【 以下文字转载自 History 讨论区 】
发信人: sgld (sgld), 信区: History
标 题: Re: 时间是绝对的吗？
发信站: BBS 未名空间站 (Sat Sep 3 17:51:42 2011, 美东)
转一篇文章。
Ten Things Everyone Should Know About Time
by Sean
“Time” is the most used noun in the English language, yet it remains a
mystery. We’ve just completed an amazingly intense and rewarding
multidisciplinary conference on the nature of time, and my brain is swimming
with ideas and new questions. Rather than trying a summary (the talks will
be online soon), here’s my stab at a top ten list partly inspired by our
discussions: the things everyone should know about time. [Update: all of
these are things I think are true, after quite a bit of deliberation. Not
everyone agrees, although of course they should.]
1. Time exists. Might as well get this common question out of the way. Of
course time exists — otherwise how would we set our alarm clocks? Time
organizes the universe into an ordered series of moments, and thank goodness
; what a mess it would be if reality were complete different from moment to
moment. The real question is whether or not time is fundamental, or perhaps
emergent. We used to think that “temperature” was a basic category of
nature, but now we know it emerges from the motion of atoms. When it comes
to whether time is fundamental, the answer is: nobody knows. My bet is “yes
,” but we’ll need to understand quantum gravity much better before we can
say for sure.
2. The past and future are equally real. This isn’t completely accepted,
but it should be. Intuitively we think that the “now” is real, while the
past is fixed and in the books, and the future hasn’t yet occurred. But
physics teaches us something remarkable: every event in the past and future
is implicit in the current moment. This is hard to see in our everyday lives
, since we’re nowhere close to knowing everything about the universe at any
moment, nor will we ever be — but the equations don’t lie. As Einstein
put it, “It appears therefore more natural to think of physical reality as
a four dimensional existence, instead of, as hitherto, the evolution of a
three dimensional existence.”
3. Everyone experiences time differently. This is true at the level of both
physics and biology. Within physics, we used to have Sir Isaac Newton’s
view of time, which was universal and shared by everyone. But then Einstein
came along and explained that how much time elapses for a person depends on
how they travel through space (especially near the speed of light) as well
as the gravitational field (especially if its near a black hole). From a
biological or psychological perspective, the time measured by atomic clocks
isn’t as important as the time measured by our internal rhythms and the
accumulation of memories. That happens differently depending on who we are
and what we are experiencing; there’s a real sense in which time moves more
quickly when we’re older.
4. You live in the past. About 80 milliseconds in the past, to be precise.
Use one hand to touch your nose, and the other to touch one of your feet, at
exactly the same time. You will experience them as simultaneous acts. But
that’s mysterious — clearly it takes more time for the signal to travel up
your nerves from your feet to your brain than from your nose. The
reconciliation is simple: our conscious experience takes time to assemble,
and your brain waits for all the relevant input before it experiences the “
now.” Experiments have shown that the lag between things happening and us
experiencing them is about 80 milliseconds. (Via conference participant
David Eagleman.)
5. Your memory isn’t as good as you think. When you remember an event in
the past, your brain uses a very similar technique to imagining the future.
The process is less like “replaying a video” than “putting on a play from
a script.” If the script is wrong for whatever reason, you can have a
false memory that is just as vivid as a true one. Eyewitness testimony, it
turns out, is one of the least reliable forms of evidence allowed into
courtrooms. (Via conference participants Kathleen McDermott and Henry
Roediger.)
6. Consciousness depends on manipulating time. Many cognitive abilities are
important for consciousness, and we don’t yet have a complete picture. But
it’s clear that the ability to manipulate time and possibility is a crucial
feature. In contrast to aquatic life, land-based animals, whose vision-
based sensory field extends for hundreds of meters, have time to contemplate
a variety of actions and pick the best one. The origin of grammar allowed
us to talk about such hypothetical futures with each other. Consciousness
wouldn’t be possible without the ability to imagine other times. (Via
conference participant Malcolm MacIver.)
7. Disorder increases as time passes. At the heart of every difference
between the past and future — memory, aging, causality, free will — is the
fact that the universe is evolving from order to disorder. Entropy is
increasing, as we physicists say. There are more ways to be disorderly (high
entropy) than orderly (low entropy), so the increase of entropy seems
natural. But to explain the lower entropy of past times we need to go all
the way back to the Big Bang. We still haven’t answered the hard questions:
why was entropy low near the Big Bang, and how does increasing entropy
account for memory and causality and all the rest? (We heard great talks by
David Albert and David Wallace, among others.)
8. Complexity comes and goes. Other than creationists, most people have no
trouble appreciating the difference between “orderly” (low entropy) and “
complex.” Entropy increases, but complexity is ephemeral; it increases and
decreases in complex ways, unsurprisingly enough. Part of the “job” of
complex structures is to increase entropy, e.g. in the origin of life. But
we’re far from having a complete understanding of this crucial phenomenon.
(Talks by Mike Russell, Richard Lenski, Raissa D’Souza.)
9. Aging can be reversed. We all grow old, part of the general trend toward
growing disorder. But it’s only the universe as a whole that must increase
in entropy, not every individual piece of it. (Otherwise it would be
impossible to build a refrigerator.) Reversing the arrow of time for living
organisms is a technological challenge, not a physical impossibility. And we
’re making progress on a few fronts: stem cells, yeast, and even (with
caveats) mice and human muscle tissue. As one biologist told me: “You and I
won’t live forever. But as for our grandkids, I’m not placing any bets.”
10. A lifespan is a billion heartbeats. Complex organisms die. Sad though it
is in individual cases, it’s a necessary part of the bigger picture; life
pushes out the old to make way for the new. Remarkably, there exist simple
scaling laws relating animal metabolism to body mass. Larger animals live
longer; but they also metabolize slower, as manifested in slower heart rates
. These effects cancel out, so that animals from shrews to blue whales have
lifespans with just about equal number of heartbeats — about one and a half
billion, if you simply must be precise. In that very real sense, all animal
species experience “the same amount of time.” At least, until we master #
9 and become immortal. (Amazing talk by Geoffrey West.)
