Does the aether really exist? This question challenges our understanding of wave propagation in the universe. Some signals, like sound waves, need a medium to travel through, while others, such as light and gravitational waves, seem to travel effortlessly through the vacuum of space. This raises the question of whether it's feasible for these waves to propagate without a medium. Wade Campbell poses the inquiry:

> Back in the late 1800s, an “aether” was proposed as the medium that light travels through. We now don’t believe that is the case. What is the evidence and/or proof that no aether exists?

This assumption is straightforward, yet proving it false is quite complex. Here’s the background.

In the early days of science, long before Newton, our focus was limited to observable, macroscopic phenomena. We encountered various types of waves, including:

• Wind-induced ripples on clothing or sails
• Ocean waves
• Seismic waves from earthquakes
• Vibrations in a plucked string
• Sound waves, which behave differently in air, water, or solid media.

All these wave types required matter as a medium for propagation. This medium either compresses and rarefies in the direction of wave travel (longitudinal waves) or oscillates perpendicular to it (transverse waves), enabling signal transfer.

As we delved deeper into wave studies, we identified a third category: surface waves, which involve particles moving in circular paths, as seen in water ripples.

All these mechanical waves transport energy through a material medium. For example, a wave travels through a slinky, water, or air, requiring an initial displacement to propagate energy to its destination.

Given our discoveries, it was only natural to presume that newly identified waves would share properties with known types. The concept of aether emerged as the medium of space through which celestial bodies moved. Tycho Brahe’s 1588 work, De Mundi Aetherei Recentioribus Phaenomenis, translates to "On Recent Phenomena in the Aethereal World."

Aether was thought to be the medium through which all entities, including light, traveled. The debate over whether light was a wave or a particle persisted for centuries. Newton advocated the particle theory, while Christiaan Huygens supported the wave theory. Ultimately, the 19th-century experiments confirmed light's wave-like behavior.

Understanding electricity and magnetism further reinforced the wave theory. Accelerating charged particles demonstrated their interaction with magnetic fields, and the bending of such particles produced light. This led to the conclusion that light is an electromagnetic wave traveling at a finite speed, denoted as c.

If light is an electromagnetic wave and requires a medium, then aether must be that medium. The challenge became identifying aether's properties.

Maxwell, who derived the electromagnetic nature of light, highlighted that aether could not be molecular. In a letter to Lewis Campbell in 1874, he stated:

> It may also be worth knowing that the aether cannot be molecular. If it were, it would be a gas...

This implies that aether, whatever it was, lacked the properties of traditional media. It couldn't consist of particles, transfer heat, or convey energy. Essentially, it could only serve as a passive medium for light.

This led to the pivotal Michelson-Morley experiment, designed to detect aether. If aether existed, Earth's motion through it would affect light's interference pattern. Michelson's initial attempts in 1881 yielded inconclusive results, but in 1887, he and Morley produced a null result, finding no evidence for aether.

Aether supporters proposed various explanations for this null result, such as the possibility of aether being dragged by moving objects or existing in a stationary state that caused length contraction and time dilation.

However, these theories were set aside with the advent of Einstein's relativity, which established that the laws of physics remain consistent across all frames of reference, making the concept of an absolute frame, like aether, unnecessary.

Today, physics operates without the need for aether. Our understanding of both Special and General Relativity shows that electromagnetic and gravitational waves can traverse the vacuum of space without a medium.

This does not definitively disprove aether's existence; it merely indicates that if it does exist, it has no detectable properties. Aether does not influence light or gravitational waves under any circumstances, suggesting that our observations align with its non-existence.

If a phenomenon has no measurable impact on our universe, we consider it non-existent. Although we lack evidence for aether, we cannot definitively prove it doesn't exist. What we can ascertain is that if aether is real, it does not affect the matter and radiation we observe.

As Carl Sagan famously remarked, “absence of evidence is not evidence of absence.” While we have yet to prove the non-existence of aether, numerous unresolved questions regarding the nature of empty space and the quantum vacuum continue to challenge physicists today.

What causes the intrinsic energy of empty space, or dark energy? If space has a discrete nature, does this suggest a preferred reference frame? Can light or gravitational waves propagate without any spatial medium?

For any inquiries for Ask Ethan, reach out to startswithabang at gmail dot com!

Starts With A Bang is featured on Forbes and republished on Medium after a week's delay. Ethan has authored two books: Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.