Sentence Transformers capture semantic meaning by leveraging advanced neural network architectures and training techniques that focus on understanding relationships between words and sentences. Unlike keyword-matching approaches, which rely on surface-level word overlap, these models generate dense vector representations (embeddings) that encode the context and intent behind text. For example, the sentence “The cat sat on the mat” and “A feline rested on the rug” would have similar embeddings despite sharing no keywords, because the model recognizes their semantic equivalence through learned patterns.
The key mechanism enabling this is the transformer architecture, which uses self-attention to weigh the importance of each word in a sentence relative to others. This allows the model to capture nuances like negation (“not good” vs. “good”), polysemy (“bank” as a financial institution vs. a riverbank), and long-range dependencies between words. Additionally, Sentence Transformers are often trained using contrastive learning objectives, such as the Multiple Negatives Ranking (MNR) loss or triplet loss. These techniques teach the model to cluster semantically similar sentences (e.g., questions with their answers) in vector space while pushing unrelated ones apart. For instance, during training, the model might see pairs like ("How old are you?", “I am 25 years old”) and learn to map them to nearby embeddings.
Practical implementation details also play a role. Pre-trained language models like BERT or RoBERTa are fine-tuned on domain-specific datasets using sentence-level tasks. For example, the model might process Wikipedia sections where headings and their corresponding paragraphs are treated as positive pairs. Unlike traditional embeddings that average word vectors, Sentence Transformers use pooling layers (e.g., mean-pooling of token embeddings) or cross-encoders to create a unified sentence representation. This allows them to handle paraphrasing, summarization, and cross-lingual tasks effectively. Developers can verify this by using the sentence-transformers library to compute cosine similarity between embeddings of semantically related but lexically distinct sentences, observing high scores even without keyword matches.