It's on.

It's that time of year when we have to think about spooky things.

And Brady, you've set, is the challenge in physics of coming up with something physics See that can represent Halloween.

Not easy.

But in the world of particle physics, which is a rather unusual world, we have ghost particles.

What ghost particles?

They're bad things, right?

Really bad things that can be good.

And that could be bad.

The difference between the good ones and bad ones is that the good ones don't exist.

Israel particles they just virtual partners.

He can grab one.

Where's the bad ones do on dhe?

Some of them make a theory consistent on others, completely destabilize a theory and said the good ones make the theory consistent.

And if you have the bad ones than the fear becomes inconsistent on usually is thrown out.

Let's talk about the good ones first.

In particle physics, we deal with very small things, were interested in interactions and so we might have a group of particles initially which interact to give me a group of particles at the end, and they don't have to be the same.

But What we're interested in is a way of describing that interaction to go from this initial state to the final state on.

There are a number of approaches to that, and one of the most famous that's been developed initially by direct but then really pushed forward by a fine man was called The Path Integral formalism.

So we have various ways of doing it.

There's might be one path that will take me this way, but perhaps it's not unique.

Maybe there's another way of getting to that state and goes up here and comes down.

Baby goes down here and comes up.

In fact, there's an infinite number of ways of doing this on what Fineman said was, If you want to work out the actual amplitude or the probability of starting here and ending here, you can't just take what is called the classical path.

The path that were used to from classical mechanics like Newton told us, You have to include all of these other weird and wonderful trajectories.

Now we want to use the Path Central to describe real physics.

We want to be able to use it to describe the interactions the forces that were used to electromagnetism.

We want to understand the weak force, You know.

Why is the sun burning?

We want to understand the strong force wire.

Why is the nucleus of the atom bound together?

Why don't the protons blow themselves apart?

Why they bound together, wire quarks bound together.

This requires us to understand that an area of particle physics called gauge theories.

Well, what the good ones do is they eat up other things that are all physical.

So in physics you have and theories which have lots of symmetry.

So you want lots of things look the same.

So there's lots of sort of redundant ingredients on what the air, the good ghost come along and do is they eat them off essentially.

But they don't exist.

Israel particles, so they don't actually screw with the With the real world, they just they just help us.

The quantity, certain types of three gauge theories.

You introduced the ghost particles and they cancel out all the extra ones leaving you with just the physical ones left.

And so it's a way of regularizing your theory and making itself consistent.

So these are the good particles.

Yeah, I guess there.

Really?

In this case, they really are just a mathematical trick.

Because, as I say, you've got You got these theories which have the symmetries, these redundancies of information on dhe You've got to when you actually contacted there, you've got to remove those redundancies and use this mathematical trick of introducing this ghost particle that comes along, sees the actual degrees of freedom, the extra redundant bits and eats them.

And then they got So you just left with what's physical?

Our ghost particles.

Real things?

No.

But if ghost particles do something, they make something work.

Then Julie must exist so that the ghost particles that are needed for the consistency of this model are virtual particles.

They will.

They take part in an interaction, but you'll net you never able to actually pin them down.

So they're never part of the final step.

If if you say what does an experiment measure and experiment measures an initial state and some final state on, then things happen in between.

The ghost particles pop up and disappear again in that into intervening period.

They're definitely there in sense of their virtual particles that pop up and disappear again.

but they're not.

They're in the sense of you detecting them so they actually particles while you give them the name particle because they have properties that a similar to particles, you can associate various numbers with them.

You can associate a momentum with them you can associate amassed with them.

If it turns out these, um gauge feels that we were describing acquire a mass, which they do and and what's known as the Higgs symmetry breaking phase transition, where all particles acquire mass.

These particles will also acquire a mess.

They have many properties of conventional particles.

It's just that their virtual ones that link you from your initial state your final step.

That sounds really wishy washy.

To me, that sounds like something humans have made up to cover for something they don't understand.

No, no, no.

It's not wishy washy.

Remember what the ghost particles of their T help make this path integral approach.

Consistent, Huge, huge questions about the path in tickle approach Indeed, at the foot.

For mathematicians, it's not a very well defined concept.

For various reasons, it's it's not that well defined physically, it's brilliant.

It works amazingly well.

It produces fantastic accuracy in terms of comparing theoretical predictions of quantum electrodynamics, Electra Week theory, comparing it with observations.

The bad case where they really are.

Bad things about a crucial difference between these and the other ones is that is that they basically that they can exist.

Israel particles on dhe.

What's so bad about it?

If you want it to be quantum mechanically consistent, then they have to carry negative kinetic energy on dhe.

That's really bad.

So what?

That means this is essentially, If you and I was a run along on the football field, a CZ we have in the past there, Brady and you know, the faster we run, the more kinetic energy way have.

If a ghost particle was doing that, he would have more negative kinetic Angie.

The faster he ran, the more negative it's kinetic energy would be come at school were taught that the energy is usually made up of the kinetic energy.

That's the energy of movement costs, potential energy on.

Do you always think of these is positive?

So if you just take this this little blob of space here right right in front of me, then you could produce on the one hand, you could produce their unordinary particle like an apple that's carrying negative that's moving around.

That's carrying positive energy.

But at the same time, you could produced a ghost who produced the Apple in the Ghost, The Apple, Carrie's Positive Kinetic energy.

The Ghost carries Nagel negative kinetic energy and the two together cancel each other out, all right, so you could actually produce these without any cost of energy.

These fluctuations these ghostlike particles on the cold Ghost.

Because of the negative kinetic energy, these ghostlike particles could be a couple tow other particles on.

Did you at no, at no cost.

You just keep producing ghost particles in these extra particles.

You just bombard the universe with these extra particles, so there's nothing to stop that happening.

And actually, what happens is these will constantly get produced in these two pairs.

Okay, if usually apples and goes the other thing that goes on, it'll happen everywhere, and eventually the room will be rapidly filled up with positive energy, apples and negative energy ghosts and you completely haven't unstable situation on that is that's an instability in the theory.

That solution you've got is clearly an unstable when it doesn't want to stay there.

And so people tend to use this criteria.

This existence of this ghost particle in the second version has a as an instability in the theory, and you discount the theory.

So, yeah, your theory might predict their existence.

If it does, it's a bad prediction.

It's not a prediction you want feel theory.