Masaki Shigemori

G30 Studium Generale
10 January 2020

What Is A
Black Hole?

What is a black hole?
What one gets by compressing matteras much as possibleSun:
106 km1030 kg3

3 km

Escape velocity
Minimum velocity for an object toescape a star’s gravity
Earth: 𝑣 = 11 km/s (∼ 40,000 km/h)
Sun: 𝑣 = 620 km/s (~2,000,000 km/h)
 Heavier star → 𝑣: larger
 More compact star → 𝑣: larger©pixabay4Extremely heavy and compact star
→ 𝑣 is larger than speed of light→ Nothing can escapeBlack hole
What’s inside:collapsed star?
“event horizon”
5Black holes in the sky
 Stellar

black holes

Figure removed due to copyright restrictions.
A few solar masses. Radius ~10 km

Millions ofsolar masses.

 Supermassive

black holes

6Radius:can be larger than
Pluto’s orbit
https://ja.wikipedia.org/wiki/%E3%83%95%E3%82%A1%E3%82%A4%E3%83%AB:Supermassi
veblackhole_nasajpl.jpg 2020/02/13

How do we observea black hole?

Gravitational waves

Courtesy Caltech/MIT/LIGO Laboratory8

Gravitational wave detectors
LIGO Hanford,USA

VIRGO,Italy Detects tiny distortion of spacetime (1/1000 of proton radius)

when gravitational waves reach the earth.

9First detection of grav. waves
September 14,2015


Merger of binary black holes2017 Nobel prize

LIGO

10

Current status of GW observation
BHs foundby gravitationalwaves

BH found by
conventional
method(electromagnetic
waves)
Neutronstars

https://media.ligo.northwestern.edu/

11

“Event Horizon Telescope”
 Observe BHs with an earth-sized radio telescope

12

EHT networkhttps://twitter.com/thenrao/status/940237297697677312

13

Pngimg.com

BH at the center of Milky Way Galaxy

supermassive BH(4.3 million solar
masses)Pngimg.com

10 microarcsec
https://alma-telescope.jp/column/bhimaging7 2020/2/13

26,000lightyears
earth

14

(a golf ball on the moon)

SagittariusA*©pixabay

BH at the center of M87
nasa.gov

supermassive BH(6.5 billion solar masses)earth

55 million
lightyears

Figure removed due tocopyright restrictions

Supergiant elliptic
galaxy M87
15

Image of the BH in M87
April 10, 2019 The first direct image of a BH

 Matches well with computation based on theoretical model

www.eventhorizontelescope.org

 Other black holes in the future16

Spacetime andBlack Holes

Einstein’s “general relativity”
Not only matter but its container –spacetime – is also a physical object.
It does stuff like move, bend,etc.

Albert Einstein
https://listas.20minutos.es/lista/personajesfamosos-de-la-astronomia-95863/2020/2/13

18“spacetime”

Matter bends spacetime. Conversely, bent
spacetime affects the motion of matter inside it.
https://listas.20minutos.es/lista/personajesfamosos-de-la-astronomia-95863/2019/2/13

A heavy object bends spacetime around it

Bent spacetime affects motion of matter in itBlack holes in general relativity
Extremely curved spacetime
Light rays

Karl Schwarzschild
Event horizon

Singularity
20

https://en.wikipedia.org/wiki/Karl_Schwarzschild 2020/2/13

Inside: curved spacetimeand singularity

Microphysics ofBlack HolesMacroscopic vs microscopic
Macroscopic:
Continuous matter

Microscopic:
Discrete structure (atoms)“Quantum mechanics”©pixabay

Niels
Bohr
22

https://ja.wikipedia.org/wiki/%E3%83%8B%E3%
83%BC%E3%83%AB%E3%82%B9%E3%83%BB%
E3%83%9C%E3%83%BC%E3%82%A2 2020/2/13

Macro vs. micro for black holes
Macroscopic
 Smooth configuration of spacetime

fabric (Einstein’s theory)?

Microscopic
 Microstructure (“atoms”)

for black holes?
 How does quantum

mechanics change thepicture?
23

What does Einstein’s theory miss?
No matter what you make a black hole from…

http://en.wikibedia.ru/wiki/P%C5%99%C
3%ADru%C4%8Dn%C3%AD_slovn%C3%A
Dk_nau%C4%8Dn%C3%BD 2020/2/13

All look identical… Information lost?
But according to quantum mechanics,
information can never be lost24 Information must be stored in black holes in somemicroscopic structure Need to go beyond Einstein’s theory

To understand the microscopic physics of black holes,
we need to unify:
Quantum gravity
GeneralRelativity

Quantum
Mechanics

Most promising candidate:superstring theory26

Superstring
theoryWhat is superstring theory?
What if the fundamental element of Nature is not a point
particle but a tiny vibrating“string”?

∼ 0.00000000000000000000000000000000001 m(10−35 m)
Too small to be observed with our current technology
28Superstring theory
 First developed in late 1960’s
 Found to be a theory of quantum gravity in 1980’s

 Studied very actively even now as the most promising

candidate for a theory to unify all forces in Nature Surprising predictions

29

Prediction 1: extra dimensions
 Spacetime is not 4-dimensional but 10-dimensional.

Extra 6 are “compactified” and invisible.

What is compactification?

Tiny ants perceive the hose as 2-dimensional

At large scale, it looks like 1-dimensional30

Likewise, in our universe, 6 among 10 dimensions
are very small and compactified
Extra 6 dimensions.
Too small to beobserved.
4 dimensions we can perceive31 What are extra dimensions good for?

Smooth in higher dimensions
Space abruptly ends.
“Singular”.

Appears singular in lower dimensions
Conversely, singular spacetimes not allowed in lower
dimensions are allowed in higher dimensions.
Room to store “what the black hole is madefrom”
in higher dimensions.
32

Prediction 2: supersymmetry
Symmetry ofsnow flake

Supersymmetry
particle“superpartner”

To each particle, there exist
superpartners.
33

Gravity is extended to supergravity
Gravitygraviton

Supergravity
graviton
super
partners

Richer physics, more room
to store information
34

Superstring theory
Extra dimensions + supersymmetry

Rich structure to store more information

Suggests a novel picture of black holes
totally different from the conventional one.

35

A new picture of
black holes

New picture of black holes
Conventional picture

event
horizon“bubbles ofspacetime”“inside”
singularity

New picture

Event horizon and region inside it
get removed and replaced by“bubbles of spacetime”
No “inside” in naïve sense37

“Bubbles of spacetime”
 Smooth microstructure Possible in higher dimensions
 Stabilized by various

excitations in supergravity
 Called“fuzzballs”

Samir D.Mathur
www.physics.ohio-state.edu

 Increasing evidence,but still conjectural
38

Information loss problem?

39

Information loss problem?

Different microstructure depending on what BH was made from

Information about what BH was made from isstored in fuzzballs and never lost40

Life of a black hole (conjecture)
Star collapses

BH produced

Information about original
star stored in fuzzballs

Radiation comes out of fuzzballs
and thus contains information
about original star

Can reconstruct the original starfrom the information in radiation.

Evaporate & disappear
All information
carried away inradiation
41

Becomes smaller
by radiation

Just like burning a dictionary.

What happens if you fall into a BH?
Spaceship falls into
BH

Spaceship melts and becomespart of fuzzball

Comes backout as radiation

Can reconstruct
information about spaceshipfrom the radiation.

42

Matter takes various forms depending on mass
White dwarf

atoms such as He,C,O
1.4 solar masses(Chandrasekhar limit)

Neutron starneutrons~3 solar masses(Tolman-Oppenheimer-Volkoff limit)

Black holefuzzball

Above critical value, ordinary matter cannot support mass.
Spacetime “melts” into a new form of existence (fuzzball) to support it.
43Can we observe
fuzzballs?

Can we observe fuzzballs?
Gravitational wave echoes

 Allow us to probenear-horizon structure Bubbles of spacetime

observable?

http://cerncourier.com/cws/article/cern/67457

45

Can we observe fuzzballs?
“Extreme Mass Ratio Inspiral” (EMRI)LISA/DECIGO

gravitational
waves
Small
objectGravitational waves containdetailed information about
spacetime near BH

May be able to observe
deviation from conventional
theory46

Conclusions

Conclusions (1) Black holes are ubiquitous in our universe.

They are now objects of direct observation
 The conventional picture of BHs is macroscopic andapproximate
 According to string theory, BHs have microscopic

structure called“fuzzballs”
 In future, microstructure of

BHs may also become observable
48

Conclusions (2)Q. What’s inside a black hole?
(What is behind the event horizon?)A.

There is no “inside” in naïve sense.
Spacetime has “melted” into bubbles of spacetime(“fuzzball”) just before the event horizon.

49