Tom H (Yahoo! 2007-05-15)
What is time ?
Here's one of my favorite quotes
It's a translation by John A. Wheeler of a provocative statement made by
J. Henri Poincaré:
Time is defined so that motion looks simple.
In other words,
"time" is defined as the independent variable which makes the equations of mechanics take on a simple form.
This is an operational definition which was designed in a simpler era of "classical" physics.
It still holds for nonrelativistic quantum theory, where time remains an old-fashioned "independent variable".
However, at a deeper level of understanding, time cannot be simply such an "independent" parameter
against which events are recorded. Instead, it's a component of spacetime
(to a degree, time and space can be traded for each other).
This has profound implications for our modern descriptions of the physical world.
Especially in the quantum realm.
Tom H (Yahoo! 2007-07-08)
The Beginning of Time
What was there before the Big Bang took place?
Time is just another coordinate of spacetime, so it has to unfold together with the other dimensions.
Time is created with the rest of space; there was simply no "before".
There was no "instant" of creation and there was no "location" for the primordial explosion either.
The center was everywhere.
It still is.
A geometrical analogy might help:
Think of the surface of a sphere and imagine latitude is "time".
There's nothing north of the North Pole, is there?
This analogy with a sphere has other nice features.
In particular, the North Pole is not very different from nearby points;
it's just an artifact created from the way we measure things.
So too, the "instant" of creation is not well defined; it depends on the speed and location
of the vantage point from which the (theoretical) mesurement is made.
All of this is without even considering the quantum aspects
which nobody really understands (yet?).
Does this blow you mind? Well, it should.
It blows everybody's mind.
Concerning, the "stuff" the Universe was made from, the answer is also weird...
The key remark is that gravitation has more negative energy when everything is packed tight.
Think about everyday experience: energy is released when an object is dropped,
so there's less energy (more "negative energy") when the Earth and the object are closer together.
At the scale of the entire Universe, the numbers are mind-boggling:
The positive energy in the Universe today
(the energy of radiation and matter according to
E=mc2 )
seems to balance exactly the negative energy of gravitation.
Therefore, it looks like the Universe could have been created from zero energy,
from absolutely nothing!
Come to think of it, it MUST have been so,
or else how would you explain the "manufacture" of the original stuff itself?
This framework makes the Universe explainable
(in principle, at least)
without violating physical laws.
Ultimately, we can hope to be left with only one question:
"Why?" or "What caused it?"
That last question, however, is not a scientific one
(no matter how interesting it might be).
blue22op (Yahoo! 2007-05-15)
Unavoidable Time Machines
Is there a time machine in the process of being made?
I grieved to think how brief the dream of the human intellect had been.
H.G. Wells (1866-1946)
The Time Machine (1895)
Like perpetual motion, time travel is
both unavoidable and impossible.
Microscopically, time-travel is unavoidable.
Elementary particles routinely go backward in time;
there's no difference between an antiparticle moving forward in time
and its antiparticle moving backward in time.
A particle-antiparticle
creation may also be described as a particle reversing its direction in time.
Now, can we harness this basic mechanism to make coherent systems
consisting of many particles
(and carrying definite information with them) go back in time?
The answer is as much of a "no" as what applies to the related question of
whether it's possible to transform brownian motion into coherent motion
(that would be called perpetual motion "of the second kind").
If you don't believe in one, you don't believe in the other...
Of course, science is not supposed to be about beliefs, but it is (to some degree).
It's a much more productive belief
(from a scientific standpoint)
to assume that perpetual motion can't exist than the opposite...
In one case, you'll refine the basic laws of thermodynamics.
In the other case, you may waste your life on doomed tinkering.
Similarly, the impossibility of time-travel imposes useful constraints
on the very laws of fundamental physics we are aiming to formulate.
It's almost certainly the more useful of two possible beliefs,
to put it in provocative terms.
This does not mean you can't have fun thinking about the paradoxes of time-travel.
However, those very paradoxes should be an indication that attempts at building
an actual time-machine are as doomed as attempts to build a perpetual motion machine.
Or vice-versa.
(2003-11-03)
Laplace's Demon
In a predictable Universe, the past and the future are alike.
[For an intellect which would know all positions and velocities]
nothing
could be uncertain and the future,
just like the past, would be present before its eyes.
Pierre Simon de Laplace (1749-1827)
The "intellect" so introduced by the Marquis de Laplace
(Essai Philosophique sur les probabilités, 1814) has since been dubbed
Laplace's Demon.
It is an icon of the concept of determinism,
which can be traced back to Socrates and which used to be unseparable from modern science,
from the early days until the dawn of Quantum Theory.
Interestingly, Laplace himself first discussed this in a treaty about probabilities,
the practical approach for dealing with the uncertainty Laplace's Demon
would never have to face.
In fact, Laplace's Demon cannot possibly exist, within this world or outside of it.
From a philosophical or religious standpoint, its existence would preclude
free will, which is the one thing that makes the Creation different
from an uninteresting clockwork toy.
To be the greatest of creators, it would seem, God has to allow His creation
some life of its own, that would escape even Him.
So to speak,
He did make a stone so heavy that He couldn't lift it:
The quantum Universe.
From a scientific standpoint, even the theoretical possibility of Laplace's Demon
condradicts our best understanding of fundamental observations.
In particular, quantum logic is incompatible with so-called
hidden variables,
which would make Quantum Theory deterministic and allow the
enire past and future of the Universe to stand "before the eyes" of the Demon.
Actually, what is ruled out by observation is only the existence of local
hidden variables,
since this would violate Bell's inequalities (John Bell, 1964) which have been
confirmed experimentally.
The possibility remains that some set of nonlocal hidden variables exist,
which makes the whole Universe knowable at once.
However, if time can't be separated from the other dimensions (motion of the observer trades
space for time) we may push the argument of nonlocality to its extreme conclusion
and state that the entire knowledge of the past and future of the Universe
would require no part of its history to be unknown to begin with.
Thus, there may be a God who knows everything that ever was and ever
will be, but there's no such thing as an entity that can deduce such knowledge from
anything but prior knowledge of the same.
Laplace's Demon is a fallacy.
The demons of classical Physics
