Understanding Absence of a Microscopic Arrow of Time

In summary: The weak interaction is symmetric under time inversion, yet the entropy of the universe increases as it expands and cools down.In summary, at the particle interaction level, we cannot distinguish a preferred direction of interaction, an arrow of time as they say. However, in annihilations, there is a manifest disparity between particles before (massive fermions) and after (photons) an interaction, resulting in an inferred directionality. The process of pair production, which is the reverse of annihilation, is possible but highly improbable due to the precise timing and energy requirements. The weak interaction, while violating time-inversion invariance, does not have an effect on the thermodynamic arrow of time. Temperature plays a significant role in particle creation and ann
  • #1
Islam Hassan
233
5
At the particle interaction level, we cannot distinguish a preferred direction of interaction, an arrow of time as they say.

I do not understand this if i) in annihilations, there is a manifest disparity between particles before (massive fermions) and after (photons) an interaction and therefore an inferred directionality and ii) in weak interactions there is an asymmetry in the interaction process.

What am I missing here?IH
 
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  • #2
Islam Hassan said:
there is a manifest disparity between particles before (massive fermions) and after (photons) an interaction and therefore an inferred directionality
How so? That reaction is reversible: google for "pair production"/
 
  • #3
Islam Hassan said:
in annihilations, there is a manifest disparity between particles before (massive fermions) and after (photons) an interaction
Annihilations are not the only process. There is also two photon pair production, which is the time reverse of anhilation
 
  • #4
To take annihilations, their reversal would I believe require a number of sufficiently energetic photons to converge to a single, very precise microscopic location with very precise microscopic timing. Their combined energy must then be exactly equal to the energy need to produce one given particle only and its antimatter particle only.

If I understand correctly what you are saying is that this process remains possible despite a very low, perhaps infinitesimal probability and that this is why there is no microscopic arrow of time?IH
 
  • #5
Ok, I have read the wiki on pair production...only a single boson is needed to do this. I had thought that annihilation produces many more photons than one, a big burst of photons.

Thanks for the feedback.

What about the weak interaction though?IH
 
  • #6
Islam Hassan said:
Their combined energy must then be exactly equal to the energy need to produce one given particle only and its antimatter particle only.
This is false. As long as the energy is sufficient to be above threshold, any excess energy will go into kinetic energy of the resulting electron and positron.

Islam Hassan said:
Ok, I have read the wiki on pair production...only a single boson is needed to do this. I had thought that annihilation produces many more photons than one, a big burst of photons.
A single photon cannot pair produce by conservation of energy and momentum. The energy and momentum is typically taken form a nearby nucleus by exchange of a virtual photon.
 
  • #7
Orodruin said:
This is false. As long as the energy is sufficient to be above threshold, any excess energy will go into kinetic energy of the resulting electron and positron.A single photon cannot pair produce by conservation of energy and momentum. The energy and momentum is typically taken form a nearby nucleus by exchange of a virtual photon.
Thanks for the clarification. Is pair production the same as annihilation? Or can annihilation produce a multiple photon burst?IH
 
  • #8
Temperature plays a very big role. At any temperature, an equilibrium will be reached between particle creation and annihilation. The equilibrium point varies with temperature.

Susskind's course on Cosmology, lectures 7 and 8, discuss genesis of particles as a function of temperature during the origin of the universe around the time of the big bang. He also disusses the asymmetries.

Here is lecture 7, watch both 7 and 8.


Islam Hassan said:
Is pair production the same as annihilation? Or can annihilation produce a multiple photon burst?
Annihilation of what? Not all particles are alike. Not all decay or creation events are alike.

Islam Hassan said:
What about the weak interaction though?
Search for CPT symmetry.
 
  • #9
Islam Hassan said:
Their combined energy must then be exactly equal to the energy need to produce one given particle only and its antimatter particle only.
No. Any additional energy in the photons will become KE in the particle pair.
 
  • #10
The exact inverse reaction to e.g. ##e^- e^+ \to \gamma \gamma## is ##\gamma \gamma \to e^- e^+##. While it is quite easy to get the first reaction the second one is much more challenging for various technical reasons, but it exists and it has been measured in accelerators (with some caveats about the photon sources).

The inverse reaction to ##\gamma + N \to e^- e^+ N## where N is a nucleus would be ##e^+ e^- N \to \gamma N##. While possible, it would need the three particles to come together at the same time and with suitable conditions for momentum and energy. That is extremely unlikely.

Edit: Some more.
If you want to reverse a decay, e.g. of the Higgs boson, ##H \to \gamma \gamma##, then you have to carefully tune the photon energies for ##\gamma \gamma \to H## (or try often enough with a larger range of energies). To make that more realistic, particle accelerators will often look for final states with more than one particle, e.g. ##e^- e^+ \to H Z## as this doesn't require a tuning that precisely (it also has some other advantages not relevant here). This was the reaction LEP hoped to find, but it turned out that the Higgs was just a few percent too heavy to be produced at LEP.
 
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  • #11
Islam Hassan said:
What about the weak interaction though?
Even though it violates time-inversion invariance, this violation has nothing to do with the thermodynamic arrow of time, according to which entropy increases rather than decreases.
 

1. What is the concept of a microscopic arrow of time?

The concept of a microscopic arrow of time refers to the idea that at the microscopic level, there is a preferred direction of time. This means that microscopic events, such as the movement of particles, are more likely to occur in one direction than the other.

2. How does this differ from the macroscopic arrow of time?

The macroscopic arrow of time, which is more commonly known, refers to the phenomenon of time moving forward in a specific direction. This is based on our perception of events, such as aging and the flow of time on a larger scale. However, the microscopic arrow of time deals with the behavior of particles at a very small scale.

3. What evidence supports the absence of a microscopic arrow of time?

One of the main pieces of evidence for the absence of a microscopic arrow of time comes from studies of subatomic particles, such as electrons and protons. These particles have been found to exhibit symmetrical behaviors in both the forward and backward directions of time, suggesting that there is no preferred direction at the microscopic level.

4. How does this concept relate to the laws of thermodynamics?

The absence of a microscopic arrow of time is closely related to the second law of thermodynamics, which states that entropy, or disorder, always increases in a closed system. If there were a preferred direction of time at the microscopic level, this law would not hold true. The absence of a microscopic arrow of time supports the idea that the laws of thermodynamics apply equally in both directions of time.

5. What are the implications of understanding the absence of a microscopic arrow of time?

Understanding the absence of a microscopic arrow of time has significant implications for our understanding of the fundamental laws of physics and the nature of time itself. It challenges our traditional understanding of cause and effect, and may lead to new discoveries and theories in the field of physics.

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