Observer (physics)

In physics , the observer is the one who observes a phenomenon. It can be a real person, a suitable measuring device or - in a thought experiment - an imaginary person. The observer usually describes the phenomenon in his rest system . A change to a different observer therefore generally also means a change to a different reference system and thus to a different description of the same phenomenon. Variables whose values ​​do not depend on the observer's state of motion are called invariant .

Formal status of the observer concept

Although the observer concept is used to illustrate physical facts, especially in books and works on the theory of relativity, it is not an actual part of the theoretical structure of physics. Physical statements that use the observer concept are therefore considered pragmatic formulations that are used in particular in didactics and thought experiments. The basic equations and axiomatizations of physical theories, however, are not based on observer concepts. Physical facts can therefore basically be formulated without using the observer concept. Instead of the sentence scheme

"The observer z in the reference system I finds the value y for the property P of the object x."

then the sentence scheme occurs

"The property P of the object x has the value y relative to the reference system I."

Some physicists and philosophers, such as B. JS Bell , K. Popper , or M. Bunge are critical of the use of observer-based formulations, especially in quantum mechanics, as they can lead to misunderstandings and ambiguities.

Classical physics

In classical physics the observer has a purely passive role. In other words: a physical process does not depend on whether it is observed or not. However, the way in which the phenomenon is described depends on the state of motion of the observer. A distinction must be made here whether the observer is in an inertial system or in an accelerated reference system .

Inertial systems

In an inertial system, the law of inertia applies . This means that all force-free bodies move evenly and in a straight line. A cause can be specified for all forces: actio = reaction . Any observer who moves uniformly relative to an inertial system is also in an inertial system. In other words: All inertial systems are equal. A Galileo transformation can be used to switch from one inertial system to another.

Often an observer is arbitrarily referred to as a "resting" observer, although such a definition does not make sense, because theoretically every observer could claim to be at rest. An observer who moves with the constant speed relative to the stationary observer describes all movements differently than the other: All speeds change , all places change . A vivid example is a conductor who moves against the direction of travel of the train at walking speed . Its movement appears to different observers in different ways: ${\displaystyle +{\vec {v}}}$ ${\displaystyle -{\vec {v}}}$ ${\displaystyle -{\vec {v}}t}$ ${\displaystyle {\vec {v}}}$

• For a cow standing in the pasture, the conductor moves forward from their point of view, but slower than the train by his walking speed.
• The passenger sitting in the train observes the conductor in relation to the train, so that the conductor moves backwards at his walking speed.
• The conductor himself does not move to his own frame of reference (himself), the train seems to be moving past him, namely with the speed .´ ${\displaystyle -{\vec {v}}}$

Another example is the acoustic Doppler effect : the frequency heard depends not only on the frequency of a sound source, but also on the movement of the observer. (In both cases, “movement” means movement relative to the medium of propagation, which is regarded as stationary). If the observer moves towards the sound source, he hears a higher tone than if he moves away from it.

Accelerated observer

If the observer is accelerated, inertial forces appear in his reference system which do not exist for a "resting" observer and are therefore referred to as apparent forces. For example, the moving observer in a rotating carousel must accept an outwardly directed centrifugal force in order to interpret the movement in his reference system. The “resting” observer does not need any additional apparent forces to explain the observed phenomena. In its inertial system, all bodies obey the principle of inertia . This means, among other things, that a body only follows the circular movement of the carousel if it is forced to do so by an external force.

In classical physics, the following quantities are invariant, among others: time, length, mass, electrical charge, potential energy, etc. Examples of non-invariant are: velocity, momentum, kinetic energy, etc.

theory of relativity

Special theory of relativity

In the special theory of relativity , in addition to the principle of relativity , the invariance of the speed of light also applies . This means that the speed of light always has the same value regardless of the observer's reference system. This has far-reaching consequences:

• Relativity of simultaneity: Two events are only then consistently viewed as simultaneous by two different observers if the two observers are at rest relative to one another. The discrepancy between their statements is greater, the faster they move relative to each other and the further they are apart.
• Time dilation : If the time interval is measured for a physical process, the observer who is at rest relative to this process measures the shortest interval. This is also called proper time . Any observer who is moved relative to it will measure the same process for a longer period of time. This is often expressed boldly with the words: "Moving clocks go slower".
• Length contraction : The observer who is at rest relative to an object always measures the greatest value for its length, the so-called intrinsic length . Every observer who moves relative to him measures a shorter distance. In short: "Moving lengths are shrinking".

In the theory of relativity, an observer who is at one space-time point cannot, from his point of view, make any observations at another space-time point. He can only collect information about events that are within his event horizon . In the special theory of relativity there are preferred "families of observers". Such a system consists of observers who are at rest in relation to one another and are distributed over the entire space-time with synchronized clocks and thus forms an inertial system. A Lorentz transformation can be used to switch from one inertial system to another.

In the theory of relativity, the quantities light speed, space-time interval, mass etc. are invariant. In contrast, the quantities length, time, momentum, energy etc. are not invariant.

general theory of relativity

The general theory of relativity is based on the principle of equivalence , i.e. H. on the principle that an observer cannot distinguish between inertial forces and gravitational forces in his frame of reference. This immediately leads to the equality of inert and heavy mass, which cannot be justified in classical physics. In addition, the general theory of relativity results in the gravitational time dilation. This means the phenomenon that clocks go slower the lower the gravitational potential at their location.

There is no preferred family of observers in general relativity. Whether two observers are at rest relative to one another or whether their clocks are synchronized depends on the way in which they communicate with one another. An observer who does not observe at his space-time point, but rather writes down a fixed path at another space-time point, is also referred to as an “accountant”.

FIDO (fiducial observer, stationary stationary), FFO (free falling observer, free falling observer), ZAMO (zero angular momentum observer, locally moving observer) are common, unless an observer is assumed to be outside the gravitational field.

The influence of measurements

In classical physics, it is conceivable that an observer perceives a process and measures it with any high degree of accuracy without influencing or changing the process. For example, it is assumed that it is only a question of the appropriate measurement technology how accurately the speed of a vehicle or the temperature of a cup of coffee can be measured. But even with coffee, the limits of an uninfluenced measurement become clear: If we dip a cold thermometer into the coffee, the coffee thermometer warms up and the coffee becomes colder. The measurement result is falsified by the use of the measuring apparatus. Similar problems can be found in many measurement processes.

In the field of quantum physics , the influence of the measuring apparatus on the observed process turns out to be a very fundamental phenomenon. Each type of measurement implies a mutual interaction between the system under investigation and the measuring instrument. Therefore, the mere fact of the measurement already has an effect on quantum mechanical effects. For example, in an experiment at the Weizmann Institute, only isolated electrons could behave like waves.

The Heisenberg uncertainty principle states that two physical quantities that are linked in a certain way, such as B. place and impulse or time and energy, can not be measured at the same time with any accuracy. In popular scientific literature in particular, this effect is often explained by the fact that the measurement of one variable disrupts the other. For example, one can determine the location of a particle by scattering a photon on it. The interaction of the photon with the particle changes its momentum. In order to measure the location of the particle as precisely as possible, radiation of the shortest possible wavelength must be used. However, short-wave photons have a particularly large impulse, so that they change the size of the impulse particularly strongly. This type of explanation is usually not given in textbooks, since the quantum mechanical uncertainty is a fundamental property of quantum objects and not an artifact caused by the measurement method.

In quantum physics, quantum objects are described by wave functions . These indicate, among other things, how likely it is to find a particle at a certain point in time in a certain state. It is not possible to make a clear prediction of the value that the measurement will result in in advance. The exact state is only determined at the moment of observation. If several states are possible in principle , the quantum object is in superposition until a measurement is carried out. Erwin Schrödinger took this amazing consequence of quantum physics to extremes in his famous thought experiment Schrödinger's cat : Is it possible that a cat, whose life depends on a quantum physical process, is both alive and dead until it is observed? In the history of quantum physics there have been various approaches to understand how this supposed paradox is to be understood, see Interpretations of Quantum Physics and Quantum Mechanical Measurement .

Individual evidence

1. ↑ M. Bunge, Philosophy of Physics, D. Reidel Publishing Company, 1973, p. 30. (google books)
2. ↑ M. Bunge, Philosophy of Physics, D. Reidel Publishing Company, 1973, p. 49 ff. (Google books)
3. ↑ JS Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge University Press, 2004.
4. ^ KR Popper, Quantum Theory and the Schism in Physics, Routledge, 1989.
5. ↑ M. Bunge, Philosophy of Physics, D. Reidel Publishing Company, 1973, chap. 3.2.
6. ↑ ^{a b} Tevian Dray: Differential Forms and the Geometry of General Relativity . CRC Press, 2014, ISBN 1-4665-1000-5 , pp. 39 ( limited preview in Google Book search).
7. ↑ Observation influences reality. Weizmann Institute, accessed on April 14, 2014 (from idw-online.de). 

Web links

• Weltbild Physik on the website of the University of Tübingen