Although it is commonly accepted that learning and memory occur via enduring changes in neuronal properties such as synaptic strength within a network of neurons, many details of these processes remain unknown, including the mechanisms responsible for the persistence and maintenance of memory over long periods of time."

Source : Learning & Memory journal (special issue) 2021. 28:

There is no agreed theory to account for how memories can come to be forgotten.

The 'Decay Theory' - or the 'Law of Disuse' - states that if a person does not access and use the memory they've formed, the memory trace will fade or decay over time. It was first proposed in 1914. At the time, there was no physical evidence that this was the case, and the evidence is still patchy even today. Since long-term memories can be extremely stable, it was also proposed that they retain their stability by being (somehow) reinforced, or 'rehearsed' over time.

But, as researchers were pointing out as early as 1932, there isn't any hard evidence that disuse causes memories to fade.

The thesis is advanced that the law of disuse cannot account for the major phenomena of forgetting; first, because it lacks generality, since disuse often fails to produce forgetting; second, because even where forgetting and disuse are correlated, there is no evidence that it was the disuse that caused the forgetting, instead of other important factors which were present; third, because the principle of passive decay has no analogue anywhere else in science, and is illogical; and fourth, that experimental work with retroactive inhibition shows that forgetting varies with interpolated conditions rather than with disuse."

Source : Psychological Review, (1932) 39(4), 352–370.

The forgetting of short-term memories also has no direct physical explanation.

Is forgetting in the short term due to decay with the mere passage of time, interference from other memoranda, or both? Past research on short-term memory has revealed some evidence for decay and a plethora of evidence showing that short-term memory is worsened by interference. However, none of these studies has directly contrasted decay and interference in short-term memory in a task that rules out the use of rehearsal processes."

Source : Exp Psychol Learn Mem Cogn. 35(2): 317–333.

It's now known that the many of the neuro-active molecules involved in the maintenance of memories only have an active operational window of a few hours, leading to the current theory that there must be some mechanism - known as Long-Term Potentiation (LTP) - which 'refreshes' or preserves the neuronal connections involved in memories. Nevertheless :

How molecules lasting only hours to days can maintain memory that persists weeks to years is a long-standing fundamental question in neuroscience."

See : Science Advances, Vol. 10, No. 26.

The study cited above found that chemicals which block the activity of a highly specific molecule involved in memory reinforcement - called PKMzeta - can result in memory loss (at least for short and medium term memories in rodents).

Also see : Memoryplugin-autotooltip__plain plugin-autotooltip_bigMemory

inexplicable

"Although it is commonly accepted that learning and memory occur via enduring changes in neuronal properties such as synaptic strength within a network of neurons, many details of these processes remain unknown, including the mechanisms responsible for the persistence and maintenance of memory over long periods of time.

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