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.

不管觀察者運動多塊，他們應(yīng)測量到一樣的光速介时。這簡單的觀念有一些非凡的結(jié)論敬锐，如質(zhì)量和能量等價/沒有任何東西可能行進得比光還快。

由于能量和質(zhì)量的等價壁畸，物體由于它的運動具有的能量應(yīng)該加到它的質(zhì)量上去。換言之珠洗，要加速它更為困難坡倔。這個效應(yīng)只有當(dāng)物體以接近于光速的速度運動時才有實際的意義衔肢。例如庄岖，以10%光速運動的物體的質(zhì)量只比原先增加了0.5%，而以90%光速運動的物體角骤，其質(zhì)量變得比正常質(zhì)量的兩倍還多隅忿。當(dāng)一個物體接近光速時，它的質(zhì)量上升得越來越快邦尊，這樣它需要越來越多的能量才能進一步加速上去背桐。實際上它永遠(yuǎn)不可能達(dá)到光速，因為那時質(zhì)量會變成無限大蝉揍，而根據(jù)質(zhì)量能量等價原理链峭，這就需要無限大的能量才能做到。由于這個原因又沾，相對論限制了物體運動的速度：任何正常的物體永遠(yuǎn)以低于光速的速度運動弊仪，只有光或其他沒有內(nèi)稟質(zhì)量的波才能以光速運動。

由于光速一致且空間不是絕對的（即光行進的距離不一致）杖刷，因此光行進花費的時間不一致励饵。相對論終結(jié)了絕對時間的觀念，每個觀察者都一定有他自己的時間測度滑燃，不同觀察者攜帶的同樣的種的讀數(shù)不必要一致役听。

相對論迫使我們從根本上改變了我們的時間和空間觀念。時間不能完全脫離和獨立于空間表窘，而必須和空間結(jié)合在一起形成所謂的時空的客體典予。

一個事件是在特定時刻和在空間中特定的一點發(fā)生的某件事。人們可以用四個數(shù)或坐標(biāo)來制定它蚊丐。坐標(biāo)系的選擇是任意的，人們可以使用任何三個定義好的空間坐標(biāo)和任何時間測度艳吠。在相對論中麦备，在時間和空間坐標(biāo)之間沒有真正的差別。

從一個事件散開的光在四維時空里形成了一個三維的圓錐昭娩，這個圓錐成為時間的將來光錐凛篙。以同樣的方法可以畫出另一個成為過去光錐的圓錐，它表示所有可以用一個光脈沖傳播到該事件的事件集合栏渺。對于給定的事件P呛梆，人們可以將宇宙中的其他事件分成三類。從事件P出發(fā)由一個粒子或者波以等于或小于光速的速度行進能到達(dá)的哪些事件成為屬于P的將來磕诊。它們處于從事件P發(fā)射的膨脹的光球面之內(nèi)或之上填物。這樣纹腌，在時空圖中它們就處于P的將來光錐的里面或上面。因為沒有任何東西比光行進得更快滞磺，所以在P所發(fā)生的東西只能影響在P的將來中的事件升薯。P的過去可被定義為下列所有事件的集合，從這些事件可能以等于或小于光速的速度行進到達(dá)事件P击困。這樣涎劈，它就是能夠影響發(fā)生在P的事件的所有事件的集合。不處于P的將來或過去的事件別稱之為處于P的他處阅茶。在這種事件初所發(fā)生的東西既不能影響發(fā)生在P的事件蛛枚，也不受發(fā)生在P的事件的影響。我們看到的從很遠(yuǎn)星系來的光是在幾百萬年之前發(fā)出的脸哀，至于我們看到的最遠(yuǎn)物體蹦浦，光是在大約80億年前發(fā)出的。這樣企蹭，當(dāng)我們看宇宙時白筹，我們是在看它的過去。

當(dāng)我們看宇宙時谅摄，我們看的是它的過去徒河。

8分鐘前的太陽真美麗啊送漠！