WEBVTT 00:00:10.240 --> 00:00:12.840 Imagine holding a party and then sending out 00:00:12.840 --> 00:00:15.880 invitations after the party. 00:00:15.880 --> 00:00:18.414 Wait a minute, that doesn't make any sense. 00:00:18.414 --> 00:00:20.330 Are my friends going to be able to time travel 00:00:20.330 --> 00:00:22.270 back just to attend my party? 00:00:22.270 --> 00:00:24.290 In 2009, Stephen Hawking actually 00:00:24.290 --> 00:00:27.710 conducted such an experiment to disprove time travel. 00:00:27.710 --> 00:00:31.180 What exactly is time travel, and how does it work? 00:00:31.180 --> 00:00:34.075 It actually has a lot to with speed. 00:00:34.075 --> 00:00:36.200 A lot of what we know about time traveling actually 00:00:36.200 --> 00:00:39.540 comes from Einstein's Theory of Special and General Relativity. 00:00:39.540 --> 00:00:41.040 We are actually time traveling right 00:00:41.040 --> 00:00:43.210 now but not in the manner that sci-fi films have 00:00:43.210 --> 00:00:44.440 depicted them. 00:00:44.440 --> 00:00:48.640 We are, in fact, time traveling at a pace one hour per hour. 00:00:48.640 --> 00:00:51.770 In other words, every hour I experience, the world around me 00:00:51.770 --> 00:00:54.010 experiences a singular hour too. 00:00:54.010 --> 00:00:57.124 Now, it may sound simple and trivial, but stay with me. 00:00:57.124 --> 00:00:58.540 This is where it gets interesting. 00:01:02.340 --> 00:01:05.680 Now, the last part of Einstein's Theory of Special Relativity 00:01:05.680 --> 00:01:08.790 is actually a phenomenon known as time dilation. 00:01:08.790 --> 00:01:11.100 Effectively, this means that the faster you travel, 00:01:11.100 --> 00:01:13.290 the slower time passes around you. 00:01:13.290 --> 00:01:14.930 It potentially means that I could 00:01:14.930 --> 00:01:18.810 travel at maybe more than one hour per hour. 00:01:18.810 --> 00:01:21.090 This phenomenon, however, is only noticeable 00:01:21.090 --> 00:01:25.110 at really, really high speeds, speeds near the speed of light. 00:01:25.110 --> 00:01:29.230 This rocket here, not even close. 00:01:29.230 --> 00:01:31.210 Say I do have a spaceship now that 00:01:31.210 --> 00:01:33.550 travels at 90% the speed of light, 00:01:33.550 --> 00:01:36.070 and I have a pair of newborn twins. 00:01:36.070 --> 00:01:39.610 I bring one of them with me on this journey for 10 years. 00:01:39.610 --> 00:01:43.400 When I return, one of them will have turned 23 years old, 00:01:43.400 --> 00:01:46.117 while the one with me will have only turned 10 years old. 00:01:46.117 --> 00:01:48.200 While we have experienced 10 years on a spaceship, 00:01:48.200 --> 00:01:50.440 23 years have actually passed on earth. 00:01:50.440 --> 00:01:53.700 This amount of time actually increases exponentially 00:01:53.700 --> 00:01:57.095 as you get closer to the speed of light. 00:01:57.095 --> 00:02:00.550 So we could potentially travel forward in time, 00:02:00.550 --> 00:02:04.478 but what about backwards in time? 00:02:04.478 --> 00:02:08.406 That's a whole other issue. 00:02:08.406 --> 00:02:10.861 Now, remember our previous graph. 00:02:10.861 --> 00:02:14.390 The time that it took the [INAUDIBLE] 00:02:14.390 --> 00:02:15.812 to the speed of light. 00:02:15.812 --> 00:02:17.770 Looks like this thing may be the secret to time 00:02:17.770 --> 00:02:21.177 travel lies on the other side of the line. 00:02:21.177 --> 00:02:23.010 In other words, we may have to travel faster 00:02:23.010 --> 00:02:27.674 than the speed of light just to be able to travel back in time. 00:02:27.674 --> 00:02:31.132 Now, when you do travel at speeds near the speed of light, 00:02:31.132 --> 00:02:34.096 something called relativistic mass comes into play. 00:02:34.096 --> 00:02:37.924 In other words, as your speed goes up, your mass increases. 00:02:37.924 --> 00:02:40.506 As your mass increases, you require more energy 00:02:40.506 --> 00:02:41.506 to move the same object. 00:02:41.506 --> 00:02:45.581 This might mean that we need an infinite amount of energy 00:02:45.581 --> 00:02:48.422 just to move an object beyond the speed of light. 00:02:48.422 --> 00:02:51.139 However, the more energy you put into an object, the more likely 00:02:51.139 --> 00:02:53.362 you are to increase its mass rather than 00:02:53.362 --> 00:02:54.815 to increase its speed. 00:02:54.815 --> 00:02:58.490 Therefore, to get an object past the speed of light, 00:02:58.490 --> 00:03:01.240 quite impossible at this point in time. 00:03:01.240 --> 00:03:05.514 So why are we still so obsessed with time traveling? 00:03:05.514 --> 00:03:09.430 Time travel does come with its own set of problems. 00:03:09.430 --> 00:03:11.100 Take for example, my party before. 00:03:11.100 --> 00:03:14.310 If I were to send out my invitations after the party, 00:03:14.310 --> 00:03:16.518 my friends would have come back in time to attend it. 00:03:16.518 --> 00:03:18.780 And if they did come back in time to attend it, 00:03:18.780 --> 00:03:21.550 I wouldn't have to have sent out my invitations. 00:03:21.550 --> 00:03:23.690 And if I didn't send out my invitations, 00:03:23.690 --> 00:03:25.580 they wouldn't be at my party. 00:03:25.580 --> 00:03:28.464 It doesn't make any sense at all that they are both at my party 00:03:28.464 --> 00:03:30.150 and not at my party. 00:03:30.150 --> 00:03:32.380 This is what we call the grandfather paradox. 00:03:32.380 --> 00:03:34.230 It's one of three time travel theories 00:03:34.230 --> 00:03:36.182 that we currently have. 00:03:36.182 --> 00:03:38.720 Now, so it seems that we may not be 00:03:38.720 --> 00:03:43.210 able to time travel yet based on what physics tells us at least. 00:03:43.210 --> 00:03:46.560 For now, if you are planning a party, 00:03:46.560 --> 00:03:48.650 be sure to send out the invites first. 00:03:48.650 --> 00:03:51.780 Your friends can't time travel yet.