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.If light always travels at the same speed relative to absolutespace, common sense tells us this ought to show up in the form ofseasonal changes in the speed of light measured from the Earth.Itdoes not.Einstein resolved the dilemma with his special theory.This saysthat all frames of reference are equally valid and that there is noabsolute reference frame.Anybody who moves at a constantvelocity through space is entitled to regard himself or herself as sta-tionary.They will find that moving objects in their frame of refer-ence obey Newton s laws, while electromagnetic radiation obeysMaxwell s equations and the speed of light is always measured to bethe value that comes out of those equations, denoted by the letter c.Furthermore, anybody who is moving at a constant speed relativeto the first person (the first observer in physicists jargon) will alsobe entitled to say that they are at rest and will find that objects intheir laboratory obey Newton s laws, while measurements alwaysgive the speed of light as c.Even if one observer is moving towardthe other observer at half the speed of light and sends a torch beamout ahead, the second observer will not measure the speed of thelight from the torch as 1.5c: it will still be c!Starting out from the observed fact that the speed of light is aconstant, the same whichever way the Earth is moving through 29Classical Cosmologyspace, Einstein found a mathematical package to describe thebehavior of material objects in reference frames that move with con-stant velocities relative to one another so-called inertial framesof reference.Provided the velocities are small compared with thespeed of light, these equations give exactly the same answers asNewtonian mechanics.But when the velocities begin to become anappreciable fraction of the speed of light, strange things happen.Two velocities, for example, can never add up to give a relativevelocity greater than c.An observer may see two other observersapproaching each other on a head-on collision course, each travel-ing at a speed of 0.9c in the first observer s reference frame, butmeasurements carried out by either of those two fast-movingobservers will always show that the other one is traveling at a speedless than c but bigger (in this case) than 0.9c.The reason why velocities add up in this strange way has to dowith the way both space and time are warped at high velocities.Inorder to account for the constancy of the speed of light, Einsteinhad to accept that moving clocks run more slowly than stationaryclocks and that moving objects shrink in the direction of theirmotion.The equations also tell us that moving objects increase inmass the faster they go.Strange and wonderful though all these things are, they are onlyperipheral to the story of modern cosmology and to the search forlinks between quantum physics and gravity.We stress, however, thatthey are not wild ideas in the sense that we sometimes dismiss crazynotions as just a theory in everyday language.To scientists a the-ory is an idea that has been tried and tested by experiments and haspassed every test.The special theory of relativity is no exception tothis rule.All the strange notions implicit in the theory the con-stancy of the speed of light, the stretching of time and shrinking oflength for moving objects, the increase in mass of a moving objecthave been measured and confirmed to great precision in very many 30 STEPHEN HAWKINGexperiments.Particle accelerators atom smashing machines likethose at CERN, the European Center for Nuclear Research, inGeneva simply would not work if the theory were not a good one,since they have been designed and built around Einstein s equations.The special theory of relativity as a description of the high-speedworld is as securely founded in solid experimental facts as isNewtonian mechanics as a description of the everyday world; theonly reason it conflicts with our common sense is that in everydaylife we are not used to the kind of high-speed travel required for theeffects to show up.After all, the speed of light, c, is 300,000 kilo-meters a second (186,000 miles a second), and the relativistic effectscan be safely ignored for any speeds less than about 10 percent ofthis that is, for speeds less than a mere 30,000 kilometers asecond.In essence, the special theory is the result of a marriage ofNewton s equations of motion with Maxwell s equations describingradiation.It was very much a child of its time, and if Einstein hadn tcome up with the theory in 1905, one of his contemporaries wouldsurely have done so within the next few years.Without Einstein sspecial genius, though, it might have been a generation or morebefore anyone realized the importance of a far deeper insight buriedwithin the special theory.This key ingredient, to which we have already alluded, was the fruitof another marriage the union of space and time.In everyday life,space and time seem to be quite different things.Space extendsaround us in three dimensions (up and down, left and right, for-ward and backward).We can see where things are located in spaceand travel through it more or less at will.Time, although we allknow what it is, is almost impossible to describe.In a sense, it doeshave a direction (from past to future), but we can look neither intothe future nor into the past, and we certainly cannot move through 31Classical Cosmologytime at will.Yet the great universal constant, c, is a speed, and speedis a measure that relates space and time.Speeds are always in theform of miles per hour, or centimeters per second, or any other unitof length per unit of time.You cannot have one without the otherwhen you are talking about speed.So the fact that the fundamentalconstant is a velocity must be telling us something significant aboutthe Universe.But what?If you multiply a speed by a time, you get a length
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