# I Time dilation and wormholes?

1. May 13, 2017

### sleeperservice

Hi,

I've been trying to work this out for quite a while and I just can't seem to get anywhere. My question is, if I were to travel between two points, say from earth to a planet in the Trappist system, I know that if I traveled in a vehicle I would experience time dilation, but if I were to travel instantaneously ( i know that itself is not exactly a straight forward concept, instantaneous relative to what) by whatever theoretical method (i.e. Wormhole, magic...), would I still experience time dilation? So, if I just stepped through a portal or some kind of mechanism, and to me the experience was instantaneous, and then I just hung out for a day and had a cotton candy dance party and the returned using the same method, would just a day have passed on earth?

2. May 13, 2017

### phinds

It is unfortunate that you "know" this, since it is incorrect. It is an extremely common misconception but it's commonness doesn't make it right.

No. You NEVER experience time dilation.

Depends on how you define the "day" in which you "hung out", but if I understand your intent correctly, then yes.

You need to read about the difference between time dilation (and what time dilation actually is) and differential aging, since you clearly are conflating the two in a way that is not meaningful.

3. May 13, 2017

### Staff: Mentor

There is no general answer to this question; it depends on the particular spacetime geometry involved. (This is also true for more mundane ways of traveling, btw. When you say you would "experience time dilation" if you traveled by ordinary means like a rocket ship--a better way of putting it, in accordance with phinds' objection, would be to say that when you returned to Earth you would have experienced less time than the time elapsed on Earth clocks--you are implicitly assuming a certain kind of spacetime geometry.)

4. May 13, 2017

### Mister T

You can't transport faster than light, but you can easily imagine using a transporter beam that moves at the speed of light. Suppose the transporter somehow transports all the information needed to re-constitute you in Trappist, and it does so at the fastest speed possible, the speed of light. Then one day later, after the cotton candy and the hanging out, you return using the same technique. When you get back only one day will have elapsed on your wristwatch, and you will be one day older. But a larger amount of time will have elapsed on Earth during your absence. If Trappist is, say, 10 light years away, then twenty years and a day will have elapsed here on Earth.

5. May 14, 2017

### sleeperservice

Thank you for the helpful replies. The reason i've found it difficult to work out in the past is that descriptions of time dilation due to relative velocity always involve constant acceleration at some % of c. I can see that the closer to c you get the stronger the difference on your 'ship's clock' would be relative to the clocks at your starting point. But in the scenario I describe above you would not travel the distance in the usual manner so I was curious if the same would apply. Equally, I thought it possible that if you were to step through a 'wormhole' or whatever, you would in theory be travelling far faster than c, so we don't really have a notion of what would happen as nothing should be breaking that particular speed limit?

6. May 14, 2017

### Mister T

You need to first study the simpler case of time dilation with zero acceleration. In fact, the phrase "acceleration at some percent of $c$" makes no sense. $c$ is a measure of speed, acceleration is something altogether different. You can't compare a speed to an acceleration. One way to see this is to observe that the two can never be measured with the same units.

7. May 14, 2017

### sleeperservice

Thanks Mister T, so acceleration is irrelevant in this case, all that would matter is your velocity relative to your original inertial reference frame? So someone who stepped through a portal instantaneously and traveled 30 light years from their original inertial reference frame would be traveling way faster than c, and I'm guessing that there isn't enough energy in the universe to allow that to happen, if they had actual velocity in relation to their original reference frame while traveling.

If, for arguments sake, they did step through a portal from earth to a planet over 30 light years away instanataneously, when they arrived on that planet would they have a different velocity relative to their original frame of reference? I guess they would, especially if they had stepped to another galaxy which was travelling at a percentage of c relative to the Milky Way/Earth, the original reference frame?

8. May 14, 2017

### Staff: Mentor

Why would you think that? As I said before, there is no general answer to this question; it depends on the specific spacetime geometry involved. "A portal" does not describe a precise model that can be used to make predictions; it's way too vague.

Again, there is no general answer to this question; it depends on the specific spacetime geometry involved.

You seem to think that "a portal" is some specific thing that we have precise theoretical knowledge of. It isn't. It's just a vague sci-fi concept that doesn't correspond to anything in actual science.

9. May 14, 2017

### sleeperservice

I'm not implying that a portal or wormhole or whatever sci fi device you want to insert has any basis in any model of physics (although in my extremely limited understanding, there do appear to be mathematical models for traversable wormholes, such as the Ellis Drainhole, albeit not necessarily useful for humans to pop between planets) and I do realise that as such there is no basis to come up with an actual answer to an enquiry like this. But considering how prevelant these devices are in popular culture, it's strange that relativity is always ignored when they are employed. My goal was only to get a better understanding of what the issues surrounding such a thing might be in relation to a person using them's experience of time, and everyone has actually been very helpful in pointing out what these may be.

10. May 14, 2017

### Staff: Mentor

Not that strange when you consider that the way they're used in popular culture requires something between ignoring and outright rejecting the actual science behind the concept.

Yes, the popular culture stuff is fun and not a lot of work, but the real thing is incomparably more exciting and rewarding. It's the difference between watching someone else's GoPro video and taking the race car out on the track yourself.

11. May 14, 2017

### Staff: Mentor

If it doesn't, then it is off topic here--it belongs in the sci-fi forum.

Yes, there are, and if you want to formulate meaningful questions about wormhole travel that can be discussed in this forum, you should try to do so in terms of one of those models. They are probably not realistic since making one would require using a kind of stress-energy that we don't know how to make or if it even exists. But mathematically the models are perfectly well defined and can be used to formulate thought experiments.

Again, that will depend on the specific model. There is no unique answer that applies to all wormholes. The Ellis drainhole is a particularly simple model; in that model, AFAIK, if the two mouths of the wormhole are at rest relative to each other, time "flows" for someone traveling through the wormhole the same as it does for an observer who remains outside one of the mouths and at rest relative to it.