It would seem that counting the number of photons in the universe is an impossible task. But physicists at Clemson University, USA, don’t think so. To understand at least an approximate number of photons in our world, it is necessary to take into account the whole time of existence of the Universe – about 13.7 billion years. After all, the light emitted by long dead stars still flies through space.

For the basis of calculations, scientists have taken indicators of the so-called extragalactic background radiation (extragalactic background light – EBL), which accumulates in the infrared, visible and ultraviolet ranges after the formation of stars. Even with the most modern technology, EBL is very difficult to spot – it is easily damped out by other radiations that literally permeate our universe. But scientists have found an original way, which still helped to analyze the extragalactic background radiation – blazars.

This is interesting: blazars are active galactic nuclei that emit huge jets of plasma from their center. It is believed that these jets (or jets) arise from the interaction of matter inside the accretion disk of a giant black hole inside the galaxy.

The jets emitted by blazars spread out over hundreds of millions of light-years, and their emission weakens as they pass through the EBL. It turns out that blazars scattered throughout the cosmos are an excellent opportunity to study the intensity and amount of light emitted by stars at different stages in the life of the universe as it ages and expands. After analyzing 739 blazars, physicists were able to name the approximate number of photons that exist in our universe: 4 x 1084. Or, if you’re too lazy to count zeros: 4 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 photons.

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Our solar system is quite small by cosmic standards. We can say this with certainty after astronomers at a distance of 100 light years from Earth discovered a single, seemingly lonely planet – a huge gas giant called 2MASS J2126-8140, which is 12-15 times the size of Jupiter we know.

Further research has shown that the found planet is not an outcast, thrown into interstellar space by gravitational forces, but has its own orbit around the star, distant from it at a distance of about 1 trillion kilometers. This is 6,900 times greater than the distance from the Earth to the Sun. At this distance, the star looks like a small, dim light in the night sky, about the same way we observe other stars after sunset. Interestingly, it takes about a million years for a planet to complete one revolution around its luminary.

Scientists are trying to figure out how a planetary system could be formed at such a considerable distance. It probably emerged not from a gas-dust cloud, like our solar system, but from a directional gas-dust jet.

The largest planetary system found in the cosmos is very young, 10-45 million years old. It is predicted that the life of this system will not last long. A planet and a star are so loosely coupled that any gravitational influence from another cosmic body could upset this delicate balance.

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This is interesting: Einstein’s original work was called “Toward the Electrodynamics of Moving Bodies. The theory of relativity it became later, when the scientific world realized how accurately the work of a scientist describes the principle of relativity, which has plagued scientists since ancient times: for example, standing on the deck of a stationary ship and throwing a stone toward its bow, you will not feel any difference when you throw a stone if the ship sailed.

Without going into complicated calculations and formulas, we will recall the basic postulates of Einstein’s theories concerning the properties of space-time (and space and time, according to the Theory of Relativity, are inseparable from each other). In this case we are interested in two conclusions of the theory: space-time is curved under the influence of gravitational fields, and in any moving object we can observe the effect called relativistic time dilation. It turns out that in a body moving with non-zero velocity, all physical processes will go slower than if the body were at rest. That is, if you, for example, fly in an airplane, and your friend stayed at home, your time will go slower. Of course, in practice neither you nor your friend will feel the difference: it will be a billionth of a second.

But if you accelerate to a speed much greater than that of an airplane, the difference in time for you and your friend will be much greater. One year on a space rocket traveling at near-light speed can be equal to several hundred Earth years.

That’s interesting: but that doesn’t mean that if you were to get in such a rocket and accelerate to tremendous speed, you would experience a slo-mo effect. For you, time would flow as usual. But if an observer standing on Earth could see the clock in the cockpit of a flying rocket, it would seem to him that time would go slower on it. On the other hand, if you could see through the porthole of an ordinary Earth dweller’s watch, it would seem to you that it went slower than yours. And that’s because if you were in a rocket, it would be the Earth with all its inhabitants moving relative to you. But why would not all the inhabitants of Earth experience the time dilation effect, but only the astronaut? This can be explained by the fact that he experienced the processes of acceleration, being in the rocket, which means that the frames of reference for the Earth and the spacecraft were unequal (the Earth flew uniformly and straightly, and the rocket experienced the effects of acceleration).
gravity

But what if we are talking about more massive objects, such as our Earth? Indeed, its mass is enough to warp space-time around itself so strongly that we can see this difference using modern instruments. The closer to a massive body – the stronger is its gravitational influence, and therefore the slower goes time. This statement has been verified in numerous experiments, and time shifts are taken into account in information transmissions between the Earth and communication satellites.

This is interesting: in fact, you can check this for yourself at any time. One of the conclusions of the Theory of Relativity is that in a gravitational field, a free-falling body moves uniformly and in a straight line. Hit a soccer ball – first it flies up, and then, it falls down – to the Earth. In fact, the trajectory of the ball is perfectly straight, and it falls to the surface due to the curvature of space-time: at some point, the trajectories of the Earth and the ball will intersect.

It turns out that the unambiguous statement that time in space always goes slower or always goes faster is incorrect. In different parts of space, it will go differently. Somewhere faster and somewhere slower. Near, for example, black holes, it will slow down significantly, and in intergalactic space, away from stars and planets, on the contrary, go faster. In addition, when calculating the time for any object, it is important to consider its velocity parameters as well.

This is interesting: now we can say for sure that time should go faster on the Earth’s orbit than on the surface – because we are at a greater distance from a massive object, i.e. our planet. To confirm, let’s give out absolutely synchronously running atomic clocks to the cosmonaut and you, reconciling them before launching the rocket. Where to send the astronaut? To the ISS, the International Space Station, of course. Imagine that after living for a year in orbit and returning home the first thing the astronaut did was not to go through medical checks and see his family – he checked the time against your atomic clock. You will be surprised to find that the cosmonaut’s watch… is lagging – his time went slower! How is this possible: after all he was at a greater distance from the massive object than we were?

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Of course, man-made vehicles have been to the Moon and other planets of the solar system, and the Pioneer and Voyager stations even went beyond its limits. But even if they manage to reach the nearest stars, we are unlikely to know about it. After all, such a flight would take about 2 million years, and communication with the spacecraft would be cut off much earlier. It is obvious that interstellar flights require new principles of motion in space – the traditional rocket engines are not very suitable for this purpose. Meanwhile, man could find many interesting things for himself in neighboring star systems. Now more than 1000 exoplanets have already been discovered, and it is possible that some of them are suitable for life. More and more often in the ranks of scientists hear calls to secure humanity from cosmic catastrophes. According to them, sooner or later the conditions of life on Earth may become uninhabitable, and only expansion into space will help save our species. The whole question is how to carry it out.

According to cosmologists’ estimates, the size of the visible Universe is about 93 billion light years, and it is known to keep expanding. Against this background, not only the Solar System, but the entire Milky Way (about 100 thousand light years across) looks like a tiny grain of sand. The situation is complicated by the fact that the speed of movement of material objects, according to the special theory of relativity (STR), can not exceed the speed of light (about 300 thousand km / c). But even it takes many thousands of years to cross a single galaxy.

In principle, it is possible to create a jet engine capable of imparting near-light speed to the vehicle, even with the current level of technology. The authors of the Daedalus and Ikarus projects, perhaps the most elaborate plans for interstellar flight today, propose just such an engine. But it is unlikely that they can be used to colonize other worlds: there is not enough fuel even for braking at the end point, so the flight will be, as they say, “one-way”.

So far, only characters from science fiction novels are able to travel through the Universe, with FTL spaceships, teleports, and other future physics achievements at their disposal. So isn’t it time for us to start developing them? Back in 2006, NASA launched the Breakthrough Propulsion Physics (BPP) program, designed to develop radically new engines for interstellar travel. Despite the fact that one of the engines is named after the famous scientist and popularizer of science Stephen Hawking, the world-famous physicist did not participate in it: he relies on simpler annihilation engines.

The ideas of the BPP participants, on the other hand, were much more daring. So daring that many of them are only possible mathematically: the physical principles involved are not yet known to science. Others, while not violating known laws of nature, would require enormous expenditures of energy or the development of materials with unusual properties. Most of the proposed engines are based on “games” with gravity. According to modern concepts, gravity is nothing but the curvature of space-time. It is not difficult to guess that antigravity should curve space in the opposite direction. By placing an antigravity substance in the stern of the ship, it would be possible to give it constant acceleration without any expenditure of energy. The only difficulty is related to the fact that particles with negative mass have not yet been found, and it is unknown whether they exist at all.

However, perpetual motion can also be obtained with ordinary gravity. For this purpose it is necessary to somehow divide the mass into a source of the gravitational field and a part interacting with it, and then to fix them motionless relative to each other. It remains to figure out how to do this: one might as well propose, for example, to separate the electric field from the charge that creates it. Another way of traveling in space is based on a local change in the laws of nature. Isaac Newton – the author of the first mathematical theory of gravitation – established that the force of attraction depends on the mass of interacting bodies and the distance between them. There is also a constant in the equation – the gravitational constant (G). If we somehow increase this constant at the front of the spaceship and decrease it at the stern, there will be an effect essentially similar to antigravity. But the value of G is called a constant for a reason: it is believed that its value is the same throughout the Universe.

However, there are also alternative cosmological concepts, in which the gravitational constant is a variable. One way or another, it is not yet clear how to change it artificially. The Alcuberrier engine is perhaps the most attractive of the proposed projects. It proposes to create a kind of spatial bubble that would surround the ship by compressing space-time in front of its bow and expanding in the stern. Such a “bubble” could even exceed the speed of light without violating STO – because the restrictions in speed concern only particles of matter, not space itself. But, unfortunately, this would again require negative mass, which so far exists only in theory. Other projects suggest exploring the stellar expanses with the help of a sailing fleet. Once upon a time seagoing ships gave up sails in favor of engines: it is possible that their space “brethren” will someday make the return trip. And this is not fiction. Solar sails accelerate the vehicles due to the pressure created by the flow of ionized particles or photons. The magnitude of this pressure is very small, so the sails must have a very impressive area. Now they are being actively developed in different countries, including Russia. The BPP researchers have much more original and effective solutions in their stock. For example, they suggested creating something like a solar diode. Such a sail should transmit light only in one direction and reflect it in the other. Alternatively, one side of the sail could reflect photons and the other side could absorb them. The difference in light pressure would create thrust even when there is no “space doldrums” – when there is no photon tail flow. “Casimir’s sail” makes it possible not to depend on stellar radiation at all. The effect, predicted by Hendrik Casimir in 1948, is related to vacuum fluctuations, during which short-lived particles are formed. These particles are called “virtual”, but they exert quite real, though very weak pressure. If you somehow strengthen it on one side of the sail, the ship would gain constant acceleration without any fuel costs. How exactly to do this, the inventors are silent.

The BPP program ran for 6 years, after which it was discontinued. There is no doubt: the proposed ideas are very entertaining, but do they justify the 1.2 million dollars invested in the development? Because of the significant costs and the complete absence of practical results, some media outlets have even called the program “the biggest scientific scam of the century. However, this is hardly fair: breakthrough results are impossible without a long theoretical preparation. After all, the first plans for a flight to the Moon also had little in common with reality… Time will tell whether the development of NASA specialists will remain a curiosity of science or become the first step to interstellar travel.

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The unfortunate person will immediately turn into an ice cube

One can answer with certainty that one is not destined to turn into an ice block instantly. Space, of course, is very cold, but its density is extremely low. Therefore, the human body will not be able to transfer its heat – because there is nothing around. By the way, one of the main problems on the ISS is not to protect the crew from the cold, but, on the contrary, to remove heat from the station.

Man will be incinerated by cosmic radiation

Space radiation is certainly dangerous. Charged particles pierce an astronaut, causing radiation sickness. But to get a lethal dose, you need to spend quite a long time in space, during which time you will have time to die under the influence of other factors. Normal clothing covering your body can protect your body from most burns. But if you end up in space completely naked, the effects of even a short stay can be bad.

Blood boils due to low pressure

But what if a person’s blood boils and ruptures blood vessels? After all, there is very low pressure in space, which helps reduce the boiling point of the fluid. But the blood inside the body will continue to be under its own pressure and in order to boil it will need to reach a temperature of 46 ° C, which, as we know, is not observed in living people. But if you stick your tongue out, you will feel your saliva boiling. But in this case there will be no burns, because it will boil at a low temperature.

An astronaut will explode because of the pressure drop

The pressure in space is dangerous in another way: because of its difference the human’s internal organs may enlarge and the body will swell up to two times. But you won’t be able to burst and “splash” your insides into space: our skin has enough elasticity to restrain such a strong expansion, and if you wear tight clothes your dimensions will remain the same as before.

You won’t be able to breathe

Pressure is a great danger to our respiratory system. As far as we remember, there is no oxygen in space, so the amount of time a person can live without a spacesuit depends on how long they can go without breathing. But it would not be as if we were underwater, where all we have to do is hold our breath and try to swim out. If you hold your breath in a vacuum, the difference in pressure will simply rupture your lungs, and then it will be impossible to save the person. The only way to prolong your life is to allow the gases to escape from your body rapidly (this can cause trouble, such as emptying your bowels or stomach). When the oxygen rapidly leaves your body, you will have about 14 seconds while the oxygenated blood continues to feed your brain, and then you will lose consciousness. But does this mean inevitable death? No! Our seemingly fragile organism can survive even in such an alien and hostile environment. Scientists believe that if a person after a minute and a half stay in outer space is taken to a safe place, he will not only survive, but also fully recover his functions in a few days.

Experiments on animals have shown that chimpanzees, even after a three-minute stay in conditions close to a vacuum, in a few hours come back to normal. At the same time they had the above described symptoms: body expansion and loss of consciousness due to lack of oxygen. Experiments with dogs showed that our four-legged friends endure vacuum much worse than chimpanzees: the survival limit is no more than two minutes.

Experiments with chimpanzees and dogs should not be completely trusted: the human and animal bodies may react differently to their environment. And, although no one will do such experiments on humans, we can judge the effects of the vacuum on the human body by the accidents that have happened to astronauts. In 1965, technician Jim LeBlanc tested in a vacuum chamber the tightness of a new spacesuit that was intended for lunar expeditions. During one of the tests, when the pressure in the chamber was close to the space pressure, the suit depressurized and the person lost consciousness after 14 seconds. The standard procedure to restore pressure to normal took 30 minutes, while the team of scientists risked speeding up the process and restored pressure in just a minute and a half! Consciousness returned to Leblanc when the pressure in the pressure chamber corresponded to the earth pressure at an altitude of 4.5 km above sea level. Another incident is the flight of the Soyuz-11 spacecraft. During the descent of the spacecraft to the ground, the spacecraft depressurized. A small vent valve, one and a half centimeters in size, which did not open in time, caused the death of three cosmonauts. According to the recording equipment, all three lost consciousness 22 seconds after depressurization, and vitality stopped registering after 2 minutes. In total, the crew spent about 11.5 minutes in near vacuum space. When the spacecraft landed and the welcoming team opened the hatch, it was already too late to resuscitate the astronauts.

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But in order to leave the limits of a black hole, an object must accelerate to a speed greater than the speed of light. Modern physics believes that nothing in the Universe can move faster than the speed of light – and neither can the quanta of light itself, of course. That is why we can claim that nothing, not even light, can leave a black hole.

Or can it? The amazing world of quantum mechanics claims that a black hole can radiate into space, too. To understand how this becomes possible, we need to talk about what our space represents at the level of elementary particles.

Quantum field theory states that all space-time of the Universe at any point is different energy fields. If we take empty space – physical vacuum – measure it with the most accurate modern instruments and see that not a single photon is observed in this space, then we can say that the field is in the state of the lowest energy, that is not able to give energy. So the energy of the field is zero? Not at all. Even in this case it is impossible to accurately (definitely) measure the energy of the field, otherwise it would violate the uncertainty principle (or Heisenberg principle), the basis of quantum mechanics. It turns out that even in the state with the lowest energy we can set the value of the field energy only by the probability distribution. And this means that in physical vacuum various fluctuations will always occur.

Quantum theory explains their existence by constant birth and annihilation of virtual particles and antiparticles. Why virtual? Because it happens in such short time intervals (about 10-24 sec.), that we simply can not register these particles. Initially the existence of virtual particles was found on paper – during the derivation of formulas – and for a long time was questioned as only a mathematical description of reality. However, scientists now know for sure that virtual particles exist – they react with ordinary real particles, changing characteristics of the latter, which has been repeatedly confirmed by various experiments. Yes, the world at the quantum level looks quite different from our everyday world, but is a kind of boiling broth, in which new particles are constantly born and destroyed out of nowhere. Theoretically, when an external field affects the vacuum, a pair of virtual particles can be transformed into a pair of real ones by applying energy.

Now let us imagine that pairs of virtual particles are born on the very event horizon of a black hole. Among innumerable such pairs there may arise one which under the influence of the gravitational field will turn into a real state. There will come a moment when one of the particles will fall into the black hole and the other one will be able to avoid the fall by taking a lucky trajectory of flight, which as if from a sling will “kick” the particle back into space, giving it a huge acceleration.

Note that the real particles were not born by themselves – a black hole created them with its energy, radiating then one of the particles into space. It is possible to calculate that the first particle which has fallen over the event horizon could not compensate to a black hole loss in energy which it spent for transformation of virtual particles into real ones, and then for giving impulse to the second particle. It turns out that the black hole not only radiated a particle into space, but also lost part of its energy, and hence mass, because of it. Theoretically, with time it should simply evaporate – after all every instant countless virtual particles are born, and matter near a black hole sooner or later ends.

This radiation is named after the famous theoretical physicist Stephen Hawking and is called the “Hawking radiation”. It is possible to prove or disprove Stephen Hawking’s theory by measuring the thermal spectrum of radiation near the event horizon of the black hole, but modern technology has not yet reached the necessary level for such complex observations. There is still a fierce debate about the existence of Hawking’s radiation.

This is interesting: some physicists believe that it is Hawking’s radiation that vaporizes those microscopic black holes that could theoretically arise during experiments at the Large Hadron Collider.

The discovery of vanishing black holes could put an end to the debate – each vaporization should theoretically end in a grand explosion. However, so far no traces of such occurrences have been found – most likely, the age of the Universe is still too small for even the first black holes formed in it to come to the end of their lives.

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