Observer in Physics

By Santosh Chaudhary, M.Sc (Physics)

Nalanda Open University

Observer

In physics, an "observer" refers to any device or entity that records an event in a way that touches so many other atoms and particles, such as by radiation or vibrations, that reversing the event becomes statistically nearly impossible. 

An observer in physics is any entity that makes a measurement of a physical system. This can be a human being, a scientific instrument, or even another physical system. 

In classical physics, the observer is typically assumed to be passive, meaning that their presence does not affect the system being observed. This is a good approximation for many macroscopic systems, but it breaks down at the microscopic level.

Here are some  examples of the observer effect in physics:

When you measure the voltage of a battery, you are drawing a small current from the battery, which changes its voltage slightly.

When you weigh yourself on a scale, the scale is compressing your body slightly, which changes your weight slightly.

When you look at a star through a telescope, the telescope is blocking some of the light from the star, which changes the image of the star that you see.

The act of observation can have a significant impact on the system being observed, particularly in the realm of quantum mechanics. In quantum mechanics, the observer plays a more active role. The act of observing a quantum system necessarily interacts with it, and this interaction can change the state of the system. This is known as the observer effect.

The term "observer" is particularly significant in the context of the "observer effect", which is the disturbance of an observed system by the act of observation. This effect often results from using instruments that, by necessity, alter the state of what they measure in some manner. A common example is checking the pressure in an automobile tire, which causes some of the air to escape, thereby changing the pressure.

In quantum mechanics, a notable example of the observer effect occurs as demonstrated by the double-slit experiment. One of the most famous examples of the observer effect is the double-slit experiment. In this experiment, a beam of light is shone on a barrier with two slits in it. If the light is observed as it passes through the slits, it behaves like a stream of particles. However, if the light is not observed, it behaves like a wave. Physicists have found that observation of quantum phenomena can change the measured results of this experiment. 

However, it's important to note that the need for the "observer" to be conscious is not supported by scientific research and is often pointed out as a misconception rooted in a poor understanding of quantum mechanics.

The observer effect has led to some profound philosophical questions about the nature of reality. Some physicists believe that the observer plays a fundamental role in the creation of reality, while others believe that the observer effect is simply a consequence of our limited ability to measure quantum systems.

Regardless of one's philosophical views, the observer effect is a real and important phenomenon in physics. It is essential to take into account the observer effect when designing and interpreting experiments.

Here are some  examples of the observer effect in the quantum physics:

The Heisenberg uncertainty principle states that it is impossible to know both the position and momentum of a particle with perfect accuracy. This is because the act of measuring one of these properties necessarily disturbs the other.

The collapse of the wave function is the process by which a quantum system transitions from a superposition of states to a single state. It is thought that the collapse of the wave function is triggered by the act of observation.

The quantum Zeno effect is a phenomenon in which the repeated observation of a quantum system prevents it from decaying.

Quantum eraser experiment: This experiment shows that the observer effect can be undone. In the quantum eraser experiment, photons are passed through a double slit and then through a device that allows them to interfere with themselves, even if a detector has been placed at one of the slits. The results of the experiment show that the act of observation does not cause the wave function to collapse irreversibly.

The observer effect is a reminder that physics is not just about studying the natural world. It is also about studying the relationship between the observer and the observed.

Observer in Theory of Relativity

There is no observer effect in the theory of relativity. However, there is a sense in which the observer's frame of reference plays an important role in relativity.

In relativity, the observer is simply a frame of reference from which measurements are made. Different observers will measure different values for space and time, but this is not because the act of observation is changing the system being observed. It is simply because different observers are moving at different speeds and are in different gravitational fields.

There is some confusion about the observer effect in relativity because the word "observer" is used differently in relativity and quantum mechanics. In relativity, an observer is simply a frame of reference.

In special relativity, all inertial frames of reference are equivalent, meaning that the laws of physics are the same in all of them. However, different observers in different inertial frames of reference will measure different things, such as the length of objects, the time it takes for events to happen, and the mass of objects.

For example, if two observers are moving relative to each other, they will measure different lengths for the same object. The object will appear shorter to the observer who is moving relative to it. This is known as the length contraction effect.

Similarly, two observers moving relative to each other will measure different times for the same event. The event will take longer to happen for the observer who is moving relative to it. This is known as the time dilation effect.

The observer effect in relativity is not as profound as the observer effect in quantum mechanics. However, it is still an important concept to understand, as it shows that our observations of the universe are dependent on our frame of reference.

One example of the observer effect in relativity is the twin paradox. In this paradox, one twin travels on a relativistic spaceship at high speed, while the other twin stays on Earth. When the traveling twin returns to Earth, they find that they have aged less than the stay-at-home twin. This is because the traveling twin has spent time in a different frame of reference, where time flows more slowly.

Another example of the observer effect in relativity is the Doppler effect. When a source of sound or light is moving relative to an observer, the observer will measure a different frequency for the sound or light. This is because the motion of the source and observer compresses or stretches the waves.

The observer effect in relativity is a reminder that our observations of the universe are not absolute. They are always dependent on our frame of reference.