In physics there are several fundamental assumptions that we have become accustomed to accepting as truths; But physics, being an experimental science, requires us to make sure that these assumptions have a hold in reality, and to continue to perfect the tools necessary to test their limits. Below is a review of two experiments that test the "obvious" in the theory of relativity
Alluvial container, Galileo
1. Is a mass a mass a mass?
If we throw a stone and a feather from a tall tower, which will fall faster? Day-to-day experience teaches us that the answer is "the stone", but in the absence of the effect of air, it is common to think that the two bodies will fall with equal acceleration - the gravitational acceleration of the Earth, denoted by the letter g. This claim is based on the basic premise of the theory of general relativity: the equality between two quantities called "inertial mass" and "gravitational mass";
The mass of a body has two separate measurable consequences: one is its inertia (persistence), which is usually described as "the resistance of the body to a change in its velocity" - the greater the body's mass, the more difficult it is to accelerate or stop it (the meaning here is acceleration by a force that is not gravity, for example, by pushing manually or applying an electric field).
The other property of mass is gravitational: mass creates a gravitational field around it, which causes other bodies to attract it and be attracted to it. The greater the mass, the greater the field and the greater the gravitational force.
In all equations in physics, simply "mass" appears, based on the assumption that the principle of equivalence holds: the gravitational mass is equal to the inertial mass, in terms of a mass is a mass is a mass. Another and broader formulation of the equivalence principle means that it is not possible to distinguish by any experiment between a physical system suspended in a gravitational field and a system that is accelerating under the influence of a non-gravitational force.
We will not be able to tell if we are in a closed room on Earth or in a structure located in space, and being pulled upwards (in the eyes of those sitting in the room), with a constant acceleration equal to g by a cable connected to a spaceship.
We will now take two bodies with different masses and drop them in the Earth's gravitational field; Let's assume that a gravitational mass is indeed equal to an inertial mass: the larger the mass, the greater the gravitational force acting on it, and therefore it must accelerate at a greater rate than the smaller mass - but its inertial mass is also greater, and therefore it is more difficult to change its speed.
The equivalence between gravitational mass and inertial mass will mean that both bodies, even though different forces act on them, will move with equal acceleration. This principle of equivalence is a basic assumption in general relativity, as well as in the Newtonian formulas.
Today, many attempts are being made to develop a theory that would combine general relativity and quantum mechanics. There are theories that hold that on a sufficiently small scale, where quantum phenomena become significant, the equivalence between gravitational mass and inertial mass is violated. Some theories take into account a mutual influence between the gravitational properties of a particle and its spin. It is possible that the internal arrangement of the particles in an atom will affect how the atom will accelerate in a gravitational field.
Scientists from Germany conducted an experiment with the aim of looking for a difference between the gravitational mass and the inertial mass of individual atoms. Sebastian Frey (Fray) and colleagues dropped individual atoms in the gravitational field of the NASA: a rubidium atom, which has 85 nucleons (protons and neutrons), and a heavier isotope of rubidium, which has 87 nucleons.
The researchers measured the acceleration of the fall using a sensitive optical system, which allowed an accuracy of one ten-millionth of the standard gravitational acceleration g. They found that even for these tiny masses the principle of equivalence is fulfilled, within the precision possible in the experiment. Also, no dependence was found between the acceleration of the atoms and their internal arrangement. This is good news for general relativity, but unfortunately, these findings provide no clues as to where unification theory will come from.
2. Does the information carried by a light wave really move at the speed of light?
It is known that information cannot be transmitted at a speed higher than the speed of light in a vacuum, denoted by the letter c. This assumption underlies special relativity, and is true for any reference system. Transmitting information faster than the speed of light is equivalent, in a sense, to sending the information back in time, and therefore to a violation of causality.
Recently, experiments were conducted in various laboratories around the world, in which they made light move slower than the speed of light in a vacuum, stop altogether or move faster than the speed of light in a vacuum. On the face of it, it appears that these experiments contradict the basic assumption that it is not possible to exceed the speed of light c. In practice, there is no contradiction. A wave can be created that appears to be moving faster than the speed of light; The information carried by the wave is that it cannot move faster.
For example: it is possible to create a "green wave" at the traffic lights, which will advance faster than the speed of light: this is possible if we program computers in advance that will turn on the green light at the various traffic lights at predetermined times; But in such a case we do not transmit information - all the necessary information was transmitted ahead of time.
The well-entrenched assumption that the speed of light has an upper limit has been tested experimentally by Nicolas Gisin and colleagues at the University of Geneva. In an article published in Physical Review Letters, a fairly simple experiment was described in which photons were launched into an optical fiber, the arrival times at the end of the fiber were measured, and their distribution was examined.
Since the light pulse has a certain temporal width - the time is not uniform, and depends on the shape of the pulse and the medium through which the light passes. The pulse consists of a large number of frequencies; The medium may transmit certain frequencies faster than others. This leads to a distortion in the shape of the signal which is characterized by the shift of the peak, which was initially located in the center of the signal. If such a distortion occurs, the peak of the pulse may reach the end of the fiber faster than c (if it is shifted forward on the signal) or slower (if it is shifted backward).
The researchers ran a beam of light through an optical fiber and determined the properties of the system so that the speed of the peak of the signal, referred to as the "bunch speed", was greater than the speed of light in a vacuum. In addition to measuring the speed of the group, the speed of photons located at the front of the beam was measured only.
Their speed was given the nickname "signal velocity" since it is customary to refer to them as the photons that carry the information. It was found that despite the high group speed, the signal speed did not exceed c. In the diagram you can see several pulses moving at different bunch speeds, but distorted so that the fronts all move at the same speed.
Individual photons may arrive even before the main front, but they also do not exceed the speed of light c. This is the first time a direct measurement of signal speed has been made, and the result is consistent with the decades-old theoretical claim that this speed cannot exceed c.
Einstein knew
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