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  • [MUSIC PLAYING]

  • And all of you here are in for a treat today.

  • Sam Gralla, who I'm very impressed with,

  • is going to tell us about one of the most amazing things

  • that science has done in a long time.

  • And that's the measurement of gravity waves.

  • And he will describe it to us, and at the end of the evening

  • we'll have a quiz.

  • [LAUGHTER]

  • But Sam is a theorist working on gravitational physics

  • and relativistic astrophysics.

  • He's interested in the strongest gravitational

  • and electromagnetic fields in the universe,

  • which occur near black holes and neutron stars.

  • And you heard a little bit about this last week.

  • Sam applies techniques from diverse communities,

  • relativity, astrophysics, particle physics

  • to study the physical processes occurring

  • in these extreme environments.

  • His PhD is from Chicago, and his postdoctoral work

  • is from Maryland and Harvard.

  • And we're really lucky to have Sam as one of our faculty

  • members.

  • Sam, where are you?

  • [APPLAUSE]

  • Well, what a pleasure to be here in this beautiful hall,

  • in this great university in this wonderful city.

  • I'm a newcomer to Tucson.

  • I've been here less than two years.

  • So I thought I'd start by telling you

  • the story of how I came to the University of Arizona.

  • When you're a young theoretical physicist, you apply widely

  • and you hope you get some interviews--

  • which, fortunately, I did.

  • Now, a job interview for a professor job

  • is a little different from a normal job interview.

  • It's really more like 20 or 30 interviews right

  • after each other.

  • You meet with everybody-- professor

  • after professor, half an hour meetings.

  • You barely have time to pee.

  • If you're lucky, in the midst of all this

  • you get to meet with the dean.

  • Now, I had a thing I did with deans on job interviews.

  • The first thing I would do when I got in the dean's office

  • is I'd take out a book and I'd start reading it.

  • That would get the conversation started, usually

  • with something like, what are you doing?

  • And then I'd say, you tell me one thing--

  • if you're in a vehicle moving at the speed of light

  • and you turn on your headlights, do they work?

  • [LAUGHTER]

  • And Dean [INAUDIBLE] said the U of A

  • was the only one who did not immediately

  • throw me out of the room.

  • OK, I made that one up.

  • That's a recycled Steven Wright joke.

  • But the comedian Steven Wright has his interviewee in his joke

  • ask a really deep question.

  • You go at the speed of light, you

  • turn on your headlights, what happens?

  • This is a good question for two reasons.

  • First, it pushes us outside of our comfort zone, right?

  • You think you kind of know how light

  • works from living with light all around you,

  • but as soon as you ask a question involving where

  • you're moving like the speed of light,

  • you get a little confused.

  • Maybe you don't understand light so well,

  • because you're used to not moving that fast.

  • The second reason Steven Wright's question is a good one

  • is because it's reasonably precise.

  • It's not some vague, can you make light stop?

  • It's some pretty precise sequence of actions.

  • You get in your car, you get up to light speed,

  • and you pull the lever that would normally

  • turn the lights on.

  • What happens?

  • What happens?

  • These kinds of questions that are reasonably precise

  • yet push us outside our comfort zone

  • are called thought experiments.

  • And Einstein really was the pioneer

  • of the thought experiment.

  • He called them gedanken experiments in German.

  • This is not exactly the kind of thought experiment

  • Einstein did, but it's close.

  • And the answer to this particular conundrum

  • was given to us by Einstein.

  • The answer is you can't go that fast.

  • [LAUGHTER]

  • Seems a little unsatisfying, but wait.

  • Einstein didn't mean your car won't go that fast.

  • That's not what he meant.

  • He meant you just can't go that fast,

  • nothing can go that fast-- not your car, not a truck,

  • not a motorcycle, not a plane, not a spaceship,

  • not an ambulance, not a backhoe--

  • we have lots of those in my neighborhood right now--

  • not a garbage truck, not a fire truck--

  • I have a two-year-old, he really likes trucks.

  • He knows about suction excavators and so forth.

  • None of your favorite desert animals

  • can go that fast-- coyotes, road runners.

  • You know those wolf spiders that seem like they're

  • going the speed of light?

  • Those ones can't go that fast.

  • You can't hit a baseball that fast, a hockey puck.

  • No elementary particle can be accelerated that fast--

  • absolutely nothing can go the speed of light.

  • That's what Einstein taught us.

  • There's a universal speed limit.

  • [LAUGHTER]

  • Now, this speed limit is not Einstein's limit

  • where he'll come give you a ticket if you violate it.

  • This is a law of nature.

  • You simply cannot go this fast, no matter how hard you try.

  • That seems well and good.

  • We have the authority of Einstein to back it up.

  • But if you were a physicist around Einstein's time,

  • you might say, hey, wait a minute, mister.

  • That doesn't make any sense, because of the following.

  • I don't know about you guys, but I

  • have memories from when I was a kid riding

  • in the backseat of my car, parents

  • driving me on the freeway.

  • You're watching the scenery go by, and then

  • all of a sudden, whoosh, a car whizzes by you.

  • Of course, they're just driving 60 miles an hour or so too,

  • but they're going in the opposite direction.

  • So to you, they're going 120 miles per hour.

  • Then there's a formula, v equals v1 plus v2,

  • that if you wanted to, will tell you how to compute this.

  • And it's really kind of indisputable, this formula.

  • What is speed?

  • It's how far you go in how much time.

  • So if the car up here on the left is going 60 miles an hour,

  • and in an hour then he'll be 60 miles to the right,

  • and the other car will then in that same hour

  • be 60 miles to the left, so in one hour

  • the relative distance is 120 miles.

  • So they're going 120 miles an hour--

  • seems obvious.

  • The person who is skeptical of Einstein

  • now says, well, what if I'm going

  • at 75% the speed of light?

  • Now this obvious formula tells me

  • that I measured the other car, [WHOOSH] the one going

  • by me, moving at 150% the speed of light,

  • which is supposed to be impossible.

  • So here's another thought experiment

  • that seems to reveal something self-contradictory

  • about this universal speed limit.

  • We now have a situation that theoretical physics

  • calls a paradox.

  • We have two laws that we really believe,

  • but they're incompatible.

  • You can't have the universal speed limit

  • and the formula v equals v1 plus v2.

  • If you were a lesser physicist you might think,

  • OK, well we know v is v1 plus v2 That's

  • utterly trivial and obvious, so Einstein must be wrong.

  • But if you're Einstein, you think,

  • we don't really have any good experimental evidence

  • that v equals v1 plus v2 when you're

  • moving near the speed of light.

  • We've only done those experiments

  • at 60 miles an hour, which might as well be standing still

  • compared to light.

  • So which of these do we keep and which do we discard?

  • We keep the speed limit.

  • It turns out, that formula is wrong.

  • Now, as Professor Dienes emphasized in the first lecture

  • in this series, if you were here,

  • physics doesn't discard laws .