【拒绝原创】Einstein之“运动物体的电动力学”-挑战相对论-西陆网

【拒绝原创】Einstein之“运动物体的电动力学”

[楼主] 作者:scaling 发表时间:2009/04/07 22:09

点击:372次

ON THE ELECTRODYNAMICS OF MOVING BODIES

By A. Einstein June 30, 1905

It is known that Maxwell's electrodynamics--as usually
understood at the present time--when applied to moving bodies,
leads to asymmetries which do not appear to be inherent in the
phenomena. Take, for example, the reciprocal electrodynamic
action of a magnet and a conductor. The observable phenomenon
here depends only on the relative motion of the conductor and the
magnet, whereas the customary view draws a sharp distinction
between the two cases in which either the one or the other of these
bodies is in motion. For if the magnet is in motion and the
conductor at rest, there arises in the neighbourhood of the magnet
an electric field with a certain definite energy, producing a current
at the places where parts of the conductor are situated. But if the
magnet is stationary and the conductor in motion, no electric field
arises in the neighbourhood of the magnet. In the conductor,
however, we find an electromotive force, to which in itself there is
no corresponding energy, but which gives rise--assuming equality
of relative motion in the two cases discussed--to electric currents
of the same path and intensity as those produced by the electric
forces in the former case.

Examples of this sort, together with the unsuccessful attempts to
discover any motion of the earth relatively to the ``light medium,''
suggest that the phenomena of electrodynamics as well as of
mechanics possess no properties corresponding to the idea of
absolute rest. They suggest rather that, as has already been shown
to the first order of small quantities, the same laws of
electrodynamics and optics will be valid for all frames of reference
for which the equations of mechanics hold good.1 We will raise
this conjecture (the purport of which will hereafter be called the
``Principle of Relativity'') to the status of a postulate, and also
introduce another postulate, which is only apparently irreconcilable
with the former, namely, that light is always propagated in empty
space with a definite velocity c which is independent of the state of
motion of the emitting body. These two postulates suffice for the
attainment of a simple and consistent theory of the
electrodynamics of moving bodies based on Maxwell's theory for
stationary bodies. The introduction of a ``luminiferous ether'' will
prove to be superfluous inasmuch as the view here to be developed
will not require an ``absolutely stationary space'' provided with
special properties, nor assign a velocity-vector to a point of the
empty space in which electromagnetic processes take place.

本帖地址：http://club.xilu.com/hongbin/msgview-950451-184309.html[复制地址]

[楼主] [3楼] 作者:scaling 发表时间: 2009/04/07 22:13

[加为好友][发送消息][个人空间]

回复修改来源删除

§ 1. Definition of Simultaneity
Let us take a system of co-ordinates in which the equations of
Newtonian mechanics hold good.2 In order to render our
presentation more precise and to distinguish this system of coordinates
verbally from others which will be introduced hereafter,
we call it the ``stationary system.''
If a material point is at rest relatively to this system of coordinates,
its position can be defined relatively thereto by the
employment of rigid standards of measurement and the methods of
Euclidean geometry, and can be expressed in Cartesian coordinates.
If we wish to describe the motion of a material point, we give
the values of its co-ordinates as functions of the time. Now we
must bear carefully in mind that a mathematical description of this
kind has no physical meaning unless we are quite clear as to what
we understand by ``time.'' We have to take into account that all our
judgments in which time plays a part are always judgments of
simultaneous events. If, for instance, I say, ``That train arrives here
at 7 o'clock,'' I mean something like this: ``The pointing of the
small hand of my watch to 7 and the arrival of the train are
simultaneous events.''3
It might appear possible to overcome all the difficulties
attending the definition of ``time'' by substituting ``the position of
the small hand of my watch'' for ``time.'' And in fact such a
definition is satisfactory when we are concerned with defining a
time exclusively for the place where the watch is located; but it is
no longer satisfactory when we have to connect in time series of
events occurring at different places, or--what comes to the same
thing--to evaluate the times of events occurring at places remote
from the watch.
We might, of course, content ourselves with time values
determined by an observer stationed together with the watch at the
origin of the co-ordinates, and co-ordinating the corresponding
positions of the hands with light signals, given out by every event
to be timed, and reaching him through empty space. But this coordination
has the disadvantage that it is not independent of the
standpoint of the observer with the watch or clock, as we know
from experience. We arrive at a much more practical determination
along the following line of thought.
If at the point A of space there is a clock, an observer at A can
determine the time values of events in the immediate proximity of
A by finding the positions of the hands which are simultaneous
with these events. If there is at the point B of space another clock
in all respects resembling the one at A, it is possible for an
observer at B to determine the time values of events in the
immediate neighbourhood of B. But it is not possible without
further assumption to compare, in respect of time, an event at A
with an event at B. We have so far defined only an ``A time'' and a
``B time.'' We have not defined a common ``time'' for A and B, for
the latter cannot be defined at all unless we establish by definition
that the ``time'' required by light to travel from A to B equals the
``time'' it requires to travel from B to A. Let a ray of light start at
the ``A time'' from A towards B, let it at the ``B time'' be
reflected at B in the direction of A, and arrive again at A at the ``A
time'' .
In accordance with definition the two clocks synchronize if
We assume that this definition of synchronism is free from
contradictions, and possible for any number of points; and that the
following relations are universally valid:--
1. If the clock at B synchronizes with the clock at A, the clock
at A synchronizes with the clock at B.
2. If the clock at A synchronizes with the clock at B and also
with the clock at C, the clocks at B and C also synchronize
with each other.
Thus with the help of certain imaginary physical experiments we
have settled what is to be understood by synchronous stationary
clocks located at different places, and have evidently obtained a
definition of ``simultaneous,'' or ``synchronous,'' and of ``time.''
The ``time'' of an event is that which is given simultaneously with
the event by a stationary clock located at the place of the event,
this clock being synchronous, and indeed synchronous for all time
determinations, with a specified stationary clock.
In agreement with experience we further assume the quantity
to be a universal constant--the velocity of light in empty space.
It is essential to have time defined by means of stationary clocks
in the stationary system, and the time now defined being
appropriate to the stationary system we call it ``the time of the
stationary system.''

[楼主] [5楼] 作者:scaling 发表时间: 2009/04/14 21:10

[加为好友][发送消息][个人空间]

回复修改来源删除

§ 2. On the Relativity of Lengths and Times
The following reflexions are based on the principle of relativity
and on the principle of the constancy of the velocity of light. These
two principles we define as follows:--
1. The laws by which the states of physical systems undergo
change are not affected, whether these changes of state be
referred to the one or the other of two systems of co-ordinates
in uniform translatory motion.
2. Any ray of light moves in the ``stationary'' system of coordinates
with the determined velocity c, whether the ray be
emitted by a stationary or by a moving body. Hence
On the Electrodynamics of Moving Bodies Page 4 of 31
http://www.fourmilab.ch/etexts/einstein/specrel/www/ 8/18/2003
where time interval is to be taken in the sense of the
definition in § 1.
Let there be given a stationary rigid rod; and let its length be l as
measured by a measuring-rod which is also stationary. We now
imagine the axis of the rod lying along the axis of x of the
stationary system of co-ordinates, and that a uniform motion of
parallel translation with velocity v along the axis of x in the
direction of increasing x is then imparted to the rod. We now
inquire as to the length of the moving rod, and imagine its length to
be ascertained by the following two operations:--
(a) The observer moves together with the given measuring-rod
and the rod to be measured, and measures the length of the
rod directly by superposing the measuring-rod, in just the
same way as if all three were at rest.
(b) By means of stationary clocks set up in the stationary system
and synchronizing in accordance with § 1, the observer
ascertains at what points of the stationary system the two
ends of the rod to be measured are located at a definite time.
The distance between these two points, measured by the
measuring-rod already employed, which in this case is at rest,
is also a length which may be designated ``the length of the
rod.''
In accordance with the principle of relativity the length to be
discovered by the operation (a)--we will call it ``the length of the
rod in the moving system''--must be equal to the length l of the
stationary rod.
The length to be discovered by the operation (b) we will call
``the length of the (moving) rod in the stationary system.'' This we
shall determine on the basis of our two principles, and we shall
find that it differs from l.
Current kinematics tacitly assumes that the lengths determined
by these two operations are precisely equal, or in other words, that
a moving rigid body at the epoch t may in geometrical respects be
perfectly represented by the same body at rest in a definite
position.
On the Electrodynamics of Moving Bodies Page 5 of 31
http://www.fourmilab.ch/etexts/einstein/specrel/www/ 8/18/2003
We imagine further that at the two ends A and B of the rod,
clocks are placed which synchronize with the clocks of the
stationary system, that is to say that their indications correspond at
any instant to the ``time of the stationary system'' at the places
where they happen to be. These clocks are therefore ``synchronous
in the stationary system.''
We imagine further that with each clock there is a moving
observer, and that these observers apply to both clocks the criterion
established in § 1 for the synchronization of two clocks. Let a ray
of light depart from A at the time4 , let it be reflected at B at the
time , and reach A again at the time . Taking into
consideration the principle of the constancy of the velocity of light
we find that
where denotes the length of the moving rod--measured in the
stationary system. Observers moving with the moving rod would
thus find that the two clocks were not synchronous, while
observers in the stationary system would declare the clocks to be
synchronous.
So we see that we cannot attach any absolute signification to the
concept of simultaneity, but that two events which, viewed from a
system of co-ordinates, are simultaneous, can no longer be looked
upon as simultaneous events when envisaged from a system which
is in motion relatively to that system.

作者:
密码:
	游客来访
请输入验证:		刷新验证码
更换马甲注册用户提交

精彩推荐>>