How detrimental is involvement in politics to a scientific career?

I will direct my answer at an American academic career in the sciences.

You certainly have a right to make your political opinions known. Unfortunately the internet makes it possible (indeed, trivial) to search through every public remark someone has ever made, and this means that everyone can be held to a higher level of scrutiny in this regard than in the past. As others have said, there are situations in which remarks people have made have had a negative impact on their academic career. Most academics believe that "academic freedom" should ensure immunity from retribution for a range of such political remarks...but certainly not all of them.

In fact, it is not entirely clear what constitutes a political remark as opposed to something else. For instance, one of Steven Salaita's tweets was

Israel: transforming 'antisemitism' from something horrible to something honorable since 1948.

This is taken from this article in which he explains the context. The additional context he provides convinced me to view it is as a truly political remark that should come under the protections of "free speech" and "academic freedom". However, without that context....in that it contains the claim that antisemitism is honorable, it looks pretty bad. If my colleague had posted this and asked me to defend his right to keep it there, I would on the contrary ask him to take it down.

Here are some thoughts on how to be politically active in a way which is not to the detriment of one's academic career:

• Don't post on twitter.

Really, "don't post on twitter" seems close to being universally good advice. [Added: David Z disagrees. I can see that if you want to post academic/scientific content only then the twitter effect would be at least non-negative. I cannot agree that twitter has had a significant effect in disseminating work in my field, especially in comparison to so many other electronic media. But maybe it is different for others.] But the combination of telling the whole world and strict character limits make it anathema to academic discourse or even, I would advise, to discourse by academics. In general, young people need to use social media carefully: political articles are different, but one's off-hand political, social and religious comments are best not shown to the entire world.

• Make sure that your political remarks can only be construed as political.

This is the moral from the above example. Political remarks advocate policy, support or criticize governments, or support or criticize political figures on political matters. Salaita's tweet (intentionally, and even rather cleverly) plays on the distinction between criticizing the Iraeli government and criticizing Jewish people. But don't play with that. Don't criticize or denigrate any ethnic group. When you want to criticize a group of people aligned around a certain practice, make sure you are criticizing the practice, not the people as people. For instance, if you are pro-life, don't (publicly) paint women who get abortions as immoral or unclean: that's not a political statement. Don't (publicly) criticize conservative politicians who are against gay rights by saying that they must either have terrible sex lives, be latently gay or both.

• Try to have a clear separation between your political activities and your academic ones.

The OP says that he has political articles published online. Sounds fine to me. I would think at least three times about incorporating these articles into your science classes. As a general rule, I feel free to discuss politics and religion in my (mathematics) courses because I feel these should not be taboo topics among human beings, but I always characterize them as digressions from the class, I never push a position, and in fact I try not to enunciate my own stance or position in a classroom environment. If someone wants to hear how I really feel about Islam or the midterm elections, they can talk to me after class.

This "clear separation" should work just as well in the sciences as it does in math. Academics in certain other fields might have more trouble with this: e.g. women's studies.

• Hiring committees doing significant digging into candidates' extra-academic life is the exception rather than the rule.

The OP specifically mentions graduate admissions and googling. I have done lots of graduate admissions and I cannot specifically remember ever having googled an applicant (and I often google their letter writers or their home institution). For faculty and such: sure, sometimes I get curious, but I don't feel like such googling is part of the faculty search process. If I found some political activity about a faculty candidate through the job search, it would have to be extremely significant or specifically problematic in some way for me to bring it up to my colleagues at all. To give two examples of googling academics: I have learned for instance that someone had been a union organizer and someone else was a leader of a campus pro-life organization and had run for political office. If these people applied for jobs I would keep this information to myself.

• On average, it is a little safer to be on the left than on the right.

The majority of American academics that I know are not very politically active but are considerably left of the center of American politics. This applies to me. If I learned that a potential job candidate was very active in Tea Party politics, I would take a moment to steel myself not to let this affect my decision. If I learned that a potential job candidate had been active in Obama's campaign, I would think "Well, that will make for a fun story sometime." I have colleagues whose political views are very different from my own, including one whom I respect the most, because of his great personal integrity and selflessness. But I still have to think and act a little more carefully around this colleague because of this; often I cut off a "humorous remark" just before it leaves my mouth because I remember that he will not be amused, and I don't want to make him uncomfortable.

I put this point last because it is purely contingent on "local phenomena", but I think it would be naive to expect exactly the same academic reception for political activism on the two sides of the spectrum. On the other hand, at some state universities the local politics can be very different from the politics of the university and the faculty. This is really beyond the scope of my answer, so I'll just say: surely it is best if state employees who run the university system know as little about a faculty member's political activity as possible, at least until tenure.

The recent case of Steven Salaita (who was offered a tenured position at the University of Illinois and then had the offer pulled after there were complaints about some of his political remarks on twitter) shows that being politically outspoken can have an effect on one's academic career in the United States. Although Salaita doesn't work in the physical or life sciences, I think the same thing could very easily have happened to (for example) a chemist who had made the same statements.

Unlike most professionals working for large corporations, faculty are public figures. If an employee of a large corporation makes political statements, it's very clear that these don't represent the views of the corporation and it isn't likely to cause a significant problem to the employer (although individuals within the organization might well object to the statements and respond in ways that might be unfair.) When a faculty member makes political statements the press is likely to pick these statements up and amplify them and the administration of a university is more likely to be concerned about the effect of these statements on the reputation of the university. For example, your political statements might cause a wealthy donor to stop a planned donation to the university. This is especially true when the university is a public university that depends on the state and federal governments for financial support.

A growing problem in the sciences is that even making non-political statements about scientific issues (evolution, natural resources, global climate change, etc.) can result in politically motivated attacks against a researcher. If your research interests happen to be in one of these areas and you're also politically active, you're even more likely to become the target of such an attack.

It will very much depend on the country you intend to work in.

In Britain, academia has long been a home for radical thought: for example, the foundation of University College London in the early 19th Century was an open act of defiance against the establishment at the time.

It's far from universally true, but academia does strive to reward merit for merit's sake; so people will strive to assess you on the basis of your research. Having said that, recruitment decisions are made by us humans, with all our foibles, prejudices and weaknesses.

There are political movements that get associated with large-scale crimes against humanity, including mass murder: Khmer Rouge, Stalinism, Fascism. If you were involved with those, that would indeed be likely to be devastating to a career in academia.