# If a strong gravitational wave hit Earth would clocks record it?

1. Aug 2, 2013

### wolram

If a strong gravitational wave hit Earth would clocks record it? as if i understand it it is space time that is oscillating so the time component must be changing.

2. Aug 2, 2013

### Simon Bridge

You mean, would two clocks go out of sync?
I think that would depend on the geometry of the wave.

3. Aug 3, 2013

### wolram

You mean, would two clocks go out of sync?

That is what i am thinking, as for the geometry of the wave am not sure.

4. Aug 3, 2013

### Simon Bridge

Two clocks in different strength gravity will go out of sync.
If a strong pulse passes by, and it was shaped so that one clock spent time in stronger gravity than the other, then they'd go out of sync. So it is possible ... the short answer to your question is "probably".
You'd have the usual trouble working out whose clock was "correct" (answer: neither of them) but you'd theoretically see the difference.

5. Aug 3, 2013

### Staff: Mentor

Hmm... I think that would need a source very close to earth (or extremely directed).
And even then, the required signal strength to see it would be absurd.

6. Aug 3, 2013

### Simon Bridge

Yeah - I don't think we'd be noticing the clocks being a few milliseconds out so much as the sudden appearance of a pair of huge masses at the edge of the solar system.

I don't think there is anything in the math to prevent pretty much any shape pulse - but that doesn't mean the source can exist in nature.

7. Aug 4, 2013

### timmdeeg

Imagine 2 clocks emitting light pulses at a fixed distance. I think, they will see each other periodically ticking slower, rep. faster, due to the locally expanding and contractiing space-time in case a gravitational wave passes by.

8. Aug 4, 2013

### Staff: Mentor

Length changes are easier to detect - we have gravitational wave detectors sensitive to relative length changes of 10-22 - less than the diameter of a proton over a kilometer. That is more precise than our best clocks (~10-17).

9. Aug 4, 2013

### Chalnoth

Yeah. I'd honestly be shocked if the collision of a pair of supermassive black holes very close to the Earth would be enough to cause a detectable difference.

10. Aug 4, 2013

### Tanelorn

I beg to differ, I wouldn't want be anywhere near something like that! :)

There again since they are both black holes perhaps nothing gets ejected or emitted and they just join and become one?

Last edited: Aug 4, 2013
11. Aug 4, 2013

### Simon Bridge

12. Aug 4, 2013

### Tanelorn

Interesting facts:

"The Milky Way has a black hole at its core with a mass of four million times that of the Sun.

A billion solar mass black hole would have an event horizon 3 billion km in radius — roughly the distance of Neptune to the Sun.

See where I’m going here? If you were to rope off the solar system out past Neptune, enclose it in a giant sphere, and fill it with air, it would be a black hole!

That, to me, is by far the oddest thing about black holes. Sure, they warp space, distort time, play with our sense of what’s real and isn’t… but when they touch on the everyday and screw with that, well, that’s what gets me."

I thought the gas would collapse to a near singularity?

Last edited: Aug 4, 2013
13. Aug 5, 2013

### Simon Bridge

-- Phil Plait

I had a big argument about that in these forums a while ago ... the trick to thinking about this sort of thing is to ask "who is doing the measuring?" Would an infalling observer ever see the gas cloud?

Remember also that the physical singularity at the center of a black-hole is most likely an artifact of our maths rather than something that actually happens.

14. Aug 5, 2013

### Tanelorn

Perhaps if it was rotating... no I don't think that would prevent collapse at the poles?

If the gas molecules were stationary and there was just normal space beyond, wouldn't gravity cause the molecules to collapse to something much more dense?

Much smaller gas clouds collapse to something the size of a star.
Would the event horizon in some way prevent the collapse?

Last edited: Aug 5, 2013
15. Aug 5, 2013

### Chalnoth

I'm sure you wouldn't :)

Just stating that I don't think you'd find two clocks at different places on Earth becoming noticeably out of sync. There probably will be other significant effects, but not that one.

I think it depends upon whether or not any matter gets caught up in the merger. But the gravity wave emissions of the event would be quite massive. I'm not sure how noticeable it would be, but obviously our gravity wave detectors would go nuts.

16. Aug 5, 2013

### Staff: Mentor

The gas cloud would collapse and form a star long before you have 3 billion solar masses inside. If that is not possible (because you use iron or similar elements where fusion does not release energy), you quickly get a white dwarf, then a neutron star, and then a smaller black hole, slowly growing as you put in more gas.

17. Aug 5, 2013

### Simon Bridge

Aside:
@mfb: you could, in principle get enough mass inside it's Schwarzschild radius while it is still very diffuse... unless a star can form at low densities?

The comment comes off Phil Platt's popular black hole lecture. Perhaps we should ask him for a reference?
He observes that a "solar-system filled with air" would be a black hole.

Presumably, IRL, we'd expect such a large condensing cloud to contain irregularities so some parts of it would form protostars etc first.... before the entire thing got that small.
i.e. via: Begelman M., Volonta M. (2006) Formation of supermassive black holes by direct collapse in pre-galactic haloes (RAS Monthly Notices)

Perhaps this is a topic for another thread?

18. Aug 5, 2013

### Staff: Mentor

In principle, you can probably shoot gas in from everywhere at the same time in the right way to avoid collisions, assuming magic outside controlling all that mass. Just slowly pumping mass inside (and 1 solar mass per second is slow here!) won't work, however.

1 solar mass per second needs a flow velocity of 100km/s.
100 solar masses per second need 10000km/s... ;).

19. Aug 7, 2013

### Lino

Why would that be a black hole? I assume that we would have to fill the sphere with a billion solar masses of compressed air to have that effect ... wouldn't we?

Regards,

Noel.

20. Aug 7, 2013

### Staff: Mentor

You don't have to compress the air to get a black hole.

If you don't fill that sphere quickly, it will compress itself due to gravity, however.

21. Aug 7, 2013

### timmdeeg

Whether the solar system being filled with air would be a black holes depends on the mass M of the air and the radius R of the system. If the Mass is within (R <= r) it's Schwarzschild radius according to r = 2 GM (G is the gravitational constant) then it is a black hole.

22. Aug 7, 2013

### Lino

Thanks timmdeeg. I accept that.

timmdeeg / mfb, are you saying that a mass of any material, for example air, will collapse under its own gravity such that its "size" will be less than its Schwarzchild radius? If so, when the solar system was forming from a gas cloud, what prevented its collapse to a BH and instead caused the sun to start shining?

Regards,

Noel.

23. Aug 7, 2013

### Staff: Mentor

No. How did you get that impression?

24. Aug 7, 2013

### Lino

Apologies mfb, I don't know how else to read "You don't have to compress the air to get a black hole". If you start with a volume like that in the earlier posts, and don't compress it, other than with its own gravity, how do you get all the volume within the required radius?

25. Aug 7, 2013

### Staff: Mentor

There is no need to get "volume in", the volume is always there and determined by the radius of the region. You can get all the air in if you let it flow in at standard pressure and temperature (=like in our atmosphere). You don't need a high density, that is the key point of the large black hole.