Momentum–Point-Perplexity Mechanics in Large Language Models

The project is ambitious and cool, with obvious physics and AI safety relevance. If conserved energy really does measure the trade-off between perplexity (in a sense, model uncertainty) and kinetic energy (in terms of the ‘velocity’ of the hidden state), these results can be turned into actionable control methods, but the idea needs a comb through and some serious stress-testing.

The write-up is overall well done, but there are some important edits that should be made. Some examples: ‘time’ is never defined (I assume this is the position in the context window, but it could also be through the layers, or after every training step). If it is the context position, this picture is somewhat muddied by the parallel processing that transformers carry out by design (though maybe there is the potential to treat this statistically instead of as a literal trajectory of the inference process). Sometimes the language (Lagrangian which contains log terms, or ‘log lagrangian) was confused. This led to some issues, including a bug in the code that treated the resultant energy as additive rather than multiplicative. While this doesn’t necessarily make the project worse, it does make the results harder to judge. I look forward to the follow up.

The appendices were also very repetitive in content, adding to an already pretty hefty problem of length. I would have liked to have seen one of these directions carefully fleshed out within the body of the paper, and left the rest for follow-up work.

Interesting to define velocity as change in hiddent state across sequence position. One confounder is that a (potentially) large fraction of the difference can be explained by the token, especially at early layers. This work trains a per-token SAE, where the relevant part is training a per-token bias term to explain the residual stream (given the current token of course). It'd be good to see if the difference between trained & untrained isn't just the embedding. This could be checked by training this per-token bias &subtracting that from h_t at every step.

Although looking at your K/V definitions, it seems your log-kinetic energy is a measure of velocity (change in hiddent state) & your potential energy is a measure of perplexity. Obviously the perplexity will be different between trained & untrained, however, you state:

"

• On trained models, Kt ≈ 7, and Vt typically 0.2 to 2.5

• On untrained models, Vt may reach 6–8, with Kt relatively unchanged"

Meaning that the change in velocity is ~the same between the both. If it's the same, then why is it included in a ratio to differentiate them? The only difference is the perplexity, so I'm unsure on what results which rely on both of these metrics tells us about the difference between these models.

Why is SmolLM2-360M an outlier for the random weight models?

Unsure on protocal for testing minimality of effect of jacobian steering. For the stated protocol, I'd appreciate an example of the original output & the steered output, so I can understand what "higher semantic quality" of the top-10 tokens might mean.

But what do we actually want? I assume it's to "preserve semantic coherence" while steering in a direction, like steering towards spanish words, while maintaining meaning w/o devolving into repeating the same words. There is a paper that benchmarks steering by KL-divergence with proposed completions (eg finishing a sentence in spanish, or the original english equivalent) as well as other steering benchmark papers that might work. (Sorry I cannot find that paper atm).

This is potentially very good work. However, the attached code (kudos for including a repo) has a fundamental bug that makes the conservation results incorrect as tested. I don't know if these results will survive fixing bugs - but the idea is interesting.

The bug is that in the paper, the energy is defined as the product of a kinetic and potential term, whereas in the experiment (line 275 of their momentum_perplexity_phase_space_experiment.py) a sum is used: total_energy_point = kinetic_energy + point_potential

(where the LHS is the "full" kinetic energy and the RHS is the log of the probability). I think this is responsible for the unreasonably large CV scores (given that entropy of text has >1 *additive* standard deviation, it is not reasonable that the multiplicative energy would have the exact conservation properties implied by the CV). I'd be very interested in a corrected calculation here, which should be clearly baselined wrt the CV of the PPL and the logit gradient by themselves.

The paper claims to find a certain approximate conservation law in trained model token-to-token behavior. The paper also looks at random-weight models, where I strongly suspect the effects are spurious law-of-large-numbers style effects that don't depend on the specific energy function chosen; I'll ignore this for the present review.

For trained models, the claimed energy conservation implies language models trade off uncertainty of text prediction (roughly correlated with next-token entropy) vs. speed of token-to-token movement in the residual stream. When a model is unsure about the next token (it is claimed), its final-layer residual stream moves less than when it is confident of the completion. Moreover, the paper claims that the terms (probability assigned to the output token) and (movement in the residual stream) are roughly proportional in a single text generating instance, with the proportionality constant playing the role of a conserved energy-like parameter. This is very surprising, not least since (by construction) transformers are highly parallel and so one would not expect the difference between adjacent tokens to be a very meaningful measurement (especially since the last layer is to a large extent aligned with the embedding of the output token, which one would expect to be pretty stochastic).

Given these observations, the CV scores given are unreasonably large for the multiplicative energy, and are almost certainly a result of the bug. Indeed, entropy of text has >1 *additive* standard deviation, so it is not reasonable that the multiplicative energy would have the conservation properties implied by the (additive) CV; perhaps the claim is different, but it would have to be fixed. I'd be very interested in a corrected calculation here, noting that it should be clearly baselined wrt the CV of the PPL by and the logit gradient by themselves. For example I would like to see log graphs of the logit gradients and PPL of a few representative text samples.

One could imagine that there is some relationship between last-layer-embeddings and logit entropy which could contribute to some directional effect analogous to the paper's findings, but a stronger relationship that links token dynamics to prediction entropy would be genuinely surprising and interesting - it's interesting that their steering method improves quality. Again, this might be confounded in some way, but I'm looking forward to seeing the corrected results.

Very exciting, really interesting result, clear physics analogy and new ideas which seem to work. I think the conservation is really interesting and generally one of the best papers.