TOE 모든 것의 이론 Theory of Everything

표준 모형을 넘어서래 .. 이야말로 '후덜덜.'

beyond the Standard Model

모든 것의 이론, 그 꿈의 실현을 방해하는 우주의 불협화음

박효진기자기자2009.09.23 14:52:46 / 조회: 584

전 우주의 다양하고 복잡한 자연현상들을 단 하나의 원리로 설명할 수 있다면 얼마나 좋을까? 이는 물리학자들의 오랜 꿈이다. 하지만 20세기 물리학의 이론은 거시세계와 미시세계를 설명하는 각각의 이론이 전혀 통합될 수 없는 상태에 있었다. 거시세계를 설명하는 상대성이론과 미시세계를 설명하는 양자역학이 각자의 영역에서만 들어맞을 뿐 서로 상통하지 않았던 것이다. 
우주가 두 부분으로 나눠진 것이 아닌 만큼 우주를 설명하는 이론도 하나로 통합될 수 있어야 한다. 물리학자들은 언젠가 이런 이론을 발견할 수 있을 것으로 생각하고 그 이론을 ‘모든 것의 이론(T.O.E : Theory of Everything)’이라 이름 붙이고 연구를 해오고 있다.

거시세계와 미시세계의 충돌
모든 것의 이론은 구체적으로는 통일장이론(Unified Theory of Field)이라고 불린다. 통일장이론은 자연계에 존재하는 기본적인 4가지 힘 - 중력, 전자기력, 강한 핵력, 약한 핵력- 을 통합하고자 하는 이론이다. 현재 중력을 제외한 나머지 3개의 힘은 ‘표준모형(Standard Model)’이라는 이론으로 통합한 상태이다.
표준모형에 의하면 각각의 힘은 장(場)을 이룬다. 예를 들어 전기력은 전기장을, 자기력은 자기장을 이루고 있고 그 장들은 공간에 보이지 않는 선으로 흐르고 있다. 힘의 장에는 입자들이 퍼져있는데, 그 입자들은 힘을 전달하는 매개체로 작용한다. 우리가 사물을 보려면 빛 입자가 우리 눈을 때려야 하는 것처럼 어떤 힘이 전달될 때에도 매개 입자에 의해 전달된다.
구체적으로는 전자기장에는 ‘광자’, 강력장에는 ‘글루온’ 입자, 약력장에는 ‘W, Z보존’이라는 입자가 퍼져있다. 이 입자들은 실험에서도 이미 발견이 됐다. 물리학자들은 같은 맥락에서 중력장에도 중력을 매개하는 입자가 있을 것으로 예상하고 이를 ‘중력자’라고 이름 붙였지만 아직까지 실험에 의해 발견되지는 않고 있다.
이론적 계산에 의하면 중력자는 질량이 0이 되어야 한다. 중력이라는 힘은 미칠 수 있는 범위가 아주 넓은데, 질량이 0이 되어야 무한대의 거리까지 힘이 미칠 수 있기 때문이다. 질량이 없다는 것은 공간이 평평하다는 의미를 가진다. 일반상대성이론에 따르면 질량이 있는 물체는 공간을 휘게 하기 때문에 반대로 질량이 없으면 공간이 평평하다. 하지만 거시세계에서 평평하게 보이는 공간을 미세한 영역까지 확대해보니 아주 광폭하게 요동치고 있었다. 확대된 공간이 왜 요동을 칠까, 이는 초미세영역의 세계를 설명하는 이론인 양자역학의 원리에 의해서 설명될 수 있다.
양자역학에서 양자의 개념은 ‘에너지는 불연속적으로 증가한다.’이다. 우리가 생각하기에는 50이라는 에너지가 55로 증가하려면 에너지를 5만큼 가해주면 된다. 그리고 에너지는 50에서 55가 되기까지 순차적으로 증가한다고 생각한다. 하지만 에너지는 연속적이지 않고 어느 한 값으로 고정되어 있다는 사실이 발견되었다. 에너지가 증가할 때는 서서히 연속적으로 증가하는 것이 아니라, 한 지점에서 한 지점으로 도약하듯이 증가한다는 것이다. 이는 기존의 뉴턴 물리학을 뒤흔드는 발견이었다.
양자역학에서의 또 한 가지 기본 원리이자 핵심은 어떤 입자의 위치와 속도를 동시에 정확하게 측정할 수 없고, 입자가 언제 어디를 통과하는 지는 확률로만 알 수 있을 뿐이라는 것이다. 또한 속도와 위치는 동시에 측정할 수 없을 뿐만 아니라, 위치를 정확하게 알 게 되면 속도오차가 매우 커지고 속도를 정확히 측정하면 위치오차가 매우 커진다. 이것이 하이젠베르크의 ‘불확정성의 원리’다. 이 원리는 입자의 크기가 아주 작은 미세영역에서 보이는 현상이고, 일상세계에서는 입자의 크기가 아주 크기 때문에 양자역학의 효과를 알아차릴 수 없다. 만일 미시세계에서 보이는 현상이 일상세계에서도 적용된다면 우리는 정상적인 생활을 할 수 없을 것이다. 예를 들어 두 사람이 악수를 할 때 손과 손을 맞잡은 위치가 정확하기 때문에 속도오차가 커져 상대방의 손을 따귀를 때리듯 아주 세게 칠 수도 있다. 또는 사람이 한 곳에 서 있는 모습은 속도가 0임을 측정할 수 있는 대신 위치오차가 커져 윤곽이 흐릿하게 퍼져 보이게 된다.
이렇게 위에서 살펴 본 양자역학의 기본 원리에 의해, 확대해서 본 공간이 요동치게 되는 것이다. 이 공간의 요동을 양자역학에서는 양자적 요동(quantum fluctuation)이라고 한다. 이 양자적 요동 때문에 미시세계에는 상대성이론의 부드럽게 휘어진 곡면 기하학을 전혀 적용할 수 없다. 이것이 거시세계와 미시세계의 충돌이다.

Copyright @ 2006 경북대신문. All rights reserved. http://www.knun.net/news/article.html?no=12945

자료: http://en.wikipedia.org/wiki/Theory_of_everything

A theory of everything (ToE) or final theory is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle.^[1]

Many candidate theories of everything have been proposed by theoretical physicists during the twentieth century, but none have been confirmed experimentally. The primary problem in producing a TOE is that general relativity and quantum mechanics are hard to unify. This is one of the unsolved problems in physics.

Initially, the term 'theory of everything' was used with an ironic connotation to refer to various overgeneralized theories. For example, a great-grandfather of Ijon Tichy—a character from a cycle of Stanisław Lem's science fiction stories of the 1960s—was known to work on the "General Theory of Everything". Physicist John Ellis^[2] claims to have introduced the term into the technical literature in an article in Nature in 1986.^[3]Over time, the term stuck in popularizations of quantum physics to describe a theory that would unify or explain through a single model the theories of all fundamental interactions and of all particles of nature: general relativity for gravitation, and the standard model of elementary particle physics — which includes quantum mechanics — for electromagnetism, the two nuclear interactions, and the known elementary particles.

While loop quantum gravity attempts to unify quantum field theory and general relativity, string theory and its successor M-theory, remain the only prominent candidates as a theory of everything.^{[citation needed]}

Modern physics

[edit]Conventional sequence of theories

A Theory of Everything would unify all the fundamental interactions of nature: gravitation, strong interaction, weak interaction, and electromagnetism. Because the weak interaction can transformelementary particles from one kind into another, the TOE should also yield a deep understanding of the various different kinds of possible particles. The usual assumed path of theories is given in the following graph, where each unification step leads one level up:

Theory of Everything

Gravitation

Electronuclear force (GUT)

Strong interaction SU(3)

Electroweak interaction SU(2) x U(1)

Weak interaction SU(2)

Electromagnetism U(1)

Electricity

Magnetism

In this graph, electroweak unification occurs at around 100 GeV, grand unification is predicted to occur at 10¹⁶ GeV, and unification of the GUT force with gravity is expected at the Planck energy, roughly 10¹⁹ GeV.

Several Grand Unified Theories (GUTs) have been proposed to unify electromagnetism and the weak and strong forces. Grand unification would imply the existence of an electronuclear force; it is expected to set in at energies of the order of 10¹⁶ GeV, far greater than could be reached by any possible Earth-based particle accelerator. Although the simplest GUTs have been experimentally ruled out, the general idea, especially when linked with supersymmetry, remains a favorite candidate in the theoretical physics community. Supersymmetric GUTs seem plausible not only for their theoretical "beauty", but because they naturally produce large quantities of dark matter, and because the inflationary force may be related to GUT physics (although it does not seem to form an inevitable part of the theory). Yet GUTs are clearly not the final answer; both the current standard model and all proposed GUTs are quantum field theories which require the problematic technique of renormalization to yield sensible answers. This is usually regarded as a sign that these are only effective field theories, omitting crucial phenomena relevant only at very high energies.

The final step in the graph requires resolving the separation between quantum mechanics and gravitation, often equated with general relativity. Numerous researchers concentrate their efforts on this specific step; nevertheless, no accepted theory of quantum gravity – and thus no accepted theory of everything – has emerged yet. It is usually assumed that the TOE will also solve the remaining problems of GUTs.

In addition to explaining the forces listed in the graph, a TOE may also explain the status of at least two candidate forces suggested by modern cosmology: an inflationary force and dark energy. Furthermore, cosmological experiments also suggest the existence of dark matter, supposedly composed of fundamental particles outside the scheme of the standard model. However, the existence of these forces and particles has not been proven yet.

[edit]String theory and M-theory

Unsolved problems in physics

Is string theory, superstring theory, or M-theory, or some other variant on this theme, a step on the road to a "theory of everything", or just a blind alley?

Since the 1990s, many physicists believe that 11-dimensional M-theory, which is described in many sectors by matrix string theory, in many other sectors by perturbative string theory, is the theory of everything. However, there is no widespread consensus on this issue, because M-theory and superstring theory is not a completed theory but rather an approach for producing one. All these theories attempt to deal with the renormalization problem by setting up some lower bound on the length scales possible.

String theories and supergravity (both believed to be limiting cases of the yet-to-be-defined M-theory) suppose that the universe actually has more dimensions than the easily observed three of space and one of time. The motivation behind this approach began with the Kaluza-Klein theory in which it was noted that applying general relativity to a five dimensional universe (with the usual four dimensions plus one small curled-up dimension) yields the equivalent of the usual general relativity in four dimensions together with Maxwell's equations (electromagnetism, also in four dimensions). This has led to efforts to work with theories with large number of dimensions in the hopes that this would produce equations that are similar to known laws of physics. The notion of extra dimensions also helps to resolve the hierarchy problem, which is the question of why gravity is so much weaker than any other force. The common answer involves gravity leaking into the extra dimensions in ways that the other forces do not.^{[citation needed]}

In the late 1990s, it was noted that one problem with several of the candidates for theories of everything (but particularly string theory) was that they did not constrain the characteristics of the predicted universe. For example, many theories of quantum gravity can create universes with arbitrary numbers of dimensions or with arbitrary cosmological constants. Even the "standard" ten-dimensional string theory allows the "curled up" dimensions to be compactified in an enormous number of different ways (one estimate is 10⁵⁰⁰ ) each of which corresponds to a different collection of fundamental particles and low-energy forces. This array of theories is known as the string theory landscape.

A speculative solution is that many or all of these possibilities are realised in one or another of a huge number of universes, but that only a small number of them are habitable, and hence the fundamental constants of the universe are ultimately the result of the anthropic principle rather than a consequence of the theory of everything. This anthropic approach is often criticised^[9] in that, because the theory is flexible enough to encompass almost any observation, it cannot make useful (i.e., original, falsifiable, and verifiable) predictions. In this view, string theory would be considered a pseudoscience, where an unfalsifiable theory is constantly adapted to fit the experimental results.

[edit]Loop quantum gravity

Current research on loop quantum gravity may eventually play a fundamental role in a TOE, but that is not its primary aim.^[10] Also loop quantum gravity introduces a lower bound on the possible length scales.

There have been recent claims that loop quantum gravity may be able to reproduce features resembling the Standard Model. So far only the first generation of fermions (leptons and quarks) with correct parity properties have been modelled by Sundance Bilson-Thompson using preons constituted of braids of spacetime as the building blocks.^[11] However, there is no derivation of theLagrangian that would describe the interactions of such particles, nor is it possible to show that such particles are fermions, nor that the gauge groups or interactions of the Standard Model are realised. Utilization of quantum computing concepts made it possible to demonstrate that the particles are able to survive quantum fluctuations.^[12]

This model leads to an interpretation of electric and colour charge as topological quantities (electric as number and chirality of twists carried on the individual ribbons and colour as variants of such twisting for fixed electric charge).

Bilson-Thompson's original paper suggested that the higher-generation fermions could be represented by more complicated braidings, although explicit constructions of these structures were not given. The electric charge, colour, and parity properties of such fermions would arise in the same way as for the first generation. The model was expressly generalized for an infinite number of generations and for the weak force bosons (but not for photons or gluons) in a 2008 paper by Bilson-Thompson, Hackett, Kauffman and Smolin.^[13]

[edit]Causal dynamical triangulation

Causal dynamical triangulation (abbreviated as "CDT") invented by Renate Loll, Jan Ambjørn and Jerzy Jurkiewicz, and popularized by Fotini Markopoulou and Lee Smolin, is an approach toquantum gravity that like loop quantum gravity is background independent. This means that it does not assume any pre-existing arena (dimensional space), but rather attempts to show how thespacetime fabric itself evolves. The Loops '05 conference, hosted by many loop quantum gravity theorists, included several presentations which discussed CDT in great depth, and revealed it to be a pivotal insight for theorists. It has sparked considerable interest as it appears to have a good semi-classical description. At large scales, it re-creates the familiar 4-dimensional spacetime, but it shows spacetime to be 2-d near the Planck scale, and reveals a fractal structure on slices of constant time.

By far the greatest advantage of this theory is that it derives the observed nature and properties of spacetime from a minimal set of assumptions, and needs no adjusting factors. The idea of deriving what is observed from first principles is very attractive to physicists, as it often indicates a concept that is close to the truth, or offers powerful tools for investigating the nature of reality.

[edit]Other attempts

Any TOE must include general relativity and the standard model of particle physics.

A recently very prolific attempt is called Causal Sets. As some of the approaches mentioned above, its direct goal isn't necessarily to achieve a TOE but primarily a working theory of quantum gravity, which might eventually include the standard model and become a candidate for a TOE. Its founding principle is that spacetime is fundamentally discrete and that the spacetime events are related by a partial order. This partial order has the physical meaning of the causality relations between relative past and future distinguishing spacetime events.

Outside the previously mentioned attempts there is Garrett Lisi's E8 proposal. This theory provides an attempt of identifying general relativity and the standard model within the Lie group E8. The theory doesn't provide a novel quantization procedure and the author suggests its quantization might follow the Loop Quantum Gravity approach above mentioned.^[14]

[edit]Present status

At present, no convincing candidate for a TOE is available. Most particle physicists state that the outcome of the ongoing experiments – the search for new particles at the large particle acceleratorsand for dark matter – are needed in order to provide theoretical physicists with further input for a TOE.

[edit]Theory of everything and philosophy

Main article: Theory of everything (philosophy)

The philosophical implications of a physical TOE are frequently debated. For example, if philosophical physicalism is true, a physical TOE will coincide with a philosophical theory of everything.

The "system building" style of metaphysics attempts to answer all the important questions in a coherent way, providing a complete picture of the world. Plato and Aristotle could be said to have created early examples of comprehensive systems. In the early modern period (17th and 18th centuries), the system-building scope of philosophy is often linked to the rationalist method of philosophy, which is the technique of deducing the nature of the world by pure a priori reason. Examples from the early modern period include the Leibniz's Monadology, Descarte's Dualism, andSpinoza's Monism. Hegel's Absolute idealism and Whitehead's Process philosophy were later systems.

Other philosophers do not believe their techniques can aim so high. Some scientists think a more mathematical approach than philosophy is needed for a TOE, for instance Stephen Hawkingwrote in A Brief History of Time that even if we had a TOE, it would necessarily be a set of equations. He wrote, “What is it that breathes fire into the equations and makes a universe for them to describe?”.^[15]