# Is information lost if a particle is annhilated by it's antiparticle

#### BernieM

If a particle (let's call it Z) is happily minding it's own business, and a particle-antiparticle pair spawns in it's vicinity (let's call them particle A and B, created from 'zero point' energy), and it just so happens that the antiparticle (let's say B) collides with particle Z before it is annhilated by it's co-particle (A), is the information particle Z was carrying lost from the universe? Or is the information somehow passed on to the 'replacement' particle (A) that did not get destroyed?

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#### tom.stoer

No information is lost.

I am not familiar with quantum information theory, but the relevant mathematical issue is whether time evolution is unitary or not. An example where information seems to be lost is the black hole (Hawking) radiation where the time evolution is given by

"pure quantum state = single ray in Hilbert space" → "mixed state = thermal density matrix"

This is not allowed in quantum mechanics and was the starting point of the discussion regarding black hole information paradox.

In the proccess you are describing there is a perfectly unitary evolution like

"pure quantum state = single ray in Hilbert space" → "new pure quantum state = new single ray in Hilbert space"

The two states are different b/c they describe different particle content, but the time evolution is unitary and therefore no "information" is lost.

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#### Demystifier

2018 Award
When particle and antiparticle annihilate, they crate two new particles (photons), which take all the information. So information is not lost.

#### haael

The remaining particle keeps information from the original particle. What you are describing here is actually one way of looking at quantization and uncertainity, most similar to the path integral approach.

#### Meselwulf

If a particle (let's call it Z) is happily minding it's own business, and a particle-antiparticle pair spawns in it's vicinity (let's call them particle A and B, created from 'zero point' energy), and it just so happens that the antiparticle (let's say B) collides with particle Z before it is annhilated by it's co-particle (A), is the information particle Z was carrying lost from the universe? Or is the information somehow passed on to the 'replacement' particle (A) that did not get destroyed?
Charge conservation forbids any information is lost.

#### Dickfore

What is "information" and how much does a particle carry?

#### Meselwulf

What is "information" and how much does a particle carry?
Well if we are talking about two photons, the information they carry are intrinsic properties, such as angular momentum, ect.

Photon-photon collision is just a very special type of decay process which can carry on the information when it creates two new particles and it's the observable properties which we could call the information.

#### Meselwulf

Photon-photon collisions don't always create matter particles however, this has a special name, called parapositronium, in the case of creating an electron and positron.

#### Dickfore

Well if we are talking about two photons, the information they carry are intrinsic properties, such as angular momentum, ect.

Photon-photon collision is just a very special type of decay process which can carry on the information when it creates two new particles and it's the observable properties which we could call the information.
So, are you identifying "information" with angular momentum, and then deducing that "information" must be conserved because there is a law of conservation of angular momentum?

If that is the case, then I don't really see the need for the term "information" because it carries the same content as the term "angular momentum". If that is not the case, then I don't really understand your explanation of the term "information", I guess.

#### tom.stoer

I tried to explain what "information" and "information loss" could mean in the context of quantum mechanics; it's about unitary and non-unitary time evolution:

No information is lost.

I am not familiar with quantum information theory, but the relevant mathematical issue is whether time evolution is unitary or not. An example where information seems to be lost is the black hole (Hawking) radiation where the time evolution is given by

"pure quantum state = single ray in Hilbert space" → "mixed state = thermal density matrix"

This is not allowed in quantum mechanics and was the starting point of the discussion regarding black hole information paradox.

In the proccess you are describing there is a perfectly unitary evolution like

"pure quantum state = single ray in Hilbert space" → "new pure quantum state = new single ray in Hilbert space"

The two states are different b/c they describe different particle content, but the time evolution is unitary and therefore no "information" is lost.

#### Q-reeus

How does the notion of 'conservation of information' as absolute law of physics gel with the consensus belief in an inflationary phase big-bang that began presumably from a Planck scale or smaller region? One can seriously believe that the entire information content of current universe was all there in that infinitesimal embryo?

#### Meselwulf

So, are you identifying "information" with angular momentum, and then deducing that "information" must be conserved because there is a law of conservation of angular momentum?

If that is the case, then I don't really see the need for the term "information" because it carries the same content as the term "angular momentum". If that is not the case, then I don't really understand your explanation of the term "information", I guess.
No.

I am trying to be as simple as possible. No more, no less.

#### Meselwulf

Information is a gathering of observables in nature. Remove but one of these observables then you loose information in that system, and thus... in the universe at large when you consider global black holes, for instance.

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