Do Antiparticles Move BACKWARDS in Time?

Antimatter is just like normal matter - except all antiparticles have the opposite charge to matter particles. But why do some physicists think of antiparticles as travelling backwards in time?

In this video, we will be looking at two particle interaction processes that commonly occur in our universe. Then, we will see what links these two processes, despite initially looking quite different. Finally, we'll learn a very basic (but not very rigorous) way to understand the notion that antiparticles move backwards in time.

The first process we will study is Compton Scattering. This occurs when a photon interacts with an electron. A photon carries some amount of energy, related to the wavelength of the source of EM waves from which it was created. The larger the wavelength, the less energy it carries. It's worth noting though that all photons travel through space at the same speed - the speed of light.

So when a photon meets an electron, the photon can transfer some energy to the electron so that it starts moving through space. As a result, the original photon is said to be "absorbed" by the electron, and a new photon is released. This new photon has energy equal to the original photon's energy minus the energy given to the electron. This way conservation of energy is obeyed. But also, conservation of momentum is obeyed too! The two new particles (new photon and electron) move through space so that the original photon momentum matches the electron and new photon's momenta.

The second process looks at the interaction between an electron and its antiparticle, the positron. All particle that have the same values for all possible descriptors, e.g. mass, charge, spin, etc., are said to be the same type of particle. All electrons have the same mass, charge, etc. But if we take a particle and we now consider another one with all the same descriptors except the opposite sign of charge, then we are looking at the original particle's antiparticle. In other words, the positron has all the same properties as the electron except it is positively charged rather than negative.

When a particle and its antiparticle meet, they annihilate each other and two photons are released. These two photons combined have the same amount of energy as the initial two particles had. This process is known as pair annihilation. And aside from involving similar particles to Compton Scattering, it seems to be quite different to Compton Scattering.

However, if we use "Crossing Symmetry", we see that these two processes are not so different. When we write any of these processes in equation form, we can take any of the particles on either side (the reactant/before side or the product/after side) and move it to the other side of the equation, provided we turn it into its antiparticle. This is explained in more detail in the video.

If we start with pair annihilation and move the positron to the other side, while doing the same with one of the photons (using Crossing Symmetry of course), we end up with Compton Scattering! These processes are said to be inherently the same "process".

But more importantly, Crossing Symmetry allows us to consider the idea that a particle on the "after" side of any equation would turn into its antiparticle on the "before" side. Since an equation showing particle interactions allows us to see how particles behave over time, we can therefore imagine that the antiparticle is moving backwards in time relative to the original particle. This is one of the very basic explanations of why antiparticles can be though of as moving backwards in time compared to their particles.

In reality though, the mathematics of particle physics makes it difficult to differentiate between a particle moving forward in time, and its antiparticle moving backward in time. That's the true reason for this notion - though it doesn't mean antiparticles do actually move backward in time! It's just a cool thing to think about.

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Timestamps:
0:00 - Particle Physics - Two Processes That Are Surprisingly Similar
0:50 - Process 1: Compton Scattering
2:30 - Antiparticles: The Very Basics
3:56 - Process 2: Pair Annihilation
4:26 - Crossing Symmetry
6:15 - Do Antiparticles Move Backwards In Time? A Visual Analogy

[sjzara]

I have found a simpler way to show the equivalence of particles and antiparticles with time reversal is that Feynman diagrams can sometimes be valid if rotated.

[ArvinAsh]

Well done my friend. Simplification of a very complex idea.

[bxlawless]

Parth, you have a very cheat and logical mind that makes your instruction so easy to understand. You also pause at the current times so we can digest your thoughts. Very well done!

[LookingGlassUniverse]

This was a really cool idea! And you explained it so clearly in 8 minutes too

[bartpastoor]

Thanks Parth for explaining this. Always found it strange anti particles traveling back in time . The concept of crossing symmetry you explained made it clear how to interpret this.
In a state of half awake half sleeping last night, this crossed my brain few times 😊

[akbarahmed]

You have managed to explain this complex topic it in a very simple way. Nice! I am currently taking Particle Physics course in my final year of UG

[dinodinoulis]

Easier to understand than a Christopher Nolan movie. :)

[JamesMolony-um]

Excellent explanation of a tricky concept. Thank you my students found you very easy to understand!

[colchi]

It would be amazing if you did full lectures on Physics. Similar to what Walter Lewin did back in the day but with your modern animations and great explanations. Cheers!

[luispedrokraemer]

Love your videos. Greetings from Brazil

[Tyletoful]

Your videos are truly the best resource to clearly explain these ideas. I have listened to hundreds of hours of podcasts and pop physics books and was always lacking an "understanding" of what certain quantum phenomena and when I'd search on youtube I could get close to an understanding, but definitely didn't get as close as I am after watching your videos. Please keep producing content, I'm sure your channel will explode.

[BrynSCat]

Pauli exclusion principle can be interpreted as temporal direction.
A positron going backwards in time is also an electron.In Grassman Algebra
a field is a CatHole.(wormhole-wormhole with 360 twist)

[Dr_Jeff]

Alongside Feynman diagrams this gives good background to the concept of antiparticles being particles travelling backward in time; in Feynman diagrams you just rotate the diagram, here we rebalance the particle equations.
antimatter

[KaliFissure]

If time was a compact dimension, spinning, a single Planck second shared by all points in space then we can evolve via kuramoto synchrony a phase hyperplane of the present. Time spinning in the plane. Flat. One side spinning one direction, the other side spinning (apparently) the other. If the axis of spin becomes out of plane of then you have inflows and outflows. Charge. An inflow on one side is an outflow from the other. Clockwise on one side is counterclockwise on the other. Antimatter is the other side of the temporal manifold.
This explains electron(and positron) half spin.
An electron does one orbit on this side as an electron but then has an internalized orbit on the other side as a positron. An electron here is a positron there. Conservation at every point in space.
The manifold itself is like a layer of photons but propagating 90° to us. An event horizon is vacuum energy laid bare. That's why it's black. The light is 90° to us.

[apollo]

Antiparticles may be inverted particles. As an analogy an animation of a four dimensional tesseract hypercube imagining the inner expanding cube consisting of matter and electrons which twist in a space-time dimension, so that the inner cube passes through the outer cube in the fourth dimension, therefore not colliding in a neutral spin orientation in a continuous process, at the speed of light. The edges of the inner and outer cubes could be denoted by Feynman arrow fermions that would constantly changing direction, and the corners of the hypercube might be analogous to bosons.

[JxH]

"Where did all the Big Bang's primordial antimatter go?"
"Backwards in time."
"Huh? Where...?"
"27.54 billion years <- that-a-way."

Perhaps a silly explanation, but at least it's an explanation.

[PenandPaperScience]

Great video! I'm curious what software you use to make the animation :)
Keep it up!

[iburiedmyguitar]

@Parth G An anti-particle is a particle in opposite coordinate and of opposite charge.

[ruhnke]

Hey Parth! I'll realy like to understand the Feynman's Diagrams. What about videos on the subject? Thanks

[KaliFissure]

C is one Planck length per Planck second. But Planck size varies inversely to energy density. So when energy density is higher the Planck are smaller and c seems to propagate more slowly. Time dilation is an artifact of Planck size. Time is always one Planck rotation per Planck second.