I thought gravity was only supposed to affect things that have mass. But then how is it that gravitational fields can affect light (and, I assume, other forms of energy on the electromagnetic spectrum), which has no mass, by changing its trajectory for example?

I believe the short answer is that gravity warps space-time and objects with sufficient mass have gravity that distorts space-time.

This causes the light to travel over the 'curved' space time as opposed to in a straight line as if it were in 'flat' space time.

Once a light beam enters the event horizon of a black hole the curvature of space-time in that area is so intense that the light beam cannot escape.

This is an incomplete factually crappy answer, that none the less I believe is sufficiently correct to satisfy your question.

Gravity warps space. Light which travels in a straight line, is effected because its straight line path through spaces is bent because the space is warped. If the gravity is strong enough (black hole), the space is so warped that light has no path out

Can't we simply just say that this

is wrong, and gravity affects light too?

Or no, do you need the spacetime warping description?

Not necessarily because gravity isn't effecting light. It's effecting the space that the light is traveling through.

Mmmm I see. Ok that definitely makes sense.

Just curious why you think it's factually crappy? Just because the theories of gravity and space-time are incomplete? Is it a terrible model? Just curious as it's the description I hear the most and from scientific sources (for public consumption obviously).

I guess I phrased that poorly, I just mean I'm a layman, and often times the simplistic explanation has some form of errors in it but that conceptually I think that answer is good enough. I'm not sure how actually crappy that explanation was to someone who actually knows all physics. As you pointed out, there are mysteries still, so its undoubtedly incorrect in someway if you go down far enough.

This is a kind of cool explanation and I have never heard it phrased this way:

https://www.youtube.com/watch?v=pZ4giJARSyk

This ends up being an incredibly good approximation. More precisely gravity couples to something called the stress energy tensor which can be represented by a 4x4 matrix (4 comes from 3 spacial dimensions +1 time). So while light has no mass, it's stress energy tensor is non zero, so it couples to gravity as does everything. But it turns out that for pretty much all situations on earth or even in our solar system the stress energy tensor is dominated by the scalar mass term and Newtonian gravity works.

You could.

People want to say that because Einstein did.

The truth is, nobody has really seen a black hole and everything we "know" is theoretical. They just aren't highly luminous like a star, but there could be very little going on inside them to generate light anyway. For all we know they are purple or green or whatever, but the closest one is too far away to see if there's no nuclear fusion going on.

You don't need a black hole to see that gravity likely affects light though. Find "gravitational lensing" in wikipedia. Since nobody really knows what gravity is or what light is, who is anybody to say that gravity doesn't affect light?

Even if there was a lot of nuclear fusion or other luminescent processes going on, it would still be black. DUCY? (relates also to the first sentence in the quote)

Having just watched this, the question that occurs to me is probably one that is unanswerable given our present (lack of) understanding of black holes, but I'll pose it anyway. What would the threshold/boundary/perimeter of a black hole look like? Does it just sort of mesh gradually with the surrounding space, or is there a definite, identifiable edge to it, as with a star or planet for instance? Black holes have mass obviously, so that would seem to mean that they are distinct from the surrounding "massless" space. But I'm guessing a black hole wouldn't be solid to the touch, just like I assume stars aren't cause despite having enormous mass they're ultimately just burning gas. (Which reminds me, how do stars get ignited in the first place, anyway? I'm guessing the hydrogen just combusts when exposed to sufficient pressure from the immense gravity at the centre?)

I have so many questions

The theoretical identifiable edge is the event horizon....but I think "mesh gradually" is a reasonable description of what happens. You can cross the event horizon without noticing unlike say landing on a solid planet.

As you cross the event horizon shouldn't you start seeing light curving around to you from some of the stars recently gobbled by the black hole that you couldn't see before?


PairTheBoard

The last thing I remember reading on the subject I believe it as though you are frozen in time with all things that fell in before you 'below' you and if you could turn around you would see all things that fell in after you.

Gravity is so intense that your body would be stretched due to the difference in gravity between the part that crossed first and the parts that followed. It's an event that you'd be unable to survive haha, which I think we all assumed any way.

Another way you could clearly identify the boundary of a black hole is if it had an accretion disk of matter that was spiraling around it's gravitational field.

Obviously the internal components are all conjecture as we have no way of knowing exactly what happens inside an event horizon.

Interesting reading related:

https://en.m.wikipedia.org/wiki/Holographic_principle

https://en.m.wikipedia.org/wiki/Blac...mation_paradox

https://en.m.wikipedia.org/wiki/Hawking_radiation

https://en.m.wikipedia.org/wiki/Blac...thermodynamics

https://en.m.wikipedia.org/wiki/Accretion_disc

http://m.livescience.com/19683-happe...lack-hole.html

I'm not an expert....but i think there is serious debate among the actual experts related to this issue. What I said before about the event horizon not being special to an infalling observer was pretty much what everybody thought 3 years ago. Now alot of serious people believe in AMPS firewalls which says that what happens at an event horizon of an old/large black hole is radically different from what happens just outside.

I personally still like the old school argument ie nothing observably different happens when you cross the event horizon of large/old black holes because it would be really strange that general relativity/the equivalence principle gets violated so spectacularly so far away from any singularity. I;m completely fine with the equivalence principle failing well inside an event horizon because thats when you need quantum gravity which nobody yet understands. But my opinion is worth ~0 on this.

I hear we're looking for all that exotic matter like WIMPS and other really heavy stuff, so what are the reasons they wouldn't be found in a black hole?

Practical reasons abound.

Not sure i understand the question, but if WIMPS exist i imagine they should be found in as well as out of black holes.

That makes two of us - I don't understand my question either! I guess, my understanding is the theories for WIMPS and other exotic matter were formed in attempt to explain what's keeping galaxies from from flying apart, because there isn't enough regular matter to account for all the gravity needed. But we haven't been able to detect any of this exotic matter because it doesn't interact with anything or something, which I don't quite get because it's supposed to have such mega gravity. But anyway, could one reason we haven't been able to detect it be because most or all of it sits in the middle of these super massive black holes at the center of galaxies? Or have they been able to rule that out?

Dark matter is definitely appearing to be outside the center (or whatever is the cause of the effect if modified gravity or other models or systematic errors forced by poorly understood universal processes, would seem to fit such assumption also). DM is predominantly far from the central black hole of each galaxy that occupies anyway a tiny part of the galaxy only at best 1000 au in radius and most of the time far less than that say 10 au or 1 au or much less in our galaxy even.

It is the rotation curves that force the density of whatever is causing them to have a certain profile with radius that is definitely keeping it (speeds) at significant levels far from the center. In other words the rotation curves cannot be the result of just the visible matter plus other less visible compact objects, you need additional matter. A lot more in fact.

Eg see here; https://en.wikipedia.org/wiki/Galaxy_rotation_curve

and look under halo density profiles.
and https://en.wikipedia.org/wiki/Navarr...3White_profile
http://arxiv.org/abs/astro-ph/0502515 -> http://arxiv.org/pdf/astro-ph/0502515v1.pdf

https://en.wikipedia.org/wiki/Dark_matter_halo

https://en.wikipedia.org/wiki/Einasto_profile

http://arxiv.org/abs/astro-ph/0509417

Ah, that makes sense. Thx masque. Cool stuff.

New thought on crossing the event horizon.

http://arstechnica.com/science/2015/...to-a-hologram/