Is It possible to reduce the speed of light?

When people talk about "the speed of light" in a general context, they usually mean the speed of light in a vacuum. However, light need not always travel in vacuum. The concept of a refractive index describes the relationship between speed of light in vacuum vs a particular medium.

The speed of light in free space c is constant. However, a physical beam of light has a finite extent such that even in free space it is by nature dispersive. The field confinement changes its wavevector, hence, altering the light’s group velocity vg (1).

Physicists have found a way to slow light down without touching or intercepting it, simply by making it 'twist' as it moves through space. That means they were able to make beams of light take longer to get from point A to point B, a skill that will be incredibly useful when it comes to controlling the next-generation of optical computers and telecommunication systems. (1)

Light speeding through the emptiness of space moves at one speed only - 299,792 kilometers (about 186,000 miles) per second. Yet if you throw a mess of electromagnetic fields into its path, such as those surrounding ordinary matter, that extraordinary velocity starts to slow.

Most transparent materials will slow light by a tiny fraction like people mentioned in their answers here.

In optics, slow light is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. Slow light occurs when a propagating pulse is substantially slowed by the interaction with the medium in which the propagation takes place. (6)

It's the changes in speed that cause light to bend as it passes from one medium to another. But really putting the brakes on requires special materials like photonic crystals or even super-chilled quantum gases.

The new method builds on what's known as electromagnetically induced transparency (EIT), which uses a clever bit of laser trickery to manipulate electrons inside gas that's stored in a vacuum – essentially turning it from opaque to transparent.

This means laser light can pass through, but because of how it has been manipulated, it also slows down. That makes it very interesting for physicists, but the approach also means a lot of the light and energy is lost along the way.

To reduce this loss and improve the whole system's efficiency, the researchers took some of the principles of EIT in controlling light and designed a new material to slow down light. The material is a kind of metasurface - a synthetic, 2D structure with properties unlike any in nature.

The metasurfaces designed by the team were made from very thin layers of silicon – like today's computing chips – and were shown to be much better than existing options in the way they held and released energy (in this case, from light).

Based on the results obtained by the researchers, light can be slowed down by more than 10,000 times in this system. At the same time, the light loss is reduced by more than five times compared with other comparable methods.

It's not the only way that scientists have found to further slow down light, beyond the natural slowing that happens in substances like water, but its effectiveness and its scalability make it a promising option for further study. (2, 3)

Well, I am not an expert in the field, I am only reporting from the research papers I read.

And I read an interesting article too: Structured photons slow down in a vacuum (4).

It says according to a team of physicists in the UK, which has found that the speed of an individual photon decreases by a tiny amount if it is initially sent through a patterned mask. The phenomenon – which is different to other observations of slow light – should also occur for sound waves, the researchers say. (5)

The mask forced the photon to change its shape and travel slower than the speed of light.

Not by much - a few millionths of a metre - but it showed that it had not just been slowed by the mask, but had continued to travel at less than light speed even after it had returned to free space.

Light travelling at less than the speed of light! You can now read how the media interpreted it here:

Scientists slow the speed of light

And this one (7):

Lene Hau is a world-renowned physicist at Harvard University, and she has figured out a way to stop light in its path. One of the tricks to slowing light to a halt is creating a cloud of nearly motionless atoms at near absolute zero (-460 degrees Fahrenheit). This is called a Bose-Einstein condensate. (7)

By combining the coldest thing and the fastest thing on Earth, Dr.Hau was able to show that she could decrease the speed of light from approximately 186,000 miles per second down to 15 miles per hour.

After a few more experiments, Dr. Hau figured out a way to tweak some properties of the atom cloud in such a way that she could completely stop a light beam in the cloud. It is as if the light wave is frozen inside this cloud, and stays there until it is warmed up again. (7,8)

Free thinkers and experts can analyse this now and can come up with better explanations.

In science facts are only facts until someone comes and rewrites the story.

And we watch all this with a thrilled expression!

Footnotes:

1. Subluminal group velocity and dispersion of Laguerre Gauss beams in...
2. https://pubs.acs.org/doi/10.1021/acs.nanolett.3c04174
3. Scientists Slowed Down Light by 10,000 Times in an Experiment
4. Structured photons slow down in a vacuum – Physics World
5. https://www.science.org/doi/10.1126/science.aaa3035
6. Slow light - Wikipedia
7. Light speed reduction to 17 metres per second in an ultracold atomi...
8. Slowing the Speed of Light to Zero