Embedding spaces and similarity metrics

One-line definition

An embedding space is a learned vector space in which points represent objects (words, sentences, images, items, users) and geometric relationships. Distance, angle. Encode semantic relationships such as similarity or relevance.

Why it matters

Embeddings are the substrate for retrieval, recommendation, search, clustering, classification, and most LLM-adjacent products. A senior interview will check whether you can pick the right similarity metric, the right normalization, and the right index.

Common similarity metrics

For two vectors $u,v\in {\mathbb{R}}^d$:

Metric	Formula	When to use
Dot product	$u^{\top }v$	When magnitudes are meaningful (e.g., learned matrix-factorization scores). Indexable with MIPS algorithms.
Cosine	$\frac{u^{\top }v}{\Vert u\Vert \Vert v\Vert }$	Default for embeddings where direction encodes meaning and magnitude is a noise/popularity confound.
Euclidean / L2	$\Vert u-v{\Vert }_2$	When distances have physical meaning (image patches in pixel space, geographic coordinates).
Negative Euclidean²	$-\Vert u-v{\Vert }^2$	Equivalent to dot product on L2-normalized vectors plus a constant.

Cosine is the default for sentence embeddings (BERT-family), CLIP, two-tower retrieval, and most embedding APIs. Reason: training objectives (contrastive, triplet) typically L2-normalize, making magnitude meaningless.

L2 normalization

A common convention: project every embedding onto the unit sphere by dividing by its L2 norm before storing or comparing. Effects:

• Dot product equals cosine similarity (no extra division at query time).
• Vector index (FAISS, ScaNN, HNSW) can use Inner Product mode for cosine retrieval.
• Magnitude (which often correlates with item popularity or training frequency) is removed as a confound.

If you store unnormalized embeddings and compare with cosine, you’re paying the normalization cost at every query.

Geometry of learned embeddings

Empirical regularities in well-trained embedding spaces:

• Clusters form for semantically similar items.
• Linear analogies (king − man + woman ≈ queen) hold in word2vec / GloVe; less reliably in modern contextual embeddings.
• Anisotropy: contextual LM embeddings (BERT, GPT) often concentrate in a narrow cone; cosine on raw embeddings can be misleading. Whitening or mean-centering helps.
• Curse of dimensionality: in high-d, all pairwise distances concentrate. Distinguishing top-1 from top-10 becomes noisier. Useful embedding dimensions are typically 64–1024 even when the model space is much larger.

Indexing for fast retrieval

For $N$ items and queries:

Method	Build	Query	Recall
Brute force	.	$O(Nd)$	exact
HNSW (Malkov & Yashunin, 2018)	$O(N\log N)$	$O(\log N)$	tunable, ~95–99%
IVF + PQ (FAISS)	$O(Nd)$	$O(\sqrt{N})$	tunable
ScaNN (Google)	$O(Nd)$	$O(\log N)$	tunable

HNSW is the default for most production embedding stores (Pinecone, Weaviate, pgvector with hnsw index, Qdrant).

Common pitfalls

• Mixing normalized and unnormalized vectors in the same index. Cosine and dot give different rankings.
• Comparing across embedding models. Vectors from BERT and CLIP live in unrelated spaces; concatenating or comparing across them is meaningless without alignment.
• Treating embedding dimension as quality. Higher-d embeddings are not strictly better; tradeoff is recall vs. storage and query latency.
• Ignoring popularity bias. Magnitude correlates with frequency; if you don’t L2-normalize, popular items dominate top-k for everyone.