Both photons and gravitational waves propagate at the speed of light through the vacuum of empty Despite the fact that it isn't intuitive, there's no evidence that there's a physical medium, or aether, required for these entities to travel through.
All throughout the Universe, different types of signals propagate. Some of them, like sound waves, require a medium to travel through. Others, like light or gravitational waves, are perfectly content to traverse the vacuum of space, seemingly defying the need for a medium altogether. Irrespective of how they do it, all of these signals can be detected from the effects they induce when they eventually arrive at their destination.
But is it really possible for waves to travel through the vacuum of space itself, without any medium at all to propagate through? That's what Wade Campbell wants to know, asking:. Back in the late s, an "aether" was proposed as the medium that light travels through. We now don't believe that is the case. Whether through a medium, like mechanical waves, or in vacuum, like electromagnetic and In no cases is the propagation speed infinite, and in theory, the speed at which gravitational ripples propagate should be the same as the maximum speed in the Universe: the speed of light.
The waves we observed came in many different varieties, including:. In the case of all of these waves, matter is involved. That matter provides a medium for these waves to travel through, and as the medium either compresses-and-rarifies in the direction of propagation a longitudinal wave or oscillates perpendicular to the direction of propagation a transverse wave , the signal is transported from one location to another.
This diagram, dating back to Thomas Young's work in the early s, is one of the oldest pictures This is a physically identical setup to a double slit experiment, even though it applies just as well to water waves propagated through a tank. As we began to investigate waves more carefully, a third type began to emerge.
The rippling characteristics of water, which were previously thought to be either longitudinal or transverse waves exclusively, were shown to also contain this surface wave component. All three of these types of wave are examples of mechanical waves, which is where some type of energy is transported from one location to another through a material, matter-based medium. A wave that travels through a spring, a slinky, water, the Earth, a string, or even the air, all require an impetus for creating some initial displacement from equilibrium, and then the wave carries that energy through a medium towards its destination.
A series of particles moving along circular paths can appear to create a macroscopic illusion of Similarly, individual water molecules that move in a particular pattern can produce macroscopic water waves, and the gravitational waves we see are likely made out of individual quantum particles that compose them: gravitons. It makes sense, then, that as we discovered new types of waves, we'd assume they had similar properties to the classes of waves we already knew about. Even before Newton, the aether was the name given to the void of space, where the planets and other celestial objects resided.
The aether, it was assumed, was the medium inherent to space that all objects, from comets to planets to starlight itself, traveled through. Whether light was a wave or a corpuscle, though, was a point of contention for many centuries.
Newton claimed it was a corpuscle, which Christiaan Huygens, his contemporary, claimed it was a wave. The issue wasn't decided until the 19th century, where experiments with light unambiguously revealed its wave-like nature. With modern quantum physics, we now know it behaves like a particle also, but its wave-like nature cannot be denied. The results of an experiment, showcased using laser light around a spherical object, with the actual Note the extraordinary validation of Fresnel's theory's prediction: that a bright, central spot would appear in the shadow cast by the sphere, verifying the absurd prediction of the wave theory of light.
This was further borne out as we began to understand the nature of electricity and magnetism. Experiments that accelerated charged particles not only showed that they were affected by magnetic fields, but that when you bent a charged particle with a magnetic field, it radiated light. The biggest question remaining, then, was to determine what properties the aether itself possessed.
In Descartes' vision of gravity, there was an aether permeating space, and only the displacement of This did not lead to an accurate formulation of gravity that matched with observations. In an letter to Lewis Campbell, he wrote:. It may also be worth knowing that the aether cannot be molecular.
If it were, it would be a gas, and a pint of it would have the same properties as regards heat, etc. It could not be composed of individual particles. It could not contain heat. It could not transfer energy through it.
In fact, just about the only thing left that the aether was allowed to do was serve as a background medium through which things like light were permitted to travel. If you split light into two perpendicular components and bring them back together, they will produce If there's a medium that light is traveling through, the interference pattern should depend on how your apparatus is oriented relative to that motion.
All of this led to the most important experiment for detecting the aether: the Michelson-Morley experiment. If aether really were a medium for light to travel through, then the Earth should be passing through the aether as it rotated on its axis and revolved around the Sun.
With a sensitive enough interferometer, if light were a wave traveling through this medium, we should detect a shift in light's interference pattern dependent on the angle the interferometer made with our direction of motion. Michelson alone tried to measure this effect in , but his results were inconclusive.
Their experiment, however, yielded a null result; there was no evidence for the aether at all. The Michelson interferometer top showed a negligible shift in light patterns bottom, solid as The speed of light was the same no matter which direction the interferometer was oriented, including with, perpendicular to, or against the Earth's motion through space.
All of these possibilities, despite their arbitrary constants and parameters, were seriously considered right up until Einstein's relativity came along. Once the realization came about that the laws of physics should be, and in fact were, the same for all observers in all frames of reference , the idea of an "absolute frame of reference," which the aether absolutely was, was no longer necessary or tenable.
If you allow light to come from outside your environment to inside, you can gain information about The fact that the laws of physics, the speed of light, and every other observable is independent of your reference frame is strong evidence against the need for an aether.
What all of this means is that the laws of physics don't require the existence of an aether; they work just fine without one. The vacuum of space, devoid of any material entity, is enough all on its own. This doesn't mean, however, that we've disproven the existence of the aether.
All we've proven, and indeed all we're capable of proving, is that if there is an aether, it has no properties that are detectable by any experiment we're capable of performing. It doesn't affect the motion of light or gravitational waves through it, not under any physical circumstances, which is equivalent to stating that everything we observe is consistent with it's non-existence. Visualization of a quantum field theory calculation showing virtual particles in the quantum vacuum. Even in empty space, this vacuum energy is non-zero, and what appears to be the 'ground state' in one region of curved space will look different from the perspective of an observer where the spatial curvature differs.
As long as quantum fields are present, this vacuum energy or a cosmological constant must be present, too. If something has no observable, measurable effects on our Universe in any way, shape or form, even in principle, we consider that "thing" to be physically non-existent. But the fact that there's nothing pointing to the existence of the aether doesn't mean we fully understand what empty space, or the quantum vacuum, actually is. In fact, there are a whole slew of unanswered, open questions about exactly that topic plaguing the field today.
If space is discrete at some level, does that imply a preferred frame of reference, where that discrete "size" is maximized under the rules of relativity? Can light or gravitational waves exist without space to travel through, and does that mean there is some type of propagation medium, after all? As Carl Sagan famously said, "absence of evidence is not evidence of absence. All we can demonstrate, and have demonstrated, is that if the aether exists, it has no properties that affect the matter and radiation that we do observe.
I have won numerous awards for science writing. I have won numerous awards for science writing since for my blog, Starts With A Bang , including the award for best science blog by the Institute of Physics. Follow me on Twitter startswithabang. This is a BETA experience. Edit Story. Ethan Siegel Senior Contributor.
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