00:00The universe is vast. But is it truly infinite? And if it has an edge, what lies beyond it?
00:06These are the questions nearly everyone who has ever looked up at the night sky has asked.
00:10The answer, as it turns out, is far more brutal than you might expect.
00:14You have probably heard the number 46 billion light years.
00:17Many people believe this is the size of the universe. It is not. That number is only the
00:23farthest distance humans can see with their telescopes. Astronomers call it the observable
00:27universe. It does not mean the universe ends there. It means our view ends there. Picture
00:33yourself floating in the middle of the ocean. As far as you can see, there is a circular patch of
00:38water. But you would never believe the ocean is only that circle. Beyond that horizon, the ocean
00:43continues. It is the same with space. Beyond that 46 billion light year horizon, space keeps
00:49stretching onward. So how big is the real universe? Since we cannot see it, scientists decided to stop
00:55looking. Instead, they would calculate it. The key is the curvature of space. If the universe is a
01:02sphere, curving back on itself like the surface of earth, then it is finite. Keep walking in one
01:07direction, and eventually you will return to your starting point. But if the universe is flat, like
01:12an endless sheet of paper, it might have no boundary at all. The measuring tool is the oldest light in
01:17the universe. It is the leftover heat from 380,000 years after the Big Bang, called the Cosmic Microwave
01:23background radiation. The hot and cold patches on it have fixed sizes, like a ruler carefully
01:28calibrated in advance. If space is curved, that ruler will appear stretched or compressed in our
01:33view. From the Planck satellite to ground telescopes, humanity spent decades measuring it again and again.
01:38The final conclusion is that within a precision of 0.25%, the universe is remarkably flat.
01:44That means if the universe is finite, it must be almost incomprehensibly vast, so large that within the
01:49reach of our instruments, it appears perfectly flat. If it is finite, the universe is at least
01:54hundreds of times larger than what we can observe. Some calculations even suggest it may be millions
01:59of times larger. Of course, someone will now ask the obvious question, is the universe finite or
02:04infinite? The honest answer is that no one knows, and humanity will probably never know. This is not
02:10because our technology is not advanced enough. It is because the laws of physics themselves have locked
02:15away the answer. The fastest that any information or signal can travel is the speed of light.
02:20And the range in which humans can receive information is forever trapped inside the
02:23observable universe. Anything happening beyond that circle can never influence us, and can never send
02:28us evidence. You can guess that the universe is infinite. You can guess that it is finite but
02:33unimaginably enormous. You can even guess that beyond it lies an infinite array of parallel universes.
02:39But all of these can only ever remain theoretical speculation.
02:43This is the trap the universe built for us. It gave us a mind hungry for answers, then wrote into
02:48the laws of physics a rule that says you can never gather enough information to answer the question.
02:52And finally, here is a thought that should truly haunt you. If the universe is expanding,
02:57what exactly is it expanding into? If you say nothing, then what is that nothing itself?
03:03Another thing.
03:30Here we have a good idea.
03:31Thank you very much.
