A vacuum is a region of space where there are no particles. It can be thought of as empty space.

A vacuum is not technically a totally empty space though. There is still energy associated with it. This is because the vacuum contains virtual particles that have an energy associated with them.

So how much energy is there in the vacuum?

It turns out that the answer to this question depends on whether you are talking about quantum field theory or general relativity. But we’ll get into that.

First and foremost, before finding out the energy density of a vacuum, we must first understand what is meant by energy density and a vacuum.

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**What Is Energy Density?**

Energy density is defined as the amount of energy per unit volume. For example, if I had some mass inside a box, then the total energy would be equal to E = mc2.

If I took the mass outside the box, however, the total energy would be zero. In other words, the energy inside the box was greater than the energy outside the box.

We want to know the energy density of the vacuum. As mentioned earlier, the vacuum is a region of empty space. So when we talk about the energy density of the vacuum, we mean the energy density of the empty space.

The energy density of the vacuum is given by:

E = mc2.

Where C is the permittivity of free space. Note that the units for energy density are joules/meters cubed.

Vacuums can have a high energy density, but why is this?

Remember, a vacuum is a place where all matter is absent. And so, since there is nothing present, the energy should be zero.

However, since the vacuum is full of virtual particles, they give off energy, thus making up for the fact that the vacuum is empty.

**What Is A Vacuum?**

A vacuum is a region of spacetime that has a very low probability of containing any sort of particle, but the operative term here is low probability.

That means that there may well be some little particles hanging around up there in the void of empty space.

Although the definition of a vacuum implies that there is nothing within it, outer space isn’t truly a vacuum.

Well… it’s not an absolute vacuum in any case. An absolute vacuum would indeed be completely barren, but up in space, there are a very small number of particles in the void.

These are predominantly made up of plasma, helium, hydrogen, and a few other things like electromagnetic radiation and neutrinos.

The truth of the matter is, although a vacuum appears empty, they aren’t really empty at all, in the strictest scientific sense, anyway. And it’s from these particles that the energy of the vacuum is created.

However, this energy is extremely small compared to the rest of the universe. Still, the amount of energy it does have is jaw dropping considering there isn’t supposed to be anything going on up there.

**How Much Energy Does The Vacuum Have?**

To find out how much energy the vacuum contains, we need to calculate its energy density.

To do this, we use the equation E mc2. Where m is the mass of an object (in kilograms) and c is the speed of light in meters per second.

Now, the problem here is that we don’t know the mass of the vacuum.

So instead, we will assume that the mass of the vacuum is equal to the sum of the masses of all the particles that fill the vacuum. This is called the Casimir effect.

So now that we know the mass of the entire vacuum, we can plug it into the equation above. Doing this gives us the following result:

E 1.3 x 10-19 J / m3.

Where J is the unit of energy (Joule).

That’s right. The energy density of the vacuum turns out to be around 1.3 x 10^-19 Joules per cubic meter.

**What Does This Mean?**

The energy density of the vacuum is incredibly tiny. In fact, it’s about 100 million times smaller than the energy density of water. That’s some very minor energy, huh?

**Why Does It Matter?**

If the energy density of the vacuum was not as small as it is, then the vacuum wouldn’t be able to hold together. If the vacuum wasn’t holding things together, then it couldn’t form stars or planets.

And if it couldn’t form stars and planets, then it wouldn’t be possible for life to exist on Earth.

In short, the existence of the vacuum is absolutely necessary for life to exist. Without it, there would be no stars and no planets. And without these two things, it’s impossible for life to exist anywhere.

So when physicists talk about the “energy density” of the vacuum, they’re talking about the total amount of energy that exists inside the vacuum.

That means that the energy density of the whole universal vacuum is also included.

Doesn’t this mean that the universe is full of empty space?

No, because the universe is full of matter. Not empty space. There is no such thing as a perfect vacuum. Even nothingness is rife with some kind of invisible goings on.

**Is There More Than One Type Of Vacuum?**

Yes. There are three different types of vacuums:

- The quantum vacuum (the most common type)
- The classical vacuum (which is used in the standard model of physics)
- A hypothetical vacuum called the true vacuum

**Quantum Vacuum**

The quantum vacuum is the most common type of vacuum. It’s the one we live in. It contains virtual particles.

It’s the vacuum that fills up our entire universe.

**Classical Vacuum**

The classical vacuum is the vacuum that scientists use in their calculations. It doesn’t contain any virtual particles. Instead, it contains only real particles.

Scientists use this vacuum to calculate how much energy is present in the universe.

**True Vacuum**

The true vacuum is the vacuum that exists at absolute zero temperature. At absolute zero, there is no heat or motion. So there are no virtual particles.

There are no real particles either. All of the atoms and molecules have been destroyed.

In fact, the entire universe has been reduced to nothing more than an almost perfectly uniform sea of energy.

**Final Thoughts**

The energy density of a vacuum is very important. It can tell us whether or not something actually exists in the universe.

For example, if the energy density of the universe turned out to be higher than what we expected, then we could conclude that some kind of force is acting on the universe.

But if the energy density of a vacuum turns out to be lower than what we expect, that might mean that something isn’t actively happening.

We don’t yet understand why the energy density of the quantum vacuum is so low. But we do know that it’s extremely low. This is because the quantum vacuum is filled with virtual particles.

These particles pop in and out of existence all the time. Whenever they appear, they can both add and detract from the energy of the vacuum.

Thanks to energy density we know a lot more about the universe, and are constantly learning a lot more!

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