The subject of time travel captures the imagination.
The subject of falling into a black hole has been treated seriously by theoretical physicists, such as the American Kip Thorne.
Time travel is a feature in many science fiction dramatizations, but the consensus is that it is not possible in theory.
It appears that quantum gravity effects inside a black hole prevent time travel due to the creation of particle pairs.
Direct evidence is hard to come by.
At extremely high energies and early in the universe, quantum fluctuations may make time intervals meaningless.
The time at which all forces were unified may be the reason for this.
It would be meaningless to consider the universe at times earlier than this.
The crucial time may be short.
quantum gravity suggests that there is no such thing as a vanishingly short time.
Time may be grainy with no meaning to time intervals shorter than some tiny but finite size.
Not only is quantum gravity in its infancy, no one knows how to get started on a theory of graviton and unification of forces.
Only indirect evidence can provide clues as to the validity of TOE, since the particle separation is small and the energies are high.
The approach many theorists have taken is called Superstring theory and is the topic of the Superstrings.
The principle of Superstring theory is that fundamental particles, including the graviton, act like one-dimensional vibrating strings.
Superstring theory is an attempt at a Theory of Everything since gravity affects time and space.
The dimensions of space are independent of one another and are represented by a different type of Superstring.
Some of the dimensions of superspace are thought to have curled up as the universe evolved.
At particle separations, forces are expected to be unified only at extremely high energies.
It's possible that Superstrings must have dimensions or wavelengths of this size or smaller.
Just as quantum gravity suggests that there are no time intervals shorter than a finite value, it also suggests that there may be no sizes smaller than a finite value.
Superstrings are the smallest possible size and can't have any more substructure.
The answer to the question the ancient Greeks considered would be this.
There is a limit to space.
Superstring theory deals with dimensions about 17 orders of magnitude smaller than the details we have been able to observe directly.
There are a number of theoretical possibilities to choose from, and it is relatively unconstrained by experiment.
Figuring out what is the most elegant theory is subjective, with less hope than usual, because of this.
It has led to speculation of alternate universes, with each new universe having a random set of rules.
Since an alternate universe is not doable, these speculations are not tested in principle.
It is similar to exploring a self-consistent field of mathematics, with its axioms and rules of logic that are not consistent with nature.
Such endeavors have given insight to mathematicians and scientists alike, and sometimes have been related to the description of new discoveries.
There is more matter in the universe than we can see, which is one of the most exciting problems in physics.
There is about 10 times as much mass in the stars as in the objects we can see.
We don't know what it is.
There is a chance that it is of a completely unknown form, a stunning discovery if verified.
Particle physics has implications for dark matter.
It is1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556 There may be enough dark matter to stop the expansion of the universe.
We don't know how much dark matter there is.
The first clues that there is more matter than meets the eye came from the American astronomer Vera Rubin, who did some initial work.
The velocities should decrease as the square root of the distance from the center increases if the mass is concentrated in the center.
The velocity curve is almost flat, implying that there is a lot of matter in the halo.
It wasn't immediately recognized for its significance, but it has now been made for many galaxies with similar results.
The mass distribution of the galaxies has been shown to be greater than that obtained from the number of stars.
Radio waves and X rays have confirmed the existence of dark matter.
There was a question of whether there was enough gravitation to stop the expansion of the universe.
Einstein created a constant to prevent expansion or contraction of the universe.
Einstein considered that an illogical possibility when he developed general relativity.
The constant was discarded after Hubble discovered the expansion.
The amount of slowing down is not known, but it is slowing the expansion of the universe.
The constant can counteract the effect of gravity.
The universe is expanding faster now than in the past, possibly due to the dark energy causing the expansion of the present-day universe to accelerate.
If the expansion rate was affected by gravity alone, we should be able to see that it was once greater than it is now.
It was less than it is now.
We can use the average density of matter to calculate the amount of slowing.
There is less visible matter than needed to stop expansion.
Due to uncertainties in the expansion rate of the universe, this estimate is only good to a factor of two.
The critical density is small and indicative of how empty the space is.
Luminous matter seems to account for less of the critical density than needed.
Taking into account the amount of dark matter we detect indirectly and all other types of normal matter, there is only what is needed to close the case.
If we are able to refine the measurement of expansion rates now and in the past, we will be able to answer the question of the curvature of space.
The most recent measurement of the CMBR has implications for the cosmological constant, so it's not just a device for a single purpose.
Most researchers feel that the universe should be barely open after the recent discovery of the cosmological constant.
You can travel an unlimited distance in any direction.
You will return to your starting point if you travel far enough in any direction.
The universe is very close to flat and will expand forever.
The flatness of the microwave background is explained by the inflationary scenario.
There is a special symmetry to a flat universe since general relativity suggests that matter creates the space in which it exists.
We can measure the velocities of stars relative to their galaxies by using the hydrogen spectrum.
The rotation of a spiral galaxy is indicated by these measurements.
A massive halo of dark matter extending beyond the visible stars is implied by the flatness of the curve.
The X rays show hot clouds of ionized gas in the regions of space that were previously thought to be empty.
There is no doubt that dark matter exists, but there are two facts that are still being studied.
We want to explain new observations in terms of known principles.
It is becoming more and more difficult to explain dark matter as a known type of matter as more discoveries are made.
Jupiter is too small to ignite fusion in its core and become a star, but we can see sunlight reflected from it.
We wouldn't be able to see it directly if Jupiter flew by another star.
There is a question as to how many planets or other bodies are smaller than the mass of the Sun.
If such bodies pass between us and a star, they will not block the star's light, being too small, but they will form a gravitational lens.
The huge amount of mass that seems to be there is what makes searches for dark matter particularly interested.
A few MACHOs have been observed, but not in the numbers needed to explain dark matter.
MACHOs are one of the most conventional objects proposed to explain dark matter.
Red dwarfs, which are small dim stars, are being actively pursued, but too few have been seen so far.
Old remnants of stars called white dwarfs, which are small as the Earth, are also under consideration since they contain about a solar mass.
Old dim dwarfs are not known.
There is no evidence for the existence of large numbers of smaller than stellar mass black holes.
There is a chance that dark matter is composed of small, but finite, neutrinos.
neutrinos are thought to be massless, but we only have upper limits on their mass, rather than knowing they are zero.
The upper limits come from the measurement of total energy emitted in the decays and reactions in which neutrinos are involved.
It is possible to prove that neutrinos have mass in a completely different way.
There are three flavors of neutrinos and the weak interaction could change their flavor.
If there is a mass, this can happen.
If neutrinos are massless, they must travel at the speed of light and time will not pass for them, so that they cannot change without an interaction.
In 1999, the results began to be published with convincing evidence.
Using the Super-Kamiokande detector in Japan, the oscillations have been observed and are being verified and explored.
The low number of observed solar neutrinos may be explained by Neutrino oscillations.
The detectors for observing solar neutrinos are specifically designed to detect electron neutrinos produced in huge numbers by fusion in the Sun.
A large fraction of electron neutrinos may be changing flavor to muon neutrinos on their way out of the Sun, possibly enhanced by specific interactions.
There is a discrepancy in the amount of neutrinos observed.
The showers of radiation produced by extremely energetic Cosmic rays should contain twice as many s as s, but their numbers are nearly equal.
The muon flavor may be explained by the electron flavor.
The existence of massive neutrinos is consistent with a large body of known information and explains more than dark matter.
At this time, the question is not settled.
The most radical proposal to explain dark matter is that it consists of previously unknown leptons.