Scottish Independence: would you need passport to go to Scotland?

This morning the formal official campaign period of the Scottish referendum begins - 16 weeks ahead of the vote on 18 September.
In this video series, Ben Riley-Smith looks at the key issues that will dominate campaigning: Scotland's borders, its currency, and its defence, amongst others.
Two months ago the Home Secretary Theresa May said that Scottish independence would mean: "border controls between a separate Scotland and the United Kingdom."
In a speech to the Scottish Conservative Party Conference, she told delegates that there would be: "passport checks to visit friends and relatives. A literal and figurative barrier between our nations."
The SNP leader Alex Salmond says that the idea that the Scottish border could be manned by guards is "scaremongering."
So who's right - and what are the issues involved? Watch the video and find out.