What Is Spacetime? - Scientific American
The Relationship of Space and Time To make an ultimate theory of physics one needs to understand the true nature not only of – from A New Kind of Science. Doreen Massey -- Space-Time, 'Science' and the Relationship between salonjardin.info - Download as PDF File .pdf), Text File .txt) or read online. Scientific American is the essential guide to the most awe-inspiring advances Even to speak of “there” is problematic because the very spacetime that . The relations are dictated by quantum theory or other principles, and.
In the Cartesian coordinate systemthese are called x, y, and z.
A position in spacetime is called an event, and requires four numbers to be specified: Spacetime is thus four dimensional. An event is something that happens instantaneously at a single point in spacetime, represented by a set of coordinates x, y, z and t.
The word "event" used in relativity should not be confused with the use of the word "event" in normal conversation, where it might refer to an "event" as something such as a concert, sporting event, or a battle.
These are not mathematical "events" in the way the word is used in relativity, because they have finite durations and extents. Unlike the analogies used to explain events, such as firecrackers or lightning bolts, mathematical events have zero duration and represent a single point in spacetime. The path of a particle through spacetime can be considered to be a succession of events.
The series of events can be linked together to form a line which represents a particle's progress through spacetime. That line is called the particle's world line. It was only with the advent of sensitive scientific measurements in the mids, such as the Fizeau experiment and the Michelson—Morley experimentthat puzzling discrepancies began to be noted between observation versus predictions based on the implicit assumption of Euclidean space.
Each location in spacetime is marked by four numbers defined by a frame of reference: The 'observer' synchronizes the clocks according to their own reference frame.
In special relativity, an observer will, in most cases, mean a frame of reference from which a set of objects or events are being measured.
This usage differs significantly from the ordinary English meaning of the term. Reference frames are inherently nonlocal constructs, and according to this usage of the term, it does not make sense to speak of an observer as having a location. Any specific location within the lattice is not important. The latticework of clocks is used to determine the time and position of events taking place within the whole frame.
The term observer refers to the entire ensemble of clocks associated with one inertial frame of reference. A real observer, however, will see a delay between the emission of a signal and its detection due to the speed of light. Because black holes can warm up and cool down, it stands to reason that they have parts—or, more generally, a microscopic structure.
And because a black hole is just empty space according to general relativity, infalling matter passes through the horizon but cannot lingerthe parts of the black hole must be the parts of space itself. As plain as an expanse of empty space may look, it has enormous latent complexity. Even theories that set out to preserve a conventional notion of spacetime end up concluding that something lurks behind the featureless facade.
For instance, in the late s Steven Weinberg, now at the University of Texas at Austin, sought to describe gravity in much the same way as the other forces of nature. He still found that spacetime is radically modified on its finest scales. Physicists initially visualized microscopic space as a mosaic of little chunks of space. If you zoomed in to the Planck scale, an almost inconceivably small size of 10—35 meter, they thought you would see something like a chessboard.
But that cannot be quite right. For one thing, the grid lines of a chessboard space would privilege some directions over others, creating asymmetries that contradict the special theory of relativity. For example, light of different colors might travel at different speeds—just as in a glass prism, which refracts light into its constituent colors.
Whereas effects on small scales are usually hard to see, violations of relativity would actually be fairly obvious. The thermodynamics of black holes casts further doubt on picturing space as a simple mosaic.
By measuring the thermal behavior of any system, you can count its parts, at least in principle. Dump in energy and watch the thermometer. If it shoots up, that energy must be spread out over comparatively few molecules. In effect, you are measuring the entropy of the system, which represents its microscopic complexity. If you go through this exercise for an ordinary substance, the number of molecules increases with the volume of material.
Spacetime - Wikipedia
That is as it should be: If you increase the radius of a beach ball by a factor of 10, you will have 1, times as many molecules inside it. But if you increase the radius of a black hole by a factor of 10, the inferred number of molecules goes up by only a factor of The black hole may look three-dimensional, but it behaves as if it were two-dimensional. This weird effect goes under the name of the holographic principle because it is reminiscent of a hologram, which presents itself to us as a three-dimensional object.
On closer examination, however, it turns out to be an image produced by a two-dimensional sheet of film. If the holographic principle counts the microscopic constituents of space and its contents—as physicists widely, though not universally, accept—it must take more to build space than splicing together little pieces of it.
The relation of part to whole is seldom so straightforward, anyway. An H2O molecule is not just a little piece of water. Consider what liquid water does: An individual H2O molecule does none of that: Likewise, the building blocks of space need not be spatial. The geometric properties of space are new, collective, approximate properties of a system made of many such atoms.
In loop quantum gravity, they are quanta of volume aggregated by applying quantum principles. In string theory, they are fields akin to those of electromagnetism that live on the surface traced out by a moving strand or loop of energy—the namesake string. In M-theory, which is related to string theory and may underlie it, they are a special type of particle: In causal set theory, they are events related by a web of cause and effect.
In the amplituhedron theory and some other approaches, there are no building blocks at all—at least not in any conventional sense.
Although the organizing principles of these theories vary, all strive to uphold some version of the so-called relationalism of 17th- and 18th-century German philosopher Gottfried Leibniz. In broad terms, relationalism holds that space arises from a certain pattern of correlations among objects.
In this view, space is a jigsaw puzzle. You start with a big pile of pieces, see how they connect and place them accordingly. If two pieces have similar properties, such as color, they are likely to be nearby; if they differ strongly, you tentatively put them far apart.
Physicists commonly express these relations as a network with a certain pattern of connectivity. The relations are dictated by quantum theory or other principles, and the spatial arrangement follows.
Phase transitions are another common theme. If space is assembled, it might be disassembled, too; then its building blocks could organize into something that looks nothing like space. In this view, black holes may be places where space melts.
Known theories break down, but a more general theory would describe what happens in the new phase. Even when space reaches its end, physics carries on. Entangled Webs The big realization of recent years—and one that has crossed old disciplinary boundaries—is that the relevant relations involve quantum entanglement. An extrapowerful type of correlation, intrinsic to quantum mechanics, entanglement seems to be more primitive than space.
For instance, an experimentalist might create two particles that fly off in opposing directions. If they are entangled, they remain coordinated no matter how far apart they may be. That changed when black holes forced the issue. Over the lifetime of a black hole, entangled particles fall in, but after the hole evaporates fully, their partners on the outside are left entangled with—nothing.
What Is Spacetime?
Even in a vacuum, with no particles around, the electromagnetic and other fields are internally entangled. If you measure a field at two different spots, your readings will jiggle in a random but coordinated way.
And if you divide a region in two, the pieces will be correlated, with the degree of correlation depending on the only geometric quantity they have in common: In Jacobson argued that entanglement provides a link between the presence of matter and the geometry of spacetime—which is to say, it might explain the law of gravity. Several approaches to quantum gravity—most of all, string theory—now see entanglement as crucial. String theory applies the holographic principle not just to black holes but also to the universe at large, providing a recipe for how to create space—or at least some of it.
For instance, a two-dimensional space could be threaded by fields that, when structured in the right way, generate an additional dimension of space. The original two-dimensional space would serve as the boundary of a more expansive realm, known as the bulk space.
And entanglement is what knits the bulk space into a contiguous whole. Suppose the fields at the boundary are not entangled—they form a pair of uncorrelated systems.