## Separation of Physics and Theology?

Down in the comments section of this post, reader St. TY has the following kind thing to say about me:

What an excellent blog. I have been looking for one like this for a long time. I tell what I like about it: Although we all know St. Aron’s Christian bias, but he does not let it intrude into his physics and, as one with a mathematical background, I like that separation of Church and State.

As for the format I’m old fashioned and I like the written word because good writing demands clarity and coherence I must add honesty, and so I like reading Aron’s pieces and the comments.

I would like Aron to put all of this meaty stuff in a book.
Would you, Aron?
Thank you.

Thanks so much for your gracious compliments about my blog!  It's too bad really, that I must strongly disagree with you when you say that

Although we all know St. Aron’s Christian bias, but he does not let it intrude into his physics and, as one with a mathematical background, I like that separation of Church and State.

Your proposal that I keep a separating wall is not really very undivided, is it?  I expressed a different aspiration in my About page:

"Undivided Looking" expresses the aspiration that, although compartmentalized thinking is frequently helpful in life, one must also step back and look at the world as a whole. This involves balancing specialized knowledge with common sense to keep both kinds of thinking in perspective.

So in response I would say, that one's physics views can and should be influenced by one's theological views (or vice versa), if there is a legitimate reason why it should do so.  There is, after all, only one universe, and therefore no compartments can be kept completely watertight.  For example, most economists don't need to know much about chemistry, but if they're talking about buying things that might explode then there needs to be some cross-talk.

Christianity is not a "bias", but a "belief", one which happens to be true.  Deducing things from one's beliefs is not bias unless it is done in an irrational and capricious manner.  But perhaps you were speaking in a semi-humorous way, in the way that we might say that all scientists seek to be biased towards the truth!

Reasonable physicists will probably have similar intuitions about how physics should be done (I'm excluding unreasonable people like Young Earth Creationists), regardless of whether they are atheists or theists.  Or rather, people have different intuitions about physics but they mostly don't correlate with religious views!  But if on a particular matter (e.g. the universe having a beginning in time) somebody happens to be influenced by their religion (or lack thereof) to think that one viewpoint is more likely than another, I don't think that should be taboo.

Far from corrupting the scientific process, I think science usually works better when people explore a variety of intuitions and options.  As I said in discussing the importance of collaboration in science:

Healthy scientific collaboration encourages reasonable dissent.   Otherwise group-think can insulate the community from effective criticism of accepted ideas.  Some people say that scientists should proportion their beliefs to the evidence.  However, there's also some value in diversity of opinion, because it permits subgroups to work on unpopular hypotheses.  I suppose things work best when the scientific community taken as a whole proportions its research work to the evidence.

It doesn't necessarily matter whether the source of the original intuition is something that could be accepted by all scientists.  What matters is that the resulting idea can be tested.  Sometimes, the original motivation for a successful scientific theory is rather dubious (e.g the Dirac sea motivation for antimatter), but nevertheless the resulting theory is confirmed by experiment and later is motivated by a different set of considerations.

So I don't believe in the complete separation of Physics and Theology, hence the blog.  But maybe I believe in something else which has some similar effects on my writing.  You must after all be detecting something about what I am doing which provoked your favorable statement.

Perhaps it is this: I believe in being honest.  I must to the best of my ability weigh the evidence on fair scales, and be open about what I am doing.  It would be dishonest if, because I want to prove the truth of Theism, I were to report the relevant Physics data in an imbalanced way, playing up anything which might seem to help my case and playing down anything which does not.  People often do this kind of thing reflexively when they argue, even to the extent of first deceiving themselves before they deceive others.  But it's still unfair tactics, especially when deployed by the expert against the layman.

It is not dishonesty for me to have my own views about what's important in Physics and what's not, but it would be dishonest if I implied that all physicists agreed with me about that when they don't.  Nor would it be dishonest if my views about speculative physics are influenced to some extent by my theological views—I think this is inevitable, and possibly not even fully conscious—but to pretend that a view is based on purely physical considerations when it is not, or to distort the data about Physics to match a preconceived agenda (theological or otherwise) is repugnant to me.

So I'll do the best I can to be honest, and hopefully that will tilt the scales in the right direction.

Once upon a time, a college friend and I planned to write a book about Science-and-Religion topics, but that never got off the ground.  A few of the ideas from that time are being recycled here.

I originally started this blog because an elder Christian whom I respect back in Maryland told me (and gave me to understand that it was a divine revelation to him, and I trust him to know the difference) that I should not neglect my gift of teaching when I went to Santa Barbara.  At first I tried to start a Bible study with my church, but it already had lots of other groups, and it kept not working out for various reasons; then I thought of the idea of blogging instead.

Once I reach a critical mass on the blog, perhaps some of them could be organized into book format.  But I don't need to decide that yet.  For the time being, the informal blogging environment seems more fruitful for developing ideas.

Posted in Blog, Ethics, Scientific Method, Theological Method | 14 Comments

## Server back up

The blog was down for several days due to computer troubles.  There was a power failure in Mountain View while my parents were out of town, and then when it ended the wall.org computer booted up with the wrong operating system.  Just wanted to let everyone know that the problem is fixed now.

Posted in Blog | 1 Comment

## Did the Universe Begin? VII: More about Zero Energy

A reader who wishes to be anonymous writes in with the following question:

I heard your paper referenced in the Carroll vs Craig debate, attempted to read it, then looked you up and found your blog (which I really like!!).  I’m fascinated by the origin of the universe and think it is a great argument for a creator.  I have a question I’m hoping you can help me with, or better yet, do a blog post on so I have something to reference!

Frequently when I debate an atheist online, they will bring up the argument that the net energy of the universe is zero and so the First Law of Thermodynamics was not violated at the origin of the universe since energy was still conserved.  As they explain it, the positive energy of matter is countered by the negative energy of gravity.  Our universe formed from a freak quantum fluctuation and is the ultimate free lunch.  I understand this at a very simple level, but what I do not understand is how a zero-energy universe matches what we observe.  If matter only makes up ~5% of the universe, 30% if you include dark matter, then how does the universe have a net energy balance of zero if 70% of it is dark energy pushing the universe apart through repulsive gravity?  It seems the expansion of the universe indicates a net positive energy.  Could you please give a simple layperson explanation for why folks like Hawking, Krauss, Guth, etc claim the universe has a net energy of zero?  It feels like there is a slight-of-hand going on and dark energy is being excluded, but I don’t know enough or have any sources to point to that say otherwise.

Dear Reader, thanks for your question.  I notice there's an interesting inversion here from the Carroll-Craig debate.  In that debate, St. Craig was trying to argue that the universe had a beginning, and Carroll was trying to outmaneuver him with the "Quantum Eternity Theorem", saying that the universe couldn't have begun unless its total energy is zero.  He then opened himself up to the retort that the energy probably is zero.

On the other hand, in your debate, it's the atheist who seems to be championing the position that the energy of the universe is zero.  Presumably this is because he wants to say that the universe emerged from a Nothing somewhat like the one Krauss' has in mind (though all this talk of Nothing doing things as if it were Something keeps reminding me of "The Nothing" in The Neverending Story...) and therefore no room for a Creator' etc.  In this case the theist might argue that Energy Conservation makes this impossible (absent a miracle), opening herself up to the retort that the energy probably is zero.

So perhaps if you and Craig were locked in a room together, you might discuss whether a physics-type beginning of the universe is helpful or unhelpful, when arguing for Theism.  Alternatively, there could be a Krauss-Carroll debate about whether there's less "room" for a Creator with or without a beginning of time (both of them granting that the idea is absurd either way).  One could more or less construct such a debate just from their remarks directed against Theism already linked to on this blog.  Carroll could argue that in models like Aguirre-Grattan:

There is no room in such a conception [an eternal universe with the entropy lowest in the middle] for God to have brought the universe into existence at any one moment.

and Krauss could respond that:

It has become clear that not only can our universe naturally arise from nothing, without supernatural shenanigans, but that it probably did.

and Carroll could retort that:

That is not what the universe does even in models where the universe has a beginning, a first moment. Because the verb popping, the verb to pop, has a temporal connotation, is the word I'm looking for. It sounds as if you waited a while, and then, pop, there was the universe. But that's exactly wrong. The correct statement is that there are models that are complete and consistent in which there is a first moment of time. That is not the same as to say there was some process by which the universe popped into being.

Apologies to Krauss and Carroll for wrenching their remarks totally out of context, but I believe I have not done any violence to their actual views.  If you'd rather see what the real Carroll actually said about Krauss' conception, you can find that on his blog here.

But that wasn't your question.  Setting aside which team benefits more from it, what does physics say about whether the energy is zero?

As I said when discussing the "Quantum Eternity Theorem", there are lots of different concepts of energy in General Relativity, and even the experts sometimes find the relationships between them tricky to think about.  It's no wonder laypeople get confused when the "experts" make definitive sounding pronouncements about the subject.  If the energy at every point in the universe is positive, how could it possibly be true that the total adds to zero?

Well, the simple layperson'' explanation is that in cosmology, there's contributions to the energy both from 1) matter (baryons, dark matter, dark energy, etc.) and 2) from spacetime, stored in the gravitational fields.  There's a notion of energy density where you only count category #1, and then the energy density is positive.  But this notion isn't very useful for discussing things like energy conservation, since it isn't conserved in situations where space is changing with time (e.g. expanding).  There's another notion where we count both #1 and #2, and then it turns out that the contribution from #2 is negative and (in a finite sized "closed" universe).

That's the best I can do without launching into technicalities.  But I can't resist trying to say more about the real story, even if what follows may not really count as a simple layperson explanation.

Perhaps it would be easiest to explain if we start with a theory that's simpler than GR.  GR is in many ways quite similar to an easier theory of physics, namely Maxwell's equations.  Like the gravitational field, the electromagnetic field is sourced by a particular type of matter.  Gravitational fields are produced by the flow of energy and momentum through a spacetime, while electric and magnetic fields are produced by the flow of charge.

Let's just think focus on one of the Maxwell equations right now, the Gauss Law.  This is a special type of Law of Physics called a constraint.  That means, instead of telling you how things change with time, it places restrictions on what is allowed to be the case at a single moment of time.

The Gauss Law is written in equations like this:

Here $E$ is the electric field vector at any given point, and $\rho$ is the rate at which charge is flowing through time at a given point.  Which is a really fancy way of saying, the charge density.  $\nabla \cdot E$ means $\nabla_x E^x + \nabla_y E^y + \nabla_z E^z$, where $\nabla_i$ means taking the derivative with respect to the $i$-th spatial coordinate.

But maybe you hate equations: if so you are in good company.  When I was at St. John's College we read a funny letter in which St. Faraday wrote to St. Maxwell, saying that he loved his work, but why did he have to write it using math?   St. Faraday, you see, lived in the time where you could still be a respectable scientist and explain everything using words.  Very carefully chosen words, expressing precise quantitative relationships.

Anyway, Faraday figured out this brilliant way to visualize the Gauss Law, which we still use as a crutch today.  Instead of thinking of $E$ as a vector, you can think of it as a density of electric field lines passing through a point.  The direction of the vector says which direction the lines are going in, and the magnitude says how many there are.  I'm sure you've seen electric and magnetic field lines before, but if not, here are some pretty pictures on Google.

The Gauss Law says that electric field lines can only begin or end on charges.  The number of electric field lines coming out of (into) a charge, is proportional to the positive (negative) charge of the particle.  (We say "number" to make it easy to visualize, but in fact the field lines form a continuum.)

This means that if you have a region of space $R$, you can do a census of the total charge in that region, simply by measuring the total amount of electric field lines coming into or out of that region.   One can write this as an equation too:

Here $Q_R$ is the total charge inside the region $R$, $\partial R$ is fancy-schmancy notation for the boundary of $R$, $E_n$ is the number of electric field lines poking out per unit area, and $\int dA$ tells you to integrate that over the whole area to get the total number of electric field lines poking out.  (Faraday would have said, why work so hard to invent these silly symbols when you could just say "count the number of electric field lines poking out"?)  We physicists call an integral like this a boundary term, because—go figure—it's the integral over a boundary of a region.

We are now in a position to appreciate the following interesting truth.  Suppose the universe is closed.  (That means, finite in size but without any boundary.  For example, space at one time could be shaped like a giant hypersphere; as we all know a sphere is finite in size but has no end.  Or like one of those video games where if you go off the edge of the screen on one side, you "wrap around" and appear on the other side, so that there isn't really an edge there.)  In a closed universe, the total electric charge is always EXACTLY ZERO.

If you're Faraday, that's because each electric field line has to either circle around in loops, or else begin on a positive charge and end on a negative charge.  So everything has to balance out.  If you're Maxwell, it's because if you take the region $R$ to be the whole universe, then $\partial R$ is the empty set, and so the Gauss Law just says $Q_R = 0$.

This doesn't necessarily have to be true if space is infinitely big.  You could just have a single electric charge sitting in infinite empty space, and this would be OK because the field lines beginning at the charge would go out to infinity, so they don't need another endpoint.

Now what about GR?  It turns out that things work in a very similar way, only using energy instead of charge.  If the universe were a single star or a galaxy sitting in an otherwise empty infinite space, then the gravitational field lines'' coming out of the mass extend out to infinity.  This allows the total "ADM" energy of the spacetime to be nonzero.  In fact, there is a Positive Energy Theorem in GR which says that, for reasonable types of matter, this energy is always positive for any state besides the vacuum (which has 0 energy).

On the other hand, if the universe is closed, then the total energy is zero because there's no boundary for gravitational field lines to go off to.  But how can this be, when the cosmologists tell us that the universe consists of about 5% ordinary matter, about 25% dark matter and 70% dark energy, and each of these components of energy is positive?

(I hate the term dark energy'', by the way, since it makes people think it's related to dark matter.  The two are nothing alike.  Dark matter is just some other kind of stuff, which clumps into structures.  The so-called dark energy is most likely just a cosmological constant, i.e. a constant positive energy density throughout all of space.)

To answer this, I need to remind you of how Einstein's equation of GR works.  The Einstein equation says how energy and momentum lead to spacetime curvature.  It can be written like this

The symbol $G_{ab} = R_{ab} - \tfrac{1}{2} g_{ab} R$ is called the Einstein tensor; basically it's a 4x4 symmetric matrix which encodes certain properties of the curvature of spacetime.  On the other hand, $T_{ab}$ is the stress-energy tensor of matter.  This is also a 4x4 symmetric matrix, which encodes the rate at which momentum in the $a$-direction is flowing in the $b$-direction.  (The $T_{tt}$ component, where both indices are chosen to be time, is just the energy density, since energy is momentum in the time direction.)

A key point here is that $T_{ab}$ only counts the energy and momentum in matter.  It does not count the energy and momentum stored in the gravitational field (although by convention, these days most people include the cosmological constant or dark energy'' in $T_{ab}$).  When the cosmologists tell you about the "energy budget" of the universe, they are only really talking about $T_{tt}$.  They are ignoring the contribution from the gravitational field, which also contributes to the total energy of the universe.  It turns out that in a closed universe, the gravitational part (due to $G_{tt}$) counts negatively and this exactly cancels the matter contribution.

Defining the total energy of the universe is, as I said, quite tricky, since in the Hamiltonian formalism energy is related to time, and you have to make an arbitrary decision about what counts as the `time'' direction.  You have to decide this separately for every single point, so there's actually a lot of arbitrariness here.  Once you've picked a time coordinate, if you want to evaluate the total energy on $t = \mathrm{constant}$ slice $\Sigma$, the total energy $H$ ends up being given by something like the following integral over the volume $V$ of space:

(If you don't know about tensor notation, just don't worry about the fact that one of the t's moved upstairs.  If you do, I've raised an index using the inverse metric $g^{ab}$.)  The boundary term is an integral $\int_{\partial R}$ of something I'm not bothering to write down.

Now the t-t component of the Einstein equation, a.k.a. the Hamiltonian constraint, tells us that $T^t_t = G^t_t$.  So the whole thing boils down to a boundary term, and in a closed universe that has to be zero.  Thus, the ambiguity about time doesn't matter in the end, since "0" is conserved no matter what.

Posted in Physics, Reviews | 10 Comments

## Did the Universe Begin? VI: The Generalized Second Law

Last time I discussed the cosmological implications of the regular-old Ordinary Second Law.  Now I want to discuss what happens if you use the Generalized Second Law instead—this being a generalization of the Second Law to situations involving black holes and other horizons, which seem to have an entropy proportional to their surface area.

I started thinking about this issue after Sean Carroll gave a colloquium at U Maryland about the Carroll-Chen model, and the Second Law, back when I was a grad student.

From my perspective, the important thing about that colloquium was that it got me thinking about refining the classic argument that the Second Law predicts a beginning.  I said to myself something like the following:

"Self, for the past couple years you've been spending all of my time thinking about the Generalized Second Law (GSL), that wild new version of the Second Law which applies to causal horizons.  Well, there are horizons which appear in our own cosmology (because of the accelerating expansion of the universe).  So can we make this argument using the GSL instead of the Ordinary Second Law (OSL)?  And if we do, will it make the argument stronger or weaker?"

Well, when I thought about it a little bit, I realized that you could use the GSL in two distinct ways to argue for a beginning.  One of them is a quantum generalization of the Penrose singularity theorem, which I discussed here.  The other way is a generalization of the Argument from the Ordinary Second Law, described above.  Both of these uses of the GSL are discussed in my article, but it is important to realize that they remain two distinct arguments!

Fine-grained vs. Coarse-grained.  The reason is that there are actually two subtly different ways to formulate the GSL.  You see, the entropy is a measure of our ignorance about a system.  To exactly define it, you need to make a list of the things you are allowed to measure about the system (e.g. the  pressure and temperature of a box of gas), and then the entropy measures how much information content is in the things you can't measure (e.g. the positions or velocities of individual molecules).  The procedure of ignoring the things we can't measure is called coarse-graining (because it's like looking at a grainy photograph where you can't see all of the information in the object).

Technically then, there's some ambiguity in the definition of the entropy, since the intitial step where we list what we can measure is a little bit ambiguous.  Fortunately, since the amount of information we can't measure is much larger than the information we can, this doesn't usually matter very much.  Quantitatively, the different ways of defining entropy give pretty close to the same numerical answers.

But we could pretend that we could measure everything about the box of gas to arbitrary accuracy.  The only uncertainty allowed which could produce a nonzero entropy is uncertainty about the initial conditions.  This is called the fine-grained entropy, and while it has the property that it neither increases nor decreases as time passes.  Since the fine-grained entropy can't decrease, it technically obeys the Second Law, but in a really boring and stupid way.

The distinction becomes important when you start talking about black holes and the GSL.  Suppose you have a star orbiting a black hole.  Matter from the star is slowly getting sucked off the outer layers of the star, and getting sucked into the black hole.  (This is a realistic scenario which is believed to really occur in some solar systems, by the way!)

Well, we have a choice.  We could use a coarse-graining to describe the entropy of the star.  In that case, the entropy would go up for 2 distinct reasons: A) because stuff is falling into the black hole making its area increase, and B) because ordinary thermodynamic processes are happening inside the star, making the entropy increase for usual non-black-holey reasons.

Or, we could take the fine-grained point of view, and pretend we know everything about the matter outside the event horizon.  In that case, the entropy increases only because of (A), things falling across the horizon.  Stuff happening inside the star doesn't make a difference.  This would be the fine-grained GSL, and it is nontrivial—the entropy defined in this way can go up, but not down.  You could say, that the only coarse-graining we use is to forget about anything that fell across the horizon, and this is enough to get a nontrivial result.  (This was pointed out by Rafael Sorkin.)

In my dissertation research, I proved the GSL in the fine-grained sense.  This was very useful since there are still some thorny and unresolved issues of interpretation with the Ordinary Second Law due to the exact meaning of coarse-graining.  The fact that one can avoid this issue in discussing the GSL made my life much easier!

Also, coarse-grained versions of the Second Law are only true if you have a history with a well-defined arrow of time—i.e. a universe that is constrained to begin with low entropy, but has no particular constraint on how it has to end up.  The fine-grained GSL, on the other hand, appears to be true for all states and therefore has no dependence on the arrow of time.  As a result, you can even apply the fine-grained GSL backwards in time if you want to, and this is perfectly OK, even though we normally think of the Second Law as something which only works in one time direction.

The forwards-in-time GSL applys when you have a worldline (an "observer", if you feel like anthropomorphizing) which extends infinitely far to the future.  It says that the boundary of what the observer can see (called a "future horizon") has increasing entropy.  The backwards-in-time GSL, says that if you have a worldline which extends infinitely far to the past (if there are any), then boundary of what they can be seen by (called a "past horizon") has decreasing entropy.  Equally true.

How to apply the GSL.  If you want to use the GSL as a singularity theorem to show that time ends in the middle of a black hole, you'll want to use the forwards-in-time GSL.  But if you want to use it to argue that there was an initial singularity at the Big Bang, you need to use the backwards-in-time GSL.  That's what I did to generalize the Penrose singularity theorem in my paper (like the original, it only works if space is infinite).

But I also considered the possibility that you might use the forwards-in-time GSL to argue for a beginning.  In this case, it would be a substitute not for the Penrose theorem, but for the OSL.  The details are in section 4.2 of my article, but the upshot is pretty much the same as before, that there probably had to be a beginning unless either (i) the arrow of time reverses, or else (ii) the universe was really boring before a certain moment of time.

Why even bother?  Other than the fact that the GSL has deeper connections to quantum gravity, the main technical advantage of using the (forwards) GSL is that it is more clear that the entropy reaches a maximum value in our universe (due to the accelerating expansion of the universe at late times, there is a de Sitter horizon at late times whose entropy is about $10^{120}$).  This makes it harder to play games with infinities.

Also, if we want to reverse the arrow of time, the GSL indicates that the universe probably has to also be finite in size.  That's because, if the universe is finite in size, there is the possibility that before a certain time $t_0$, everything is visible to an observer, and before that time there would be no horizon at all.  That would make the fine-grained GSL trivial, forcing us to use the coarse-grained GSL.  That's important because it's only the coarse-grained GSL which depends on the arrow of time.

But mainly I just wanted to see if the standard argument from thermodynamics would still work when I rephrased it in terms of horizon thermodynamics.  Not surprisingly, it does.

(On the other hand, the connections to the Penrose singularity theorem are much more surprising, and I believe that it is telling us something deep about the laws of quantum gravity.)

Posted in Physics, Reviews | 5 Comments

I have moved my comments policy, with help entering equations to its own page, accessible from the top bar above.

Recently, a few people have had trouble leaving comments on the site, due to incorrectly formatting equations, and some capricious comment mulilation by the WordPress software.  It took me a little while to figure out the exact rules since apparently the comment box which you would enter text into, has slighly different rules than the ones I enter text into as a logged-in user.

So here's the deal.  WordPress uses < and > to enclose html tags.  For example, if you write "<b>this is bold</b>" in a comment, you'll see "this is bold".  Unfortunately, this means that if you include an < followed by an > in your comment, WordPress will interpret whatever is in between as an html tag, and—even if it is not a valid html tag—will simply delete everything in between them!  Even I will be unable to see what you originally wrote.  So do NOT use > or < to mean greater than or less than (unless you use just one kind, or only >'s followed by <'s... but the safest rule is just to avoid them entirely...).

If you want to include greater than or less than symbols, you can write them as ${\mathrm \\\verb|\|\mathrm{gt}\\}$ or ${\mathrm \\\verb|\|\mathrm{lt}\\}$.  That will look like > or < respectively.  Or better still, put your entire equation inside of the double dollar signs using LaTeX notation.

[quietfanatic points out in a comment below that you can also use the html escapes $\&$gt; or $\&$lt; if you don't want to use the double dollar signs.]

Also, do NOT try to use wordpress.com latex notation (which has a single dollar signs, and the magic word "latex").  It won't work!  There is a difference between wordpress.com and wordpress.org.  The former is a website used to host WordPress blogs, while the latter is where you download software to host your own WordPress blog.  This is a WordPress.org blog which is hosted on my family server, wall.org.

In order to put LaTeX in a wordpress.org blog, you have to install a special plugin to do so.  You might think that this plugin would use the same notation as wordpress.com, but no it doesn't.  Instead you type $\\\mathrm{E = mc\verb|^|2}\\$ to get $E = mc^2$, and type $\\!\mathrm{E = mc\verb|^|2.}\\$ to break it out into a separate line like so:

It's best not to put any spaces after the dollar signs in this case, or they'll make a weird indentation in the next line, as shown here.

Got it?  Good.

Posted in Blog | 4 Comments