Time Dilation : An Experiment With Mu - Mesons (1962)

could I have the top piece of plastic day here yeah thanks it's for the multiplier that clear enough oh yeah I'm secret nine three one of the most startling prediction of the theory of special relativity is that moving clocks run slow by a factor the square root of 1 minus V squared over C squared where V is the speed of the clock relative to the observer and C is the speed of light for example if we had three identical clocks at rest with respect to an observer and at one moment they read the same then of course we expected a somewhat later time they will all read the same again but if we have the same three identical clock but one of them is in motion with respect to the observer then at the later time let's say when this one has passed the others it will read a left a lesser elapsed time a shorter elapsed time than the ones that are at rest as read by the observer the moving clock runs slow this effect is called the Einstein time dilation and ordinarily it's a mighty small effect because V over C for most things is a very small number so that this whole expression is very close to 1 for example a clock moving bias in a rocket at 5 miles a second it loses only one second in a hundred years but we do have some clocks that really go fast let's go 99 percent to speed of light and they run at a ninth of the rate they do when they're at rest with respect to us these clocks were going to show you these are mu meson charged radioactive particles in the cosmic rays first we have to detect them so detect MU meson then we'll stop some of these new Madam's to measure the distribution of their radioactive decay time as you'll see this will enable us to use these meson to measure time interval so distribution of decay times used as clock finally we'll use these meds on decay clocks to measure time dilation we'll compare two measurements of a time interval the measurement by massan's going at high speeds with respect to us and the measurement by mesons at rest with respect to us the MU mesons will use our produced high in the atmosphere and come shooting down toward the earth as they come down some of them disintegrate in flight the number arriving at a high altitude is greater than the number surviving to reach sea level right now we're up on top of a mountain not Washington in New Hampshire we will count the number of meson that arrive here and then we'll go down later to sea level and count the number that survived to arrive there by comparing these numbers we will show that these moving clocks run slow to start we need to detect these mu meson every time one of those new mesons which professor fish has told you about passes through their special plastic scintillator a flash of light is emitted in fact the plastic is simulating right now from the MU mesons that are passing through it we can't see them because those flashes are much too dim to be photographed so we detect them with this photo multiplier which is just an extremely sensitive photo cell which turns the flashes of light into usable electrical signals of course when working we have to put steeped plastic up next to the photo cell like that and then to keep out stray light we have to cover it with a light tight cover first it's aluminum that's shiny to reflect the light around inside and then finally with this finally with this light type piece of ply now I can turn on the high voltage to the photomultiplier the electrical signal from the photomultiplier comes out on this cable which I take and plug into this amplifier I take the output of this amplifier and put it into the trigger of this high-speed oscilloscope so every time the MU meson goes through our plastic scintillator and is picked up by the photomultiplier the electron beam is the oscilloscope starts moving and sweeps very rapidly across the scope phase we also take that same signal from the photomultiplier and put it on the vertical deflection plates of the oscilloscope so that the starting pulse is visible at the beginning of the trace most of these sweeps indicate the passage of immune as on clear through the scintillator this spot is where the oscilloscope beam rests until a mu meson enters the scintillator and starts the scope beam sweeping the pulses that starts the suite are displayed over here notice that their heights vary that's because some of the mesons go through just a corner of the scintillator making a small flash of light and hence a small signal whereas others go all the way through the sweep passes by each one of these major divisions in a microsecond right now it's sweeping across seven and a half microseconds later it will sweep across all nine minutes to the second which you can see that may sound pretty fast but one of our men's arms enters the scintillator passes clear through and leaves in only about two billions of a second which is about 1/100 of the width of one of those starting pulses so the width of the pulse has nothing to do with how long it takes our mesons to go through the plastic scintillator but is determined entirely by the electronics most of the mesons shoot right on through the scintillator we can stop some of them in the scintillator however by slowing them down in this two and a half foot thickness of iron incidentally that's a big pile of iron it weighs more than ten tons we want to stop some of the new mesons in the scintillator in order to observe their radioactive decay that brings us to our second point we already know how to detect new meson now we want to find out how long it takes them to decay so let's use them as clocks when a positive Yuma's on decays it gives off a neutrino and antineutrino and one positive charged particle called a positron when the moon is on enters our scintillator it makes a flash of light if it stops and decay in the satellite ER the positron takes a second flash of light that's what's the second false on our oscilloscope trace at the instant the MU meson decays so now we're ready to stop a lot more of these new Mazon and measure the time distribution if the radioactive decay so they push the scintillator under the iron and I'll get back to the scope let's have a look you may notice that there are fewer sweets than before that's because the iron not only slows down the meson but it actually stops them before they can get to our scintillator you also see that most of them still go all the way through thus giving just one pulse but there did you see that one about two divisions out from the starting fall let's see some more there there now I want to look at the next one in a little more detail there that one still has the first pulse on it where the mez are entered the scintillator that Mazon had just the right speed to be slowed down to the iron so that it would stop in the simulator it then sat around for a while before X disintegrated thus giving the second pulse when the positron was given out the time it took the meson to stop was completely negligible on our scale we can tell how long the Mazon got around at rest in our simulator before it blew up because we know that it takes just 2.9 microseconds for the beam to sweep from the starting pulse to the decay pulse now let's look at someone there there you see that these pulses are occurring at different places across the face of our scope and that indicates that the meson take different lengths of time sitting around in our simulator before they blow up so if we're going to use these measures as timing devices or clocks we're going to have to accumulate quite a stack of data to make data taking easy I'm going to move the starting pulse over here to a predetermined position behind the mask on the scope face and then I'm going to move the main trace up behind the second mask now you will only be able to see the decay pauses there there and there's another these are the only process which we need to record you saw the decay pulse is peeking out from behind the mask here let me show it to you we're only interested in where those decay pulses occur on our time scale and the easiest way to record that is to use this Polaroid camera if we leave the shutter open we can record up to about 20 separate events one after the other on a single exposure we also have a photo multiplier which looks through this aperture in the camera mat at the decay pulses but before we can use it I need to seal out the room light the light from the decay pulses that show out from behind the mask is picked up by this photo multiplier here's the high voltage input for the photo multiplier this cable has the signal output from the photo multiplier it comes up into this amplifier it enters in the back here comes the output comes out in the back here comes up to this circuit which is actually a scaling circuit which is just used as a driver for this register this register counts individually the new Mazen's which have entered the scintillator decayed and their decay pulses have counted on the photomultiplier if you're ready with the taping gym I'll turn on the high voltage it's okay go ahead I'll turn on the counter and open the camera shutter there's one remember this counter gives a running record of the number of MU mesons that have stopped and decayed an articulator how about should we take five over 1005 the best day I'll go over and get the chart ready well here's our picture let's take a look at it where you see the pulses which are counter recorded on all five of them the starting pulse is over here just behind the mask now what I want to do is to measure the length of time which each one of these measure on live so that professor fresh can plot it on his chart there's a convenient ruler to use simply another exposure as a grid on the scope page now I'm measuring from the edge of the mask and I find that this particular math on lift for a one two three four one eight five microseconds four point eight five I'm growing on this expanded scale a line representing the length of the decay time of that Mazon in microseconds now professor fresh is plotting those times vertically downwards for a reason that will be obvious later but I don't want you to think that that has anything to do with the distance from here to here it has to do with this distance the length of time then there's our lived in this little later now here's another one this one only lived for 0.659 or second 0.65 and this one 2.90 Oh microseconds this one 0.8 0.8 Oh Oh my hand Oh microseconds and the last one three point four five oh no that's close to the 3.50 320 500 microseconds okay here are the lines showing the decay times of these five mesons we got we put them side by side just to display them together but this coordinate doesn't mean anything it's only the time which we plotted in that direction that means something in order to see the pattern of the distribution of the decay times we will need to take many hundreds of counts so Jim would you start an hour's run you got filming camera yes there's only one you see our first count coming in so show you a complete hour we will compress time so that you will see it in the next 25 seconds also will show you samples of the polarized which we touch during now [Applause] 568 ok ok 1 2 3 ok 1 point 1 2 3 0 X 1.8 another 106 58.85 okay here okay so here's our finished chart based on the 568 counts which we got during this hour so 568 counts we have platters on our chart the result of one hours run previously we had taken five different runs each for an hour and their average comes out 564 so that the 568 we have plotted are indeed a good sample of the number of mesons decaying in arson later every hour so you can think of the MU mesons is coming down in a steady ring from an hour to hour many marathons live for as long as one microsecond you are live for two microseconds by the time we get to five microseconds only a small fraction of the original number remains and by the time we get down to eight and a half 90 seconds which is the longest time we can see on the scope face only a very few madhan's are decaying that chart is a pictorial representation of the decay time of those mesons which had stopped in the simulator before we began to measure their decay time they were at rest with respect to us during the time shown on the chart now we come to the heart of the experiment what happens to the decay times of these new Mazon when instead of being at rest with respect to us they're shooting on down by us with speeds nearly that of light let's look at the chart in more detail for instance here's Amazon which lived for three and a half microseconds sitting in our satellite suppose we hadn't stopped it where would it have gone here we are up on top of Mount Washington 60 300 feet above sea level we stop that mess on up here if we hadn't it would have gone on for three and a half microseconds down below before the King now how far would that have been it would have gone a distance three point five times ten to the minus six seconds times its speed V so in order to know just how far it would have gone before the King we need to know it's the C fortunately we do know the speed of the mesons would stop in our scintillator because we know how far a charged particle of a given mass and speed can go through matter a Mumma is on entering the top of the iron with 0.995 o times the speed of light gets just into the top of the scintillator before it stops with any less than point 9 9 5 o z it stops in the iron with slightly greater 0.995 for C it goes down and stops in the bottom of the scintillator with greater speeds than that it goes on through and into the mountain so the measure ons we count have speeds between point 9 9 5 au and 0.995 for the speed of light for our present purposes we don't need that accuracy let's just say that they have approximately the speed of light and go close to a thousand feet per microsecond that is Z is 1,000 feet per microsecond or this particular messin would have gone on 3,500 feet before decaying now this one which lasted only 2.4 microseconds would have gone on only 2400 feet before the king this one which lasted nearly 5 microseconds would have gone on nearly 5000 feet I might in fact simply reel able this axis over here in thousands of feet and then we can simply ask how many of our Mazon would live to reach sea level which is 63 hundred feet down let me put a string across here at 6300 feet and simply come how many of our mother arms would have gone down that fly reached this level there's 1 2 3 4 5 6 7 8 9 10 minutes fourth and five twenty six twenty seven so if we take our equipment down to sea level we expect to find 27 or about twenty-seven meson stopping and dying in our plastic each hour that calculation is based on the assumption the numa's on decay that is keep time when they are in flight in the same way as they do when they are at rest with respect to us that assumption gives us Z twenty-seven meson per hour when we go down to sea level but if we count appreciably more than that say this many it means the mesons have decayed as if only this much time has passed in other words we calculate this time interval for their flight down six point three microseconds they would measure this time interval so that's our experiment to go down to sea level and see how many new measures per hour are left okay pull the cable through this okay Jim you can roll the counter under we're down here at sea-level and the equipment seems to be in pretty good shape as soon as I get this together let's try to see how many meson survived the trip down from Mount Washington we turn on again boy that was a quick one while we're counting let's look at the situation we counted the 568 mesons we have on our chart up on top of Mount Washington now we've come down to sea level to see how many are left of course we won't be able to count the survivors of the exact same meson we counted up on top of Mount Washington because we stopped them all in the citta later up there fortunately it doesn't matter because there was nothing special about these particular Mazon indeed we never could have said exactly when any particular one of them would decay what we use as Amazon clock is the average distribution of decay times of a large number of Mazon so we can count the survivors of another hours worth of massan's instead because as we saw up there the average number is the same from hour to hour we also can't set up at sea level right below Mount Washington we'd have to excavate a hole all the way down to let the Mazon through all that rock and the Appalachian Mountain Club wouldn't like it so we're over here at Cambridge Massachusetts 150 miles away and we're counting the survivors of the megan's incident at the 6,000 foot level above us here the average intensity of cosmic rays is the same over horizontal distances of many hundred miles across the earth so there are approximately 568 Muslims coming down 6,000 feet above us during this hour now there's one thing different about our equipment down here on top of Mount Washington we use to a layer of rayon about two and a half feet six to select our mezzo but between here and 6,000 feet there's a layer of their equivalent in flooring down power to one foot of iron so to compensate we've removed one foot of iron from our stack we've removed that iron in order that the mess on switch stop in our simulator here have a speed 0.995 see when they pass the six thousand foot level above it that's the same speed which the mez arms had when they entered the iron on top of Mount Washington and stopped in our simulator now this is only one of many effects we've had to take into account in this experiment for instance not all new measures come straight down also a few mu mesons are produced by other cosmic rays between six thousand feet and sea level in addition some you mess on switch stop in our simulator make nuclear interactions we believe that we've properly taken all these effects into account and that their effect on our final result is small now let's take a look and see how the counts are coming in 13 that's a lot more than we expected let's go on and take the four hours count and get some good statistics 409 410 411 or twelve okay that's the hour instead of 27 we have 412 massan's left of sea-level that's way up here on our chart I've been tabulating here the number of mesons surviving as a function of time on our mess on decay distribution clock and 412 corresponds to only about 0.7 microseconds point seven microsecond elapsed flight time as measured by the decay distribution of the moving Mazon divided by 6.3 microseconds flight time as measured on clocks at rest with respect to us point seven divided by six point three equals 1/9 these mess on moving by us at 0.99 the speed of light keep time at 1/9 the rate they do when they're at rest with respect to us we used radioactive particles numa's on to show you that moving crocs are in flow but I don't want you to make the mistake of thinking that it has anything to do with a particular kind of croc we use equivalent experiments have been done with other clocks Adams for instance and the results are invariably the same in fact if we could only move them fast enough we believe we could do the same experiment with alarm clocks for instance if we're moving alarm clock past the first of two fixed alarm clocks when all three read exactly the hour and then 15 minutes later past the second of those two alarm clocks that is 15 minutes as read by the fixed alarm clock then the results of this experiment make us believe that that moving alarm clock if it had been moving with the same speed as our MU meson would have read only 1 9 to 15 minutes or a minute and 40 seconds past the hour now that's how it looks to us at rest with respect to these alarm clocks what would it look like to someone riding on the moving alarm clock or riding with one of our measurements what then to someone riding along with a mess on the Mazon would seem to be at rest first the top of Mount Washington would come shooting up past him at 0.99 the speed of life and then sometime later he would see sea-level come shooting up past him at the same speed the mez on he's sitting on when Mount Washington passes him has some chance of surviving until sea-level passes our distribution of decay times tells us that if he rides on 568 mesons one after the other about 412 of them will survive at least until sea level passes so he reads from this distribution of decay times point 7 microseconds as the most probable elapsed time between when the top of the mountain fascism and when sea level passes in so this observer on the Mazon the distance between the top of Mount Washington and sea level is the distance anything goes at 0.99 speed of light in point 7 microseconds which according to our distance scale is only about 700 feet so my Washington appears to be only about 700 feet high just hop a fast measure on to make a molehill out of the mountain this is an example of the Lawrence Fitzgerald contraction in which a moving length is contracted by a factor the square root of 1 minus V squared over C squared along the direction of its motion when it's moving with a speed V relative to an observer this is just the same factor as in the Einstein time dilation the experiment we've done can be interpreted as showing that moving clocks run slow or that moving lengths are contracted I say clock and length in order to emphasize once again that what we've done is to of all clocks and all lengths not merely meson and Mount Washington