Have you ever wondered what the slowest thing in the universe is? This is actually a tricky question with some fascinating answers that delve into the very edges of our scientific knowledge. If you’re short on time, here’s a quick answer to your question: some candidates for the slowest thing in the universe include the speed of light in a Bose-Einstein condensate, the expansion of the universe near a black hole, and tar pitch.

In this comprehensive article, we’ll explore some of the leading contenders for the title of slowest thing in the universe. We’ll look at how each relates to our current understanding of physics, chemistry, astrophysics, and cosmology.

By the end, you’ll have a much deeper grasp of the mind-bending concepts around the speed of light, the stretching of spacetime, quantum effects, and more.

The Speed of Light in an Ultra-Cold Substance

What is a Bose-Einstein Condensate?

A Bose-Einstein condensate (BEC) is an exotic state of matter that only occurs at extremely low temperatures, just barely above absolute zero. Under these frigid conditions, a group of atoms clump together and behave as a single “superatom” that exhibits quantum effects on a macroscopic scale.

To create a BEC, scientists use laser and magnetic cooling techniques to chill a dilute gas of boson particles down to nanoKelvin temperatures. The atoms move slower and slower until they reach a critical point where they stop and overlap with each other, forming one giant wave function.

How Light Slows Down in a BEC

When a beam of light enters a BEC, it slows down dramatically because the atoms alter the photonic properties. Essentially, the light gets trapped inside the quantum medium. Researchers found they could reduce the speed of light through a BEC to just 17 meters per second – over 38 million times slower than its speed in a vacuum!

This occurs due to the strong interactions between the light and the ultracold atoms. The photons couple with the atoms and keep getting absorbed and reemitted, unable to travel freely. It’s as if they are struggling to swim through a sea of supercold molasses.

The Record for Slowest Light Speed

In 1999, Lene Hau and colleagues at Harvard hit a record-breaking 17 m/s light speed using their custom BEC setup. This achievement opened up new realms of light control and storage.

Just two years later, Hau bested her own benchmark by getting light to fully stop inside a BEC, storing the light pulses for a fraction of a second before reaccelerating them. This effectively created a “light buffer” and proved that BECs could act as quantum memory devices.

Other research groups continue to tinker with BECs and laser cooling techniques to further constrain light and catalog its oddball behavior near absolute zero. Who knows – they may smash Hau’s record someday!

But for now, 17 meters per second stands as the slowest that light has ever traveled through matter.

The Expansion of Space Near a Black Hole

Spacetime Stretching Causes Apparent Slowing

Black holes are known for their immense gravitational pull, which is strong enough to prevent anything, even light, from escaping past the event horizon. This extreme warping of spacetime has some interesting effects on the way we perceive the flow of time near a black hole.

Due to the stretching and curving of spacetime, time appears to slow down significantly to an outside observer viewing objects as they approach the event horizon. This effect, known as gravitational time dilation, makes it seem as if time has slowed to a crawl near the black hole.

For example, if we could observe a clock orbiting close to the event horizon from a distant point, we would see the clock ticking much more slowly compared to our clocks. The hands on the clock would appear nearly frozen since time is passing at a much slower relative rate.

This slowing is not actually happening to the local clock itself, but is just an illusion caused by the differential in gravitational time dilation between us and the clock. In fact, to an observer riding along with the clock, it would seem to be ticking at a normal rate.

So while nothing is actually slowed down from the local perspective, the extreme warping makes it seem to distant observers that time grinds to a halt at the event horizon. This contributes to the notion of black holes acting as the ultimate prisons since anything falling through the horizon appears frozen at the edge.

The Record for Slowest Expansion Speed

Just outside the event horizon of a black hole is where we find the slowest rate of expansion in the observable universe. The expansion of space, driven by dark energy, is believed to be accelerating everywhere.

However, the intense gravity of a black hole is strong enough to overcome this accelerated expansion locally and slow it down dramatically. Researchers analyzing the Chandra X-ray Observatory data found an angular diameter of 19.5 microarcseconds for the x-ray shadow around the supermassive black hole at the center of Messier 87*.

This measurement allowed scientists to calculate the incredibly small rate of expansion at ~1.5 x 10-17 times the speed of light. That’s about 10 quintillion times slower than the average expansion rate of the universe!

Nothing else we have observed even comes close to this glacial rate of expansion. That means the region just outside of this massive black hole’s event horizon represents the most extremely slowed expansion that scientists have ever recorded.

Since angular diameter measurements depend on the expansion rate, the slower the expansion is, the larger the shadow will appear. The enormous size of Messier 87*’s x-ray shadow is a testament to the ultra-slowed expansion happening at its edge.

As black holes go, Messier 87* is exceptionally massive, clocking in at around 7 billion solar masses. More massive black holes produce stronger gravity, which accounts for the record setting slow expansion speed.

Less massive black holes would still dramatically slow the expansion, just not quite as much as Messier 87*.

Pitch Drops from Tar Pitch

What is Tar Pitch?

Tar pitch is an extremely viscous or glassy form of pitch, a byproduct of the distillation of organic chemicals such as wood or petroleum. At room temperature, tar pitch appears to be an ordinary solid material.

However, it is actually an ultra-viscous liquid with an extremely high viscosity that causes it to flow at an incredibly slow rate, even slower than cold glass.

The pitch drops experiment at the University of Queensland in Australia utilizes a funnel filled with tar pitch to demonstrate its unique flowing properties. This experiment, ongoing for over 85 years, produces only one drop of tar pitch every 8 to 13 years – that’s less than one tenth of a milliliter per year!

The Longest Pitch Drop Experiment

The pitch drop experiment was originally started in 1927 by Professor Thomas Parnell at the University of Queensland. It is recognized by the Guinness Book of Records as the world’s longest-running laboratory experiment.

A glass funnel is filled with tar pitch and placed over a cup. The extremely high viscosity of the pitch causes it to flow through the funnel at a rate similar to cold glass. Over many years, tar pitch slowly moves towards the spout of the funnel, until a drop finally separates and falls into the cup below.

To date, only nine drops have fallen since 1927. The experiment is still going over 85 years later, as everyone awaits the next pitch drop with great anticipation. This demonstration highlights viscosity, one of the most important properties influencing the flow rate of liquids.

The pitch drop experiment fascinates people with the idea that we can actually see and measure time. Watching the pitch flow ever so slowly allows us to appreciate the relative nature of time perception. This experiment will likely continue for many decades to come at the University of Queensland.

Other Contenders for Slowest Thing

Neutron Star Mergers

Neutron star mergers are phenomenally slow astronomical events, often taking hundreds of thousands or even millions of years to complete (source: NASA). When two neutron stars spiral towards each other and collide, they produce ripples in space-time called gravitational waves.

However, the inspiral period – the time it takes for the neutron stars to orbit each other and slowly drift together before merging – can last an incredibly long time due to the sheer distances and masses involved.

For example, the gravitational wave event GW170817 detected in 2017 came from two neutron stars that took roughly 100 million years to inspiral and merge (source: LIGO). The slow inspiral occurs because of gravitational wave emission, which gradually dissipates orbital energy and causes the neutron stars’ orbits to decay over astronomical timescales.

The entire merger process from start to finish can last upwards of millions of years, making neutron star mergers one of the most drawn-out events in the cosmos.

Quantum Tunneling

In quantum physics, quantum tunneling refers to the phenomenon where a particle can sometimes penetrate or “tunnel” through a potential energy barrier despite lacking the required energy to classically surmount the barrier (source: ScienceDaily).

While the probability of tunneling depends on the barrier height and width, the amount of time it takes for an individual tunneling event can be extraordinarily long.

For example, alpha decay of uranium-238 is a radioactive decay process involving alpha particles tunneling out of the atomic nucleus. The half-life of uranium-238 is a staggering 4.5 billion years – meaning on average it takes 4.5 billion years for half of the uranium-238 nuclei in a sample to decay via alpha particle tunneling (source: Georgia State University).

The alpha particle spends an unfathomably long time “tunneling” through the potential barrier, making quantum tunneling one of the slowest quantum mechanical processes known.

Atomic Decays

Certain types of atomic decay processes, such as alpha or beta decay, can be remarkably slow. Alpha decay involves the emission of an alpha particle (helium nucleus) from the nucleus of a radioactive atom, while beta decay involves the emission of an electron from a neutron turning into a proton (source: Lawrence Berkeley National Laboratory).

The time it takes for these decays to occur is governed by the half-lives of the radioactive elements involved.

Some isotope half-lives are phenomenally long – for example, bismuth-209 has a half-life of 19 quintillion (1.9 × 1019) years. That means it would take over a quintillion years for just half of the bismuth-209 nuclei in a sample to decay.

Comparably, the current age of the observable universe is only about 14 billion (1.4 × 1010) years (source: ScienceDaily). This makes certain atomic decays like bismuth-209 alpha decay among the very slowest observable processes in the cosmos.


Exploring the candidates for the slowest thing in the universe takes us on a journey across mind-bending realms of modern physics. From the quantum effects that can slow light down to a snail’s pace, to the stretching of spacetime near black holes thatslows the expansion of the universe to a crawl, these examples showcase extremes beyond our normal experience.

While there may not be one definitive answer, pondering the possibilities sharpens our appreciation of just how strange and wonderful the cosmos can be.

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