I just learned something new. Did my brain just gain weight?

Or, what is the literal weight of a thought?

Assuming for argument that thoughts have weight, would long term memory have more weight than short term? Would all thoughts in long term weight the exact same, or would similar thoughts / concepts share a basic foundation, and piggyback ?


If you assume that, then you already know the answer under that assumption. The assumption isn’t true in the real world however.

Ony you would know. You are the one making cretaing the assumptions for your imaginary world. You are making all this up, make up some weights while you are at it. It won’t be true in reality of course.

That was quite the snark Blake.

I don’t think mere thoughts do much besides shuffle neurotransmitters and ions around, but aren’t memories formed from neurons growing new connections? Those would weigh something.

It used to be commonly accepted that the brain didn’t make new cells starting sometime in early childhood. But now it’s known that the adult brain does in fact create new cells. These new cells seem to be associated with memory, particularly memory of new things rather than the same old things.

So the answer is, yes, your brain did gain weight when you learned something new.

Can we see some evidence for this claim?

Bear in mind that neurons are one of the least dense substances in the human body, much less dense than water, and the skull of an adult is a fixed size. So if a new neuron is produced, it has to displace either a liquid or a solid that was already in the skull. So why why would the growth of a new neuron increase the mass of the brain? If it’s displacing connective tissue, cerebrospinal fluid or glial cells, then it will actually *reduce *the weight of the brain because a neuron is less dense than all those things. If it’s displacing another neuron, there will be no change in mass. It’s hard to imagine what else it could be displacing.

Is CSF considered part of the brain?

Even if it is, it doesn’t change anything. Neurons are less dense than CSF. I just can’t see any way that adding a neuron to the brain can do anything except make it lighter.

This is similar to the “weight of a full/empty MP3 player/flash drive/hard drive” question. Learning something new creates information, and relativity tells us that information has energy, and energy is mass, and the mass value can be calculated.
On a flash drive, we know how many electrons are required for each bit of data, and how they rearrange themselves to store data, and how many bits are available. We are a lot less sure about how this happens in the human brain. We know how much energy the brain uses during activities, but not how much of that is devoted to new memories/new learning/neuronal connection forming.

But, just like the MP3 player question, the point is moot - the information mass of the device is very small, and (practically) immeasurable. The same applies to the brain, and any real changes in mass due to new information would be swamped by the continual mass changes that occur as chemicals are consumed and excreted, water absorbed and flushed, heat produced and removed.


It’s a valid distinction because the question is whether the brain gains weight, not the head.

If CSF is not considered part of the brain and is displaced when a neuron is added, then the brain gains weight, but the total weight of the head decreases.

Good point.

Although there’s still a whole lot of ‘If’ in there - we probably ought to examine some citation on when and whether the brain really does grow new cells.

Cells multiply by division, and this isn’t something that happens instantaneously, so I don’t see how it could be the case that learning something would cause new cells to grow - they’d have to be growing already, or else they would not be ready to facilitate memory of the new information.

The jury is still out on this, but it is looking increasingly unlikely - from a recent article in New Scientist:


Ignoring increases in cell number, what about changes in individual cells? If ion gates or channels have a change in their selective permeability properties, that change may involve a change in weight. Right?

The one that passed through my mind was the “What is the weight of a shadow”, but I concede that yours is closer to my OP.

Not to hi-jack my own, but per the MP3 player, wouldn’t it be a simple matter of flipping bits that are already there? same weight, but it just looks and acts differently? Could any parallel be drawn to the brain?

Relativity isn’t telling us anything about the mass of information when stored in any given medium though - and it’s trivial to define a storage medium in which the mass decreases as the information increases, in at least a couple of ways:

  1. Posit a pair of storage media that differ only in that one uses inverted symbology to the other - if the mass of one increases when information is added, the same information must effect a decrease in the other.

  2. Punch cards.

I have seen reports that neurons are capable of more reproduction and renewal than we previously thought possible; I have not seen reports linking this reproduction to learning or memory.

That is correct. But from a statistical thermodynamics viewpoint, the particular arrangement of bits that represents Helter Skelter, say, is different from the bits that represent 4’33", and that difference is the energy (and thus mass value) of the information stored.

Of course, as Mangetout points out, if I stored those songs on punch cards, 4’33" would be heavier (no punchouts). However, if I shuffled the stacks, I will still have a 4’33" stack, but I’m not getting Helter Skelter back, and it is not turning into While my Guitar Gently Weeps, either.

N.B. I am deliberately oversimplifying, but I hope you get the idea. 4’33", Helter Skelter, and While my Guitar Gently Weeps are all about the same length, and I know that recordings of 4’33" are not completely silent, but for the purposes of this discussion, it works. And I am considering uncompressed raw audio data for simplicity.


Learning probably involves the pruning of synaptic connections just as much as, or more than, establishing new ones. On that basis, one might expect the brain to lose weight rather than gain it. But this is all largely speculation anyway. The fact is that not all that much is yet known, in detail and for certain, about the physical basis of learning in the brain.

In any case, all the hypothetical changes in brain weight due to learning that are being discussed in this thread are bound to be trivial compared to fluctuations in brain weight that are occurring all the time for other quite unrelated reasons.

The OP’s question falls into the category of “silly questions”, that, if you pursue them, are likely to lead you into ever deeper confusion and ignorance about the domain you are concerned with (in this case, how learning happens, and how the brain works). Much of the most creative work in science consists in figuring out which questions are “silly” in this sense, and which are valid and worth trying to answer.

This is preposterous. Thoughts don’t weigh anything. However, some decisions weigh heavily.

i started looking in some books for the answer. i learned some new stuff in doing so.

i will have to resume composing my answer later. typing has become difficult, my neck can no longer support the weight of my head and has plopped down on the table so i’m having to view things at a right angle to type and read the screen. i’ll be back after some neck exercises.