1 00:00:10,240 --> 00:00:12,840 Imagine holding a party and then sending out 2 00:00:12,840 --> 00:00:15,880 invitations after the party. 3 00:00:15,880 --> 00:00:18,414 Wait a minute, that doesn't make any sense. 4 00:00:18,414 --> 00:00:20,330 Are my friends going to be able to time travel 5 00:00:20,330 --> 00:00:22,270 back just to attend my party? 6 00:00:22,270 --> 00:00:24,290 In 2009, Stephen Hawking actually 7 00:00:24,290 --> 00:00:27,710 conducted such an experiment to disprove time travel. 8 00:00:27,710 --> 00:00:31,180 What exactly is time travel, and how does it work? 9 00:00:31,180 --> 00:00:34,075 It actually has a lot to with speed. 10 00:00:34,075 --> 00:00:36,200 A lot of what we know about time traveling actually 11 00:00:36,200 --> 00:00:39,540 comes from Einstein's Theory of Special and General Relativity. 12 00:00:39,540 --> 00:00:41,040 We are actually time traveling right 13 00:00:41,040 --> 00:00:43,210 now but not in the manner that sci-fi films have 14 00:00:43,210 --> 00:00:44,440 depicted them. 15 00:00:44,440 --> 00:00:48,640 We are, in fact, time traveling at a pace one hour per hour. 16 00:00:48,640 --> 00:00:51,770 In other words, every hour I experience, the world around me 17 00:00:51,770 --> 00:00:54,010 experiences a singular hour too. 18 00:00:54,010 --> 00:00:57,124 Now, it may sound simple and trivial, but stay with me. 19 00:00:57,124 --> 00:00:58,540 This is where it gets interesting. 20 00:01:02,340 --> 00:01:05,680 Now, the last part of Einstein's Theory of Special Relativity 21 00:01:05,680 --> 00:01:08,790 is actually a phenomenon known as time dilation. 22 00:01:08,790 --> 00:01:11,100 Effectively, this means that the faster you travel, 23 00:01:11,100 --> 00:01:13,290 the slower time passes around you. 24 00:01:13,290 --> 00:01:14,930 It potentially means that I could 25 00:01:14,930 --> 00:01:18,810 travel at maybe more than one hour per hour. 26 00:01:18,810 --> 00:01:21,090 This phenomenon, however, is only noticeable 27 00:01:21,090 --> 00:01:25,110 at really, really high speeds, speeds near the speed of light. 28 00:01:25,110 --> 00:01:29,230 This rocket here, not even close. 29 00:01:29,230 --> 00:01:31,210 Say I do have a spaceship now that 30 00:01:31,210 --> 00:01:33,550 travels at 90% the speed of light, 31 00:01:33,550 --> 00:01:36,070 and I have a pair of newborn twins. 32 00:01:36,070 --> 00:01:39,610 I bring one of them with me on this journey for 10 years. 33 00:01:39,610 --> 00:01:43,400 When I return, one of them will have turned 23 years old, 34 00:01:43,400 --> 00:01:46,117 while the one with me will have only turned 10 years old. 35 00:01:46,117 --> 00:01:48,200 While we have experienced 10 years on a spaceship, 36 00:01:48,200 --> 00:01:50,440 23 years have actually passed on earth. 37 00:01:50,440 --> 00:01:53,700 This amount of time actually increases exponentially 38 00:01:53,700 --> 00:01:57,095 as you get closer to the speed of light. 39 00:01:57,095 --> 00:02:00,550 So we could potentially travel forward in time, 40 00:02:00,550 --> 00:02:04,478 but what about backwards in time? 41 00:02:04,478 --> 00:02:08,406 That's a whole other issue. 42 00:02:08,406 --> 00:02:10,861 Now, remember our previous graph. 43 00:02:10,861 --> 00:02:14,390 The time that it took the [INAUDIBLE] 44 00:02:14,390 --> 00:02:15,812 to the speed of light. 45 00:02:15,812 --> 00:02:17,770 Looks like this thing may be the secret to time 46 00:02:17,770 --> 00:02:21,177 travel lies on the other side of the line. 47 00:02:21,177 --> 00:02:23,010 In other words, we may have to travel faster 48 00:02:23,010 --> 00:02:27,674 than the speed of light just to be able to travel back in time. 49 00:02:27,674 --> 00:02:31,132 Now, when you do travel at speeds near the speed of light, 50 00:02:31,132 --> 00:02:34,096 something called relativistic mass comes into play. 51 00:02:34,096 --> 00:02:37,924 In other words, as your speed goes up, your mass increases. 52 00:02:37,924 --> 00:02:40,506 As your mass increases, you require more energy 53 00:02:40,506 --> 00:02:41,506 to move the same object. 54 00:02:41,506 --> 00:02:45,581 This might mean that we need an infinite amount of energy 55 00:02:45,581 --> 00:02:48,422 just to move an object beyond the speed of light. 56 00:02:48,422 --> 00:02:51,139 However, the more energy you put into an object, the more likely 57 00:02:51,139 --> 00:02:53,362 you are to increase its mass rather than 58 00:02:53,362 --> 00:02:54,815 to increase its speed. 59 00:02:54,815 --> 00:02:58,490 Therefore, to get an object past the speed of light, 60 00:02:58,490 --> 00:03:01,240 quite impossible at this point in time. 61 00:03:01,240 --> 00:03:05,514 So why are we still so obsessed with time traveling? 62 00:03:05,514 --> 00:03:09,430 Time travel does come with its own set of problems. 63 00:03:09,430 --> 00:03:11,100 Take for example, my party before. 64 00:03:11,100 --> 00:03:14,310 If I were to send out my invitations after the party, 65 00:03:14,310 --> 00:03:16,518 my friends would have come back in time to attend it. 66 00:03:16,518 --> 00:03:18,780 And if they did come back in time to attend it, 67 00:03:18,780 --> 00:03:21,550 I wouldn't have to have sent out my invitations. 68 00:03:21,550 --> 00:03:23,690 And if I didn't send out my invitations, 69 00:03:23,690 --> 00:03:25,580 they wouldn't be at my party. 70 00:03:25,580 --> 00:03:28,464 It doesn't make any sense at all that they are both at my party 71 00:03:28,464 --> 00:03:30,150 and not at my party. 72 00:03:30,150 --> 00:03:32,380 This is what we call the grandfather paradox. 73 00:03:32,380 --> 00:03:34,230 It's one of three time travel theories 74 00:03:34,230 --> 00:03:36,182 that we currently have. 75 00:03:36,182 --> 00:03:38,720 Now, so it seems that we may not be 76 00:03:38,720 --> 00:03:43,210 able to time travel yet based on what physics tells us at least. 77 00:03:43,210 --> 00:03:46,560 For now, if you are planning a party, 78 00:03:46,560 --> 00:03:48,650 be sure to send out the invites first. 79 00:03:48,650 --> 00:03:51,780 Your friends can't time travel yet.