Why is "do" needed in this sentence?

Question

Why is DO needed in the sentence below? (would it read well without it?)

Although thunder and lightning are produced at the same time, light waves travel faster than sound waves do, so we see the lightning before we hear the thunder.

Thanks

Answer 1

It's not needed. Than can take different kinds of complements:

1. Light waves travel faster than sound waves.

   Here, the noun phrases light waves and sound waves are compared.

2. Light waves travel faster than sound waves travel.

   Here, the clauses light waves travel and sound waves travel are compared.

3. Light waves travel faster than sound waves do.

   This is the same as example 2, except that the verb travel has been replaced with the pro-verb do to avoid repetition.

All three of these examples are perfectly grammatical and have the same meaning.

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Historical note:

Some observers would explain example 1 via ellipsis, claiming that do is ellipted:

Light waves travel faster than sound waves do.

This is not correct as a description, but the analysis has been around for some time, dating back to the prescriptive grammarian Robert Lowth in the 18th century, who claimed that than was always a conjunction and never a preposition. We can see this is false because than me is perfectly grammatical, and has been in common use since before Lowth made his analysis.

Lowth's claim was that it should instead be than I, while Joseph Priestly claimed the opposite, that it should always be than me, and grammarians argued fiercely over the topic. However, both were shown to be descriptively incorrect by their contemporary William Ward, who noted in 1765 that than was commonly used both as a preposition and as a conjunction.

For more information about this point of grammatical contention, I recommend the entry for than in Merriam-Webster's Dictionary of English Usage, which can be found on pages 892–893.

Answer 2

In formal comparative utterances, auxiliary verbs are often repeated:

• You speak faster than I do.

• He jumps higher than John does.

• We like chocolate more than our families do.

• Light waves travel fasterer than sound waves do.

In everyday conversation, the less formal is:

• You speak faster than him. [notice him instead of he does]
• We like chocolate more than them. [notice them instead of they do]
• I like this exercise more than him. [notice him instead of he does]

Please note: Some people do use the more formal comparative forms. The informal one is not acceptable in formal writing. Your piece is formal writing.

And that's the pattern for formal comparatives versus colloquial usage.