# What is this “time travel” these popular blogs refer to?

I am getting confused by some "popular science" about time travel, such as

It is indeed an interesting property of physics as we know it, and a great gift from the theory of relativity. It leads to useful perspectives on the nature of time as exemplified in the Twin Paradox example: When you travel for instance at a Lorentz factor of γ = 100 (99.995% the speed of light) for 10 years, time "dilates" to thousand years. For instance it has been proven experimentally that muons would "tick" slower, and therefore for a measure of time in seconds. Similarly, I would assume that extending this property at a macroscopic scale, your whole body would slow down...

My question is to know if will you "live for a thousand years" (internet abounds of such statements as in this example forum and may be interpreted as such in the headlines above), or rather that during the 10 years of your travel (in your spaceship) and which would be measured as 1000 years in another frame of reference (the earth), you would have aged by "only" 10 years in your spaceship - time "froze" during the travel.

• I'm not at all clear what you are asking. If you are asking "does special relativity apply to humans" then the answer is "surely yes". If you are asking "Do we possess the technology to leverage this fact, then the answer is "surely no". If you are asking "what should bloggers write about" that is a matter of opinion (and so not a good question for this format) – James K Dec 11 '18 at 10:59
• Twin paradox and time travel are not the same. None of the twins time-travel! So you got it right. There is just time dilation, or living slower. – Alchimista Dec 14 '18 at 11:48
• Even after the edit, it's still not completely clear what your actual question is asking. If your round trip takes 10 years according to your shipboard clocks, with a $\gamma$ relative to Earth of 100, then you will age 10 years, but 1000 years will have passed on Earth when you return. – PM 2Ring Dec 20 '18 at 12:11
• We all travel into the future at a rate of 1 second per second in our various frames of reference. The only thing which is even slightly surprising here is that these frames of reference can be more varied than most people assume. – Steve Linton Dec 21 '18 at 10:38

My question is to know if will you "live for a thousand years" (...) or "only" 10 years in your spaceship - time "froze" during the travel.

If you were to travel to close the speed of light (not doing the math, plainly speaking), you would appear to live for a thousand years to the people that are outside of your frame of reference. Time would actually pass the same to you and you would observe the time outside your frame of reference to pass significantly faster.

Is it a scientific fact?

Human time travel is a very real scientific fact, but the implications are too complex to be presented to the public sphere since it has no practical applications yet. If we were to sent people to another star system (Alpha Centaury A/B - at 4.3 light-years from Earth), accelerating their ship close to the speed of light would make the time in their frame of reference pass slower relative to the outside of their frame of reference, thus allowing them to reach the destination in a shorter time from their perspective. It would be useful when managing supplies on this vast journey.

• Calling that "time travel" may be a bit of a stretch, no pun intended. I think that when people think of "time travel", they generally think either of travel to the past, reversible travel (to the future and back), or at least a discontinuous jump. Rip Van Winkle isn't usually described as a time travel story, and when it is, it's because of the discontinuity in his conscious experience. – Jacob C. says Reinstate Monica Feb 13 '19 at 8:55

Saying that time "froze" is a poor way to put it. To "freeze" means to stop entirely, but time only slows down. Also, saying that you could "live for a thousand years" is a very poor way to put it, because that makes it sound like you could experience a thousand years of life, which is false. What is true is what you say here, assuming you travel at the correct speed:

during the 10 years of your travel (in your spaceship) and which would be measured as 1000 years in another frame of reference (the earth), you would have aged by "only" 10 years in your spaceship

If you travel faster than that, even more time would pass on earth compared to ten years on the ship, and if you travel slower, less time would pass on earth. A trip like this would go far past Alpha Centauri, since you would be traveling at nearly the speed of light.

No one knows if we will ever be able to travel at these speeds.

They're referring to Time Dilation. The faster you go the slower time is moving for you. If you were to reach the speed of light for you time would come to a dead stop.

• Except it's impossible to "reach" the speed of light. Either the speed of light is the only speed you travel at in a vacuum (because you're a massless particle), or it's the speed you can never reach (because you have mass). – Chappo Hasn't Forgotten Monica Dec 28 '18 at 23:38
• Right, you can not reach speed of light, but the answer rightly captures the fact that it is a dilation not a travel... – meduz Jan 28 '19 at 14:37

These popular books refer to the moving of people from the present, past or future to any other willingly or intentionally. The world we are currently living in is the present. We move towards the future at an almost constant rate that is 1min/minute(say). If by some reason some people found out how to travel slower or faster than this normal rate, they are able to travel in time. Physical laws of physics have no such limitation or rule that this cannot happen, its the engineering and machinery which lacks the ability to do so. Even so, there are some quite interesting stuff which might happen if you went in future or past. These movements through time introduce paradoxes which are hard to wrap our heads around. Like, the very famous of them all--Grandfather Paradox, where you travel back in time and somehow prevent your grandparents from the meeting. This generates a question, who was the person who prevented the meeting in the first place? There is a very good explanation thought up by our scientists that, the time you go which you regard as past is actually a parallel universe and in no way affect the reality of the time from which you came from. Its said that we will not be able to travel to the past, but we can travel to the future quickly. This can be achieved by living inside a high gravitational field where time slows down making you travel through time slowly with respect to others. These ideas are thought upon and modified a bit according to the imagination of the authors to provide a new version to it, making it interesting to read.