clump of matter to be physically indistinguishable because it is not possible for the wave states of these two macroscopic objects to overlap, no matter how physically similar they may otherwise be.  A cat’s physical identity cannot be lost by trying to make a physical copy of it.  It is not enough to somehow assemble a set of ≈10^23 atoms that are physically identical to, and in a physically identical arrangement as, the ≈10^23 atoms comprising the cat.  Each of those constituent atoms also has a history of interactions and entanglements that narrowly localize their wave functions to such an extent that overlap of those wave functions between corresponding atoms of the original cat and the copy cat is physically impossible.  (See note below on the Myth of the Gaussian.)

Imagine that someone has claimed to have made a “perfect copy” of me in order to prove that conscious identity is just an illusion.  He claims that Andrew-copy is indistinguishable from me, that no one else can tell the difference, that the copy looks and acts just like me.  Of course, I will know that he’s wrong: even if no one else can distinguish the copy from me, I can.  And that alone is enough to establish that Andrew-copy is not a perfect copy.  But now I understand that my conscious identity implies physical identity – that my ability to distinguish Andrew-copy from me also implies physical distinguishability.  There is no such thing as a perfect physical copy of me.  Even if the atoms in Andrew-copy are in some sense the same and in the same configuration as those in my body, and even if some arbitrary person cannot distinguish me from Andrew-copy, the universe can.  The atoms in Andrew-copy have a history and entanglements that are distinguishable from the atoms in my body, the net result being that the two bodies are physically distinguishable; their separate physical identities are embedded as facts in the history of the universe.

So if the universe can distinguish me from Andrew-copy, then why should it be surprising that I can distinguish myself from Andrew-copy and that I have an enduring conscious identity?  The question is not whether some evil genius can make a physical copy of my body that is indistinguishable to others.  The question is whether he can make a copy that is indistinguishable to me or the universe.  And the answer is that he can’t because making that copy violates special relativity. 


Note on the Myth of the Gaussian:

Physicists often approximate wave functions in the position basis as Gaussian distributions, in large part because Gaussians have useful mathematical properties, notably that the Fourier transform of a Gaussian is another Gaussian.  Because the standard deviation of a Gaussian is inversely related to the standard deviation of its Fourier transform, it clearly demonstrates the quantum uncertainty principle whereby the commutator of two noncommuting operators is nonzero.  An important feature of a Gaussian is that it is never zero for arbitrarily large distances from the mean.  This treatment of wave functions often misleads students into believing that wave functions are or must be Gaussian and that: a) an object can be found anywhere; and b) the wave states of any two arbitrary identical objects always overlap.  Neither is true. 

Regarding a), physics students are often given the problem of calculating the probability that his/her body will quantum mechanically tunnel through a wall, or even tunnel to Mars; the calculation (which is based on the simple notion of a particle of mass M tunneling through a potential barrier V) always yields an extremely tiny but nonzero probability.  But that’s wrong.  Setting aside the problem with special relativity – i.e., if I am on Earth now, I can’t be measured a moment later on Mars without exceeding the speed of light – the main problem is physical distinguishability.  The future possibilities for my body (and its physical constituents) are limited by their histories and entanglements. 

While some electron may, due to quantum dispersion or being trapped in a potential well, develop a relatively wide quantum wave packet over time whose width “leaks” to the other side of the wall/potential barrier, this requires that the electron remain unmeasured (i.e., with no new correlations) during that time period.  But the particles and atoms in a human body are constantly “measuring each other” through decoherence so that their individual wave packets remain extremely tightly localized.  In other words, my body doesn’t get quantum mechanically “fuzzy” or “blurry” over time.  Thus none of the wave packets of the objects comprising my body get big enough to leak through (or even to) the wall.  More to the point, the QM “blurriness” of my body is significantly less than anything that can be seen... I haven’t done the calculation, but the maximum width of any wave packet (not the FWHM of a Gaussian, which extends to infinity, but the actual maximum extent) is much, much, much smaller than the wavelength of light. 

As I showed above, physical distinguishability is an inherent feature of the physical world.  An object that appeared on the other side of the wall that happened to look like my body would be physically distinguishable from my body and cannot be the same.  That is, there is no sense in which the body that I identify as mine could quantum mechanically tunnel to Mars or through a wall – that is, there is ZERO probability of me tunneling to Mars or through a wall.  If I have just been measured in location A (which is constantly happening thanks to constant decohering interactions among the universe and the objects comprising my body), then tunneling to location B requires an expansion of the wave packets of those objects to include location B – i.e., my tunneling to B requires a location superposition in which B is a possibility.  But past facts, including the fact that I am on Earth (or this side of the wall) right now have eliminated all configurations in which my body is on Mars (or on the other side of the wall) a moment later.

Thursday, February 25, 2021

“Interaction-Free Measurements” in Quantum Mechanics are Not Surprising

Let’s say I write a paper logically showing why the fragments of a detonating nuclear bomb cannot exceed the speed of light.  Would that be interesting?  Perhaps the nuclear bomb aspect might make the paper a little sexier, but clearly the paper wouldn’t add anything new to our understanding of special relativity (SR).  Nobody who understood SR would be surprised by the paper.  In the unlikely event that I managed to get the paper published, nobody would cite it, right?  And anyone who did cite it as a “surprising” result clearly doesn’t understand SR.  Right?  Having said that...

Quantum mechanics is all about one thing: negative probabilities.  Everything about it, particularly why it’s weird, can be summarized in the following very simple point about double-slit interference experiments.  It was found, empirically, that when we send a certain kind of stuff (“particles,” such as photons or electrons) through a very narrow slit in a plate, and we detect them on a screen that is parallel to and far away from the plate (called the far-field approximation), we find that individual particles are detected, and if we detect enough of them, their distribution forms what is called the Fraunhofer diffraction approximation:

(Please ignore the axis units.)  In the above example, the probability of detecting a particle at, for example, location A is relatively high.  It was also found, empirically, that if we redo the experiment using two closely-spaced narrow slits (say, a left slit and a right slit), we find that the detected particles form what is called an interference pattern:

Notice that the interference pattern seems like it could fit inside the diffraction pattern shown earlier; we call this the diffraction envelope.  In the above example, the distance between the slits is about four times the slit width, and the greater this ratio, the narrower the distance between peaks inside the diffraction envelope.  Notice also that the likelihood of detecting a particle at location A is now zero. 

That’s right.  If only one slit had been open, the probability of detecting a particle at this point would have been nonzero.  So how is it that by adding another slit – by adding another possible path through which a particle could reach location A – we decrease its likelihood to reach location A?  The answer, mathematically, is that by adding probability amplitudes of waves prior to taking their magnitude, terms that are out of phase can cancel each other, resulting in a negative probability.  The answer, conceptually, is that the “particle” isn’t really a particle until it is actually detected.  It is only by assuming that there is a particle that traversed either the left slit or the right slit that we run into trouble.

And that’s it.  That’s the very essence of quantum mechanics. 

Now, let’s say that you’re about to do a double-slit interference experiment on electrons.  Just before you start, you have to use the bathroom so you put your lab partner in charge.  When you return, your lab partner says, “I was messing around with the double-slit plate and a foreign object – maybe a speck of dust – might have gotten stuck in the right slit.  But the left slit is fine.”  You go ahead with the experiment and send a single electron through, which you happen to detect at location A.  What does this tell you?

It tells you that an object must be in the right slit, because if they were both fully open, then interference would have prevented the detection of the electron at location A on the screen.  It also tells you that because the electron was in fact detected on the screen, it was not absorbed (or scattered) by the object in the right slit.  In that sense, you have managed to figure out that an object is in the right slit without actually hitting the object with an electron. 

There is absolutely nothing interesting or surprising about the above point.  In other words, once you’ve accepted that quantum mechanics allows negative probabilities, then of course you can set up a quantum mechanical interference experiment in which the detection of a particle in a particular place (or by a particular detector) renders information about the presence or absence of another object that obviously did not absorb or scatter that particle.

In 1993, a famous paper was published in which the above example was characterized as an “interaction-free measurement.”  (The Wikipedia entry on it is terribly written but at least gives the general idea.)  It described what came to be known as the Elitzur-Vaidman bomb tester, in which a bomb would go off if its sensor absorbs a single photon, but defective sensors (of defective bombs) would allow photons to pass through unaffected.  The general idea is nothing more than what I described above – you can set up the experiment so that detection of a photon in a particular place (such as location A) tells you that the sensor/bomb is operational even though the sensor did not absorb the photon. 

The whole “bomb detection” notion was just a way to make the paper a little bit sexier but didn’t add anything to our understanding of quantum mechanics.  To be fair, the paper wasn’t completely useless... it did explain how to increase the efficiency of detection to 50%.  (A paper published in 1995 showed how to push the efficiency much higher.)  In my example above, the likelihood of detecting an electron at location A is of course very low, yielding a very low efficiency, but the fact that it is nonzero is what clearly demonstrates that an object can be “measured” in the right slit without it absorbing or scattering the electron. 

And there is nothing interesting or surprising about that fact over and above the fact that quantum mechanics allows negative probabilities. 

So why did I write this post about a 1993 paper whose conclusion should have been obvious to anyone who understood quantum mechanics?  Because it has been cited over 800 times by publications, many of which continue to characterize “interaction-free measurement” as some kind of inexplicable paradox within quantum mechanics.  What might that tell us about the credibility of those papers or their authors as experts on quantum mechanics?

Part of the confusion is the incorrect notion that an “interaction” only occurs if the object being detected (bomb sensor, speck of dust, etc.) actually absorbs or scatters a particle.  Quantum mechanical waves are constantly interacting with other objects.  In the double-slit interference experiment above, the waves emanating from only the left slit (when the right slit is clogged with a dust speck) are different from waves that would emanate from both the left and right slits, which is why the screen detection distributions differ.  Therefore, the electron wave did interact with the speck of dust in the right slit even if the entirety of the electron wave ultimately collapses onto the screen and not the speck of dust.  In other words, to say that the electron didn’t interact with the right slit presupposes that the electron is a particle, but it does not assume a particle form until it is detected!  The entire misnomer of “interaction-free measurement” assumes that only “particles” can interact, but photons and electrons do not take on particle-like qualities until they are measured!  (Specifically, the particle- and wave-like characteristics of an object are complementary.)

Some of this confusion is clarified by Vaidman himself (such as here) and by other papers (such as this).  I am not criticizing the discussion.  I am simply pointing out that “interaction-free measurements” should never have been surprising in the first place.

Monday, February 22, 2021

Does Consciousness Cause Collapse of the Quantum Mechanical Wave Function?


First, at this point I am reasonably confident that collapse actually happens.  Either it does or it doesn’t, and non-collapse interpretations of QM are those that have unfounded faith that quantum wave states always evolve unitarily.  As I argued in this paper, that assumption is a logically invalid inference.  So given that we don’t observe quantum superpositions in the macroscopic world, I’d wager very heavily on the conclusion that collapse actually happens.

But what causes it?  Since we can’t consciously observe a (collapsed) quantum mechanical outcome without being conscious – duh! – many have argued that conscious observation actually causes collapse.  (Others have argued that consciousness and collapse are related in different ways, such as collapse causing consciousness.)  In this blog post, I discussed the consciousness-causes-collapse hypothesis (“CCCH”) in quantum mechanics.  I pointed out that even though I didn’t think CCCH was correct, it had not yet been falsified, despite an awful paper that claimed to have falsified it (which I refuted in this paper).

Two things have happened since then.  First, I showed in this paper that the relativity of quantum superpositions is inconsistent with the preparation of macroscopic quantum superpositions, which itself implies that CCCH is false. 

Second, this paper was published a few days ago.  Essentially, it’s a Wigner’s-Friend-esque thought experiment in which a poison-containing breaks or does not break at 12pm, per a QM outcome, but the person in the room will be unconscious until 1pm.  That’s it.  If CCCH is correct, then collapse of the wave function will not occur until the person is conscious at 1pm... but if he is conscious at 1pm, how could the wave state possibly collapse to an outcome in which the person dies at noon?  It’s a very simple logical argument (even though it is not explained well in the paper) that is probably valid, given some basic assumptions about CCCH.

So when does collapse actually occur?  I’ve been arguing that it happens as soon as an event or new fact (i.e., new information) eliminates possibilities, and the essentially universal entanglement of stuff in the universe (due to transitivity of correlation) makes it so that macroscopically distinct possibilities are eliminated very, very quickly.  For example, you might have a large molecule in a superposition of two macroscopically distinct position eigenstates, but almost immediately one of those possible states gets eliminated by some decoherence event, in which new information is produced in the universe that actualizes the molecule’s location in one of those position eigenstates.  That is the actual collapse, and it happens long before any quantum superposition could get amplified to a macroscopic superposition.

Monday, February 8, 2021

Bitcoin, Speculation, and Legal Tender Laws

Note: I took Banking Law, and received one of only two As, at Georgetown University Law Center under Prof. Daniel Tarullo who, from 2009 to 2017, was a Governor of the Federal Reserve Board.

Bitcoin is currently at $43,389.

And everything you need to know about Bitcoin is contained in that one sentence.  In other words, beyond its cost, there is nothing interesting about Bitcoin (or any other electronic “currency”).

Oh, there are interesting facts about where it comes from (and the monumental waste in “producing” it), as well as the utility of block chain technology (which is actually independent of electronic currencies).  But there is nothing interesting about a Bitcoin itself, which is just a seemingly random string of bits.  This isn’t true of gold or silver or wine or emeralds or cars or real estate.  Sure, one of my rental houses might have a market value of $150,000, but there are lots of interesting facts about it other than its “exchange rate” in dollars.  For example, it provides my tenant shelter, modern plumbing, electric conveniences, a big back yard, etc.  And we can debate all day about the intrinsic value of gold, but it is a good electrical conductor and people like wearing it as jewelry.  At least it does something. 

In sharp contrast, Bitcoin doesn’t do anything.  And it’s not because it’s a string of bits.  Hell, software is just a string of bits and so is the information in your favorite movie or Netflix show.  Unlike these, 1BTC is literally a useless string of bits that is simply recognized as “one Bitcoin” by the open-source Bitcoin algorithm.  Its only value is that ascribed by those who own it and/or want it.

“OK, so what?” asks the enthusiast.  “That’s also true of fiat money like the U.S. dollar.”

My three-word answer: LEGAL TENDER LAWS.

Look, there are a thousand reasons to hate Bitcoin, so I’m not going to mention any of them except the one that no one else seems to be talking about – namely, the fact that governments extract wealth from their citizens in the form of taxation, and taxes will always be payable in the governments’ chosen currency.

There is a common fear among Bitcoin enthusiasts that the government will eventually act to shut down electronic currencies.  Sure, that’s a possibility, but that’s not the main problem with Bitcoin.  The real problem – which almost no one seems to realize – is that the government is never going to accept Bitcoin in payment for taxes.  There is no government on Earth that accepts Bitcoin as payment or as legal tender.  Why?  Because accepting payment in an alternative currency devalues their own state-sanctioned currency.  Historically, there are a few shitty rogue governments that have been so incompetent with their own monopoly over currency issuance that their economies are either effectively or legally dollarized.  Zimbabwe springs to mind with its moronic (and fascinating!) $100 trillion bills.  Within the states and territories of the United States, the U.S. dollar is legal tender, which means that all debts, particularly debts to local, state, and federal governments, are payable in this currency and nothing else. 

You cannot pay your New York property taxes in British Pounds.  You must pay it in U.S. dollars or else the state will foreclose on your property.  If you happen to have a bunch of British Pounds, luckily there are 67 million people on an island across the Atlantic who need British Pounds to pay their property taxes to their government.  The meeting of supply with demand creates an exchange rate.  You cannot pay your U.S. income taxes in Indian Rupees.  If you happen to find yourself awash in Rupees, there are 1.4 billion people who need Rupees to pay taxes to their government, and the resulting currency market will allow you to exchange your Rupees for Dollars so you can pay your income tax bill.  The government has a monopoly on the use of force, and it will ultimately use that force to collect taxes on income, sales, property, value-added, etc.

So let’s say you use 2BTC, which you bought ten years ago for a nickel (or whatever) to buy a Tesla automobile, as Tesla apparently plans to start accepting it in payment.  Under the Internal Revenue Code, that is a realization event that makes you liable for taxes on $86,778 in gains.  But you cannot pay this tax in Bitcoin, nor will you ever be able to.  (The Federal Reserve owns the planet.  OK, it sort of shares it with a few other central banks, like the Bank of England, the European Central Bank, etc.)  And since no other government forces its citizens to pay taxes in Bitcoin, there is no “exchange rate” for Bitcoin.  To pay your taxes, you have to get U.S. dollars either by earning them or selling more Bitcoin, which means that the value of Bitcoin must always be denominated in some other country’s currency.

In other words, because Bitcoin is not and never will be legal tender in any country, it will never stand on its own.  The question will always – always, always, always, always, always – be “How much is Bitcoin today?” 

And that’s a problem... an insurmountable problem for Bitcoin enthusiasts.  When you are about to make a purchase in a store in Paris, the clerk doesn’t have to ask, “How much is the Euro today?”  In fact, to most Europeans, that question wouldn’t even make sense.  After all, 1 Euro is 1 Euro!  The store clerk does not need to look up the “value” of the Euro in terms of other currencies or commodities.  She doesn’t care.  She knows she needs Euros to pay her rent, her bills, and – most importantly – her taxes.  But Bitcoin is different.  A price will NEVER be fixed in Bitcoin... every transaction involving Bitcoin will ultimately involve some person or computer asking the question, “How much is Bitcoin today?”

I don’t want Bitcoin because it has no intrinsic value or use.  Governments don’t want Bitcoin because it devalues their monopoly on currency issuance.  And here’s the thing.  Even Bitcoin owners and enthusiasts don’t want Bitcoin. 

“Andrew, shut up.  Of course they do – that’s why they bought it!” 

Wrong.  They bought it because they think others want it.  (Conversely: if they did not think others wanted it, then no one would buy it.)

I used to collect old U.S. coins because I thought they were fascinating and I loved the history.  When I would share my collection with other numismatists, occasionally one of us would say something like, “Can you believe how much this coin is worth?!”  But that wasn’t the focus of our conversation.  We talked about minting, and history, and coin material and condition, and fascinating mint errors like double-struck coins, etc.  The point is that there was substance to the conversation because we actually enjoyed and valued and appreciated the asset, with “dollar exchange rate” a secondary consideration.

Not so with Bitcoin.  After countless conversations with Bitcoin enthusiasts (who tend to show up in droves at Libertarian conventions), I have learned that conversations revolve almost entirely around these two general topics:

* “The price of Bitcoin is $_____... can you believe it?!”  (Sometimes it’s way up, sometimes it’s way down – the only apparent consistency in the Bitcoin price is its volatility.)

* “Death to the Dollar (or Pound or Yen or Transnistrian Ruble)!”

In other words, even Bitcoin owners and enthusiasts don’t value Bitcoin per se – of course they don’t!  It’s just a useless string of bits!  Rather, they value it in terms of its selling price in dollars.

Let that sink in.  Bitcoin enthusiasts hate the U.S. dollar so much that they purchase a useless string of bits whose value – as judged by their own conversations – is determined by the number of those hated U.S. dollars they can sell it for.  That is madness.

The last thing I want to mention is speculation.  If I can pick a booger and manage, through suave argumentation, to convince a handful of people that it is worth a million dollars – is it actually worth a million dollars?  Value is a very subjective thing and the phrase “market value” only has meaning in an efficient and rational market.  The fact that Bitcoin is at $43,389 is exciting to a lot of people.  There are people who will pay this amount and more for 1BTC.  There may very well be people who, under the right conditions, would pay $1 million for 1BTC.  Just keep in mind that it is pure speculation.  Unlike a tulip, which at least offers the tiny subjective value of being easy on the eyes, Bitcoin’s only “value” is its price as denominated in fiat currencies.

And as much as one might despise the U.S. dollar for its lack of intrinsic value, it at least has the ability to prevent IRS agents from confiscating one's property.  Bitcoin cannot do that.  It cannot do anything.

Bitcoin is currently at $43,389.

And that’s all there is to say. 

Saturday, January 30, 2021

Fucking COVID

First, let me say that there are many, many, many, many people in the world who are suffering far worse than I am.  But invalidating my own feelings just makes my suffering worse.  Here are some ways I am suffering and feeling frustrated, irritated, and disconnected as a direct result of this virus.

I am sick of decisionmakers only considering the fatality rate of the actual virus when making strong pronouncements, rules, and/or edicts.  I made this point back in April in my only Facebook post on this topic.  There are lots of other factors that should be considered in the analysis, such as:

·         People who are dying of starvation because the slowdown in the world economy, thanks in part to shutdown orders, is literally depriving the millions of people in the world who already can barely feed themselves.  Even among those who aren’t literally dying of starvation, there are untold millions who are dying from otherwise preventable or treatable diseases and problems because of their falling real incomes.

·         People who are dying of suicide, alcoholism, drug overdoses, and complications of depression and poor physical and mental health brought on or exacerbated by this crisis.

·         The overall decrease in happiness (“utility”) in the world, linked to economic slowdowns, physical and emotional isolation, etc., etc.

I hate that it has only recently become somewhat socially acceptable to mention these other factors.  Yes, public policy must consider lives potentially lost to COVID, but it should not ignore lives and livelihoods that may be lost as a direct result of these policies. 

I am sick of being told what to do by authoritarians acting unilaterally.  Yes, governors and mayors (as heads of their respective executive departments) have short-term emergency powers, but this situation is no longer short-term.  Long-term legal solutions should be effected by legislatures and tempered by courts.  There is absolutely no good reason or explanation for why, a year after the virus first hit U.S. soil, decisions about which New York City restaurants can open and at what capacity are still being decided at the arbitrary whim of a single person.

I am sick of the fatigue and the ever-changing goal post.  In March of last year, we were told that shutdowns and closures were temporary so that we can “flatten the curve” so as to not overwhelm the healthcare industry.  We did that.  And then the curve started dropping.  But then the goal changed to something like, “We need to stay locked down until we reduce the positivity rate below X%.”  Where did that come from?  More importantly, such a number is absolutely meaningless unless there is consistency in the rate at which people are tested.  If New York’s positivity rate is 3%, for example, and South Dakota’s is 30%, does that imply that South Dakota is far worse off than New York?  Of course not.  If New York freely tests anyone and everyone (and if people’s jobs there depend, as they often do, on regular testing), then a 3% positivity rate might actually mean that, say, 1% of the population has the virus... while if South Dakota only tests people who are hospitalized and who have COVID symptoms, then their 30% positivity rate might actually correspond to, say, 0.1% of the population having the virus.  (On a recent trip to Florida, I discovered just how complicated, expensive, and inconvenient it was to get a COVID test, while in NYC I can easily get a free test any day of the week just a couple blocks from my apartment.)  The fact that I have to explain this very simple example to show how statistics can deceive – and which no one seems to be talking about, including the condescending intellectual elite who have weaponized the word “science” – is irritating and mind-boggling.  (As a side note, the weaponization of the word “science” is especially infuriating when scientists whom we should trust, like Dr. Fauci, simply make up numbers to manipulate public perception.  I think his intention was good, but that’s irrelevant – it makes him lose credibility as a scientist.)

I am sick of masks.  I hate wearing them.  They are not just uncomfortable, but I hate that they impede communication.  They muffle voices and hide facial expressions and moving lips, all of which are used in human communication.  I hate having to speak more loudly to people because of the muffling of my mask, and I hate that my facial expressions are similarly muffled.  It just makes me less likely to interact and communicate with people at all.  It makes me not want to go anywhere or do anything, which is fine because there is nowhere to go and nothing to do anyway.

I am sick of never knowing what is required, legally or socially, and I am sick of feeling pressured to act in ways that I know are baseless or even irrational.  Are we supposed to wear a mask AND “socially distance”?  What is the protocol for inside versus outside?  No one suggests (I don’t think!) that spouses should wear masks around each other, but what if my wife works closely with a small group of people who eat together out of necessity?  Does that mean she can invite them and their spouses over to our house for a mask-free game night... or must we play an awkward game of cards in which we’re all separated by six feet and breathing through N95 masks?  Literally the social fear of not knowing all the rules (because there is no consistent set of rules) is enough to want to avoid gatherings at all, no matter how desperately we might need them for our emotional survival.  I can’t even imagine being a single college student today and being told to wear a mask and not kiss while having sex.  No normal human would do this, right?  It sometimes makes me wonder if those in charge are playing a massive prank to see just how far they can push the rules.  (“Do you think we could get restaurants to agree to a 6% maximum occupancy and serve only pureed entrées that are sucked through a straw attached to their mask?”  “Yeah!  And all male patrons should be required to wear N95 condoms!”)

Moreover... Do these rules arise from a responsibility to others or to myself?  For example, the virus is spread by saliva and mucus, so if I don’t talk or even open my mouth... if I cover my face when I sneeze... then how can I spread it to anyone, particularly if I’m already standing six feet away?  This issue has come up a few times when I was wearing a mask over my mouth but not my nose.  (Apparently I have a big nose, which makes a mask even more uncomfortable.)  If I have to sneeze, I pull my mask up – which is disgusting, of course, but I guess that’s what others have to deal with.  Beyond sneezing, there is essentially no risk of me spreading a virus to others from my nose, yet I’ve been ordered by various people out in public to cover my nose.  I usually do, to avoid a confrontation, but why should I?  One might reply, “Because you can get the virus with your nose exposed, and you have a responsibility not to get the virus that you can then spread to others.”  But that doesn’t make sense.  If I’m already careful about not spreading germs to others, then what right does anyone have to tell me that I can’t assume a risk to myself?  Do I have the right to acquire the virus if I want?  If I decided that I just wanted to get the virus for, say, the antibodies it would provide, and I knew someone infected who was happy to provide the requisite saliva, is it my right to choose?  I think the answer in a genuinely free country is obviously yes, but I can hear the protests already!  Anyway, my point is that there is room for actual debate about the pros and cons of each rule and how they depend on situation, but what actually happens – at least in my case – is that the social complications and awkwardness of trying to balance my own comfort with my aversion to offending people with my attempt to read social cues with my unwillingness to let bullies dictate rules for everyone else with my efforts to prevent spreading COVID to my older relatives and friends with my desire to actually have fun while other people have fun... ugh... all this just makes me say “Fuck it” and stay at home.

I am sick of the phrase “socially distance.”  Whoever coined it was either a complete idiot who doesn’t know what the word “social” means, because the requirement is actually to physically distance, or was brilliant, because they knew that the constant physical distancing would, over time, emotionally wear people down and cause them to socially isolate from others.

I am sick of the polarized extremes that force people to pick a “team” and prevent them from thinking for themselves.  Do I really have to choose being either a condescending elitist mask nazi or a confederate-flag-toting anarchist?  Can’t I just say that this forced economic shutdown and physical isolation must end soon without being accused of wanting people to die of the virus?

Personally, here is how I am suffering from COVID... and I am suffering:

My primary hobby is travel.  I’ve been to 96 countries.  I canceled three trips last year, including a six-week round-the-world trip with my wife who had just finished a grueling four-year physician residency during which I barely spent any time with her.  Not only have I not traveled anywhere in the past year, but there appears to be no end in sight to this “pandemic” (another word I am sick of), with prospects of returning to any semblance of normalcy diminishing by the day.  I can’t plan anything.  The rules keep changing.  Do I need to wait until after I’m vaccinated?  At this rate, it might not be until summer, and by then, will there be a hundred new vaccine-resistant variants/mutations?  Will I be traveling in some country that was “open” when their government suddenly closes their borders?  Will I be walking around some foreign city but all its museums and restaurants are closed and I’m staying in an otherwise deserted hotel?  This all just makes me feel very hopeless and depressed.

This “virtual” learning is a disaster.  I don’t learn well this way and I suspect that few others do.  I have absolutely no idea how kids are dealing with this.  Oh, that’s right – they’re not.  I started a graduate physics program at NYU specifically for the opportunity to make friends, collaborate with colleagues, have interesting discussions, and learn with others.  The in-person time with professors and fellow students has always helped me to learn and to feel connected, but there is NONE of that now.  Death to Zoom.  Yes, some classes are offered “in-person” (or “hybrid,” which is almost worse than just plain “virtual”) and I took General Relativity last semester in part because it was in-person.  To do so, I satisfied all the requirements but what was the reward?  On average, there was only one other student in class, and even if there were more, there were no rooms or areas in which we could actually collaborate, so what was the point?  I am taking one class this semester, in part to keep my affiliation with NYU, but if normal in-person learning does not resume by Fall (and by “normal” I mean being able to go through one fucking day without thinking about masks, socially distancing, vaccines, nasal swabs, reduced capacity, etc., etc.), then I don’t see the point of continuing in the program. 

I am not connecting with people.  As an INTJ, I can certainly enjoy plenty of alone time, but I need connection, as do all humans.  It is depressingly ironic that I am calling and emailing people less now that I am connecting with them less in person.  That’s due to several factors:

·         We need in-person connection, in part because of body language and other nonverbal communication; spontaneous ideas and adventures happen when people are physically together that can’t happen when we’re staring at each other over Zoom; and we experience new things together that are actually worth talking about later!

·         In-person connection inspires deeper connection and a desire for follow-up phone calls and emails.

·         Right now, we aren’t, as a society, doing much, so there’s not much to talk about.  I have little interest in talking to a friend over the phone about COVID, my lack of connection in physics, my lack of traveling... my lack of anything!

Setting aside my dearth of social connection, I am experiencing the additional detriment of lack of connection in physics.  A little over two years ago, I started working essentially full-time in the field of the foundations of physics and the physics of consciousness, etc.  I have made significant and rewarding progress, both in learning as well as innovating and contributing.  Not only do I now have a relatively deep understanding of the foundations of quantum mechanics that has allowed me to make several contributions, I’ve also come to a deeper understanding of the relationship between physics and consciousness, free will, and other deep philosophical issues.  So that’s good.  However, it’s hard to do anything alone for a long time.  Even Henry David Thoreau eventually had to leave Walden Pond and rejoin society.

And in large part I’ve been working on physics entirely alone.  I’ve tried really, really, really hard to connect with people.  I started two different physics masters programs (ECU and NYU); I prepared to attend two conferences (one canceled and the other almost uselessly virtual); I’ve reached out directly to hundreds of professors, paper authors, graduate students, etc.; I’ve written and posted my own papers and YouTube videos; yada yada yada.  Despite these efforts, I feel almost completely disconnected and alone in this field.  Almost no one understands me, what I’m working on, or what I’ve figured out.  Among my friends and family, this work is far beyond the understanding and interest of anyone I know, although my wife and a couple other people certainly try!  Among people in the actual field – people with whom I’ve been trying to connect for several years now – maybe 1% of these contacts have actually resulted in any meaningful connection.  I am profoundly thankful for and humbled by these few contacts, and several of them have been both intellectually and emotionally supportive.  Ironically, among the tiny handful who have both understood and validated my work, a few of them happen to be the top of the top.  For instance, as I mention in this post, this paper was rejected by referees until the journal’s chief editor, Carlo Rovelli himself, took the extremely unusual step of publishing it despite the reviews because it raises “an interesting and well-argued point.”  I won’t give further details in this post, but as it turns out the reviewer I described in this post – who was one of the only people to actually understand and validate the argument I made in this paper – is one of the most intelligent, original, and influential thinkers in the field.  So I certainly am thankful for the few genuinely positive connections I’ve made, although it’s just not enough (at least right now) to emotionally and intellectually sustain me.  I need to go to conferences and attend classes and have in-person connections to keep me feeling stimulated, inspired, and connected.  Right now – and in the foreseeable future – that is simply impossible. 

So my suffering comes down to this: I am just not looking forward to anything.  I feel like my life is perpetually on hold with every day undetectably bleeding into the next.  I am sick of constantly waiting for normalcy.  I am sick of not knowing when, or even if, I will be able to travel, to efficiently learn about the physics of consciousness, to emotionally connect with friends, to intellectually connect with colleagues.  I am sick of not knowing when I will again feel engaged, inspired, in flow.

Pain is usually a sign that something’s wrong.  If I feel my hand burning, it might be because of some miswired neurons or something wrong with my skin, in which case medication might be appropriate.  But it might also be because my hand is in a fire, in which case the best solution is to pull it out!  Poor mental health – depression, anxiety, etc. – presumably evolved in humans to help us identify and fix problems and to ultimately thrive.  Sometimes, poor mental health is due to chemical imbalances that can be addressed with medication.  Sometimes it’s due to emotional trauma that can be addressed with counseling.  Sometimes it’s due to poor physical health that can be addressed with diet and exercise.  To the extent that poor mental health is the body reacting inappropriately to the world, then by all means let’s address the symptoms.  No sense in feeling the pain of anxiety when there’s nothing to feel anxious about... take a pill for crying out loud!

But sometimes, poor mental health is a direct result of circumstance and it’s the body’s appropriate response to dangerous or unhealthy surroundings.  Dulling that pain with a pill might only serve to increase one’s tolerance to what is, or should be, an intolerable situation.  I can’t speak for anyone else, but it is clear to me that the world I live in is currently intolerable.  Not that I can’t tolerate it, but that I shouldn’t. 

I don’t know what the solution is.  Like many others, fatigue is setting in.  If I knew for certain that normalcy would return by, say, July, then I could wait it out.  But if I am honest with myself, I am having growing doubts that normalcy will return at all, much less by summer.  Maybe it’s time to accept that the world I grew up in is gone forever: one in which everyone had more-or-less the same source of news and people didn’t choose their news media based on the “facts” they want to believe; one in which people could go to parties, dance clubs, restaurants, and theaters without constantly worrying about viruses; one in which human connection was almost exclusively in-person; one in which I might have 20 close friends who know my good and bad sides and with whom I can be vulnerable, instead of 2000 Facebook “friends” or 200,000 Instagram “followers” who envy my beautiful and successful life but know nothing about my doubts or sadness or despair because I only post the enviable stuff.

Maybe it’s time to mourn the loss of that world and move on.  (I suppose with such low expectations, I can only be pleasantly surprised if normalcy actually does return later this year...)  I don’t know what to do, but I know I have to do something differently.  Maybe it’s time to abandon physics for awhile and take a road trip with my dog to some national parks.  Maybe I need to build something – maybe not quite the scale of the Agora Grand, but nevertheless interesting and useful and creative.  Who knows?  Suggestions welcome! 

Thank God for my wonderful wife and my friends and family, even if I don’t connect with them as often as I should.  Thank God that I am well fed and live in a nice apartment with amenities that Cleopatra could only dream about.   Thank God I can still think with clarity.


God – please give me the grace and peace to accept the things I cannot change, the courage and strength to change the things I should, and the clarity and wisdom to know the difference.

Wednesday, January 27, 2021

Is Scalable Quantum Computing Possible? And Why Does It Matter?

Tomorrow I begin a class on quantum computing at NYU, taught by Javad Shabani.  In preparation, I am reading Scott Aaronson’s fascinating Quantum Computing Since Democritus.

The notion of quantum computing is simple.  Computers rely on bits – transistors that serve as little on-off switches.  By starting with an initial string of bits and then manipulating them in a particular way according to software (e.g., turning “on” or 1 switches to “off” or 0, etc.), a computer can essentially perform any calculation.  Computers don’t need to be made of transistors, of course, but that tends to be much more efficient than using, say, Tinker Toys.  A quantum computer is simply a computer whose bits are replaced with “qubits” (quantum bits).  Unlike a classical bit that can only take the state 0 or 1, a qubit can be in a superposition of 0 and 1 (or, more precisely, state α|0> + β|1>, where α and β are complex amplitudes and |α|2 is the likelihood of finding the qubit in state |0> and |β|2 is the likelihood of finding the qubit in state |1> if measured in the {|0>,|1>} basis). 

The reason this matters is that because there are “infinitely many” (well, not really, but certainly lots of) possible states for a single qubit, because α and β can vary widely, while there are only two states (0 or 1) for a classical bit.  In some sense, then, the “information content” of a qubit (and ultimately a quantum computer) is vastly greater than the information content of a classical bit (and corresponding classical computer).  If you think your iPhone is fast now, imagine one with a quantum computer processor!

At least... that’s the advertisement for quantum computing.  In reality, there are several problems with actual quantum computing.  I won’t dig too deeply into them, as they’re well described by articles such as this, but here are a few:

·         Nobody knows what to do with them.  There are a couple of particular kinds of software, such as Shor’s algorithm for factoring large composite numbers, that would have useful implications for cryptography and information security.  Beyond that, there don't seem to be many real-world applications of quantum computers that would be significantly faster than classical computers.

·         Qubits must remain isolated from the rest of the world (except for their entanglements with other qubits) during the computation, but this is a massively difficult problem because of decoherence.  You can have a microSD card with literally billions of classical bits... you can stick it in your pocket, use it to pick a piece of chicken out of your teeth, drop it in the toilet, probably zap it in the microwave for a few seconds... and it will probably still work fine.  (Full disclosure: I’ve never actually tried.)  But qubits are so ridiculously sensitive to influences from the world that it takes a huge multi-million-dollar system just to adequately cool and isolate even a dozen qubits.

·         Even if there were a way to adequately isolate lots of qubits, as well as entangle and manipulate them in a way necessary to execute a useful algorithm, and even if you could do this for a reasonable price on a reasonably sized device, error correction seems to be a major problem.  Errors are caused (at least in part) by decoherence, and quantum error-correction means are supposedly possible in principle, but these means (e.g., requiring 1000 additional error-correcting qubits for each existing qubit) may prove seriously problematic for the future of quantum computing.

At the end of the day, the real question is not whether a “quantum computer” consisting of a handful of entangled qubits is possible – of course it is, and such computers have already been built.  Rather, it is whether the problems of isolation, decoherence, and error-correction will prevent the possibility of "scaling up" a quantum computer to some useful size.  Aaronson famously offered $100,000 for “a demonstration, convincing to me, that scalable quantum computing is impossible in the physical world.”  I want to know the answer to this question not just because it’s such a massively important question pervading modern science and technology, but also because of its relationship to my own work on consciousness, with implications going both ways.  Specifically, what might the physical nature of consciousness tell us about the possibility of scalable quantum computing, and what might the possibility of scalable quantum computing tell us about the physical nature of consciousness?

Here’s an example.  I have been arguing for some time (e.g., in this paper and this post) that macroscopic quantum superpositions, like Schrodinger’s Cat (“SC”) and Wigner’s Friend (“WF”), can never be demonstrated, even in principle, because any “macroscopic” object (e.g., a dust particle, a cat, a planet, etc.) is already so well correlated to other objects through a history of interactions (including “indirect” interactions because of transitivity of correlation) that it can never exist in a superposition of macroscopically distinct position eigenstates relative to those other objects.  Of course, the majority opinion – practically the default position – among physicists and philosophers of physics is that WF is possible.  Nevertheless, even those who claim that WF is possible will admit that it’s really difficult (and perhaps impossible) in practice and will often resort to the plausibility of conscious AI (i.e., “Strong AI”) to save their arguments.  David Deutsch in this article, for example, spends dozens of pages with lots of quantum mechanics equations “proving” that WF is possible, but then spends a half page saying, essentially, that OK, this probably isn’t possible for an actual flesh-and-blood human but we might be able to do it on a computer and since it’s obvious that consciousness can be created on a computer... blah blah...

The problem, of course, is that not only is it not obvious, but I showed in these papers (here and here) that consciousness actually cannot be created on a computer because it is not algorithmic.  So if the possibility of WF depends on AI being conscious, and if computer consciousness is in fact physically impossible, then there must be some explanation for why WF is also physically impossible – and that explanation may equally apply to the impossibility of large quantum computers.  Further, many proponents of the possibility of computer consciousness, such as Aaronson, suspect that we’ll need a quantum computer to do the job, in which case the possibility of WF and conscious AI may hinge on the possibility of scalable quantum computing.   

Anyway, this is all to say that much of what I have discovered, innovated, and now believe about consciousness, quantum mechanics, information, Wigner’s Friend, etc., is closely related to the question of whether scalable quantum computing is possible.  Before actually beginning the class on quantum computing, here is my prediction: I think that scalable quantum computing is, in fact, impossible in the physical world.  Here’s why.

First, the possibility of scalable quantum computing, like the possibility of macroscopic quantum superpositions, follows from the assumption of “U” (i.e., the “universality” or “unitary-only” assumption that a quantum wave state always evolves linearly).  But U is an invalid logical inference as I argue in this paper; I actually think it is irrational to believe U.  In other words, it seems that the primary argument in support of scalable quantum computing is actually a logically invalid inference.  Further, I think that most of those who believe U (which is probably the vast majority of physicists) don’t even know why they believe U.  As a bettor, I would say that the smart money goes on those who actually understand (and, better yet, can justify) the assumptions they make.  The fact that so many of those who believe in scalable quantum computing also assume U leads me to doubt their claims.

Second, the possibility of scalable quantum computing depends on foundational questions about quantum mechanics, and very few scientists (including those who assert that scalable quantum computing is possible) actually understand quantum mechanics.  I know this may sound arrogant... how can I possibly claim to understand QM well enough to conclude that so few people do?  Well, that isn’t what I said – although, incidentally, I do believe I now understand QM at a level far more deeply than most.  You don’t have to understand a topic to be able to identify logical contradictions.  Unlike my brilliant physician wife, I know next to nothing about medicine or the human body, but if I heard a doctor say, “The brain does X” and then later say “The brain does not do X,” then I will know that the doctor does not understand the brain.  So it is with QM.  Here are a couple of papers in which I’ve addressed contradictions by physicists discussing QM (here, here, and here), and it drives me absolutely bonkers at the cognitive dissonance required for a physicist to say something like “Schrodinger’s Cat is both dead and alive.”

Third, and most importantly, I think that scalable quantum computing will run into the same problem as macroscopic quantum superpositions, which (as discussed above and in the cited papers) I think are impossible to create and empirically demonstrate.  I’m not sure it’s exactly the same problem, but it’s really similar.  For example, I argued here that when a tiny object measures the position of a measuring device, it inherently measures the position of other objects in the rest of the universe, whose positions are already well correlated to that of the measuring device.  Will that argument apply to, say, a million qubits that are entangled with each other but isolated and uncorrelated to other objects in the universe?  I don’t know, but it certainly suggests a similar problem. 

On a related note, I have argued that a superposition state of a single particle can grow via quantum dispersion, but as the object grows in size, a potential superposition suffers two problems: reduction in the rate of dispersion (thanks to the Uncertainty Principle) and increase in the rate of decoherence.  We can do double-slit interference experiments on objects as large as maybe a thousand atoms, although anything much beyond that seems to be impossible for all practical purposes.  I suspect the same problem, or something comparable, will arise with groups of entangled qubits.  In other words, I am reasonably confident that there is a set of quantum superpositions that are physically impossible to empirically demonstrate, even in principle – and I would bet that whatever physical mechanism prevents such superpositions would also prevent scalable quantum computing.   

But I don’t know for certain.  For example, I don’t know how an individual qubit is placed in an initial (superposition) state, nor do I know how groups of qubits are entangled and manipulated in the way necessary to perform the desired algorithm.  It may turn out that the only real limitation is decoherence, and perhaps error correction may indeed be adequate to overcome decoherence limitations.  I sincerely doubt it, but these are the sorts of questions I am looking forward to answering this semester!