128
votes
Accepted
What does it mean when people say "Physics break down"?
"Physics breaks down" is a bad way of saying what people are trying to say. It's the sort of thing that sounds cool at first, but then it starts misleading people.
What scientists mean is &...
- 7,331
56
votes
If iron can’t undergo fusion, does that mean a black hole is mostly iron?
If we are talking about stellar-sized black holes, then the object that collapses to form a black hole will have a high concentration of iron (and other iron-peak elements like manganese, nickel and ...
- 137k
51
votes
What does it mean when people say "Physics break down"?
Let me give an example of a very, very mild case of 'theory breaks down'.
Boyle's law is stated as follows:
$$ P_1V_1 = P_2V_2 $$
Expressing that for a given quantity of gas the pressure and volume ...
- 22.6k
50
votes
Accepted
Explain to a non-physicist what goes wrong when trying to quantize gravity
“What exactly goes wrong when trying to quantise gravity?”
There is no problem specific to quantum gravity! I know this isn’t conventional way to look at it, but the physics is actually not ...
- 524
49
votes
Accepted
Big Bang and the Pauli Exclusion Principle
The key confusion is the idea that Pauli Exclusion prevents any two particles from occupying the same space. The actual Pauli Exclusion Principle is slightly different: it prevents any two particles ...
- 5,136
48
votes
Accepted
I'm missing the point of renormalization in QFT
Here's the thing: renormalization and divergences have nothing to do with each other. They are conceptually unrelated notions.
Renormalization
Simply put, renormalization is a consequence of non-...
43
votes
If iron can’t undergo fusion, does that mean a black hole is mostly iron?
Iron can undergo fusion. However, iron is the point where fusions starts to cost more energy than it yields, so in a typical star it doesn't fuse.
In a supernova, and the abundance of energy ...
- 560
42
votes
Accepted
Why doesn't physics like infinity (or does it)?
There's no rule against infinity, only a rule against being empirically wrong. Historically, the infinite has hinted we're missing something, but so have plenty of other things too. Let's discuss some ...
- 25.4k
38
votes
Accepted
If black holes are just empty vacuum of space inside, then what causes the curvature?
GR describes curvature as being caused by stress-energy.
This statement is slightly wrong and is the cause of your confusion here.
Technically, in GR the stress energy tensor is the source of ...
- 109k
37
votes
How does Planck's curve solve the ultraviolet catastrophe?
The Planckian approaches zero fast enough: there can be some very high energy photons in a thermal distribution, but the probability of their presence drops fast; crucially, it drops faster than the ...
- 4,968
35
votes
Why does the Schwarzschild solution describe a black hole?
The Schwarzschild solution is not specifically for a black hole, although it does cover that. The Schwarzschild solution is a spherically symmetric vacuum solution. So it covers the vacuum region ...
- 109k
34
votes
Accepted
How does it make sense for the universe to have started from a big bang?
The singularity at the start of the universe in the Big Bang model is not supposed to be understood as part of the smooth manifold of spacetime, precisely for this reason.
The time function on ...
- 129k
29
votes
Accepted
Can a Kerr black hole become super-extremal?
TLDR; No, it cannot.
The answer to this question has a somewhat storied history.
Back in 1974 Wald considered a Gedanken experiment of what would happen if you would try to drop an object into a Kerr ...
- 12.9k
28
votes
Accepted
Do black holes exist in 1+1 dimensional spacetime?
Black holes are solutions of vacuum EFE on a space-time with singularities.
EFE in the vacuum are:
$$
G_{\mu \nu} = R_{\mu \nu} - \frac{1}{2} R g_{\mu \nu} = 0.
$$
These are sometimes written as
$$
R_{...
- 16.3k
28
votes
How do black holes move if they are just regions in spacetime?
Black holes are not just regions of space time. There was once a star there. According to the law of conservation of mass and energy, the star is still there. The star has spin and angular momentum. ...
- 393
28
votes
Explain to a non-physicist what goes wrong when trying to quantize gravity
I think, like a lot of technical questions, you can get different answers by "zooming in" to different levels of technical detail.
Here is highest level, least technical, shortest ...
- 55.4k
27
votes
Intuitively, why do attempts to delay hitting a black hole singularity cause you to reach it faster?
Actually, it turns out to be incorrect that the optimal strategy is to free fall. There is an optimal strategy for firing your rocket engine which maximizes your proper time from the event horizon to ...
- 109k
27
votes
Why does Roy Kerr claim that the Kerr black hole does not contain a singularity?
As far as I can tell, Kerr's point is that the singularity theorem doesn't specify whether or not a collapsing object will collapse to a "real" singularity. As a reminder, singularity ...
- 16.7k
26
votes
Accepted
How can electric field be defined as force per charge, if the charge makes its own, singular electric field?
It's true that a point particle with finite charge is problematic in electromagnetism because of the infinite field and associated energy near such a particle. However, we don't need that concept in ...
- 61.8k
24
votes
Accepted
Where does matter go after reaching singularity in uncharged black holes?
Strictly speaking geodesic incompleteness doesn't mean the worldline of the particle ends at the singularity, but rather that we can't predict what happens to it. The trajectory of a freely falling ...
- 363k
24
votes
Work done in assembling a point charge is infinite
“Embarrassing” is an opinion, and Griffiths generally has very well-respected opinions. However, as opinions are not facts it is not necessary for everyone to share them.
To me, it is not an ...
- 109k
24
votes
What does it mean when people say "Physics break down"?
"Physics breaks down" sounds good, but it is confusing. A better phrasing would be "known physics breaks down." Physics attempts to model reality using mathematics. In this ...
- 51.7k
24
votes
How does it make sense for the universe to have started from a big bang?
A “singularity” is shorthand for “a place where our current understanding of the laws of physics breaks down”. If we use our current model of the laws of physics (specifically general relativity) to ...
- 60.5k
23
votes
Accepted
How do physicists deal with fields at the location of charges?
This problem is nowadays referred to under the names 'self-force' and 'radiation reaction'. In classical electromagnetism it can be solved by noticing that the standard concepts (Maxwell's equations ...
- 61.8k
23
votes
What does it mean for the laws of physics to "break down" at a singularity?
The "laws of physics" are mathematical models that describe the behavior of things in the universe. We use the laws to determine things like how matter moves and how fields interact by ...
- 7,225
22
votes
Why doesn't physics like infinity (or does it)?
Infinity is a shorthand for unbounded. When we say that $\frac 1 x$ goes to infinity as $x$ approaches $0$, what we really mean is that we can make $\frac 1 x$ as large as we like by using a value of $...
- 60.5k
21
votes
Why does the Schrödinger equation work so well for the hydrogen atom despite the relativistic boundary at the nucleus?
In solving the Schroedinger radial equation there is no boundary condition applied at $r=0$. At $r=\infty$ yes, $R(r)$ must tend to zero - so we reject the positive exponential solution; any change ...
- 10.2k
19
votes
What do the poles of a Green function mean, physically?
Let me expand a little more on what Craig Thone just said :
Consider the energy/frequency-dependent Green function :
$$
\tilde{G}(\omega)=\frac{1}{\omega-(a-\mathrm{i}b)}
$$
with one single pole in $\...
- 2,598
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