Many time machines exist in the world today, but in many different senses. Here are just a couple of the awesome machines, some invented by humans and their intuitive minds, some naturally occurring in the universe. Here I will explain the scientific views on time machines, social time machines, and other things we use today to travel back to times we remember.
The scientific time machine: Albert Einstein found that time is not a constant. The only constant is light, and therefore, time can fluctuate. He proved that the faster you are travelling, the slower time will go relative to you. If you reach the speed of light (which is impossible) time will completely stop. If two identical people lived their entire lives on different parts of the Earth, the one closest to the equator would actually live slightly longer. Therefore, you cannot scientifically go back in time, but you can speed up and slow down time. It's up to scientists today to control this principle for human use.
The social time machine: Social time machines are all around us. Have you used a camera today? Have you remembered something today? Have you talked to someone today? All of these things are social time machines. They mentally take us back in time, and that counts just as much as physically being there. When you remember something, your brain is going through a time machine. These trips through time happen to us hundreds of times a day. Whenever we remember something, relive a moment, or talk to someone about what happened the other day, we're travelling through time.
Two more types of time machines I will explain are time capsules and dreams.
Time capsules: Time capsules are great things. They are containers, any size, that a person or group of people put things that are commemorative of the present inside. For example, today, a teenager might put things such as laptops, iPods, and skinny jeans. These are all items that are very popular at the moment. Many years later, either them or someone else will open the time capsule. The time capsule will capture the lifestyle of one time period, and when it is opened, many memories will flood back. This is a great example of a time machine that we don't use very often, that could bring back fond memories of the past.
Dreams: No matter what you've heard about dreams, we only know a couple things for sure. A: They happen when we are sleeping. B: They are a perfect example of a time machine that occurs naturally in the world. Scientists believe that dreams are a string of memories that we care about, jumbled up and slightly morphed. They could be things that we love, hate, fear or admire. You dream every night, so whenever you go to sleep, know that you're using nature's time machine.
As you can see, you don't need to be Doc Brown to use a time machine. Look around you. There are plenty of amazing time machines all around our everyday lives. I hope you'll use them more to relive some times in the past that you love.