nick schifrin, pbs newshour, cairo. >> sreenivasan: tomorrow night, nick schifrin will look at the centrale of egypt's courts in the revolution. >> sreenivasan: stay with us. coming up on the newshour: the supreme court halts president obama's landmark climate change plan. and "sweat," a play that doesn't shy away from hard work. but first, yesterday america's top intelligence officials were on capitol hill to discuss their updated assessment of worldwide threats to the united states. among their top concerns: cyber attacks, the islamic state group, the war in syria, north korea's nuclear activities and a resurgent russia. we're joined now by david cohen, the deputy director of the central intelligence agency. he's been at the c.i.a. for one year, after five years at the treasury department overseeing sanctions implementation and efforts to combat terrorism financing. so when i rattle off those lists of concerns that the community has, what are the top three that keep you up at night? >> well, i think that was a list of six and i think all six of those keep us up at night. i mean, obviousl

nick schifrin, pbs newshour, cairo. >> sreenivasan: tune in tomorrow night for nick's final piece on the role of women in the tumultuous five years since the revolution. >> sreenivasan: now some truly cosmic news, the sound of two black holes colliding more than a billion years ago was recorded by a team of scientists at the ligo observatory. proof of gravitational waves, or ripples in time and space, first theorized by albert einstein. we explore this monumental moment in physics with dave reitze of cal tech, executive director of the ligo labratory. now, that was a rudimentary attempt at explaining what a gravitational wave is. but what are they and why is it such a big deal to find one? >> actually, you did a pretty good job. dprafitational wraefs fluctuations in space time, and any time you have a mass, something that has matter in it, accelerating, it produces a gravitational wave. and that's a consequence of einstein's theory of general relativity. now, these particular gravitational waves, in order to be able to detect them, you need really, truly massive objects. in this case

tonight, special correspondent nick schifrin presents the final story in our series: five years on. ♪never joined any demonstrations before. once i went to the square i was really happy. a revolution is about hope, is about change, it's about being better. >> ( translated ): it was a great victory that we managed to change the way men think, because they understand we are now on the front line with them. >> ( translated ): people were trying to create a different society. >> reporter: five years ago, egyptians did create a different society. and throughout the revolution, women were at the center of the movement. >> women went to tahrir square and imposed this situation. we're gonna sleep in the street like everyone else and we are going to organize and we are part of the protests and we're gonna lead protests sometimes and lead organizations. >> reporter: in tahrir square, female protesters found a freedom they hoped the culture would echo. salma said has been protesting for 15 years. >> i've never been surrounded with so many egyptian and non- egyptian men without being harassed bef

i'm joined now by special correspondent nick schifrin. ntly on assignment for us in nairobi, kenya and reported in depth from nigeria for the newshour late last year. nick, good to talk to you again. so tell us what you know about this new u.s. engagement in nigeria. >> yes, gwen good evening. there's a new recommendation by the u.s. military's top africa command that one to three dozen special operations forces troops be deployed to north eastern nigeria. this is what the military called an advise and assist mission. they are towards the front lines but not really on the frontline advising the nigerian military in their fight, assistingu0&fige and training. those troops, now this is just a recommendation. right now this will have to go through the state department and the pent gun an pentagon and wht this is what the military in africa wants. it's what one military official described as a national progression to a real escalation what the u.s. has been doing against boko haram and helping the nigerians. the first tend is intelligence. the