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Narrator: This is Space Place Musings, with Dr. Marc Rayman, a scientist at the Jet Propulsion Laboratory, answering questions from our Space Place partners across the nation. I’m Diane Fisher of the New Millennium Program.

Marc, this time, members of Tripoli Minnesota Rocket Club #45 ask—is time travel possible?

Rayman: Time travel is one of my favorite topics! It’s fun and mind-boggling to think about time travel. What if you went back in time and prevented your father and mother from meeting? You would prevent yourself from ever having been born! But then if you hadn't been born, you could not have gone back in time to prevent them from meeting.

Narrator: But, is it physically possible to travel in time? Is it against the known laws of physics?

Rayman: Well, no. The great 20th century scientist Albert Einstein developed a theory called Special Relativity. This theory says that space and time are really aspects of the same thing—space-time. But there’s a speed limit of 300,000 kilometers per second (or 186,000 miles per second) for anything that travels through space-time, and light always travels the speed limit through empty space.

Narrator: What does the theory say about moving through time as you move through space?

Rayman: A surprising thing happens when you move through space-time, according to Special Relativity, especially when your speed relative to other objects is close to the speed of light. Time goes slower for you than for the people you left behind. You won't notice this effect until you return to those stationary people.

Narrator: So, what would happen?

Rayman: Say you were 30 years old when you left Earth in a spacecraft traveling at about 99.5% of the speed of light (which is much faster than we can achieve now), and celebrated only five birthdays during your space voyage. When you got home at the age of 35, you would find that all your classmates were 80 years old!

Narrator: That certainly sounds to me like time traveling!

Rayman: It certainly is. And Einstein’s theories about time have been shown to be valid in many many experiments.

Time travel of a sort also occurs for objects in gravitational fields. Einstein had another remarkably successful theory called General Relativity, which reveals that time passes more slowly for objects in gravitational fields (like right here on Earth) than for objects far from such fields. So there are all kinds of space and time distortions near black holes and other exotic objects where the gravity can be very intense.

Narrator: So could those space-time distortions be used to invent some sort of time machine?

Rayman: Some scientists like the idea of "worm holes," which may be shortcuts through space-time. This and other ideas are wonderfully interesting, but we don't know at this point whether they are possible for real objects.

There are many questions about traveling to the past that do not yet have solid answers in science. However, in all time travel theories that seem to be allowed by real science, there is no way a traveler can go back in time to before the time machine was built.

Narrator: Do you think humans will ever be able travel into the future?

Rayman: I am confident rapid time travel into the future is possible. We know there are no fundamental scientific reasons preventing this, but we would need to develop some very advanced technologies to do it. We could travel 10,000 years or more into the future in less time than it takes to listen to this Podcast. However, such a trip would consume an extraordinary amount of energy.

Narrator: Some of our spacecraft already travel very fast. Do these time distortions come into play at these speeds?

Rayman: Actually, scientists and engineers who plan and operate some space missions must account for the time distortions that occur because of both General and Special Relativity. These effects are far too small to matter in most human terms or even over a full human lifetime. However, very tiny fractions of a second do matter for the precise work necessary to fly spacecraft throughout the solar system.

Narrator: It’s astounding that spacecraft navigation can be so precise that these tiny effects must be taken into account.

Rayman: True. The process of flying a spacecraft to a distant target is extremely complex. But it also requires a great deal of remarkable work to send back the pictures of these strange and fascinating places. They are really what makes the journey worthwhile.

Narrator: That’s right, and people often wonder how it is even possible to send images through millions or billions of miles of space.

Rayman: I know. I often hear that question. It was fun working with The Space Place web site team to explain how it’s done.

Narrator: To see and hear this enlightening animation, our listeners can visit spaceplace.nasa.gov and enter “transponder” into the search field.

Well, that’s about all we have time for now. Thanks for joining us for another Space Place Musings!

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