The U.S. Supreme Court has struck down most of a controversial Arizona law that would have prevented undocumented immigrants from being in Arizona for any reason, including school and work.

But the Supreme Court did rule police who stop or detain suspects for other reasons can ask a person's immigration status if they have "reasonable suspicion" he or she is in the country illegally. Those migrants without papers will be turned over to the federal government for possible deportation.

Former University of Arizona law professor Gabriel "Jack" Chin – who spent eight years there and now teaches at UC Davis School of Law – discussed Monday's ruling.

What could happen to Californians visiting Arizona?

If you're lawfully stopped by police and there's reasonable suspicion you've broken a law, police can ask you questions. Police can always ask you whatever they want and you can answer or not answer. That was the law before, so it really doesn't change anything.

If you are stopped for just cause, how can you prove to Arizona authorities you're there legally?

They're not completely fooling around there. If you have a U.S. passport, that ends the issue. If you have a driver's license from a state that requires citizenship – including California – that also works.

If you're not a citizen, I would urge people to carry their immigration documents – that is, a green card or a non-immigrant visa – as required by federal law.

But you can't be stopped solely because police suspect you're an unauthorized migrant. If you are, I wouldn't recommend going to Arizona, because with or without SB 1070, cops cooperate extensively with federal immigration authorities.

What does the Supreme Court ruling mean for California?

A lot of California jurisdictions already cooperate with Customs and Border Protection and the federal Department of Homeland Security. There's a possibility some law enforcement agencies will read SB 1070 more broadly, and believe it's now constitutionally acceptable to ask about immigration status and hold people solely for immigration reasons. Then we'll see whether that's unconstitutional.

The Supreme Court said if local law enforcement held you longer than is necessary to issue you a speeding ticket, that might be unconstitutional.

The court also struck down a provision of the Arizona law that would have allowed cops to make an arrest simply because a person is deportable. The court objected because it's difficult to ascertain whether a person is deportable.

There are lots of people who are in fact never going to be deported under President Barack Obama's administration's new policies allowing unauthorized migrants brought here as kids to stay if they're 30 or under and have no criminal record. If that provision had been upheld, Arizona could have arrested a lot of people the federal government doesn't want to deport and driven them over to the federal building and dropped them off.

Does the Supreme Court ruling have an impact on other state immigration laws?

It basically shuts down all state measures to control immigration. Utah has a law that authorizes local arrests of undocumented workers, and also grants Utah work permits.

You can't punish people the federal government hasn't decided to punish, and you can't help people who the federal government has said aren't allowed to be here. You might be able to help them by giving them food and water, but you can't make their presence here lawful because the federal government says it's unlawful.

Immigration policy is a federal question: What the Supreme Court did was a plain vanilla application of existing law. The majority of the court said they're not going to break new ground. It's certainly not a pro-states' rights decision. They don't want to expand state power at the expense of federal foreign policy and national security.

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