In these lectures Roger Penrose and I will put forward our related but rather different viewpoints on the nature of space and time. We shall speak alternately and. Three illustrated lectures given by Stephen Hawking as part of a series of six lectures with Roger Penrose on the nature of space and time. physicists of the current era to carry on the debate, and who could be better qualified than. Stephen Hawking and Roger Penrose? Arguably.

The Nature Of Space And Time Pdf

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The nature of space and time - S. Hawking, R. - Ebook download as PDF File .pdf), Text File .txt) or read book online. the nature of space and time. The Nature of Space and Time [Stephen Hawking and Roger Penrose].pdf. Ahmed Shameel. This page intentionally left blank This page intentionally left blank. The Grand Design. ALSO BY STEPHEN HAWKING. A Brief History of Time. A Briefer History of Time The Grand D The Theory of

Do you not see that in such a manner the arrow would be moving faster than the air? Now what conferred this greater velocity upon the arrow? Do you mean to say that the air gives it a greater speed than its own? You know perfectly well, Simplicio, that this whole thing takes place just exactly opposite to what Aristotle says, and that it is as false that the medium confers motion upon the projectile as it is true that it is this alone which impedes it.

But when shot from the bow, since the air stands still, the sidewise arrow strikes against much air and is much impeded, while the other easily overcomes the obstacles of the tiny amount of air that opposes it. The conclusion that projectiles do not need a mover is inevitable. The motion of an object which moves on its own is now called motion by inertia.

It is controversial whether Galileo clearly realized the idea of inertial motion. What ultimately matters, however, is the essence of his arguments — that a body left on its own moves on its own and does not need a constant mover.

And this is the very core of the fundamental idea of inertia. Galileo had tried to answer the question of why free bodies would continue to move on its own forever, provided that nothing prevents them from doing so, by assuming that the continued motion of a projectile is impressed upon it by its thrower.

We have not done better than him — inertia still continues to be an outstanding puzzle in physics. Then 26 2 On the Impossibility of Detecting Uniform Motion Galileo employed the new view of motion to both the tower and ship experiments and showed that a stone dropped from the tower or the mast of the ship preserves its motion and lands at the base of the tower or the mast, respectively.

In this way he demonstrated that experiments involving a stone dropped from a tower or from the mast of a moving ship always produce null results and therefore cannot be used to detect the motion of the Earth or the ship. Therefore the motion of a body cannot be discovered by performing mechanical experiments the type of experiments Galileo considered on the moving body itself.

In throwing something to your companion, you will need no more force to get it to him whether he is in the direction of the bow or the stern, with yourself situated opposite. The droplets will fall as before into the vessel beneath without dropping toward the stern, although while the drops are in the air the ship runs many spans.

And if smoke is made by burning some incense, it will be seen going up in the form of a little cloud, remaining still and moving no more toward one side than the other. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality. Minkowski [9, p. In the previous chapter we have seen an excellent example of how this method works and how it produces results with far-reaching implications.

At that time the main results of this chapter were presented as an illustration of how this method might work. Such a result implies that special relativity could have been discovered earlier. Here we will not speculate on when it could have happened. Once this has been done, one can indeed ask the question: When could the theory of relativity realistically have been discovered?

In other words, no matter what kind of mechanical experiments we perform, we cannot detect the uniform motion of a body in space.

The latter was accepted as the correct description of the world on the basis of a combination of reasons, mostly astronomical observations, but also the requirement of logical simplicity — a single assumption that it is the Earth that orbits the Sun explains all astronomical observations without the need to introduce epicycles or other ad hoc hypotheses. The way this principle is stated — by performing mechanical experiments we cannot detect our uniform motion in space — seems to imply that there is such a motion but we cannot discover it.

Our everyday experience tells us that objects move in 3. Therefore it is natural to assume that the Earth is also moving in some kind of medium, which can be called aether, vacuum, or simply space. Since the time of Newton and Leibnitz there has been a continued debate over the nature of space. There exist two opposing views — absolutism substantivalism and relationism, which hold that space is a substance and a collection of relations between physical objects, respectively.

In this book we will not enter that debate but, in order to avoid semantic misunderstandings, we will make it clear that by space we will understand the entity in which the Earth moves.

One cannot deny the existence of that entity by claiming that it is merely nothingness or just a set of relations between the Earth and the other celestial bodies. Such a denial must obviously answer the old argument — if there were nothing between the Earth and the Moon, for example, why would they not be touching each other? The relationists should also explain how the Earth can move in a collection of relations between objects.

If space is an entity, it is obviously absolute in the sense that, being one entity, space is for everyone just the space. Then, any motion in space is an absolute motion; any rest in space is absolute rest. It was in this sense that motion and rest had been regarded as principal conditions in nature from the time of Aristotle to the time of Galileo [4, p.

Now we face another problem — if space is regarded as a medium, as in any other known medium, the natural state of objects in space should be to be at rest, and any moving object should require a mover since media impede the motion of objects. What is wrong then? What caused that problem? Imagine that in the period between the time of Galileo and twentieth century there existed a group of scientists as bright as Galileo himself.

On the other hand, however, the second interpretation seems even worse as we will see below.

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Here we will for the main part be carrying out the analyses that the second more radical research team had to perform. There are two reasons for this choice. Let us see why. It seems that a third interpretation, viz. What we have been trying to do here is to reconstruct a hypothetical analysis that could have been carried out earlier than the twentieth century.

This interpretation is more logical since if uniform motion in space does not exist it becomes immediately clear why it cannot be detected.

But if there is no absolute uniform motion, it follows that there is no uniform motion at all because absolute uniform motion is motion in space and if absolute uniform motion does not exist it means that uniform motion in space does not exist either.

In such a case the only motion possible would be accelerated motion since such a motion can be detected due to the existence of inertial forces. In terms of being detectable, accelerated motion is absolute, which seems to imply that it is motion with respect to an absolute space. Such a conclusion appears to support the traditional research team since, if absolute accelerated motion exists, absolute uniform motion should also exist — we all see that both accelerated and uniformly moving bodies move in space.

We shall return to the question of whether absolute accelerated motion requires an absolute space in Chap.

The nature of space and time - S. Hawking, R. Penrose.pdf

The conclusion that uniform motion in space does not exist is an apparent paradox because hardly anyone would deny that objects move uniformly in space. Such a claim, however, is an operationalist one, since it is based on what we can measure. We can measure the motion of an object with respect to other objects, but not with respect to space, despite the fact that the object is clearly moving in space.

The question both research teams are asking is precisely why we cannot measure the uniform motion of an object in space. The clarity and brilliance of Hawking's logic would break through in simple straightforward terms. This provided a real thrill. Stephen Hawking.

Subject Areas. Science - Physics - General Science - Astronomy. Shopping Cart Options For eBooks Many of our ebooks are available through library electronic resources including these platforms: Our eBook editions are available from many of these online vendors: Need textbooks?

View our textbook site. To request an electronic inspection copy for course use consideration, please visit one of the following services to submit your digital examination request online: Simultaneity is relative. Suppose you find two bells in different church towers striking at exactly the same time i.

If I move steadily past you, I will find that they strike at different times i. It is even possible for you to find that some event A happens before some other event B and for me to find that they occur in the opposite order.

The speed of light in a vacuum is a fundamental speed limit. It is impossible to accelerate any material object up to this speed. If these consequences seem absurd, please suspend your disbelief.

It took the genius of Einstein to realise that there was nothing illogical or contradictory in these statements, but that they describe the world as it is. Admittedly we don't notice these effects in everyday life but that is because we move slowly: relativistic effects only become significant at speeds comparable with the speed of light 2.

But not everything moves slowly. The electrons in the tube of a TV set are one example, found in most homes, where relativistic effects are significant. One of the first people to embrace Einstein's ideas was his former teacher, Hermann Minkowski He realised that although different observers experience the same events, they will describe them differently because they disagree about the nature of space and the nature of time.

On the other hand, space and time taken together form a more robust entity: 'Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality. Einstein et al. The union of space and time of which Minkowski spoke is now generally referred to as space-time. It represents a kind of melding together of space and time, and since space is three-dimensional, and time is one-dimensional, space-time is four-dimensional.

Any particular observer, such as you or I, will divide space-time into space and time, but the way in which that division is made may differ from one observer to another and will crucially depend on the relative motion of the observers. A very rough attempt at representing diagrammatically this change of attitude towards space and time is shown in Figure Before Einstein introduced special relativity, the phrase 'the whole of space at a particular time' was thought to have exactly the same meaning for all observers.

After Einstein's work it was felt that each observer would understand what the phrase meant, but that different observers would disagree about what constituted the whole of space at a particular time. All observers would agree on what constituted space-time, but the way in which it was sliced up into space and time would differ from one observer to another, depending on their relative motion. No observer had the true view; they were all equally valid even though they might be different.

Figure 25 a The pre-Einsteinian view of space and time. Not only are space and time separate and distinct, they are also absolute. All observers agree on what constitutes space and what constitutes time, and they also agree about what it means to speak of 'the whole of space at a particular time'. Different observers in uniform, relative motion will each slice space time into space and time, but they will do so in different ways.

Each observer knows what it means to speak of 'the whole of space at a particular time', but different observers no longer necessarily agree about what constitutes space and what constitutes time.

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In retrospect, special relativity can be seen as part of a gradual process in which the laws of physics attained universal significance. The earliest attempts to understand the physical world placed Man and the Earth firmly at the centre of creation.

Certain laws applied on Earth, but different laws applied in the heavens. Copernicus overturned this Earth-centred view and Newton proposed laws that claimed to apply at all places, and at all times. Special relativity continues this process by insisting that physical laws should not depend on the observer's state of motion - at least so long as that motion is uniform.But how could this be what space is made of?

What Is Spacetime?

Yes, those were the days, when theoretical physics progressed like a puzzle being assembled, where every new piece neatly fit into the growing whole. Consider dropping a stone from the top of a tower. Figure 25 a The pre-Einsteinian view of space and time.

The accidental discovery of superstrings resulted in one of the most creative outbursts in theoretical physics.