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Kaveh Daneshvar was thrilled when he was invited to speak at a molecular biology meeting next month in Banff, Canada. Daneshvar, a molecular geneticist, is finishing a postdoc at Harvard Medical School and Massachusetts General Hospital in Boston, and is preparing to go on the job market. He hoped that the conference talk would give him much-needed exposure to leaders in his field.

But that now seems impossible: if Daneshvar, an Iranian citizen, leaves the country, he may not be able to return. On 27 January, US President Donald Trump signed a sweeping executive order that blocks refugees from entering the United States for 120 days and stops Syrian refugees indefinitely. It also bans citizens of seven majority-Muslim countries “compromised by terrorism” — Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen — from entering the United States for 90 days. The US government has issued conflicting statements on whether the provisions apply to people such as Daneshvar who hold visas that would otherwise permit them to live, work or study in the United States — including those with the permanent resident visas known as green cards.

Nature spoke to more than 20 researchers affected by the new policy, who described their feelings of fear, shock and determination. Some asked to remain anonymous for fear of retaliation by the US government.

“I am really appreciative of what the US has given me and allowed me to achieve here, but at the same time this is really shocking,” says Ali Shourideh, an economist at Carnegie Mellon University in Pittsburgh, Pennsylvania. “I've always been under the assumption this is a free country, that once you immigrated they won't try to kick you out or make life hard for you.”

Shourideh, an Iranian citizen with a green card, has travelled to Iran several times recently to visit his mother, who has cancer. Now, if he leaves the United States, he may not be able to return. “You have to make a choice: do I want to see my mom or do I want to keep my job?” he says. “This is something that for sure will hurt us personally, but also the US, I think, because all these high-skilled-type professionals would not want to be here anymore.”

Legal challenge

On 28 January, the American Civil Liberties Union and other groups filed a lawsuit against the US government to overturn the order on behalf of two people with valid visas who were detained at US airports. Later that day, the group won a preliminary victory when a federal judge ruled that the government could not deport those detainees. But the ruling does not affect those who were not in transit when the ban took effect.

Scientists have already begun to organize against the immigration policy. More than 12,000 researchers — including 40 Nobel prizewinners and 6 Fields medallists — have signed a petition denouncing Trump’s actions. The American Association for the Advancement of Science and the Association of American Universities have put out statements urging the Trump administration to re-evaluate the ban.

And universities have scrambled to understand how the US policy will affect their professors, postdocs, students and other employees from the seven banned countries. Many institutions are advising these people to stay in the United States until the situation becomes clearer.

But that is little comfort to an Iranian engineering student at Wayne State University in Detroit, Michigan. The man has just bought a house with his wife, who is expecting their first child — a girl — next week. The couple were expecting their parents to come from Iran to visit the baby, but those plans are on hold. So are the green cards that the pair had expected to receive in April.

Now they are contemplating whether to start anew in Australia, where they hold permanent residency cards that will expire in May. “If you leave, you can get your life back, your parents back, your family back — but you will lose anything you did here,” the engineering student says. “We worked hard for this.”

Waiting game

The sudden nature of the ban has thrown many researchers’ professional lives into disarray.

Luca Freschi, an Italian microbial geneticist at Laval University in Quebec, Canada, had planned to move to Harvard Medical School in March. But the US immigration ban has disrupted those plans, because his Iranian wife Maryam will not be able to come with him. She has encouraged Freschi to go without her.

“It’s crazy for us because we got the visa two days before the executive order was signed,” he says.

Another couple, both scientists, are stuck in France while they wait to learn whether the woman, an Iranian, will be able to travel to the United States. They are each set to start jobs at a US university in March.

And the ban is already disrupting some international collaborations. Samira Samimi, an Iranian studying glaciology at the University of Calgary in Canada, was supposed to go to Greenland in April on a NASA-funded expedition to study snow melt. The team will depart from a US Air National Guard base in Schenectady, New York, aboard a LC-130 cargo plane to Kangerlussuaq, Greenland.

But Samimi won’t be able to cross the border to meet her colleagues in New York. And even if she purchased a commercial ticket to Greenland, she might not be allowed to fly on the cargo plane that will take the US team to its remote field sites. If Samimi can’t get to Greenland to continue the research she started there last year, it could slow her progress towards a PhD. “I thought I would be free in Canada,” she says. “I wouldn’t have to fight for my rights anymore.”

Samimi’s colleagues are exploring all options to get her on the ice. “This really upsets me,” says Mike MacFerrin, a glaciologist at the University of Colorado Boulder who is helping to organize the expedition. “None of this is right.” He adds: “There is no way this helps us or our science.”

Seeking freedom

Some of those affected by the immigration shift suffered persecution in their home countries.

Samimi, the glaciologist, was detained by Iranian police for the first time when she was 9, because she wore a T-shirt advertising the US rock band Bon Jovi. Later, she was held and questioned because she dyed her hair unacceptable colours and wore nail polish.

Ubadah Sabbagh, a doctoral student in neuroscience at Virginia Tech in Blacksburg, is a Syrian citizen who moved to the United States seven years ago, aged 16, to attend university. Because he ignored an order to serve in the Syrian army, he cannot return home or renew his passport. Now he is worried about conditions in the United States.

“This is not going to be a footnote in American history,” Sabbagh says. “We could slip into a very dark place very quickly if people just decide to be indifferent.”

Then there is Amir Haji-Akbari, a computational statistical physicist from Iran, who won a plum assistant professor job at Yale University in New Haven, Connecticut, in 2016. The position offered welcome security after his years as a postdoc, and he began planning to apply for citizenship so that he could bring his elderly parents over from Iran. His wife, who is studying quantitative and computational biology at Princeton University in New Jersey, had invited her mother from Iran to watch her PhD defence in April.

Now all of that seems impossible, says Haji-Akbari, who, as an ethnic Azeri and Sunni Muslim, faced discrimination in Iran. “I have always found the tolerance and religious freedom better here,” he says. “Why am I considered a threat? What have I done to you? I have been a second-class citizen in my own country, and now here you are treating me like garbage.”