What Is Memory? A Copy That Fades, of a Past That Doesn't

Say you and your sister are telling the story of the family holiday at the lake. You were nine. She was twelve.

In your version, the dog stole the sausages off the grill and half the beach chased him. In hers, it was a seagull, it took the sandwiches, and nobody chased anything. You tell it again, louder. She tells it again, slower. Your mother calls from the kitchen that it rained all week, and you both say it didn't.

At most one of you has the thief right. Maybe neither does.

Here's the thing about that argument: the holiday isn't in it. Whatever happened at the lake happened one way. The sausages went where they went, the rain fell or it didn't, and none of it depends on which of you tells the story better.

So what are the three of you carrying around, if it isn't the holiday? What is memory?

The short answer

Two different things go by the name memory.

The first is the past itself. Every moment you've lived is written in reality's record, exactly as it happened, and no later event changes what's written. That afternoon at the lake is still written, every minute of it.

The second is what you carry: a copy. Your body keeps a copy of some of what happened, in a store of limited size. A store of limited size can't hold everything, so it has to lose most of what it copies. And ordinary remembering isn't looking at the original. Each recall is a new event, written as a new entry after the original, and it can rewrite the copy with whatever the retelling changed. The copy can be wrong. The original never changes.

So forgetting loses the copy, never the past. A false memory is a rewritten copy, never a changed past. And you are not your memories. You are a lasting rhythm, the pattern that had the experiences (Step 5 says what that is), and a lost memory doesn't mean an experience wasn't felt.

Three links in this answer aren't derived yet. Step 1 names one and Step 6 names two, each where it comes up, beside what is derived.

Here's the reason in short. The basic laws of physics never destroy information, and our own work proves that the record only grows, so the past is kept. A store with fewer states than there are possible pasts has to give some pasts the same state, and that's counting. A recall is something that happens, so it happens after the moment it recalls. And you stay you while the material of your body is replaced, so what you are is a pattern, not material. The steps below give each reason in full.

Why this matters

Run these two things together and forgetting feels like the past dying. If your memory were the past, forgetting would be the past dying, and your memory would never be wrong. Neither is so. Forgetting is what a copy of limited size does. A false memory is a copy that got rewritten, and it leaves what happened alone. A blank in your memory is a blank in the copy, and it says nothing about what you lived or felt. And you were never your memories.

The six steps

Here's the whole answer in six steps. The rest of this post takes them one at a time and shows how we know each one.

1. The past is kept. Every moment you've lived is written in reality's record, and no later event changes it. Physics says the information is never destroyed, and our own work proves the record only grows.
2. What you carry is a copy. The past can't be wrong, lost or changed. A copy can be all three, and the case of H.M. shows the two coming apart.
3. A copy in a store of limited size has to forget. Eight possible days can't fit into four states, however cleverly you fill them.
4. Every recall is a new event. It adds an entry after the original, and it can rewrite the copy. Two experiments show it.
5. You are not your memories. You are a lasting rhythm. A lost copy isn't a lost experience.
6. The original has an address. Every moment you've lived has one, and it picks out exactly one page of the record.

Step 1: The past is kept, and nothing later changes it

Every moment you've lived is written in reality's record, and no later event changes it. Call it the record. (Other posts in this series call it the ledger.) Every event writes an entry in it, and the entries for one moment make up a page. The afternoon at the lake is a long run of pages, the best minutes and the dull ones written down the same way.

Here's how we know, in two parts.

What physics says. The basic laws of physics never destroy information. In quantum mechanics this is called unitarity: the present fixes the past exactly, and running the laws backward recovers every earlier state. That's why physicists treat the black hole information problem as a real problem: losing information would break the laws. Hawking's 1976 calculation said a black hole destroys the information that falls into it. That clashed with unitarity, which is why it became a famous problem. Most physicists now expect the information to come back out in the radiation, and the open question is how.¹

What our own work proves. The record only grows. Each new entry is added with every earlier entry kept, and writing a new entry never alters an earlier one.² So a question about an earlier stretch of the record gets the same answer however much is written after it. Take the first five entries of the lake afternoon and ask what entry 3 says. The answer is the same when entry 5 is written, after 50 more entries, and after five million more. It holds for every question about every stretch.

One link here isn't derived yet: that the physical world's change over time is this record being written, so the information the laws keep is this record.

The past still exists. It's written down, exactly as it happened. The thief at the lake is in there too, whoever it was. Part 2 of this series, Time, asks where yesterday is and what "now" is.

Next, and why: if the past is all still there, what is in your head when you remember it? Not the past. Step 2 shows how we know.

Step 2: What you carry is a copy

What you call your memory is a copy your body keeps. It isn't the past.

A copy is a pattern in a physical thing, set by an event, that stands in for the event afterward. Yours is kept in your body, above all in your brain. The original is something else: the event's own entries in the record.

Here's how we know they're two. The past can't be wrong, can't be lost and can't be changed, because Step 1 says every entry is kept and none is ever altered. A copy can be all three.

One case shows the copy and the past coming apart. In September 1953, a surgeon removed tissue from the inner part of both temporal lobes of a man's brain, including the hippocampal region, to treat severe epilepsy. The man is known in the medical literature as H.M. After the operation he no longer formed new lasting memories of events. On 26 April 1955 he was tested. He gave the date as March 1953 and his age as 27. It was April 1955, and he was 29. The researchers also describe how he ate lunch in front of one of them and, half an hour later, was unable to name a single thing he had eaten. He had no memory of eating lunch at all. Yet his early memories seemed normal, though he had lost part of the three years before the operation, and extensive tests of his perception, abstract thinking and reasoning showed no deficit.³

The part of him that makes copies was damaged, and the world kept being written. His days from September 1953 on went into the record like anyone's. They never went into his copy. The past didn't lose them. The copy never took them.

A copy of the present isn't the past. A copy is a state: how things stand at one instant. A state doesn't have to show how it was reached. Two lamps start dark. In one history the left lamp lights first, then the right. In the other, the right lights first, then the left. At the end both are lit, and a snapshot of the lamps can't tell which came first. The lamps are a drawing of a real result, which Do You Have a Soul? states: two histories can end with exactly the same final strengths and different timing, and no rule that reads only those final strengths can tell them apart. The same post shows that one state can be reached by histories that differ without limit. The order lives in the history, and a snapshot of the present doesn't have to carry it.

How the cells of a brain hold a copy is neuroscience's work, and nothing here depends on it. What matters is that the copy is a physical thing in a body. So it has a size.

Next, and why: a copy with a size is a store with limits. What do limits do to what it can hold?

Step 3: A copy in a store of limited size has to forget

Give a store of limited size more possible pasts than it has states, and some different pasts have to leave it in the same state. Once they have, nothing in the store can tell those pasts apart. That isn't a flaw in brains. It's counting.

Here's the counting, with a store small enough to see. Take a store with 2 slots, where each slot holds a 0 or a 1. Two slots make 2 × 2 = 4 states: 00, 01, 10 and 11.

Now feed it a day. This day is three independent yes-or-no facts: did it rain, was the bus late, did Sam come to lunch? "Independent" means that knowing one tells you nothing about the others, so any combination can happen. Three facts make 2 × 2 × 2 = 8 possible days.

Eight possible days, four possible states. Whatever rule fills the slots, some days have to share a state. Say the store keeps "rain" and "late bus" and drops "Sam". Then a rainy day with a late bus and Sam at lunch, and a rainy day with a late bus and no Sam, leave the very same state. Any other rule collides other days instead. Eight can't go into four without sharing.

"Can't you squeeze the day down?" Compression finds patterns. A day of a thousand coin tosses that all came up heads can be stored as "heads, a thousand times". That works because the day has a pattern. Independent facts have none: every combination can happen, so every combination needs a state of its own. The counting doesn't care how clever the rule is. In general, n independent yes-or-no facts make 2 × 2 × ... × 2 possible pasts, with n twos. A store with B slots makes 2 × 2 × ... × 2 states, with B twos. If B is smaller than n, there are fewer states than pasts, and some pasts share a state. To hold the facts exactly, a store needs at least n slots.

A photo shows the same thing. A digital photo has a fixed number of pixels, so it holds the scene down to that many dots and no finer. The scene's finer detail all merges into whichever dot it lands in.

Your brain is a store of limited size too: a finite number of cells and connections. And a life never stops feeding it. Every second brings details that don't depend on each other: the faces on the bus, the sounds on the street, what each person said. For decades. The details outnumber the room, so most of them have to go. Try it. List every car you passed yesterday, and its color. You were there for all of it. Your copy kept almost none.

Forgetting isn't a failure of your brain. It's what any store of limited size has to do.

And what the store loses, reality doesn't. The record still holds every page (Step 1). The Universe Has No Delete Key follows the same merge from the other side: a store merges two pasts into one state, reality never merges two histories, so the difference can't vanish, and the post shows where it goes. Time finds the same split at the bottom level of reality: the parts that run one way forget where they came from, and the record keeps it.

Next, and why: a copy has a size, and a size means forgetting. What happens to the copy when you go back to it?

Step 4: Every recall is a new event, and it can rewrite the copy

Remembering is an event. It adds a new entry after the one it's about, and it never touches the original. And it can reach the copy.

Here's the reason it adds an entry. Recalling the lake is something that happens, and it happens now, after the lake. Something that happens after a moment can't be part of that moment. So the recall isn't inside the original entries. It's a new entry, written at the end of the record, and the original reads exactly as it did (Step 1). Can We Ever Know Everything? works out the same point for any check of a record: the check is an event, and it adds an entry the record didn't hold.

The copy is another matter. Looking at a photo doesn't change the photo. Recalling a memory can change the memory, and two experiments show it.

A drug at the moment of recall. Researchers trained rats to fear a tone, by pairing the tone with a shock. Later they played the tone again, as a reminder, and then put a drug into the amygdala, a part of the brain involved in fear learning. The drug blocks the making of new proteins. On later tests, those rats had lost the fear memory. Rats given the same drug without the reminder kept the memory. So did rats given it six hours after the reminder.⁴ The authors' conclusion is that a stable memory, once recalled, becomes fragile again and has to be stored again, and the drug blocked the storing.

One word in a question. In 1974, Elizabeth Loftus and John Palmer showed 150 students a short film of a car accident. Fifty were asked, "About how fast were the cars going when they smashed into each other?" Fifty were asked the same question with "hit each other" in place of "smashed into each other". Fifty weren't asked about speed. A week later everyone answered a list of questions about the film, and one of them was "Did you see any broken glass?" There was no broken glass in the film. Of the students asked about cars that "smashed", 16 of 50 (32 percent) said yes. Of those asked about cars that "hit", 7 of 50 (14 percent) did. Of those never asked about speed, 6 of 50 (12 percent) did.⁵

The question didn't touch the film. It reached the copy, and the copy kept what the question put there. The authors wrote that, over time, what a person saw and what they were told afterward can be integrated "in such a way that we are unable to tell from which source some specific detail is recalled."

Back at the table, every retelling of the holiday is a new event, and events like that reach the copy. The holiday's own entries are as they were.

Next, and why: the copy is small, it loses things, and it gets rewritten when it's read. Is that what you are?

Step 5: You are not your memories

You are a lasting rhythm, the pattern that had the experiences. A memory is something your body keeps. It isn't what makes you who you are.

Many people dread losing their memories, and memory feels like what makes them who they are. It isn't. Here's the reason, in three parts.

1. What you are. Your body's material is replaced continually. Much of it turns over within months to years, and you remain you.⁶ So what stays isn't material. It's a pattern.

At the bottom level, below atoms, one small process writes the record. The smallest event is one thing told apart from another. You're doing it right now: your eyes are telling these dark marks from the white page. Telling two things apart takes information about both, and the least information there can be is one yes or no. So the smallest event takes two things and one yes or no, and the process is two units passing a bit, one yes or no, back and forth. Each pass is one step in the writing of the record.

That process can start 64 ways. Each unit is full (holding the bit) or empty, and has one of four settings, so it has 2 × 4 = 8 ways to be, and two units make 8 × 8 = 64 starts. All 64 are checked. Within three steps, 60 of them fall into random silence: a silent step, where nothing flows between the units, and from then on coins decide. Exactly 4 never do. Those four never go silent. An unbroken run like that is a rhythm, and those four are the only patterns at that level that last.⁷ You aren't a pair of units. A person is a vast web of them that never goes silent anywhere: one rhythm, one consciousness. Billions of people are billions of such webs. The four are the building pattern, not the only four minds. So you are a lasting rhythm in the record, and you are not your body. What You Are walks through the whole count.

2. Memory fails as a test of who you are. Steps 3 and 4 already show why. A store of limited size gives different pasts the same state, so two different lives can leave the same copy, and the copy can't tell them apart. A store loses things, so a stretch of a life can leave a blank copy, and the rhythm doesn't depend on the copy. Whether a copy is there tells you about the copy. It doesn't tell you who was there.

3. A blank copy isn't a blank experience. Not answering, not remembering, or a test that catches only a few moments: none of them shows that nothing was felt. You've met this one yourself. The Drive Home You Don't Remember starts with twenty-five minutes of driving that you can't recall. You stopped at every light and gave the cyclist room, and your copy has none of it. Why Your Life Flashes Before Your Eyes starts from the same point: when someone doesn't answer, or doesn't remember afterwards, you've learned something about answering and remembering. You haven't learned that nothing was felt. Where Do You Go When You Sleep? applies it to every night: a blank memory of the night doesn't show a blank night. H.M. ate lunch, and half an hour later his copy of it was gone. The lunch had happened, and so had the eating.

So a lost memory is a lost copy, and nothing more. The past is kept, nothing about the loss shows that the experience wasn't felt, and you are still the rhythm that had it. That doesn't make a lost memory nothing: the copy was yours, and it's gone. It says exactly what was lost.

Next, and why: everything you lived is in the record, and you carry only a copy. Can you read the original?

Step 6: The original has an address

Every moment you've lived has an address, and the address picks out exactly one page of the record.

If the past is all still there, why do you remember through a copy instead of reading the page? The answer has a derived part and an open part.

Derived. The record keeps every page, and later events never change an earlier page (Step 1). A reading of a page is a new event, so it adds an entry and leaves the page as it was (Step 4). And the right pair of beats gives every moment an address: a label that's different for every page and never repeats, as a page number is. Time builds the label from two beats whose speeds aren't a fraction of each other, so they never fall back into step and no two moments ever get the same one.⁸ Why Your Life Flashes Before Your Eyes works an example with numbers. A rhythm that carries such a pair, together with the length of its own record, has an address for every moment it has lived, and each address picks out exactly one page.

Not derived yet. Two links. The first is that your own rhythm carries such a pair of beats: the address of every moment you've lived rests on it. The second is how a rhythm actually performs a reading aimed at one moment of its own record. The address says which page. It doesn't yet say how a rhythm turns to that page and reads it.

Why the copy and the page stay apart. Ordinary remembering reads the copy, not the record, and Steps 2 and 4 show how we know. The record holds H.M.'s lunch, and half an hour later his recall found nothing. There was no broken glass in the film, so there is none in the record of watching it, and 16 of 50 students recalled some. Recall gave back what the copy held, gaps and additions included. The page itself is still there, it has an address, and it holds, exactly as written, every detail the copy lost, because nothing ever changed it.

What's still open

Three links aren't derived yet.

• That your own rhythm carries a pair of beats whose speeds aren't a fraction of each other. The address of every moment you've lived rests on it. Derived beside it: such a pair gives every moment a label of its own that never repeats, and the label picks out exactly one page.
• How a rhythm performs a reading aimed at one moment of its own record. Derived beside it: the record keeps every page, later events never change an earlier page, and each address picks out exactly one page.
• That the physical world's change over time is this record being written. Derived beside it: the record only grows, and writing an entry never alters an earlier one. Physics agrees on the part it covers: unitarity says the information about the past is never destroyed.

The whole trip, in one breath

Every moment you've lived is written in reality's record, and nothing written later changes it. What you carry is a copy in a store of limited size, so it has to lose most of what it copies, and that's counting, not failure. Each recall is a new event that adds an entry after the original and can rewrite the copy, so a retelling can get into the copy while the original stays as it was. So forgetting loses the copy and never the past, a false memory is a rewritten copy and never a changed past, and a blank in your memory is a blank copy and never proof of an unfelt experience. You aren't your memories. You're the rhythm that had the experiences. And every page of what you lived is still there, each with an address of its own. That's how reality is built: a record that keeps every page and changes none, and copies of it that fade.

You and your sister can argue about the holiday for the rest of your lives. The holiday won't take sides.