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Introduction

The speed of light is traditionally considered a fundamental constant. A core postulate of modern physics states that the speed of light in a vacuum is the same for all observers, regardless of their motion or the motion of the light source. This principle is embedded in both Special Relativity (SR) and General Relativity (GR), and its validity has never been questioned. However, several key aspects allow us to reconsider this postulate.

In this article, we will examine the foundations upon which the postulate of the constancy of the speed of light was introduced, the conditions required for it to remain unchanged, and the observable phenomena that may indicate its limitations. In particular, we will demonstrate that gravitational redshift can be considered evidence that the speed of light is not absolute but depends on the energy density of space.

1. The Postulate of the Constancy of the Speed of Light

In Special Relativity, the speed of light is introduced as an axiom, requiring no proof. It is formulated as follows:

The speed of light in a vacuum is the same for all observers, regardless of their motion or the motion of the light source.

This postulate was adopted following the Michelson-Morley experiment (1887), which found no variation in the speed of light due to Earth’s motion. This led Einstein to reject the concept of the ether and accept the speed of light as an absolute constant.

However, it is important to understand that SR and GR operate under the assumption of an ideal vacuum, where no material or energetic influences affect light.

In the 1887 Michelson-Morley experiment, the speed of light was measured in different horizontal directions, but possible changes in the speed of light due to variations in gravitational strength were not considered. Their setup was located on the Earth’s surface, and comparisons were made only within a single plane.

If the ether had existed, its effect was expected to manifest primarily in the horizontal direction due to the Earth’s motion through it. However, this experiment could not test the possible influence of gravity on the speed of light, as it was not designed for such measurements. It is important to note that even in modern research, this aspect remains largely unexplored, although the effect of gravity on photon frequency is well-documented.

This raises an important question: can the speed of light change depending on variations in gravitational strength? This is not a return to the concept of ether but rather an examination of how changes in spatial energy influence the propagation of electromagnetic waves. Modern hypotheses concerning the quantum structure of the vacuum, gravitational potential, and additional dimensions may lead to new experiments in this area. If the speed of light indeed depends on gravitational strength, this could indicate the existence of new fundamental laws governing the structure of space-time.

Possible Experiments to Test the Hypothesis

To confirm the dependence of the speed of light on gravitational strength, measurements of the speed of light at different altitudes can be conducted. If gravitational potential influences its value, even small differences in altitude (for example, between sea level and high mountains) could result in measurable deviations.

However, there is a challenge: standard time correction methods in modern measurements may automatically adjust the speed of light to a constant value. This creates a closed loop where the measurement method itself prevents the detection of deviations. A new approach may be required, one that excludes such corrections—for example, comparing the frequency characteristics of photons passing through regions with different gravitational strengths or using gravitational lensing effects in laboratory conditions.

An interesting experiment could involve measuring the speed of light at the same altitude above sea level but in two locations with different gravitational strengths, such as above the Mariana Trench and Mount Everest. Since the gravitational force differs in these locations, this could provide an opportunity to detect variations in the speed of light without the influence of other altitude-related factors.

2. The Vacuum Is Not Ideal

Although classical physics viewed the vacuum as absolute emptiness, modern research shows that the vacuum actually possesses energy. This is confirmed by:

• Quantum vacuum fluctuations (Casimir effect, virtual particle creation);
• Dark energy, which, according to modern cosmological models, fills space;
• The gravitational field, which exists even in the absence of matter.

Electromagnetic fields can also exist in the vacuum, influencing its properties.

Thus, the vacuum is not truly empty. If it is not ideal, this means that the properties of space can change, and possibly, so can the speed of light.

The speed of light is understood as the maximum possible speed of electromagnetic wave propagation in a vacuum. However, this does not preclude the possibility of propagation at lower values. For example, the phenomenon of light dispersion demonstrates that electromagnetic oscillations can propagate at different speeds depending on wave frequency. This clearly indicates the presence of a non-ideal vacuum. A transition between media of different densities cannot be considered an ideal vacuum, as the presence of matter contradicts the definition of an ideal vacuum. This implies that the medium may influence the speed of electromagnetic wave propagation. Interestingly, no dispersion effect is observed when light is emitted from a source, meaning light is not immediately refracted. Instead, this only occurs at the boundary where densities change.

The maximum speed of light in a vacuum is a limit determined by the properties of space itself. However, under real conditions, the speed of electromagnetic waves may depend on the density of the medium.

This does not mean that electromagnetic waves always propagate at this speed throughout the Universe. The Universe is not homogeneous, as it contains macro-objects with gravitational and electromagnetic fields that alter the local energy density. Gravitational and electromagnetic fields change the energy density of space. It is also known that changes in the gravitational field clearly affect changes in photon frequency and its energy.

3. Gravitational Redshift and Its Explanation in GR

Gravitational redshift is an effect in which light escaping from a massive object experiences an increase in wavelength (shift to the red part of the spectrum). In GR, this is explained as follows:

1. The photon moves against the gravitational potential.
2. It loses energy, but the speed of light remains unchanged.
3. The energy decrease manifests as a reduction in frequency (an increase in wavelength).

The formula for gravitational redshift in GR:

where:

• v_e — photon frequency at emission (radius r_e);
• v_r — photon frequency at reception (radius r_r);
• G — gravitational constant;
• M — mass of the gravitating body;
• c — speed of light (assumed constant in GR).

This explanation does not consider the influence of medium density and assumes that all space behaves uniformly, regardless of energy conditions.

4. Can This Be Interpreted Differently?

Now let’s consider an alternative view: if space possesses energy density, then the change in photon frequency can be explained not as an energy loss but as a change in the properties of the medium.

If we assume that the speed of light depends on the gravitational potential, we can write:

where

is the gravitational potential. If the density of space changes, then the speed of light must be a function of Φ. In this case, the photon’s frequency changes not due to energy loss but because it transitions into a region with a different speed of light.

5. How to Explain the Absence of Dispersion During Emission?

Dispersion of light during emission does not occur because:

• At the moment of emission, the photon’s frequency is determined by the surrounding medium, which «sets» the parameters of the emitted light.
• In different media, photons are emitted with different frequencies, explaining spectral differences under various conditions.
• When transitioning between media, the photon’s frequency remains unchanged, but its speed changes.

This confirms that gravitational redshift can be interpreted not as an energy loss but as a change in the properties of the medium through which light propagates.

6. Why Does This Confirm a Variable Speed of Light?

If gravitational redshift is merely an effect of changing medium density, then:

• The speed of light is lower in denser regions and higher in more rarefied regions.
• Gravitational lensing can be explained not by space curvature but by variations in the speed of light.
• Dark matter may be an illusion caused by variations in the speed of light across different regions of the Universe.

This means that while GR correctly describes the effect, its interpretation may need revision. Instead of space curvature, we can consider space as a medium with changing properties.

7. Conclusion

• The postulate of the constancy of the speed of light applies only to an ideal vacuum, which does not exist in nature.
• Gravitational redshift can be interpreted as a change in the properties of the medium, not as an energy loss by the photon.
• The absence of dispersion during emission confirms that light is initially emitted with a frequency corresponding to the medium.
• This opens the possibility of reconsidering fundamental principles of physics and proposing an alternative explanation of gravity.

Thus, GR remains valid, but its physical interpretation may change: not space curvature, but changes in its properties determine the observed effects.

This approach offers a new perspective on the nature of mass and its relationship to electromagnetic processes. A more detailed discussion of this hypothesis and its philosophical implications can be found in the following works:

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