Light is a fascinating phenomenon that has intrigued scientists and philosophers for centuries. At the heart of the intrigue is the question of whether light, which travels incredibly fast at 186,000 miles per second, ever stops or slows down. This article will examine the properties of light and what physics tells us about its motion through space and time. We’ll look at whether light can be captured, the relationship between light and time, and what Einstein’s theory of relativity revealed about the constancy of the speed of light. By the end, you’ll have a deeper understanding of one of the most ubiquitous yet mysterious aspects of our universe – light.
What is light?
To understand whether light can stop, we first need to understand what exactly light is. Light is a form of electromagnetic radiation. It is produced by vibrating electric charges – electrons within atoms and molecules. These oscillating charges create oscillating electric and magnetic fields which propagate through space as a wave.
Light is part of the electromagnetic spectrum, which includes radio waves, microwaves, infrared radiation, visible light, ultraviolet light, x-rays and gamma rays. The only difference between these types of radiation is their wavelength, or frequency. Visible light, which human eyes can detect, has wavelengths from about 400 to 700 nanometers.
So in essence, light is a self-propagating electromagnetic wave which transports energy through space. It moves at the speed of light, commonly denoted c, which is:
The speed of light in a vacuum
| Speed of light (c) | 186,282 miles per second |
| 299,792 kilometers per second |
This speed is generally rounded to 300,000 kilometers per second or 186,000 miles per second. It’s also about a billion km/h! This incredible velocity is a physical constant that is thought to be the maximum speed information and energy can travel.
But does light ever come to a halt or can its motion ever be impeded? Let’s examine the evidence.
Can light be captured or slowed down?
Our everyday experience with light might lead us to believe it can be stopped or slowed. For instance, when light enters a dense medium like water, it slows down and bends. This refraction allows lenses to focus light. We are also used to seeing light absorbed by dark surfaces or blocked by opaque objects.
So at first glance, it would seem that light can be captured, reflected, scattered or absorbed. But according to Einstein’s theory of special relativity, this is an illusion. Light only appears slowed from an observer’s perspective. In actuality, it always propagates at c. Here’s why:
Reasons light cannot be slowed
| Light is massless | So it cannot be slowed by inertia |
| It has fixed energy | Its speed is linked to its wavelength |
| Space and time dilate | To preserve the constancy of c |
Let’s go through each of these reasons:
First, light has no mass or rest mass. Without mass, light has no inertia, so there is nothing to slow it down. Attempts to measure the mass of particles of light called photons have revealed an upper limit of 10-52 kg, which is negligible.
Second, light has a fixed energy proportional to its wavelength and frequency. Decreasing the speed of light would require changing its wavelength, but this cannot happen.
Third, according to relativity theory, space and time can dilate or length contract to preserve the speed of light. From a stationary observer’s frame, light may appear slower, but it never slows down relative to an observer traveling alongside the light beam.
This preservation of the speed of light is a fundamental aspect of physics and relativity. Multiple experiments have shown that light always travels at c through any medium or vacuum.
So in summary, although light may seem slowed or stopped, it is only an illusion. Light’s speed in a vacuum is a physical constant that cannot change.
Does light slow down over distance?
If the speed of light is perfectly constant, does this mean a beam of light could theoretically travel forever? Is there any sense in which a light beam slows down or becomes weaker over vast cosmic distances?
The answer is that the speed of light does not change with distance traveled. However, there are a couple other factors that cause a light source to diminish with distance:
Why light dims over distance
| Spreading out | Light disperses over area |
| Redshift | Wavelength lengthens |
First, as light radiates outward from a source, it spreads out over an increasing surface area. This dilution reduces the light’s luminosity per unit area.
Second, the expansion of space causes distant light to be stretched to longer, redder wavelengths. This cosmic redshift makes the light dimmer and lower in energy.
Nevertheless, neither of these effects slow down light’s remarkable velocity. Each photon continues to travel at 186,000 miles per second until absorbed. The speed of light is not dependent on brightness or distance from the source.
Can light be brought to a halt?
Are there any exotic methods by which light could theoretically be stopped or its speed altered? While slowing light is impossible in everyday conditions, scientists have managed to dramatically reduce the speed of light by manipulating the medium through which it travels.
Methods used to dramatically slow light
| Passing light through ultracold gases | Bose-Einstein condensates can slow light to just 17 m/s |
| Channeling light through hot rubidium vapor | Varying laser intensity can stop and restart a light pulse |
| Firing light into extremely cold solids | Light has been brought to a complete halt inside crystal at 0 kelvin |
These methods rely on exotic mediums like Bose-Einstein condensates, extremely hot gases, and cryogenically frozen solids saturated with light. By carefully controlling conditions, researchers can dramatically decrease the speed of light pulses.
However, in all these situations, the slow-down occurs because the light transfers energy and information to the medium. The long-range propagation of light still abides by the speed limit c. Once out of the special material, any slowed or halted light returns to 186,000 mps.
So in summary, manipulating extreme conditions can make light appear slowed for limited distances. But this is just an illusion – light’s actual underlying speed remains unchanged.
Can time stop light?
If light cannot be slowed by material means, perhaps the passage of time itself could potentially stop a light beam? This ties into the intertwined nature of light and time.
In a vacuum, the speed of light is about 299,792 kilometers per second. But the denominator is as important as the numerator. The maximum speed only makes sense with respect to a particular reference of time. In essence, both space and time must expand to keep the ratio constant.
Let’s quickly walk through how light and time relate:
The interdependence of light and time
| Speed of light = distance / time | |
| Slow clocks = less time passes | |
| Contracted space = less distance | |
| Slowing time and shrinking space | Preserves the speed of light |
In non-accelerating reference frames, time progresses at a constant rate. In this “coordinate time”, the speed of light is 299,792 km/s.
But during acceleration, clocks run slower due to time dilation. Less coordinate time passes. Without adjustments to space as well, light’s speed would appear faster.
To compensate, space contracts in the direction of motion. This precisely counterbalances time dilation, keeping the speed of light constant.
So in essence, time itself ensures light propagates at the same speed, regardless of motion or gravity. Stopping time would disrupt physics as we know it!
Can light escape black holes?
Black holes represent an extreme phenomenon of gravity that gives insights into light’s motion. The immense gravitation of a black hole is thought to stop everything, even light, from escaping past its boundary or event horizon.
Light and black holes
| Light orbit | Photon spheres trap light in orbits |
| Trapped forever | Within the black hole’s event horizon |
| Escape velocity | Exceeds speed of light c |
Outside the event horizon, light can orbit in spherical paths around the black hole, caught in a delicate balance between gravity and angular momentum. These photon spheres are not fully stable though.
At the event horizon itself, light is thought to be drawn inexorably inwards, unable to escape the black hole’s extreme gravity. The escape velocity exceeds even light’s top speed.
So black holes seemingly stop light in its tracks, swallowing it up into a realm of the unknown. But another interpretation is that light can never quite reach the event horizon from an external perspective.
The speed of light at black holes
One mind-bending implication of black holes is that external viewers never actually see anything cross the event horizon, including light. Due to gravitational time dilation, the passage of time appears to slow exponentially as matter approaches the boundary.
So theoretically, light never fully stops or slows down as it nears the event horizon. The speed c is just increasingly stretched out over more distorted time. Here are some consequences of this:
Consequences of light’s constant speed at black holes
| Light never observed crossing | Time dilates to near halt at boundary |
| Light constantly slowed down | But not from infalling object’s view |
| Light might escape | Via quantum tunneling |
First, external viewers should never observe light fully stopping at the event horizon due to time dilation.
Second, light only seems slowed down from afar. To a photon or infalling observer, nothing changes locally at the event horizon.
Third, there are speculative models where light might tunnel out as a quantum effect or escape after the black hole evaporates.
So a black hole bending light to its will is only an observational effect. In actuality, light retains its constant speed. The notion of light permanently halted at an event horizon may be an illusion!
Does light stop during inflation?
The earliest moments of the universe feature an incredibly rapid expansion called cosmic inflation. This stretched miniscule quantum fluctuations into the seeds that eventually grew into galaxies. Could even this radical expansion have slowed light?
Light and cosmic inflation
| Exponential expansion | Space enlarged 10^26 times in a tiny fraction of a second |
| Smooth universe | Flattened out warps and wrinkles |
| Maintained speed of light | Horizon problem solved |
In this model, space expanded exponentially for a brief period starting 10-36 seconds after the Big Bang. This expansion was faster than light’s speed. But it did not involve objects moving through space. Rather, space itself enlarged in a uniform manner.
Consequently, inflation explains why the universe today looks smooth and uniform. Any unevenness was flattened out.
Critically, inflation also solves the horizon problem. even opposite horizons have the same temperature. Information was able to equilibrate faster than light could have transmitted across the primordial universe.
While the metric expansion of space was surpassing light speed, light itself was still locally traveling at c along warped spacetime. So inflation did not violate relativity, but became incorporated into it.
Conclusion
Does light ever slow down or stop? From all observational evidence and theoretical frameworks, the speed of light in vacuum seems inviolable. It is a fixed constraint that distorts space and time themselves.
While propagating through materials, becoming trapped in gravitational orbits, or approaching a black hole’s boundary, light may appear slowed. But this is only an observational effect due to light’s unchanging speed combined with relative time dilation.
Locally, light always travels at c along spacetime, even as the fabric itself expands and warps. The passage of light is interwoven with the passage of time itself. Stopping or slowing light would require rewriting the laws of physics as we know them.
So in short, light cannot be brought to a halt by any known means. Its speed is a fundamental constant of the universe, chosen by nature for still mysterious reasons. The motion of light seems inextricable from the truth of our reality.