Tag Archives: Physics

God Near or No Mind Hair

By Robin Hanson · September 13, 2010 10:30 am · Comments (43)

(I wouldn’t be at all surprised if the following argument isn’t original, but I haven’t seen it elsewhere yet.)

If our descendants do not destroy themselves, then over the next trillion years they may become knowledgeable and powerful enough to create new baby universes that expand to look much like the universe we can see. Such a universe might then evolve its own intelligence, which would grow powerful enough to repeat the process. A self-reproducing universe would have a chance p of evolving intelligence, which would then birth an expected number N of similar baby universes, such that p*N >1.

Our descendants might even become powerful enough to imprint themselves upon such a baby universe. An imprinted universe would somewhere contain mind(s) with important specific similarities, such as memories, personality, or values, to the minds of its creators.

One of the following two remarkable conclusions seems likely:

  1. No Mind Hair – Even if our descendants command billions of galaxies and study physics for trillions of years, they still cannot create self-reproducing baby universes, and reliably imprint their minds on them. Such a task is beyond the abilities even of such gods. They are trapped; their baby universes just cannot have “mind hair.”
  2. Gods Nearby – Somewhere out there in our universe is probably hiding the imprinted minds of our universe’s creators. If we search long and far enough and understand physics well enough, we may well find them.

Here’s why. If our descendants can make self-reproducing universes, then there’s a non-zero chance they will do so, and if so there’d be an infinity of such universes. But out of an infinity of expected universes we are quite unlikely to be in the first, making it quite likely that our universe had creators. If such creators could imprint their minds on our universe they probably would have done it. So, either imprinted versions of our creators are probably out there somewhere in our universe, or no feasible power can reliably mind-imprint a self-reproducing baby universe.  Such imprinting could at best succeed rarely. QED. Either conclusion is remarkable.

GD Star Rating
loading...
Tagged as: , ,

Future Discounts

By Robin Hanson · March 19, 2010 6:00 am · Comments (29)

In few billion years our descendants may spread across billions of galaxies. Even so, if they do not drastically change the structure of space-time, then within a trillion years they will fragment into billions of isolated galaxy-sized “universes”.  Standard physics, you see, says that in a trillion or two years all the galaxies near the Milky Way will merge into one big galaxy, and other galaxies will be too distant to see in any way.  For all practical purposes, that merged galaxy will be a separate universe.

If we do nothing to change the situation, then within ten or so trillion years, all current stars will be dead (degenerate), and no more stars will form.  Over the next billion trillion years, stars will occasionally smash in a flash, or pass close enough to each other to throw one out of the galaxy; in the end 1-10% remain in a central black hole.

What if we change the situation?  Most useful resources, such as hydrogen to turn into lead, or mass not yet dropped into the central black hole, will likely be identified and claimed within a few million years.  How fast will folks use up these resources?

In principle, most everything might be burned quickly in a few million years of party-hardy gluttony, or most might be saved to use steadily over the billion trillion trillion years or more before protons decay.  How fast resources are actually used would be determined by the discount factors of the creatures who control resources.  But what would those be?

If unused resources were completely stable and if property rights in resources were completely secure, then we’d mainly have a selection effect in discount rates.  Agents who discount fast would dominate early activity, while those who discount slowly would dominate late activity.  Even if initially only a tiny fraction of agents cared about activity in a billion trillion trillion years, those agents would dominate such late activity.

Any natural rate at which resources decay would set an upper limit on discounting.  There is no point in planning to use resources long after you expect them to decay.  Similarly, insecure property rights would increase discount rates. If you expect a 1% chance that your property will be stolen every million years, you won’t expect to still have much after a billion years, so you might as well plan to use most of it before then.   The same holds if your property is never stolen, but you have to spend 1% of your resources every million years to ensure that fact.

“Switzerlands,” from which theft is naturally harder, might be the last locations of activity in each galaxy.  These might be matter sent on very long secret orbits, to return back to galaxy central after a very long time.  Similarly, resources which simply could not be physically used until a long delay might ensure some late universe activity.

The inhabitants of a galaxy-universe could have different degrees of central coordination; some might have a strong central government, while others lived in anarchy.  With a strong central government, long term activity seems strongly influenced by the discount rate of that government. If this government taxed 1% of resources every million years, and didn’t invest those resources for the long run, then there would be little point in planning to use your resources after a billion years.  No obvious selection effect ensures that galaxy governments take a long view.

Physics may set the ultimate limits on how long resources, and life, can last, but governments and property rights will determine when they are actually used. Resources, and life, will likely die long before their physical expiration dates.

GD Star Rating
loading...
Tagged as: , ,

From Eternity To Here

By Robin Hanson · January 7, 2010 11:35 pm · Comments (28)

Out today, Sean Carroll’s new book, From Eternity to Here, is excellent.  After reading a draft in March, I wrote:

We are far from understanding thermodynamics. … The distributions we would usually use to successfully predict [physical system] futures are completely, totally, and almost maximally WRONG for predicting their pasts!  …  Worse, this “past hypothesis” is ambiguous in several ways … Only a tiny handful of physicists (and philosophers) are trying to explain this past hypothesis; … no one is even remotely close.

Here is Carroll’s proposed solution scenario:

  1. Physics is always exactly locally time-reversible.
  2. Each small region of space has bounded entropy, yet an infinite state space.
  3. So entropy has no upper bound, so systems are never in full equilibrium.
  4. Our local universe is expanding with a weak dark energy.
  5. Our distant future is a forever expanding emptiness at 10-29K.
  6. Very rarely, local fluctuations there build brains like ours.
  7. Far more rarely, local fluctuations pop a tiny new universe.
  8. Tiny new universes are very curved and thus very dense.
  9. Dense regions generically expand to get less dense.
  10. In some dense expanding regions, a dark energy starts eternal inflation.
  11. Inflation makes flat uniform local universes with scale-less fluctuations.
  12. Local universes sit in different local minima with different local physics.
  13. In some, scale-less fluctuations make galaxies etc. and brains like ours.
  14. Those local universes also get empty, then rarely pop tiny new universes.
  15. On average each tiny new universe gives rise later to several more.
  16. So there are an infinite number of local universes.
  17. A region in our past pops tiny universes in both time directions.
  18. There are overall far more brains like ours than fluctuation brains.

Many of these are far-from-proven conjectures, but still it does all hold together. Locally infinite state spaces (#2), might appear to conflict with the holographic principle:

There is a maximum amount of entropy you can possibly fit into a region of some fixed size, which is achieved by a black hole of that size.

But it doesn’t conflict; region size is neither constant nor bounded.  Even so, it is very hard to over-emphasize just how far one must project current physics beyond the accuracy with which we have verified it to talk about tiny new universes popping out of quantum fluctuations in empty space at 10-29K.  It will be truly incredible if we get that right.

On style, I’m again struck by how different is the public’s preferred style for popular physics vs. economics books.  Popular physics books, like Carroll’s, act easy and friendly, but still lecture from on high, sprinkled with reverent stories on the “human side” of the physics Gods who walk among us.  They grasp for analogies to let mortals glimpse a shadow of the glory only physicists can see directly.

The recent popular econ book Superfreakanomics is also excellent, but very different in tone.  Also easy and friendly, this is full of concrete stories about particular data patterns and what lessons you might draw from them, or you might not; hey it is always up to you the reader to judge.  Such books avoid asking readers to believe anything abstract or counter-intuitive based on the author’s authority.

The main difference, I think, is that readers don’t fundamentally care about physics, so can’t get worked up disagreeing with physics authors.  They read to affiliate with great men, and to lord their greater knowledge over lessor associates.  In contrast, people actually care about many economics topics, and our democratic culture, where everyone’s political opinions are officially equally valued, simply can’t accept opaque expertise on such things.

GD Star Rating
loading...
Tagged as: ,

Lorentz Invariance, Not

By Robin Hanson · August 12, 2009 11:00 pm · Comments (4)

New Scientist:

Major Atmospheric Gamma-ray Imaging Cherenkov Telescope. …  What MAGIC saw on that balmy June night … Lower-energy photons from Markarian 501 had outpaced their higher-energy counterparts, arriving up to 4 minutes earlier (Physics Letters B, vol 668, p 253) …

The MAGIC observations were showing just the sort of effect that quite a few models of quantum gravity predict. … A minimum size for space-time grains, as predicted by loop quantum gravity, could violate the cherished principle of special relativity known as Lorentz invariance, which states that the maximum speed of all particles, regardless of their energy, is the speed of light in a vacuum.

Here is more on the empirical issues; here is a solid and robust argument that a min size in space time implies a Lorentz violation.   We seem to be starting to see a clear quantum gravity effect!

GD Star Rating
loading...
Tagged as:

Explain This Correlation

By Robin Hanson · July 17, 2009 8:00 pm · Comments (48)

At SciFoo Camp last weekend, famed quantum gravitist Lee Smolin mentioned that he’d noticed a correlation between these beliefs:

  1. Many worlds for quantum mechanics,
  2. Anthropic arguments in physics, and
  3. Conscious computer-based AIs could be built.

This correlation seems intuitively right, but puzzling.  Any explanation for why it exists other than the obvious, that some people tend to be right about everything?

Added: Lee and most who came to the particular SciFoo session where he made this observation disagree with these beliefs, yet were creative sharp physicists, hackers, sociologists, etc.

GD Star Rating
loading...
Tagged as:

Past Hypothesis Taxonomy

By Robin Hanson · March 28, 2009 6:00 pm · Comments (43)

Let me try an experiment: using a blog post to develop a taxonomy.  Here I'll try to develop a list/taxonomy of (at least semi-coherent) answers to the question I posed yesterday: why is it harder to formally predict pasts, versus futures (from presents)? Mostly these are explanations of the "past hypothesis", but I'm trying to stay open-minded toward a wide range of explanations.

I'll start with a list of answers, and then add more and group them as I read comments, think, etc.  I'll feel free to edit the post from here on:

  • Extremely unlikely:
    • Reality isn't different; we just ask different future vs. past questions.
    • An outside "God" intervened to make our past different.
    • We live after a big local ebb (i.e., fluctuation) in matter.
  • Rather unlikely:
    • Quantum measurement has a local time asymmetry that makes big effects.
    • A weak local time asymmetry in matter accumulates to big effects.
    • A past ebb in spacetime shape (e.g., inflation) forced a big matter ebb.
    • All spacetime boundaries satisfy a law-like "low entropy" condition.
  • Unlikely:
    • Our expanding cosmos violates one-to-one state mappings across time.
    • Past and future have different spacetime law-like boundary conditions.
GD Star Rating
loading...
Tagged as:

Scandalous Heat

By Robin Hanson · March 27, 2009 6:00 am · Comments (61)
Mea Culpa: I was wrong; Eliezer was wrong; Sean Carroll was right.  

Thermodynamics is the study of heat, temperature, pressure, and related phenomena. Physicists have long considered it the physics area least likely to be overturned by future discoveries, in part because they understand it so well via "statistical mechanics."  Alas, not only are we far from understanding thermodynamics, the situation is much worse than most everyone (including me until now) admits!  In this post I'll try to make this scandal clear.

For an analogy, consider the intelligent design question: did "God" or a "random" process cause life to appear?  To compute Bayesian probabilities here, we must multiply the prior chance of each option by the likelihood of life appearing given that option, and then renormalize.  So all else equal, the less likely that life would arise randomly, the more likely God caused life. 

Imagine that while considering ways life might arise randomly, we had trouble finding any scenario wherein a universe (not just a local region) randomly produced life with substantial probability.  Then imagine someone proposed this solution: a new law of nature saying "life was sure to appear even though it seems unlikely."  Would this solve the problem?  Not in my book.

We are now in pretty much in this same situation "explaining" half of thermodynamics.  What we have now are standard distributions (i.e., probability measures) over possible states of physical systems, distributions which do very well at predicting future system states.  That is, if we condition these distributions on what we know about current system states, and then apply local physics dynamics to system states, we get excellent predictions about future states.  We predict heat flows, temperatures, pressures, fluctuations, engines, refineries, etc., all with great precision.  This seems a spectacular success.

BUT, this same approach seems spectacularly wrong when applied to predicting past states of physical systems.  It gets wrong heat flows, temperatures, and pretty much everything; not just a little, but a lot wrong.  For example, we might think we know about the past via our memories and records, but this standard approach says our records are far more likely to result from random fluctuations than to actually be records of what we think they are.

Continue reading "Scandalous Heat" »

GD Star Rating
loading...
Tagged as:

Physicists Held To Lower Standard

By Robin Hanson · November 25, 2008 1:00 pm · Comments (39)

In August I complained about vague LHC forecasts.  My oped based on that post just appeared in Symmetry, "A joint Fermilab/SLAC publication."  Its blurb: "Today’s LHC forecasts are no easier to score than the typical horoscope."  It ends:

But geez – the LHC costs more than $10 billion of public money. Shouldn’t we expect big-shot physicists who hope to crow to the public about LHC vindication to express their predictions in a more scoreable form? We don’t accept less from weather, business, or sport forecasters; why accept less from physicists?

My implicit answer: we hold physicists to lower standards.  As I posted two years ago:

Consider how differently the public treats physics and economics.   Physicists can say that this week they think the universe has eleven dimensions, three of which are purple, and two of which are twisted clockwise, and reporters will quote them unskeptically, saying "Isn’t that cool!"   But if economists say, as they have for centuries, that a minimum wage raises unemployment, reporters treat them skeptically and feel they need to find a contrary quote to "balance" their story.

That same Symmetry issue says:

Leon Lederman, a 1988 Nobel laureate and Fermilab physicist, plopped a folding table and two chairs on a busy New York City street corner and sat under colorful hand-scrawled signs offering to answer physics questions.  Even in a city of people too busy for impromptu sidewalk conversations, the sight was too tempting to resist. … Soon about 20 people formed a line down the block. They asked Lederman about the strong force, time and space, fusion, and even time travel. Some asked follow-up questions to get a clearer understanding, while others just seemed thrilled at the chance to meet a Nobel Prize winner.

I’ll bet none told Lederman he was wrong.  Imagine how a Nobel-winning economist would be received. 

GD Star Rating
loading...
Tagged as:

Self-Indication Solves Time-Asymmetry

By Robin Hanson · August 13, 2008 6:00 am · Comments (37)

This seems a deep insight simple enough to explain in a blog post (and so I’m probably not the first to see it):  the self-indication approach to indexical uncertainty solves the time-asymmetry question in physics!  To explain this, I must first explain time-asymmetry and indexical uncertainty.

A deep question in physics is time asymmetry – why doesn’t stuff happen as often "backwards" in time?  We have no idea about the tiny CP-violation in particle physics, but all the other time asymmetries are thought to arise from a very-low early-universe entropy.  The most popular explanation for this is inflation, especially eternal inflation, which says that any small space-time region satisfying certain conditions is connected to infinitely many large time-asymmetric regions much like what we see around us.  Alas, the chance that any small region satisfies these inflation conditions is extremely small.  As a recent paper puts it:

Initial conditions which give the big bang a thermodynamic arrow of time must necessarily be low entropy and therefore "rare." There is no way the initial conditions can be typical, or there would be no arrow of time, and this fact must apply to inflation and prevent it from representing "completely generic" initial conditions.  … If you can regard the big bang as a fluctuation in a larger system it must be an exceedingly rare one to account for the observed thermodynamic arrow of time.

So the question of time-asymmetry reduces to this: why does the universe have enough independently variable small regions that at least one of them gives eternal inflation?  That is: why is the universe so big?

Continue reading "Self-Indication Solves Time-Asymmetry" »

GD Star Rating
loading...
Tagged as: ,

Anthropic Breakthrough

By Robin Hanson · May 21, 2008 6:00 am · Comments (22)

If the universe is extremely large, with effective physics and cosmological conditions varying widely from place to place, how can we predict the conditions we should expect to see?  In principle we can use anthropic reasoning, by expecting to see conditions that give rise to observers, and perhaps expecting more conditions that give more observers.  But how can we apply this theory when we know so little about the sorts of conditions that produce observers?

Two recent papers suggest a simple but powerful solution:

  • “Predicting the cosmological constant from the causal entropic principle” (Phys Rev 8/07, ungated here)
  • “Predictions of the causal entropic principle for environmental conditions of the universe” (Phys Rev 3/08, ungated here)

This causal entropic principle so far successfully predicts dark energy strength, matter fluctuation ratio, baryonic to dark matter ratio, and baryonic to photon matter ratio!  I’m struggling to understand it though.

A simple reading of the principle is that since observers need entropy gains to function physically, we can estimate the probability that any small spacetime volume contains an observer to be proportional to the entropy gain in that volume.  Note:

Continue reading "Anthropic Breakthrough" »

GD Star Rating
loading...
Tagged as: ,