Theoretically, the lowest temperature that can be achieved is
absolute zero, exactly ?273.15°C, where the motion of all particles
stops completely. However, you can never actually cool something to this
temperature because, in quantum mechanics, every particle has a minimum
energy, called “zero-point energy,” which you cannot get below.
Remarkably, this minimum energy doesn’t just apply to particles, but to
any vacuum, whose energy is called “vacuum energy.” To show that this
energy exists involves a rather simple experiment– take two metal plates
in a vacuum, put them close together, and they will be attracted to
each other. This is caused by the energy between the plates only being
able to resonate at certain frequencies, while outside the plates the
vacuum energy can resonate at pretty much any frequency. Because the
energy outside the plates is greater than the energy between the plates,
the plates are pushed towards each other. As the plates get closer
together, the force increases, and at around a 10 nm separation this
effect (called the Casimir effect) creates one atmosphere of pressure
between them. Because the plates reduce the vacuum energy between them
to below the normal zero-point energy, the space is said to have
negative energy, which has some unusual properties.
One of the properties of a negative-energy vacuum is that light
actually travels faster in it than it does in a normal vacuum, something
that may one day allow people to travel faster than the speed of light
in a kind of negative-energy vacuum bubble. Negative energy could also
be used to hold open a transversible wormhole, which although
theoretically possible, would collapse as soon as it was created without
a means to keep it open. Negative energy also causes black holes to
evaporate. Vacuum energy is often modeled as virtual particles popping
into existence and annihilating. This doesn’t violate any energy
conservation laws as long as the particles are annihilated shortly
afterwards. However, if two particles are produced at the event horizon
of a black hole, one can be moving away from the black hole, while the
other is falling into it. This means they won’t be able to annihilate,
so the particles both end up with negative energy. When the negative
energy particle falls into the black hole, it lowers the mass of the
black hole instead of adding to it, and over time particles like these
will cause the black hole to evaporate completely. Because this theory
was first suggested by Stephen Hawking, the particles given off by this
effect (the ones that don’t fall into the black hole) are called Hawking
radiation. It was the first accepted theory to unite quantum theory
with general relativity, making it Hawking’s greatest scientific
achievement to date.
One prediction of Einstein’s theory of general relativity is that
when a large object moves, it drags the space-time around it, causing
nearby objects to be pulled along as well. It can occur when a large
object is moving in a straight line or is rotating, and, although the
effect is very small, it has been experimentally verified. The Gravity
Probe B experiment, launched in 2004, was designed to measure the
space-time distortion near Earth. Although sources of interference were
larger than expected, the frame-dragging effect has been measured to an
uncertainty of 15%, with further analysis hoping to reduce this further.
The expected effects were very close to predictions: due to the
rotation of the Earth, the probe was pulled from its orbit by around 2
meters per year, an effect purely caused by the mass of the Earth
distorting the space-time surrounding it. The probe itself would not
feel this extra acceleration because it is not caused by an acceleration
on the probe, but rather on the space-time the probe is traveling
through–analogous to a rug being pulled under a table, rather than
moving the table itself.
8
Relativity of Simultaneity
The relativity of simultaneity is the idea that whether two events
occur simultaneously or not is relative and depends on the observer. It
is a strange consequence of the special theory of relativity, and
applies to any events that happen that are separated by some distance.
For example, if a firework is let off on Mars and another on Venus, one
observer traveling through space one way might say they happen at the
same time (compensating for the time light takes to reach them), while
another observer traveling another way might say the one on Mars went
off first, and yet another might say the one on Venus went off first. It
is caused by the way different viewpoints become distorted compared to
each other in special relativity. And because they are all relative, no
observer can be said to have the correct viewpoint.
This can lead to very unusual scenarios, such as an observer
witnessing effect before cause (for example, seeing a bomb go off, then
later seeing someone light the fuse). However, once the observer sees
the effect, they cannot interact with the cause without traveling faster
than the speed of light, which was one of the first reasons
faster-than-light travel was believed to be forbidden, because it is
akin to time travel, and a universe where you can interact with the
cause after the effect makes no sense.
One of the longest outstanding mysteries in physics is how gravity is
related to the other fundamental forces, such as electromagnetism. One
theory, first proposed in 1919, showed that if an extra dimension is
added to the universe, gravity still exists in the first four dimensions
(three space dimensions and time), but the way this four dimensional
space curves over the extra fifth dimension, naturally produces the
other fundamental forces. However, we cannot see or detect this fifth
dimension, so it was proposed that the extra dimension was curled up,
and hence became invisible to us. This theory was what ultimately led to
string theory, and is still included at the heart of most string theory
analysis.
Since this extra dimension is so small, only tiny objects, such as
particles, can move along it. In these cases, they ultimately just end
up where they started, since the extra dimension is curled up on itself.
However, one object that becomes much more complex in five dimensions
is a black hole. When extended to five dimensions, it becomes a “black
string,” and unlike a normal 4D black hole, it is unstable (this ignores
the fact that 4D black holes eventually evaporate). This black string
will destabilize into a whole string of black holes, connected by
further black strings, until the black strings are pinched off entirely
and leave the set of black holes. These multiple 4D black holes then
combine into one larger black hole. The most interesting thing about
this is that, using current models, the final black hole is a “naked”
singularity. That is, it has no event horizon surrounding it. This
violates the Cosmic Censorship Hypothesis, which says that all
singularities must be surrounded by an event horizon, in order to avoid
the time-travel effects that are believed to happen near a singularity
from changing the history of the entire universe, as they can never
escape from behind an event horizon.
As is best shown in the equation E=MC
2, energy and matter
are fundamentally connected. One effect of this is that energy, as well
as mass, creates a gravitational field. A geon, first investigated by
John Wheeler, in 1955, is an electromagnetic or gravitational wave whose
energy creates a gravitational field, which in turn holds the wave
itself together in a confined space. Wheeler speculated that there may
be a link between microscopic geons and elementary particles, and that
they might even be the same thing. A more extreme example is a
“kugelblitz” (German for “ball lightning”), which is where such intense
light is concentrated at a particular point that the gravity caused by
the light energy becomes strong enough to collapse into a black hole,
trapping the light inside. Although nothing is thought to prevent the
formation of a kugelblitz, geons are now only believed to be able to
form temporarily, as they will inevitably leak energy and collapse. This
unfortunately indicates that Wheeler’s initial conjecture was
incorrect, but this has not been definitively proven.
The type of black hole most people are familiar with, which has an
event horizon on the outside acting as the “point of no return” and a
point singularity of infinite density on the inside, actually has a more
specific name: a Schwarzschild black hole. It is named after Karl
Schwarzschild, who found the mathematical solution of Einstein’s field
equations for a spherical, non-rotating mass in 1915, only a month after
Einstein actually published his general theory of relativity. However,
it wasn’t until 1963 that mathematician Roy Kerr found the solution for a
rotating spherical mass. Hence, a rotating black hole is called a Kerr
black hole, and it has some unusual properties.
At the centre of a Kerr black hole, there is no point singularity,
but rather a ring singularity—a spinning one-dimensional ring held open
by its own momentum. There are also two event horizons, an inner and
outer one, and an ellipsoid called the ergosphere, inside which
space-time itself rotates with the black hole (because of frame
dragging) faster than the speed of light. When entering the black hole,
by passing through the outer event horizon, space-like paths become
time-like, meaning that it is impossible to avoid the singularity at the
centre, just like in a Schwarzschild black hole. However, when you pass
through the inner event horizon, your path becomes space-like again.
The difference is this: space-time itself is reversed. This means
gravity near the ring singularity becomes repulsive, actually pushing
you away from the centre. In fact, unless you enter the black hole
exactly on the equator, it is impossible to hit the ring singularity
itself. Additionally, ring singularities can be linked through
space-time, so they can act as wormholes, although exiting the black
hole on the other side would be impossible (unless it was a naked
singularity, possibly created when the ring singularity spins fast
enough). Traveling through a ring singularity might take you to another
point in space-time, such as another universe, where you could see light
falling in from outside the black hole, but not leave the black hole
itself. It might even take you to a “white hole” in a negative universe,
the exact meaning of which is unknown.
Quantum tunneling is an effect where a particle can pass through a
barrier it would not normally have the energy to overcome. It can allow a
particle to pass through a physical barrier that should be
impenetrable, or can allow an electron to escape from the pull of the
nucleus without having the kinetic energy to do so. According to quantum
mechanics, there is a finite probability that any particle can be found
anywhere in the universe, although that probability is astronomically
small for any real distance from the particles expected path.
However, when the particle is faced with a small-enough barrier
(around 1-3 nm wide), one which conventional calculations would indicate
is impenetrable by the particle, the probability that the particle will
simply pass through that barrier becomes fairly noticeable. This can be
explained by the Heisenberg uncertainty principle, which limits how
much information can be known about a particle. A particle can “borrow”
energy from the system it is acting in, use it to pass through the
barrier, and then lose it again.
Quantum tunneling is involved in many physical processes, such as
radioactive decay and the nuclear fusion that takes place in the Sun. It
is also used in certain electrical components, and it has even been
shown to occur in enzymes in biological systems. For example, the enzyme
glucose oxidase, which catalyses the reaction of glucose into hydrogen
peroxide, involves the quantum tunneling of an entire oxygen atom.
Quantum tunneling is also a key feature of the scanning tunneling
microscope, the first machine to enable the imaging and manipulation of
individual atoms. It works by measuring the voltage in a very fine tip,
which changes when it gets close to a surface due to the effect of
electrons tunneling through the vacuum (known as the “forbidden zone”)
between them. This gives the device the sensitivity necessary to make
extremely high resolution images. It also enables the device to move
atoms by deliberately putting a current through the conducting tip.
Shorty after the Big Bang, the universe was in a highly disordered
and chaotic state. This means that small changes and defects didn’t
change the overall structure of the universe. However, as the universe
expanded, cooled, and went from a disorderly state to an orderly one, it
reached a point where very small fluctuations created very large
changes.
This is similar to arranging tiles evenly on a floor. When one tile
is placed unevenly, this means that the subsequent tiles placed will
follow its pattern. Therefore, you have a whole line of tiles out of
place. This is similar to the objects called cosmic strings, which are
extremely thin and extremely long defects in the shape of space-time.
These cosmic strings are predicted by most models of the universe, such
as the string theory wherein two kinds of “strings” are unrelated. If
they exist, each string would be as thin as a proton, but incredibly
dense. Thus, a cosmic string a mile long can weigh as much as the
Earth. However, it would not actually have any gravity and the only
effect it will have on matter surrounding it would be the way it changes
the form and shape of space-time. Therefore, a cosmic string is, in
essence, just a “wrinkle” in the shape of space-time.
Cosmic strings are thought to be incredibly long, up to the order of
the sizes of thousands of galaxies. In fact, recent observations and
simulations have suggested that a network of cosmic strings stretches
across the entire universe. This was once thought to be what caused
galaxies to form in supercluster complexes, although this idea has since
been abandoned. Supercluster complexes consist of connected
“filaments” of galaxies up to a billion light-years in length. Because
of the unique effects of cosmic strings on space-time as you bring two
strings close together, it has been shown that they could possibly be
used for time travel, like with most of the things on this list. Cosmic
strings would also create incredible gravitational waves, stronger than
any other known source. These waves are what those current and planned
gravitational wave detectors are designed to look for.
2
Antimatter Retrocausality
Antimatter is the opposite of matter. It has the same mass but with
an opposing electrical charge. One theory about why antimatter exists
was developed by John Wheeler and Nobel laureate Richard Feynman based
on the idea that physical systems should be time-reversible. For
example, the orbits of our solar system, if played backwards, should
still obey all the same rules as when they are played forwards. This led
to the idea that antimatter is just ordinary matter going backwards in
time, which would explain why antiparticles have an opposite charge,
since if an electron is repelled while going forwards in time, then
backwards in time this becomes attraction. This also explains why matter
and antimatter annihilate. This isn’t a circumstance of two particles
crashing into and destroying each other; it is the same particle
suddenly stopping and going back in time. In a vacuum, where a pair of
virtual particles are produced and then annihilated, this is actually
just one particle going in an endless loop, forwards in time, then
backwards, then forwards, and so on.
While the accuracy of this theory is still up for debate, treating
antimatter as matter going backwards in time mathematically comes up
with identical solutions to other, more conventional theories. When it
was first theorized, John Wheeler said that perhaps it answered the
question of why all electrons in the universe have identical properties,
a question so obvious that it is generally ignored. He suggested that
it was just one electron, constantly darting all over the universe, from
the Big Bang to the end of time and back again, continuing an
uncountable number of times. Even though this idea involves backwards
time travel, it can’t be used to send any information back in time,
since the mathematics of the model simply doesn’t allow it. You cannot
move a piece of antimatter to affect the past, since in moving it you
only affect the past of the antimatter itself, that is, your future.
1
Gödel’s incompleteness theorems
It is not strictly science, but rather a very interesting set of
mathematical theorems about logic and the philosophy that is definitely
relevant to science as a whole. Proven in 1931 by Kurt Gödel, these
theories say that with any given set of logical rules, except for the
most simple, there will always be statements that are undecidable,
meaning that they cannot be proven or disproven due to the inevitable
self-referential nature of any logical systems that is even remotely
complicated. This is thought to indicate that there is no grand
mathematical system capable of proving or disproving all statements. An
undecidable statement can be thought of as a mathematical form of a
statement like “I always lie.” Because the statement makes reference to
the language being used to describe it, it cannot be known whether the
statement is true or not. However, an undecidable statement does not
need to be explicitly self-referential to be undecidable. The main
conclusion of Gödel’s incompleteness theorems is that all logical
systems will have statements that cannot be proven or disproven;
therefore, all logical systems must be “incomplete.”
The philosophical implications of these theorems are widespread. The
set suggests that in physics, a “theory of everything” may be
impossible, as no set of rules can explain every possible event or
outcome. It also indicates that logically, “proof” is a weaker concept
than “true”; such a concept is unsettling for scientists because it
means there will always be things that, despite being true, cannot be
proven to be true. Since this set of theorems also applies to computers,
it also means that our own minds are incomplete and that there are some
ideas we can never know, including whether our own minds are consistent
(i.e. our reasoning contains no incorrect contradictions). This is
because the second of Gödel’s incompleteness theorems states that no
consistent system can prove its own consistency, meaning that no sane
mind can prove its own sanity. Also, since that same law states that any
system able to prove its consistency to itself must be inconsistent,
any mind that believes it can prove its own sanity is, therefore,
insane.