February 28, 2021

I, Science

The science magazine of Imperial College

Exploring the hottest, coldest, biggest and smallest things in the universe...

The most well-known scientific fact is that nothing can travel faster than the speed of light, c, which at 3×108m/s – that’s 186 thousand miles every second – is pretty fast indeed!

What other limits and extremes are there in our Universe?

fgThe biggest thing in the Universe is obviously the Universe itself! Space is big. Really big. You just won’t believe how vastly hugely mind-bogglingly big it is. But just how big is that?

The observable universe, that which can be observed (at least in principle) from Earth because light has had time to reach us from those regions, is estimated to extend around 46 billion light years out away from us in all directions.

Considering that 1 light year is equal to just over 63 thousand times the distance from Earth to the Sun, we really are a tiny planet in a truly vast cosmos.

The actual size of the Universe is actually much greater, with light from those most distant regions yet to reach us. However due to the expansion of the Universe those light rays may never reach us, so there is always going to be a limit on how much we are able to see.

From big to small. The Planck length, equal to approximately 1.6×10-35 metres, is currently the smallest scale in physics. That’s a million million million million million million times smaller than the size of atoms! We don’t have theories that describe things smaller than this and no way to probe the Universe at anywhere near this level of smallness.

Now what about ‘stuff’ in the Universe? Heisenberg’s Uncertainty Principle states that energy and time cannot both be exactly known simultaneously, meaning over even a short time period a region of empty space will always have a non-zero amount of energy, and on a really small time scale particles pop into existence from nothing. The more you compress something, squashing the atoms closer and closer together, the denser it becomes. If the pressure is great enough, electron shells get removed and the atoms packed nucleus-to-nucleus, achieving a much greater density. The average density of the human body is 1g/cm(1 gram per centimetre cubed) whereas an atomic nucleus and neutron stars are about 1×1014kg/m3 and black holes can be even denser!

iceThe lowest naturally occurring temperature ever recorded on Earth was -89.2°C in Antarctica in 1983. Bbrrr hope they remembered their wooly hat!

Theoretically the coldest temperature possible is Absolute Zero (0K or -273.15°C) however it is impossible to actually reach this temperature. This is because Heisenberg’s Uncertainty Principle means that knowing where the thing you’re cooling is located there is some uncertainty in its momentum, therefore it cannot have exactly zero atomic movement as is necessary to satisfy the definition of being at absolute zero temperature. The closest scientists have got is 100 picokelvins, that’s 0.0000000000001K, which is unbelievably cold. This could eventually be reduced a bit further through the continued refinement of experiments and development of cooling techniques, and there are physicists working on doing this, but we will ultimately always be limited by how cold we can make things.

Let’s warm up. What’s the hottest thing in the Universe? Well the hottest naturally occurring temperature ever recorded on Earth was 56.7oC at Death Valley (California, USA) in 1913. Definitely T-shirt weather!

At the centre of the Sun, our nearest star, the temperature is estimated to be around 1.5×107K, that’s a toasty 15 million oC! The highest man-made temperature was achieved by physicists at CERN’s LHC in 2012 when lead ions were collided to create a quark-gluon plasma that was measured at about 5.5 trillion C… that’s 100,000 times hotter than the centre of the Sun!

sunThe highest possible temperature is defined by a concept called Absolute Hot which theorises how much energy can be sustained in a specific region of space. The Planck temperature of 1.4×1032K is taken as the highest possible temperature, because above this conventional physics breaks down and there is no existing scientific theory for the behaviour of matter.

Whether big or small, hot or cold, science will continue to investigate these fascinating extremes of our Universe.

This post is syndicated from Scitable.

IMAGES: (Top to bottom) NASA Goddard Photo and Video, stephendepolo, NASA Marshall Space Flight Center