1.Before them people believedAristotle, who said that the natural state of a body was to be at rest and that it moved only if driven by a force or impulse.

impulse

n. an impelling force or strength

e.g. An impulse is a sudden force or desire — this could be an electrical impulse, or an impulse to get some pizza.

2.Galileo’s measurements indicated that each body increased its speed at the same rate, no matter what its weight.

measurement

n. the act or process of assigning numbers to phenomena according to a rule

e.g. Taking a measurement involves figuring out how long something is or what it weighs or how fast it is. Measurements usually require something like a ruler or a stop watch.

3.Of course a lead weight would fall faster than a feather, but that is only because a feather is slowed down by air resistance.

resistance

n. any mechanical force that tends to retard or oppose motion

e.g. If you've ever seen a dog on a leash suddenly refuse to walk any farther, you know how much resistance one little pet can put up. Resistance means "refusal to comply with or accept something."

4.In Galileo’s experiments, as a body rolled down the slope it was always acted on by the same force (its weight), and the effect was to make it constantly speed up.

constantly

adv. without variation or change, in every case

e.g. When something happens constantly, it never stops or changes. If you sister constantly complains that she's hungry, she is always insisting that she's starving.

5.This idea was first stated explicitly in Newton’s Principia Mathematica, published in 1687, and is known as Newton’s first law.This idea was first stated explicitly in Newton’s Principia Mathematica, published in 1687, and is known as Newton’s first law.

explicitly

adv. in an explicit manner

e.g. When you explain something explicitly, you do it in great detail, leaving no room for misunderstanding. You could intimidate your little brother by explicitly describing what you'll do if he reads your diary.

6.A familiar example is provided by a car: the more powerful the engine, the greater the acceleration, but the heavier the car, the smaller the acceleration for the same engine.

acceleration

n. (physics) a rate of increase of velocity

Acceleration is the act of increasing speed. When you buy a sports car, you want one that has great acceleration, so it can go from zero to 60 miles an hour in no time.

7.This is what you might expect because one could think of the new body A as being made of two bodies with the original mass.

mass

n. the property of a body that causes it to have weight in a gravitational field

Anything that has weight and shape has mass. In fact, mass is the quality that gives things weight anywhere there's gravity.

8.If the law were that the gravitational attraction of a star went down faster or increased more rapidly with distance, the orbits of the planets would not be elliptical, they would either spiral in to the sun or escape from the sun.

elliptical

adj. rounded like an egg

The word elliptical is derived from the oval shape known as an ellipse. Many comets have an elliptical orbit around the Sun that brings them closer at some times and farther away at others.

9.He was severely criticized for this irrational belief by many people, most notably by Bishop Berkeley, a philosopher who believed that all material objects and space and time are an illusion.

severely

adv. with sternness; in a severe manner

“peered severely over her glasses”

10-11.When the famous Dr. Johnson was told of Berkeley’s opinion, he cried, “I refute it thus!” and stubbed his toe on a large stone.

refute

v. prove to be false or incorrect

The verb refute is to prove that something is wrong. When the kids you're babysitting swear they brushed their teeth, you can refute their claim by presenting the dry toothbrushes.

stub

v. strike (one's toe) accidentally against an object

“She stubbed her toe in the dark and now it's broken”

12.That is, they believed that one could unambiguously measure the interval of time between two events, and that this time would be the same whoever measured it, provided they used a good clock.

unambiguously

adv. in an unambiguous manner

No signal has yet been strong enough or run long enough to be unambiguously identified as originating from anextraterrestrial intelligence.

13.A proper theory of the propagation of light didn’t come until 1865, when the British physicist James Clerk Maxwell succeeded in unifying the partial theories that up to then had been used to describe the forces of electricity and magnetism.

propagation

n. the act of producing offspring or multiplying by such production

If your dog is about to have puppies, then she's is engaged in the propagation of dogs, the creation of new life to further the species.

14-15.Maxwell’s equations predicted that there could be wavelike disturbances in the combined electromagnetic field, and that these would travel at a fixed speed, like ripples on a pond.

electromagnetic spectrum

n. the entire frequency range of electromagnetic waves

ripple

v. stir up (water) so as to form ripples

A ripple is a small wave on the surface of something, such as a ripple that forms a ring around the spot where you threw a pebble into the pond.

16.Different observers, moving relative to the ether, would see light coming toward them at different speeds, but light's speed relative to the ether would remain fixed.

ether

n. any of a class of organic compounds that have two hydrocarbon groups linked by an oxygen atom

Ether is a chemical that used to be a common anesthetic that you inhaled before undergoing surgery. In most countries, doctors have replaced it with less flammable, safer drugs.

17.The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be the same for all freely moving observers, no matter what their speed.

postulate

n. (logic) a proposition that is accepted as true in order to provide a basis for logical reasoning

Assume something or present it as a fact and you postulate it. Physicists postulate the existence of parallel universes, which is a little mind-blowing.

18.Any observer can work out precisely what time and position any other observer will assign to an event, provided he knows the other observer’s relative velocity.

velocity

n. distance travelled per unit time

If your rocket is traveling at maximum velocity, it means it can't go any faster. Velocity is quickness of motion or action.

19.Again, the choice of coordinates is arbitrary; one can use any three well-defined spatial coordinates and any measure of time.

arbitrary

adj. based on or subject to individual discretion or preference or sometimes impulse or caprice

Something that's arbitrary seems like it's chosen at random instead of following a consistent rule. Team members would dislike their coach using a totally arbitrary method to pick starting players.

20.And since the speed of light is the same at every event and in every direction, all the light cones will be identical and will all point in the same direction.

identical

adj. being the exact same one; not any other:

When you're looking for exact replicas, don't waste your time on snowflakes or fingerprints; no two are identical, or exactly the same.

不管观察者运动多块，他们应测量到一样的光速。这简单的观念有一些非凡的结论，如质量和能量等价/没有任何东西可能行进得比光还快。

由于能量和质量的等价，物体由于它的运动具有的能量应该加到它的质量上去。换言之，要加速它更为困难。这个效应只有当物体以接近于光速的速度运动时才有实际的意义。例如，以10%光速运动的物体的质量只比原先增加了0.5%，而以90%光速运动的物体，其质量变得比正常质量的两倍还多。当一个物体接近光速时，它的质量上升得越来越快，这样它需要越来越多的能量才能进一步加速上去。实际上它永远不可能达到光速，因为那时质量会变成无限大，而根据质量能量等价原理，这就需要无限大的能量才能做到。由于这个原因，相对论限制了物体运动的速度：任何正常的物体永远以低于光速的速度运动，只有光或其他没有内禀质量的波才能以光速运动。

由于光速一致且空间不是绝对的（即光行进的距离不一致），因此光行进花费的时间不一致。相对论终结了绝对时间的观念，每个观察者都一定有他自己的时间测度，不同观察者携带的同样的种的读数不必要一致。

相对论迫使我们从根本上改变了我们的时间和空间观念。时间不能完全脱离和独立于空间，而必须和空间结合在一起形成所谓的时空的客体。

一个事件是在特定时刻和在空间中特定的一点发生的某件事。人们可以用四个数或坐标来制定它。坐标系的选择是任意的，人们可以使用任何三个定义好的空间坐标和任何时间测度。在相对论中，在时间和空间坐标之间没有真正的差别。

从一个事件散开的光在四维时空里形成了一个三维的圆锥，这个圆锥成为时间的将来光锥。以同样的方法可以画出另一个成为过去光锥的圆锥，它表示所有可以用一个光脉冲传播到该事件的事件集合。对于给定的事件P，人们可以将宇宙中的其他事件分成三类。从事件P出发由一个粒子或者波以等于或小于光速的速度行进能到达的哪些事件成为属于P的将来。它们处于从事件P发射的膨胀的光球面之内或之上。这样，在时空图中它们就处于P的将来光锥的里面或上面。因为没有任何东西比光行进得更快，所以在P所发生的东西只能影响在P的将来中的事件。P的过去可被定义为下列所有事件的集合，从这些事件可能以等于或小于光速的速度行进到达事件P。这样，它就是能够影响发生在P的事件的所有事件的集合。不处于P的将来或过去的事件别称之为处于P的他处。在这种事件初所发生的东西既不能影响发生在P的事件，也不受发生在P的事件的影响。我们看到的从很远星系来的光是在几百万年之前发出的，至于我们看到的最远物体，光是在大约80亿年前发出的。这样，当我们看宇宙时，我们是在看它的过去。

当我们看宇宙时，我们看的是它的过去。

8分钟前的太阳真美丽啊！