# B Where is the slowest point in terms of time?

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1. Mar 21, 2016

### Josh_H

If moving inside the event horizon of a super-massive black hole and theoretically surviving we could see the universe pass by at millions of years per second relative to someone on earth, where could we go where time passes at a much faster rate than someone on earth? For example where 2 minutes at this place would be a minute and 59 seconds on earth e.t.c, because surely orbiting the sun and moving in our galaxy at whatever ridiculous speed compared to other galaxies gives us some time dilation, however minimal, correct? So maybe existing in a void would be the slowest point in terms of time in the universe as there are very few or no gravitational forces causing acceleration (leading to time passing slower). This is my first question on this site so apologies if I offend people with my ignorance.

2. Mar 21, 2016

### Staff: Mentor

That's not how it works. You'll find many threads about the behavior of black holes here.
Before you ask that question, you have to have a clear understanding of what it means for time to pass at different rates for different observers. Everyone, regardless of speed and gravity, experiences the same rate of time passing: one second per second. Thus, the only way that we can say that time passes more quickly for one person than another is to have them both at the same place at the same time, then separate for a while and take different path through spacetime, then meet up again so that they can compare how much time passed for each on their different journeys. (You might want to start with the twin paradox FAQ to see how this works).

3. Mar 21, 2016

### Staff: Mentor

We wouldn't. First, there is no way to compare the "rates of time flow" between these two observers. Second, someone who falls into a black hole does not see events in the outside universe pass by very quickly; someone hovering very close to the horizon does, but that's not the same thing.

Nowhere. Being able to compare "rates of time flow" at all is only possible in a limited number of cases. Of those cases, there are none in which another observer has a much faster "rate of time flow" than the observer on earth. The best you could do would be to move out into intergalactic space and sit there at rest relative to the Milky Way galaxy. Your "rate of time flow" could be compared with that of someone on earth in this scenario; but yours would only be faster by a few parts per million.

Orbiting the sun, and being in the sun's gravity well--and for that matter orbiting the center of the galaxy and being in the galaxy's gravity well--does give you some time dilation relative to the observer out in intergalactic space that I referred to above; this is one of those limited number of cases where rates of time flow can be compared.

Our motion relative to other galaxies due to the expansion of the universe, however, is not one of those cases; there is no meaningful concept of "time dilation" for us relative to distant galaxies, or vice versa.

4. Mar 21, 2016

### Staff: Mentor

Actually, there is another way: both observers can be in a stationary spacetime, and have well-defined positions and velocities relative to stationary observers in this spacetime. Then they can compare rates of time flow by exchanging light signals. But this is still, as I said in my previous post, a very limited number of cases.

5. Mar 21, 2016

### Josh_H

What does being in stationary spacetime actually mean? Does that mean when it isn't being contorted by gravity?

6. Mar 21, 2016

### Josh_H

I know that time appears to pass at the same rate for both observers, but as you say if I went off in a spaceship and went infinitesimally close to c, then you would have aged much more than me by the time I get back, experiencing the full time period that is standard on earth. I would only have experienced a fraction of the time you have experienced by the time I return, yet I was wondering is there a way to increase your relative velocity in the other direction so you age faster than the person on earth?

7. Mar 21, 2016

### Josh_H

Ok I understand now, its impossible to measure it due to the ever changing speed of our plane of existence.

8. Mar 21, 2016

### jbriggs444

Well, except for the fact that "plane of existence" is ill-defined and does not change speed.

9. Mar 21, 2016

### Staff: Mentor

No. It means that the curvature of spacetime is not changing with time. More precisely, it means that there is a family of observers in the spacetime, called "stationary" observers, who observe a spacetime curvature in their local vicinity that is not changing with time. (The curvature can be different for different observers, but it must be unchanging with time for each observer.) In the case of an isolated gravitating body like the Earth, stationary observers are observers who are "hovering" at a fixed altitude above the body.

10. Mar 21, 2016

### Staff: Mentor

No, there isn't. Of all the people who go between the same pair of starting and ending events--the event where all the travelers leave Earth while the stay-at-home person stays, and the event where all the travelers come back to Earth and compare elapsed times with the stay-at-home person--the stay-at-home person has the longest elapsed time.

11. Mar 21, 2016

### Josh_H

So you're saying space-time does not expand at different rates throughout the universe?

12. Mar 21, 2016

### Khashishi

Stationary spacetime would be space that isn't expanding or contracting or changing shape. So we don't live in stationary spacetime.
Space-time does/did/will expand at different rates at different points in history. The expansion was very fast during inflation, and is presently accelerating.
Now, if you are talking about different places at the same time, you have to be careful about how you define same time. If you tell your friend in the Andromeda galaxy to make a measurement on Monday, March 21, they aren't going to know what you mean. Moreover, if you tell them to make the measurement "now", then they will get the signal after light has had time to traverse the distance. Moreover, the distance depends on how fast you are moving relative to each other. You need some sort of absolute reference to have absolute dates. Fortunately, such an absolute reference does exist. It is the Cosmic Microwave Background. To answer the question, we think space-time expands at the same rate everywhere, using CMB reference time.

13. Mar 21, 2016

### jbriggs444

No. I am saying that "plane of existence" is not well defined. It has no standard definition nor have you provided a private definition.

Further, I am betting that whatever reasonable meaning you come up with for "plane of existence", it will be one for which the "speed" of a particular plane will fail to be a meaningful quantity.

14. Mar 21, 2016

### Staff: Mentor

The universe as a whole is not a stationary spacetime, yes. But parts of it can be very well approximated as stationary. For example, if we are studying the motion of objects around the Earth, we can consider the patch of spacetime we are working with to be stationary.

15. Mar 21, 2016

### Josh_H

Thanks this is really informative, i'll research CMB time to understand more of what you are saying.

16. Mar 21, 2016

### Staff: Mentor

A more common term for this is "comoving time" or "FRW coordinate time"; it's the time experienced by "comoving" observers in our standard cosmological models. These are observers who see the universe as homogeneous and isotropic; this includes the CMB, which is why "CMB reference time" or something similar is sometimes used to describe this time.