Hi. I'm a PhD student in physics at MIT who loves your site (especially when I'm procrastinating on doing my work... as now).

I have two points about your Lorentz Contraction entry. In brief, they are:

A) It is true that Einstein's Special Relativity predicts that the ordering of events can be viewed differently by observers moving relatively to one another. HOWEVER, Special Relativity allows an honest, accurate observer to disagree with the casual ordering that another honest, accurate observer ascribes to a pair of events ONLY WHEN the latter observer measures the two events to be separated by such a distance and such an interval of time that no signal limited to travel no faster than the speed of light in vacuum (299,792,458 meters/sec) could link the two events.
Thus, Special Relativity does NOT entail the destruction of causality as a logical concept with universally-agreed-upon status among honest, accurate observers. All it means is that one must be careful to remember that (1) no interaction (e.g., electromagnetic waves like visible light or radio) can travel faster than the speed of light in vacuum and (2) it makes no scientific sense to declare two events are "causally connected" if there's been no interaction between them.

B) The magnitudes of the Lorentz contraction and of the disagreement about simultaneity were indeed correctly calculated, I believe. However, I think it should be emphasized that the calculated length contraction is really a teeny-tiny percentage of the separation of 100,000 light years recorded by the first observer. One should always keep in mind that the effects of Special Relativity, while always present, become significant only when one considers observers travelling relative to one another at close to the speed of light (which, again, is a whopping 299,792,458 meters/sec). This is why more than 2 centuries could elapse after the seminal work of Newton before an Einstein came along and quantified that something fundamental was missed, and this is why any calculation regarding two observers moving relative to one another at normal, mundane velocities (e.g. walking!) shows only teeny-tiny percent discrepancies.

Well, yes, 100,000 light years is extremely far away. And 3 million miles--or 12 days--is a tiny magnitude compared to 100,000 light years. To put it another way, if we were thinking of an event 3000 miles away rather than 100,000 light years, the time-coordinate displacement would be on the order of five nanoseconds (but still, "now" on the other side of the continent does differ by 10 ticks of a Pentium IV between the walking and the standing observer.

The point that causality remains is a good one, and is certainly true: inside the future light cone is inside the future light cone, inside the past light cone is inside the past light cone, outside the light cone is outside the light cone. We need to move to quantum mechanics to disrupt causality...

But there is a sense in which special relativity makes it hard to believe that the universe is divided into a "past" that "was" but no longer "is," a present that "is", and a future that "will be". The idea that there is now an observer (outside my light cones) from whose perspective my entire past is still in his future is a somewhat bizarre one...

My response to this kind of argument (based either on astrophysics or quantum pysics)has always been to ask, "Are these particular effects (time-reversal or timelessness) observable on the human scale?" OR do these effects have effects that are observable on the human scale.

IE -- the timescale ranging up from a hundredth of second up to the whole history of life on earth. And the size scale ranging from the smallest virus up to the solar system.

As far as I know, the answer is always No. (The same is not true of quantum theory or astrophysics in general, with nuclear energy etc.; I just mean the time-reversal/timelessness effect derived from quantum pysics),

I have a collection of physicists books on time, which I don't read any more. The guy who changed my mind is Ilya Prigogine (Nobel Chemist), Order out of Chaos, highly recommended.

>>If you asked that second person whether the child on the other side of the galaxy had been born yet, and if so how old was she, you would get a different answer. <<

If you asked either person anything at all about what was going on "right at that instant" on the other side of the galaxy, you would *have* to get the answer "I don't know", wouldn't you?

Re: the human scale

Magnetism is a relativistic phenomenon, arising from exactly the same types of calculation as Brad used in his example, but applied to electric charges. Its human-scale effects include the computer on which you are reading this message.

isn't 900m/s a rather rapid walking speed?

Yes, many quantum and relativity effects are real at the human scale. But is the reversibility/unreality of time a real effect on the human scale? Is there any actual case when two real events, involving entities affecting humans, reverse their sequence? Or when an entity affecting humans goes backward in time? The example cited by DeLong is unreal because at the distances given, at the speed of light, perception of the events would not be received during the lifetimes of the observers.

The jump from saying that an electron or meson can move backward in time, and that a simple molecule can go backward in time, is enormous. As far as I know, no one has tried to explain how that might be possible.

Another way of saying it is that the time of geology, biology, history, practical activity, and personal life has an obscure and difficult relationship to cosmological and quantum time, and that statements true in one area are often not true in the other. So our intuitive feelings about time do not apply in these areas of physics, but the converse is also true.

At our scale what affects us is collections of googlplexes of elctrons/mesons, etc. Describing the behavior of such collections is defferent than describing the behavior of single mesons etc. Averaging processes tend to erase the most paradoxical quantum effects, producing something temporably describable.

One of Prigogine's arguments is that there could be no life forms in the timeless universe of physics.

A current hot subject in atmospheric physics is the effect of cosmic rays on climate. Satellite observations show a strong correlation between galactic cosmic ray flux and cloud cover on earth (and Neptune). The theory is: cosmic rays hit atomic nuclei in the upper atmosphere, producing showers of charged particles, which produce ionization, which promotes condensation of droplets, which means clouds. Mostly low-altitude clouds.

But cosmic ray showers reach low altitudes only because of relativistic time dilatation. Muons are the main source of low-altitude charged particles, but their lifetime isn't long enough for them to reach low altitudes in significant numbers. Except they're traveling at the better part of the speed of light, which increases their halflife in the earth frame. (See http://www2.slac.stanford.edu/vvc/cosmicrays/cratmos.html ) This is crucial, because the climate effects of low-altitude clouds are opposite those of high-altitude clouds.

Climate definitely affects humans, and climate may be dependent on the relativity of time.

It's not necessary to invoke an "obscure and difficult relationship" to explain why Modern physical concepts do not accord with our intuition. The mental toolkit we carry around with us gives results which are good enough approximations for most situations that a 1.5m tall biped might reasonably encounter in central Africa. This toolkit remains helpful when we make more careful study of largish objects moving at slowish speeds in Earthbound laboratories, though even there our intuition often fails. As we probe further, beyond the little patch of parameter space in which we evolved, we find that nature is much more accurately described by models which behave in wholly counter-intuitive ways -- but whose behaviour matches our intuition at low speeds and large scales. The relationship between physics at the human scale and more widely applicable models isn't obscure and difficult at all: mathematically, it's pretty simple. It also plays an important role in the development of more fundamental theories, where one can make progress by insisting that the theory must correspond to everyday physics in the appropriate limit. Sadly, this aspect of physics -- indeed, of nature -- is very difficult to explain in non-mathematical ways.

You write: "But there is a sense in which special relativity makes it hard to believe that the universe is divided into a "past" that "was" but no longer "is," a present that "is", and a future that "will be". The idea that there is now an observer (outside my light cones) from whose perspective my entire past is still in his future is a somewhat bizarre one..."

Well, your *entire* past can't be in anyone's future, except in the presence of a singularity. But you can't even say that an observer outside your light cones sees some of your "past" in his "future" unless you're using nonrelativistic notions of "past" and "future". If you use the relativistic definitions (my "past" is any event in spacetime that can affect me now, and my "future" is any event that I now can affect) then by definition any observer who sees part of your past in his future must be in your past as well.

Special relativity preserves causality, and defines past and future according to their causal meanings. There is a region outside the light cones (which we might choose to call the "present") which is causally disconnected from the observer; he cannot affect events there, nor can those events affect him. All special relativity says is that for causally-disconnected events there is no fundamental answer to the question of "which event happened first?"

It's also not really fair, as zizka writes above, to talk about electrons moving "backward in time". Although this is sometimes used formally in particle physics when talking about antiparticles, it's pretty misleading: the particles are *not* moving "backward in time" in the sense of being able to convey information backwards in time. Causality is preserved by these particles too.

Ian, I am NOT saying that people talking about the role of time at the astrophysical level or the quantum level are wrong, or that commonsense notions of time are applicable at those levels.

I'm just saying that, when trying to understand entities at the humanly perceptible level (virusses and molecules to solar system or galaxy, a hundredth of a second to a hundred million years), the physicist's new theory of time is irrelevant. The time of those events and things remains about what it was.

Another way to say this is that the things we concern ourselves with most (economies, societies, individuals, organisms) are not describable or distinguishable in the language of physics.

All the physicist's books about The Nature of Time that I have read say about the same thing, and they all vaguely imply that what they say has something to do with the time we live in. But as far as I can see, it doesn't. (Let me recommend Prigogine, Order out of Chaos, once more).

It reminds me of Wittgenstein's story about the guy who, after finding out that most of matter is really empty space (between the nuclei), started wearing overshoes so that he wouldn't fall through the cracks.

> I'm just saying that, when trying to understand entities at the humanly perceptible level (virusses and molecules to solar system or galaxy, a hundredth of a second to a hundred million years), the physicist's new theory of time is irrelevant. The time of those events and things remains about what it was.

But this is such a basic part of how science is done, how it is possible to make scientific progress at all, that I don't know why you're making a big deal out of it in this context.

I recommend "The Feynman Lectures On Physics", by Richard Feynman.

Because all the books I read of this kind vaguely make it seem that we should think differently about the time we live in, or that there is some broad cultural significance or meaning, and as far as I know there isn't.

I am objecting to popular and semi-popular books which endlessly repeat this great discovery which is far more than 50 years old. Obviously I have nothing to say about the significance of this at the level of quarks or galaxy clusters. I don't object to the science in any way.

I'm not likely to read Feinman, though what I have read of him leads me to believe that, besides being a great working scientist, he was a standard-issue fifties positivist, albeit with a lot of cockiness and zest.

I do recommend that everyone look at prigogine. All my reasons are there.

This thread ineluctably reminds me of certain stories of James Blish, he mused. Trivial spelling thing, Zizka: Feynman. More to the point, the note that larger theories of time and space and the nature of the universe are not observable on a casual human level seems worth making note of, and then moving on to a deeper understanding of the universe, as best we can, if one is interested.

That humans perseve what humans perseve is a rather banal point.

I believe that the time spent reading large numbers of pop-sci books about quantum mechanics and relativity would be more usefully invested in reading one or two actual physics textbooks. The point about the Feynman Lectures is that they are an undergraduate physics course, and the reader learns some real physics as opposed to goshwow handwaving. Feynman's straightforward and unpretentious exposition of scientific philosophy and scientific practice are an added bonus.

Brad wants to get all kerfuzzled about how bizarre it is that people can be outside each other's light cone of causality.

I don't see anything bizarre about this. How is it any stanger than the fact that I can't be affected for the next little while by some conversation Brad may be having right now on the other side of the continent?

-dlj.

Farber, Coleman: This thread was founded at the pop-sci level (DeLong's reading). I was responding at that level. I was specifically responding to the implicit idea that these valid ideas of physics tell us something about human time. They don't. That's my whole point.

My reading of these pop-sci books has made me acutely aware of the pop-sci idea, apparently shared by the two of you, that the time of physics is at some metaphysical level real, whereas the time of human life is illusory and subjective. This is what I deny. Two different things, each with its own, different, reality.

I'm not too interested in how the Feynman branch of the family ended up spelling the name. It's the same name. I will be good in the future. BTW, what does "perseve" mean?

Prigogine's point is not just that perceived human time is different than the time of physics. It is also that biological processes could not take place within the time of physics. The guy is a Nobel Prize winner. You ought to quit condescending to me and read his book.

Zizka:

At your recommendation, I have had a look at Prigogine's book. The central point seems to be a derivation of time-irreversibility from the dynamical behaviour of systems far from equilibrium, as opposed to the standard statistical-mechanical derivation of the time-irreversibility in equilibrium thermodynamics. While this is intrinsically interesting, it doesn't have any new philosophy which isn't already present in 19th century statistical mechanics, and doesn't solve any outstanding problems relating to the directionality of time.

You might like to look at this short article, which discusses these issues a bit more fully

http://www.usyd.edu.au/philosophy/price/preprints/ISST.html

or at this rather longer paper which goes into much more depth:

http://xxx.lanl.gov/abs/chao-dyn/9603009

I also found Prigogine's treatment of relativity and cosmology to be rather facile. Einstein is very clear and careful about his definitions of space and time in his 1905 paper on special relativity, but Prigogine doesn't seem to appreciate these fundamental points (Note: I'm not complaining that Prigogine doesn't agree with Einstein, I'm saying doesn't understand important aspects of Einstein's work.) None of Prigogine's criticisms of current cosmolological research are substantial or serious.

Basically, I think Prigogine's work is interesting and valuable within the field for which he got his Nobel Prize, and deeply flawed outwith that field.

My previous post, which you found condescending, was good advice, sincerely meant. I honestly don't think it's possible to gain a real understanding of these issues from pop-sci books.

When Prigogine got to cosmology, he lost me too.

The point of his that think is most important is that the human perception of the directionality time as "real" is NOT subjective or psychological. For example, no one doing an objective study of biology or geology can pay any attention to the time of quantum physics or cosmology. I think that what this means is that the events studied by geologists or biologists (much less historians or archeologists) simply cannot be expressed in terms of physics. While the erosion of a rock (by freezing, acid rain, the effects of wind, etc.), while it includes virtually-infinite numbers of events describable in quantum terms (breaking of various sorts of bonds, etc., is not describable by physics. So, while the time of physics may be at some level more "fundamental", there is no reason for a geoligist to care about that. And the reason isn't in the geologist's subjectivity. It's basically in the rocks. "Rocks" are not physicist's entities.

And the more so for organisms, humans, and human history.

Please excuse my nasty comment. I do that sometimes, usually to my regret. I agree that it's not possible to get a good understanding from pop-sci, but I'm not going to become a physicist. What bothers me about the pop-sci books I collect is, first, that they all say the same thing, and second, that they all present timelessness as this brilliant new idea of theirs even though it's well over half a century old (in a sense, it's Platonic or Vedantist, 2500 years old). Prigogine takes these books as a starting point and says what's wrong with them.

And yes, outside his specialty he's not so sharp. But his specialty (physical chemistry) comprehends geology, biology, history, and psychology.

If I get a chance I'll look at the articles.

One of the good things about lengthy arguments is that eventually the real point of disagreement becomes clear. I think you have an overly-narrow idea of what physics is all about. You say "The point of his that think is most important is that the human perception of the directionality time as "real" is NOT subjective or psychological. For example, no one doing an objective study of biology or geology can pay any attention to the time of quantum physics or cosmology. I think that what this means is that the events studied by geologists or biologists (much less historians or archeologists) simply cannot be expressed in terms of physics. While the erosion of a rock (by freezing, acid rain, the effects of wind, etc.), while it includes virtually-infinite numbers of events describable in quantum terms (breaking of various sorts of bonds, etc., is not describable by physics. So, while the time of physics may be at some level more "fundamental", there is no reason for a geoligist to care about that. And the reason isn't in the geologist's subjectivity. It's basically in the rocks. "Rocks" are not physicist's entities. " The idea that physicists regard the directionality of time as subjective or illusory is incorrect, as is the idea that physics doesn't deal with irreversible phenomena. Statistical mechanics and classical thermodynamics deal with time-asymmetry all the time. The standard view would be that human perceptions of time-irreversibility, and the time-asymmetries in biology, geology and so on, all fundamentally come from the Second Law of Thermodynamics, which can itself be derived from a statistical treatment of large systems. Now, thermodynamics and statistical mechanics are big parts of the field of physics: physicists aren't solely concerned with the structure of spacetime and the quantum fields. For example, where I work there are loads of physicists studying things like the behaviour of the upper atmosphere, the large-scale interactions between the Sun and the Earth, the dynamics of Antarctic ice sheets and their interactions with the ocean, and so on. (Indeed, even cosmology has a physically-meaningful arrow of time associated with it - that's what the Big Bang model is all about.)

Basically, Prigogine is flogging a wet spot where a horse used to be. The whole business about how real time-directional phenomena arise from time-symmetric laws of microscopic physics was sorted out in the 19th century, before relativity and quantum mechanics. There's no contradiction between the time of the physicist, the geologist, or the psychologist. At any rate, physicists define time as "the thing you measure with a clock", and I doubt many people would disagree, whether the clock is a pendulum, coastal erosion, or a heartbeat.

I don't think you read Prigogine very carefully. Most of his book was a critique of the history of the attempts to get thermodynamics to square with pre-existing physics. His claim is that these attempts were unsuccessful.

He was NOT saying that the people who study thermodynamics are not working for physics departments and called "physicists". He's saying that the problems he was talking about were raised, but not solved, in the nineteenth century.

Maybe there's no contradiction between the time of physics and the time of geology. But they're not the same thing. The notion of time which is most useful to me when studying geology has to be jettisoned of I'm going to study cosmology or quantum physics. You agree with me about that. But it's also true vice versa. The notion of time most useful to me studying cosmology or quantum physics would have to be jettisoned if I were to start studying geology.

Human subjectivity is not the reason. However, the time of history and human experience is more like the time of geology than the time of cosmology and quantum physics. That is pretty much the whole point I was making.

Just got the latest Scientific American, which includes two pop-sci articles by Paul Davies on this very subject. Haven't read them yet, but I'm confident that they will say about the same as the ten non-Prigogine books I have on my shelf (almost all by eminent physicists), of which I have read about a third and just checked the others.