# Is the gravitational time dilatation a real effect?

1. Jun 8, 2010

### Kamil Szot

Some people explain that gravitational time dilatation is just photons redshifting due to work they are doing against gravitational field while escaping from it, but I heard that the delay of the clock on the probe after passing near Jupiter was observed.

2. Jun 8, 2010

### D H

Staff Emeritus
3. Jun 8, 2010

### Staff: Mentor

Gravitational time dilation can even be observed in a tall building with sensitive equipment. Google the Pound-Rebka experiment.

4. Jun 8, 2010

### D H

Staff Emeritus
The Pound-Rebka experiment tested for gravitational redshift rather than time dilation. While the effects are certainly related, Kamil asked about time dilation in the OP.

To Kamil: Gravitational redshift (or blueshift) and gravitational time dilation are different effects/different ways of explaining the same phenomenon. You do not get one without the other.

5. Jun 8, 2010

### Staff: Mentor

As you say, they are the same phenomenon.

6. Jun 9, 2010

### Kamil Szot

Ok. So I understand that this is a real effect. It does affect actual clocks, probably irregardless of the principle on which they are operating. I doesn't have to involve redshifting photons.

Second question:

Difference of paces of time between two points is higher if difference of gravitational potential between them is higher, right?

7. Jun 9, 2010

### Staff: Mentor

Yes.

8. Jun 10, 2010

### Kamil Szot

At some distance from the center of mass of a sufficiently massive body there is a place with such a property that if you wanted to escape from this place (to some other spot, located, let's say 10 mld ly away) you would need to have nearly infinite energy, right?

Can I call this place as having gravitational potential equal nearly minus infinity?

Does it have nearly infinitely slower time pace than we do?

By saying that 'something is nearly equal infinity somewhere' I mean that it's very high, and if it's not high enough for purposes of my thinking I can move just a small bit to make it sufficiently higher.

I'll cut the chase to the problem that is truly bothering me.

If the answer to three questions above is 'yes' then how can anything in less than 14 mld years pass this zone of very low potential that has nearly infinitely slower time pace then we have?

If 14 mld years (by our count) from the moment of getting into slow zone something decided to use it's energy in effort to escape (this moment would be only fraction of a second after, from its point of view), we could see it out in some time in the future.

How can we know that from where we stand anything ever passed point of no return?

Can you explain to me what point of my reasoning is wrong?

9. Jun 10, 2010

### Staff: Mentor

What does "mld ly" mean? I assume ly is light-year, but I don't know what mld is.

Yes to all 3.

What is the significance of the change from 10 to 14 mld years?

The point of no return is called the event horizon. If we are outside the event horizion then we cannot know about anything past the horizon.

10. Jun 10, 2010

### Kamil Szot

Ouch. Sorry. I'm from long scale country (http://en.wikipedia.org/wiki/Long_and_short_scales ). It was supposed to be shorthand from 109, a billion in short scale. From now on I will use short scale.

Great!

Sorry again. 10 billion light years was arbitrarily chosen to be point far enough not to feel any gravitational influence of massive body. I should probably write: "escape to infinity" but I felt that I had already too many near infinities and wanted to avoid confusion. Seems that I introduced one instead.

I chose 14 billion years as time to convey suggestion that no matter how long (from our point of view) something is moving towards point of no return, we can't be sure if it won't turn back and escape to show up again some day.

I used term point of no return because it's unambiguous (I hope). Wikipedia entry about event horizon mentions absolute horizons, apparent horizons and other notions of horizons. Again, I wanted to avoid confusion.

I'm not saying anything about anything behind or at event horizon. My question is about stuff near event horizon but outside.

11. Jun 10, 2010

### George Jones

Staff Emeritus
Last edited: Jun 10, 2010
12. Jun 10, 2010

### Kamil Szot

Let me get back to my original question because it better phrases my problem:

Can anything in less than our 13 bln years pass this zone of very low gravitational potential that has nearly infinitely slower time pace then we have?

By passing I mean entering it (from outside) and leaving it (towards surface of no return). I'm not even asking how anything passed event horizon.

I'm asking how can anything (for example neutrino) pass the zone between 1 meter from event horizon and 10-15 meters from event horizon in our measly 13.7 bln of years. Please substitute 15 with sufficiently high value. I didn't really do the math on this.

Thank you for the links but I unfortunately did not find answer to my question there. Also I'm not interested in black hole formation at the time but rather with black hole growth and merging.

13. Jun 10, 2010

### Staff: Mentor

Essentially correct.

14. Jun 12, 2010

### Kamil Szot

So if nothing ever has even got close to event horizon (yet) then nothing ever passed event horizon (irregardless of whether it is possible or not) and no blackhole has since it's creation gained even one pound no matter how many stars it obliterated. Also no two blackholes ever merged. Right?

If you agree on this with me then perhaps I'm discussing that matter with wrong person. ;-)

15. Jun 12, 2010

### Staff: Mentor

All of what you say is correct in Schwarzschild coordinates or in Rindler coordinates in flat spacetime. However, in both cases the singularity at the event horizon is only a coordinate singularity which can be removed by a suitable coordinate transformation. In both cases an object free falling crosses the event horizon in a finite amount of proper time even though it takes an infinite amount of coordinate time.

16. Jun 13, 2010

### Kamil Szot

Are calculations done with Schwarzschild coordinates wrong outside of event horizon?

Don't calculations done in "coordinates after suitable transformation" indicate that there is difference in time paces just outside event horizon and in remote areas exactly same as calculations done in Schwarzschild coordinates do indicate?

Gravitational time dilatation is not some elusive mathematical glitch. It's a part of the reality we are living in and its existence should not depend on chosen system of coordinates.

I'm not even touching event horizon. I'm not trying to deal with discontinuity that occurs at that exact surface.

I just can't see how anything could get close enough to this discontinuity in mere 13.7 bln years of our time.

Let me make up a story illustrating why I think that no object falling at blackhole has passed event horizon yet.

Let's say that shortly after big bang someone wanted to have a bit fun and tossed at some black hole device that contained equal amounts of matter and antimatter and a trigger mechanism. Trigger mechanism was designed to mix matter and antimatter when whole device gets as close to event horizon as 10-6 meters. 13 bln years of outside time have past and the free falling device (after few hours of proper time) finally triggered itself and part of energy created by annihilation began ascending back from gravity well. We are going to see in in another few bln years or so.

P.S.

I really should calculate how close to event horizon can free falling object get in 13.7 bln years of remote time. Making up numbers like 10-6 probably isn't doing me any good.

17. Jun 13, 2010

### Staff: Mentor

No, they are not wrong but they are also not unique.

The key word here is "yet". It implies a choice of some simultaneity convention and therefore the answer is coordinate dependent.

18. Jun 13, 2010

### Kamil Szot

You can describe given thing in different coordinates and get different speeds, accelerations even precedence. But you cannot turn reversible event into irreversible event by changing coordinates that you use to make calculations about physical reality.

But we are drifting away from my question.

Why do you dismiss result of reasoning based on Schwarzshild coordinates if I do not go outside its domain of applicability?

Can you give me an example of some simultaneity convention by which this exact spot of spacetime at which I am at the moment is after the event of anything falling to place being 10-15 meters outside of event horizon?

19. Jun 13, 2010

### Staff: Mentor

I have never said that you could. Per your scenario we are dealing strictly with events outside the horizon, so escape is possible (I assume that is what you mean by reversible).

I don't dismiss the results. I just point out that they are coordinate dependent.

20. Jun 13, 2010

### Kamil Szot

Thank you. That was exactly what I meant.

I think I can't agree on that with you. I think if escape is possible in one set of coordinates then it also must be possible after transformation to any other sets of coordinates that are valid for given situation.

I'll ask directly. Do you think that anything of what fallen at black holes for the last 13 bln year already crossed any of the event horizons making escape impossible? And if not then when it will happen? By 'already' and 'when' I'm taking about simultaneity as seen from our frame of reference.