In 1905, Albert Einstein introduced his theory of special relativity, which established the concept of a universal speed limit—the speed of light. His long-standing curiosity about light led him to ponder what it might be like to chase after a light beam.

What would occur if you could indeed pursue a beam of light? Would you observe a stationary light wave? This notion contradicts Maxwell’s equations.

Essentially, light's velocity can be perceived as "infinite." When an object with mass approaches the speed of light, its mass approaches infinity, making it impossible to reach this speed. Time dilates, appearing to slow down relative to the surrounding universe, while objects appear contracted (Lorentz transformation).

"Everything is relative," asserted Albert, indicating that your spacetime and frame of reference differ from those of another observer. Thus, both time and space are contingent upon the observer’s perspective, marking a significant shift from the Newtonian view of a fixed spacetime.

The speed of light has its own frame of reference, independent of the observer. It always travels at the speed of light, regardless of how fast you are moving—if you are at 99% the speed of light, light will still seem to move away from you at its constant speed.

Is it feasible to surpass the speed of light?

Currently, Einstein's assertion holds true. The complete statement reads:

> Nothing can travel faster than the speed of light in vacuum through space.

Note the subtle distinction from the phrase "Nothing can travel faster than the speed of light."

While it appears almost identical, there are instances where faster-than-light travel occurs, albeit not in a vacuum or through space:

1. In a medium: Light travels slower in materials like air or water, allowing particles to exceed the speed of light specific to that medium.
2. The expansion of space: Although nothing can move faster than light in a vacuum, space itself can expand at a rate surpassing the speed of light.
3. Quantum phenomena: Quantum entanglement may appear as if information travels faster than light, as entangled particles can instantaneously share information regardless of their distance apart.

Let's delve deeper into each of these scenarios.

1. Particles exceeding light speed in a medium

In this scenario, particles exceed light speed, but this is somewhat misleading. The particles are traveling close to light speed in a vacuum; however, light itself is decelerated by the medium.

When light passes through a material, it travels slower than in a vacuum. This is why we see a straw appearing to bend when submerged in water—the different materials have distinct refractive indices.

Other particles may not experience the same deceleration, allowing high-energy particles to move faster than light in that medium. For instance, if an electron accelerates to 99% of light speed in a vacuum and then enters water, the photon will slow down while the electron continues at its rapid pace.

Thus, the electron is now traveling faster than light!

This phenomenon generates a distinct effect known as Cerenkov radiation, akin to a sonic boom when an object exceeds the sound barrier. It creates a shock front and a cone of light waves trailing behind, as the light waves collide with particles in the medium and emit light in the blue to ultraviolet spectrum.

Cerenkov radiation can also occur when high-energy cosmic rays interact with our atmosphere, resulting in an air shower. These particles collide with atmospheric molecules, creating a cascade of high-energy particles that move faster than light in air.

While one might think this scenario cannot happen in a vacuum since light travels at its maximum speed, even a vacuum is not entirely empty. It contains a "quantum foam" of virtual particle-antiparticle pairs constantly forming and annihilating, which may slightly slow light, potentially leading to Cerenkov radiation in a quantum vacuum.

2. The Universe's expansion

Regarding the vast emptiness of space, we discovered last century that the Universe is expanding at an accelerating pace. We can measure the velocity at which galaxies and other distant objects are receding, noting that more distant galaxies move away from us faster.

Hubble's observations in the 20th century established a rough rate at which galaxies are receding, known as the Hubble constant, approximately 70 km/s/Mpc (with slight variations based on measurement methods). Mpc refers to megaparsecs, or one million parsecs (3.2 million light-years). Essentially, for every 3.2 million light-years to a galaxy, it moves 70 km/s further away.

Given that light travels at about 300,000 km/s, it is calculable that at roughly 4300 Mpc (or 14 billion light-years), galaxies will recede from us at light speed.

This calculation is an oversimplification, presuming that space has consistently expanded at the same accelerating rate, which is not accurate. Prior to the influence of dark energy (the cause of this acceleration), the Universe expanded at a steadier rate, enabling us to observe further—about 46.5 billion years—before encountering galaxies receding at light speed.

The key takeaway is that galaxies at extreme distances appear to move away from us faster than light. They fall beyond our observable Universe. With each moment of expansion, new galaxies slip from our view, as they recede faster than their light can reach us.

How can this occur if nothing surpasses light speed in a vacuum? While it is true that the distance between us and these galaxies increases at a rate exceeding light speed, it is essential to note that the galaxies themselves are not moving through space at that speed. They are largely stationary, with only minor gravitational movement within their clusters. Instead, it is the fabric of space that is expanding.

A common analogy is a loaf of raisin bread. As the dough rises and expands, the raisins seem to drift apart, yet they remain stationary within the dough, which itself is stretching. Similarly, galaxies are anchored in the fabric of expanding space.

Space is not violating its own laws; the light-speed limit pertains to particles moving through space. As space expands, light waves are stretched, resulting in the redshift of light from far-off celestial objects.

3. Quantum entanglement

Finally, we arrive at the realm of quantum oddities. In quantum mechanics, two particles can become entangled such that they instantly share information, regardless of the distance separating them.

At the quantum level, the behavior of particles is often counterintuitive. They exist as probability waves, which implies they can occupy multiple locations simultaneously. This principle extends to a property known as spin, a descriptive term rather than a literal rotation.

When two fermions (like electrons) occupy the same region, they must exhibit different quantum states per the Pauli exclusion principle. For example, if two electrons share the same orbital, one must have an "up" spin while the other has a "down" spin. Until observed, both particles exist in a superposition of both spins.

Upon measurement, one particle's state collapses to a specific condition, while repeating the experiment may yield varying outcomes.

Entangled particles share properties that depend on one another when measured, similar to electrons with opposite spins. If two entangled particles are separated and sent in different directions, they will remain in a superposition of both states until one is measured. As soon as one particle's spin is determined, the other instantaneously adopts the opposite spin, regardless of distance.

This phenomenon challenges the notion of distinct particles, as no information can cross space faster than light. This perplexed Einstein, who famously referred to it as "spooky action at a distance."

One interpretation of quantum entanglement suggests that the particles act as a single entity despite the distance between them, highlighting our incomplete understanding of the quantum world.

Key Takeaways

While it remains true that nothing can exceed the speed of light in vacuum through space, the assertion that nothing can travel faster than light is not entirely accurate.

• Certain materials can decelerate light, allowing high-speed particles to outpace the slowed light within that medium.
• Space can expand rapidly enough to make distant galaxies appear to recede faster than light. They are not moving that quickly through space; it is the expansion of space itself.
• Quantum entangled particles maintain their connection even when separated by vast distances, leading to the appearance of faster-than-light communication.

Thank you for reading!

Interested in Time?

If you're curious about the physics, biology, and neuroscience of time, consider joining my Time in the Movies challenge. This initiative will challenge your perception of time and encourage better use of your personal time. The challenge includes free online live sessions from October 5th to 9th, hosted by yours truly.