# Time reversibility of electromagnetism in Incandescent light bulb

## Main Question or Discussion Point

Hi
This might seem odd but I need to explain my friend on this
Friend and I agreed that Classical physics is time reversible (i.e. laws remain same if time is reversed.. e.g. gravitational pull on a projectile.) only exception is II law of Thermodynamics (which talks of irreversible decrease in differentials of two interacting systems)

But coming to electromagnetics we hit a road block

An incandescent bulb glows by joule heating - i.e electrons from a battery are hitting ions in tungsten and that causes excited tungsten ions to realse photons.

But if we reverse time and assume the laws to hold the same, then tungsten can set electrons in motion by absorbing photons. But in real world this means that we can have light bulbs acting as photovoltaic cells !

But Classical physics is time reversible on non-quantum macro scale (isnt it ?). Which means light bulbs can be photovoltaic cells. so we are stuck there. Can we solve this without touching on quantum mechanics ?

## Answers and Replies

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bcrowell
Staff Emeritus
Gold Member
As a philosophical aside, I would not describe the second law of thermodynamics as an exception to time-reversibility. I would describe it as a statement about the initial conditions of the universe (the Big Bang was a low-entropy state), which leads to a statistical fact about which processes actually occur now.

Anyway, an incandescent bulb takes electrical energy and turns it into heat (~90%) and light (~10%). Time-reversing it violates the second law of thermodynamics for exactly the same reason that you can't time-reverse the process of burning a candle. In practical terms, you can't manipulate all the molecules and photons into a time-reversed version of the final state, because the system is too sensitive to the initial conditions.

DarioC
Gold Member
The mathematics of Physics seems to make no distinction in the direction that time flows, but the real world doesn't work that way. Some math represents the way things work, but some math doesn't. Until it is proven it is just marks on paper.

One of my favorite thoughts along that line is that the smallest "unit" of time must be the time that it takes for some physical object or energy level, somewhere in the universe, to change from condition A to condition B. Nothing else has had "time" to change yet, only this ONE item. If nothing is different, then no time has passed.
DC

Last edited:
As a philosophical aside, I would not describe the second law of thermodynamics as an exception to time-reversibility. I would describe it as a statement about the initial conditions of the universe (the Big Bang was a low-entropy state), which leads to a statistical fact about which processes actually occur now.

Anyway, an incandescent bulb takes electrical energy and turns it into heat (~90%) and light (~10%). Time-reversing it violates the second law of thermodynamics for exactly the same reason that you can't time-reverse the process of burning a candle. In practical terms, you can't manipulate all the molecules and photons into a time-reversed version of the final state, because the system is too sensitive to the initial conditions.
Thanks Ben

The mathematics of Physics seems to make no distinction in the direction that time flows, but the real world doesn't work that way. Some math represents the way things work, but some math doesn't. Until it is proven it is just marks on paper.

One of my favorite thoughts along that line is that the smallest "unit" of time must be the time that it takes for some physical object or energy level, somewhere in the universe, to change from condition A to condition B. Nothing else has had "time" to change yet, only this ONE item. If nothing is different, then no time has passed.
DC
Did u mean Plancks unit of time ? Thanks for reply