# Black holes and space-time

1. Nov 17, 2012

### stevenx

Black holes can pull photons in although they move at the speed of light. So, does this means that black holes pull space-time in faster than light and if so, why can space-time "travel" faster than light?

2. Nov 17, 2012

### Simon Bridge

Welcome to PF;
Space-time does not "travel" (not the way you seem to be talking about anyway[*]). The BH is a region of extremely curved space-time so that, when viewed from a distance, objects tend to get sucked in (they "roll down" the curve).

It is a little like a wagon rolling on the ground - a steep-sided hole can "suck down" even the fastest wagon, but the ground does not have to move to do this.

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[*] there are situations where two objects can have their separation change faster than the speed of light ... the expansion of the Universe is one example.
You have to be careful not to let this confuse you. At this stage you just need to wrap you mind around the concept of space-time.

Last edited: Nov 17, 2012
3. Nov 17, 2012

### arindamsinha

As Simon Bridge has pointed out, this is not space-time traveling.

Interestingly however, space can and does 'expand' faster than the speed of light. This is also not a 'travel', and is not prevented by relativity theory.

4. Nov 18, 2012

### DrGreg

In relativity, all speed has to be relative to something. When we talk of the "speed of light", we mean speed relative to a local particle with mass. Light trying to escape from a black hole is travelling at exactly the speed of light relative to a local observer who can't escape either and who is falling in faster than the light is.

5. Nov 18, 2012

### Simon Bridge

<ahem> if the observer and the light are both heading for the center of mass, then isn't the light falling faster than the observer?

Perhaps, "the observer is more affected than the light".
Trying to talk sensibly, and simply, about relativity is a pain.

6. Nov 18, 2012

### PAllen

No, inside the event horizon, light directed outgoing is falling slower than an arbitrarily accelerating outgoing particle, such that it remains going outwards at c relative to the struggling rocket. Yet both are actually decreasing their SC r coordinate (quite fast).

Dr. Greg said: " Light trying to escape from a black hole". The implications is outward directed light.

7. Nov 18, 2012

### Simon Bridge

:) "if the observer and the light are both heading for the center of mass" i.e. going inwards. Presumably inwards travelling light is going faster than any inwards travelling massive body?

8. Nov 18, 2012

### PAllen

No again. Dr. Greg meant outward directed light. Due to the geometry of BH interior (spherically symmetric, uncharged), outward directed light moves steadily (even rapidly) closer to the r=0 singularity. However, massive body, trying to escape, decreases r even faster - again, even though firing thrust toward r=0. So outgoing light locally appears to move outwards at c relative to struggling rocket, while both decrease in r quite rapidly.

9. Nov 18, 2012

### Simon Bridge

No? inwards travelling light does not go faster than inward travelling massive bodies?
<sigh> I know - but Dr Greg responded to my comment, saying I got it wrong, using outward directed example to illustrate. But I was talking about inwards directed light. We spend all this time telling students that velocity is a vector right? [mumble: This was supposed to be a one-comment aside mutter grumble] :( Please lets continue in private or we'll hijack the thread.

10. Nov 19, 2012

### DrGreg

I'm sorry if you got that impression, but I never actually said that. I was responding to the original questioner, and my post wasn't even consecutive to yours, and made no mention of it. As PAllen said, I was talking about light aimed outward (i.e. trying, but failing, to go outward) rather than light aimed inward.

Never mind, let's see what the questioner has to say.