What is the hottest temperature possible

Talking about the coolest possible temperature seems relatively simple. The coldest of the cold is absolute zero. As you may know, movement causes friction, which causes heat. As such, absolute zero is, in essence, when all movement stops. The temperature is reached at -459.67 degrees Fahrenheit (-273.15 degrees Celsius). We've come pretty close to reaching this temperature. Most recently, scientists at the Massachusetts Institute of technology (MIT) cooled molecules to just 500 billionths of a degree above absolute zero.

But what about the hottest possible temperature? Is there an absolute hot?

Well, things aren't really that simple. Stopping all movement is one thing, but how do we measure maximum movement? How do we take energy up to infinity? Theoretically, it is possible. But theory isn't necessarily what we observe in our physical reality.

As such, it seems that the highest possible known temperature is 142 nonillion kelvins (1032 K.). This is the highest temperature that we know of according to the standard model of particle physics, which is the physics that underlies and governs our universe. Beyond this, physics starts to breakdown. This is known as Planck Temperature.

If you are wondering, the number looks a little like this: 142,000,000,000,000,000,000,000,000,000,000 (that's a really big number). Ultimately, this can only come about when particles achieve what is known as thermal equilibrium. In order for it to be the hottest temperature, physicists assert that the universe would have to reach thermal equilibrium, with a temperature that is so hot, all the objects are at the same temperature.

The closest that scientists think we ever came to this temperature is, unsurprisingly, just after the Big Bang. At the earliest moments of our universe, spacetime expanded so fast (a period known as the inflationary period) that particles were unable to interact, which means that there could be no exchange of heat. At this juncture, scientists assert that, for all intents and purposes, the cosmos had no temperature.

No heat exchange. No temperature.

But this quickly ended. Scientists assert that, just a fraction of a fraction of a fraction of a second after our universe began, spacetime started to vibrate, which caused the universe to come to about 1,000,000,000,000,000,000,000,000,000 (1027) Kelvins.

And our universe has been growing and cooling since this moment. So. It is believed that this moment, which occurred just after the start of our universe, is the hottest moment in the universe, the time at which the hottest temperature that will ever be reached was.

Just for comparison, the hottest temperature that we have ever actually encountered is in the Large Hadron Collider. When they smash gold particles together, for a split second, the temperature reaches 7.2 trillion degrees Fahrenheit. That’s hotter than a supernova explosion.

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From what we know about our Universe, the coldest possible temperature is 'absolute' zero degrees Kelvin, or -273.15 degrees Celsius (-459.67 degrees Fahrenheit). But what about the hottest possible temperature?

Physics is a little fuzzy on what the absolute hottest of hot looks like, but theoretically speaking, such a thing does – or at least, did – once exist. It's called the Planck temperature, but, as with everything in life, it's also not that simple.

What is temperature, anyway?

First thing that might come to mind when thinking about temperature might be a description of the amount of heat an object contains. Or, for that matter, doesn't contain.

Heat, or thermal energy, is an important part of the explanation. Our intuitive understanding of heat is that it flows from sources with higher temperatures to those with lower temperatures, like a steaming cup of tea cooling as we blow on it.

In physics terms, thermal energy is more like an averaging of random movements in a system, usually among particles such as atoms and molecules. Put two objects with varying amounts of thermal energy close enough to touch, and the random movements will combine until both objects are in equilibrium. As a form of energy, heat is measured in units of joules.

Temperature, on the other hand, describes the energy transfer from hotter to colder regions, at least theoretically. It's typically described as a scale, in units like Kelvin, Celsius, or Fahrenheit. A candle's flame might have a high temperature compared with an iceberg, but the amount of thermal energy in its heated wick isn't going to make much of a difference when placed against the mountain of frozen water.

What exactly is absolute zero, then?

Absolute zero is a temperature, so it's a measure of the relative transfer of thermal energy. In theory, it marks a point on a temperature scale where no more thermal energy can be removed from a system, thanks to the laws of thermodynamics.

Practically speaking, this precise point is forever out of reach. But we can get tantalizingly close: All we need are ways to decrease the average amount of thermal energy spread among the particles of a system, perhaps with the help of lasers, or the right kind of flip-flopping magnetic field.

But in the end, there is always an averaging out of energy that will leave the temperature a fraction above the theoretical limit of what can be extracted.

What is the hottest temperature possible?

If absolute zero sets a limit on pulling thermal energy from a system, it might stand to reason there's also a limit to how much thermal energy we can shove into one. There is. In fact, there are a couple of limits, depending on precisely what kind of system we're talking about.

At one extreme is something called Planck temperature, and is equivalent to 1.417 x 1032 Kelvin (or something like 141 million million million million million degrees). This is what people will often refer to as the 'absolute hot'. Nothing in today's Universe comes close to these kinds of temperatures, but it did exist for a brief moment right at the dawn of time. In that fraction of a second – a single unit of Planck time, in fact – when the size of the Universe was just one Planck length across, the random movement of its contents was about as extreme as it could get.

Any hotter, and forces like electromagnetism and the nuclear forces would be on par with the force of gravity. Explaining what this looks like demands physics we don't have a grip on yet, one that unites what we know about quantum mechanics with Einstein's general theory of relativity.

Those are also some pretty specific conditions. Time and space will never be so confined again. Today the best the Universe can manage is the paltry few trillion degrees we create when we smash atoms together in a collider.

The opposite of absolute zero

But there is another way to look at heat, one that turns the whole question of temperature on its head.

Keep in mind that thermal energy describes an average of movements among the parts of a system. All it takes is a small percentage of its particles to be flying about chaotically to qualify as 'hot'.

So what happens if we flip this state and have far more zippy particles than sluggish ones? It's what physicists call an inverted Maxwell–Boltzman distribution, and weirdly, it's described using values that go below absolute zero.

This strange system seems to throw out the rulebook on physics. Not only do we quantify it as a negative to absolute zero, it's technically hotter than any positive value. Quite literally hotter than hot.

As a quirk of statistics, it's not something we'd find in any natural corner of the Universe. For one thing, it'd require an infinite amount of energy, and then some.

That doesn't mean we can't bend the rules a little and make something like it. In 2013 it was demonstrated by physicists at the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Germany; they used atomic gasses within very specific settings though, which impose their own upper energy limits.

The results were a stable system of particles with so much kinetic energy, it became impossible to shove any more in. The only way to describe this particular arrangement was by using a temperature scale that went into negative Kelvin, or several billionths of a degree below absolute zero.

Such a bizarre state could in theory absorb thermal energy not just from hotter spaces, but from colder ones as well, making it a true monster of extreme temperatures.

In this diabolical corner of the Universe, a machine would be able to chug away at greater than 100 percent efficiency as it fed from hot and cold alike, seeming to thumb its nose at the laws of thermodynamics.

All Explainers are determined by fact checkers to be correct and relevant at the time of publishing. Text and images may be altered, removed, or added to as an editorial decision to keep information current.

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