FAQ's

The distance between the Earth and the Sun is called an Astronomical Unit. It is roughly 150 million km.

We can estimate the mass of the Earth by using Newton’s law of gravitation. The gravitational force acting between any object on Earth with mass ‘m’ and Earth whose mass is ‘M’, separated by a distance ‘R’ which is the radius of the Earth, is given by F=GmM/(R^2). Here G is the universal gravitational constant. The force due to Earth’s gravity on the object can also be expressed as F=mg where g is the acceleration due to gravity. Equating these two forces, we get an expression M=g(R^2)/G. Using the values of g=9.8 ms^-2, G=6.674x 10^-11 m^3 kg^-1 s^-2, and R = 6.37x10^6 m, we get the mass of the Earth to be M =610^24 kg.

A leap year is a year having one extra day in the calendar and this day is added to the month of February. In a leap year, the month of February has 29 days. Earth completes one revolution around the Sun in 365.25 days and every four years this will add up to one extra day making the year to consist of 366 days. A leap year, therefore, has 366 days. To check if a year is a leap year, see if the number is evenly divisible by 4. Then, if it is evenly divisible by 4, check to see if it's evenly divisible by 100. If it's not, then it's a leap year. For e.g. 1996 is divisible by 4 and hence was a leap year and 1900 was not a leap year

Earth is spinning around its axis at a constant speed and we are moving with it. It is similar to sitting on a bus or a train or a flight that is moving at a constant speed. We do not feel the motion until the speed changes (e.g. when brakes are applied). This is the concept of inertia.

The Earth is believed to be formed approximately 4.5 billion years ago.

From space, Earth looks like a ‘blue marble’ with clouds forming white swirls on it. The blue color of the marble is due to the water bodies such as oceans. The lands are visible as brown and green patches. See the image below from NASA.

Earth From Space.

Sun is approximately a sphere of gas and plasma. It is made of 74% hydrogen, 25% helium, and 1% of other elements.

The dark spots on the sun are called sunspots where a high concentration/flux of magnetic field disrupts convection and hence transfer of heat from the solar interiors. It appears dark as it is cooler than the plasma surrounding it. The mechanism that causes the 11 year cycle is called the Babcock-Leighton dynamo model. The reason for the cycle to have a 11 year duration is not clearly known. But a recent study suggests that the tidal force of Venus, Earth and Jupiter has an influence on the period of the solar cycle.

Reference: Web-page article paper

We can use simple geometry to get a decent estimate for the size of the Sun. We know that during a complete solar eclipse, the Moon covers the solar disk (the solar atmosphere extends out much further). Applying the concept of similar triangles, we get:

radius_moon/ dist_earth-moon = radius_sun / dist_earth-sun

Plug in the values of the Moon radius (1,737 km), Earth-Moon distance (384,000 km) and Earth-Sun distance (1AU = 150,000,000 km) to get the size of the Sun!

The moon’s magnetic field is very weak and causes negligible changes to the earth’s magnetic field irrespective of the solar/lunar eclipse. But during a solar eclipse the moon blocks a portion of the solar wind directed towards the earth. Based on the activity of the sun, the ion density in the solar wind changes and hence its effect on the earth’s magnetic field.

1. 1. With numerous satellite missions probing the Moon, we have some understanding of how elements, minerals and volatiles are distributed across the lunar surface.

   2. Six missions that landed on the Moon and have brought back rocks and dust from the Moon. This helps in understanding the lunar formation.

   3. Some of the highlighted results from Chandrayaan-1 are

      • Discovered the evidence of hydroxyl/water bearing molecules on the Moon

      • First evidence of enhanced Sodium abundance in some region on the Moon’s surface

      • First evidence of complex minerals like spinel.

All the stars (point like objects) we see in the night sky belong to the Milky Way. The Milky Way is divided into different bands. When the earth revolves around the sun, we see a band of stars belonging to the Sagittarius arm during summer months and Orion arm during the winter months. The region of sky between these two bands has fewer and dimmer stars. So based on the time of night that you watch the night sky, you see fewer stars. Also light pollution reduces the number of stars you can see.

The Moon Mineralogical Mapper (M³) instrument onboard Chandrayaan-1 mission discovered the evidence of hydroxyl/water-bearing molecules on the Moon. M³ instrument measured absorption features near the polar regions on the Moon, which showed the evidence of water molecules locked in mineral grains on the surface of the Moon -- magmatic water, or water that originates from deep in the Moon’s interior.

It is no longer considered a planet, IAU categorized it as a dwarf planet in 2006.

The composition of the atmosphere of the exo-planet is determined through spectroscopy.

The Trappist-1 system is an ultra-cool red dwarf star, with 7 orbiting rocky planets, out of which 3 are thought to be in the habitable zone. This star is near to us at 40 light years away and in our own galaxy, the Milky Way. We don’t know yet if there is life on any of those planets.

Mangal Graha or Mars is the 4th planet from the sun. Humans have sent many satellite orbiters and a few landers like the curiosity rover and insight lander to mars. But no human has orbited or landed on mars.

Theoretically, it is not ruled out. But they should always travel faster than light. Their minimum velocity is that of the speed of light. For these particles, known as Tachyons, we need an infinite amount of energy to slow them down to the speed of light. Also, within relativity, space itself can expand faster than light (which supposedly happened near the beginning of the universe, during the so-called inflation era) but information cannot travel faster than light.

Space-time is a property by which a person conducting an experiment (an observer) describes the coordinates of an event in space and time. The space-time region around a massive object is not Euclidean (not connected by straight lines) but possesses a curved shape (a geometric property). Gravity according to Einstein’s general theory of relativity appears due to this curvature of space-time. It is thus not a fundamental property like time.

The outside observer loses all information about the collapsing matter beyond the horizon of the BH, mainly because of the infinite redshift and time dilation of signals emitted at the horizon. For the comoving observer (the one getting sucked into the BH) nothing changes. For a solar mass black hole at the horizon, the collapsing matter would be heated to 10¹³ K (which is what the comoving observer would measure) and this would continue to get higher as the collapse proceeds beyond the horizon.

As for the question of a portal, with the information we have, we do not know.

Reference: Link to a paper

Theoretically yes!

Black hole is a region of space-time where the gravity is so high that the escape velocity is greater than the speed of light. This is the perspective from Newtonian physics. From general relativistic consideration, around the black hole the space-time is completely warped that matter/light curves completely around. The boundary of the region from which there is no escape is called the event horizon.

One of the earliest methods was by using the Hohmann transfer orbit. Hohmann demonstrated that the lowest energy route between any two orbits is an elliptical "orbit" which forms a tangent to the starting and destination orbits. The other method is by the use of gravitational slingshot. The key aspect in interplanetary travel is to produce the very large velocity changes necessary to travel from one body to another.

Dark matter is any form of matter that does not interact with light. Hence the only way to detect them would be by their gravitational presence. (Reference: Link to a paper ).

Dark matter, not visible in the electromagnetic spectrum is believed to compose most of the matter in the Universe. It was identified due to the inability of observations (electromagnetic radiation from regular matter) to account for astrophysical phenomena such as the rotation velocity of galaxies, formation of galaxy clusters, and gravitational lensing, among others.

Mechanical waves, like sound waves, require the presence of a material medium in order to transport their energy from one location to another. On the other hand electromagnetic waves are generated by the vibration of electric charges. This vibration creates a wave which has both an electric and a magnetic component, that does not require a material medium to travel.

Earth's gravity (g) is the net acceleration acting on an object due to the mass distribution within Earth (and the centrifugal force from the Earth's rotation). With altitude (h) the Earth’s gravity is weakened, according to the formula: g_h=g(R/R+h)^2

Where R is the radius of the Earth.

And the answer to the question as to the height at which Earth’s gravity drops to zero IS infinity. But beyond a certain height the value of g drops off. The height at which the value drops to a millionth of that at the surface can be worked out from the above formula.

The accepted picture is that the Universe started with a big bang. This could have been triggered by a quantum vacuum fluctuation which exerts a negative pressure which leads to repulsive gravity. And this makes the Universe expand very rapidly from an infinitesimal region. This is the so-called inflation era of the early Universe.

There is no evidence so far, but theoretically, they can exist. Since we can’t communicate with them (as these ‘parallel universes’ are causally disconnected) there is no way of finding it out.

The origin of life on earth is an open question. The most widely accepted idea involves abiogenesis - creation of primitive life from non-living inorganic matter, aided by the prevailing atmospheric conditions on the earth roughly 4 billion years ago. The Miller-Urey (and other subsequent) experiments demonstrate this possibility, however the actual mechanism remains poorly understood. Once primitive life is formed, evolution by natural selection takes over and elegantly explains the bewildering diversity of life on the planet.

We don’t know yet of the existence of any alien life in the universe. This is not altogether surprising, we haven’t looked very far yet! But even if they do exist, they are unlikely to be any form of ‘human’, super or otherwise.

Imagine the observer (co-moving observer) falling into the black hole, for him, the time runs as usual. But if he is communicating at a regular interval with an observer outside, the time between his signals reaching the outside observer becomes longer and longer, since the time for the co-moving observer gets dilated as they get closer to the event horizon of the black hole. And eventually the signals will no longer reach the outside observer after the co-moving observer crosses over the event horizon. But before any of this, the observer falling into the black hole will be torn apart due to the tidal effects near the event horizon.

Reference: Link to a paper

Galaxies can be classified in three broad categories based on their morphology - spiral, elliptical and irregular. Lenticular galaxies are intermediary forms that connect ellipticals and spirals.

Our galaxy, the Milky Way, is a barred spiral galaxy. Spiral galaxies are the most common galaxy type in the universe. Because of our vantage point within the Milky Way, we only see it as a diffuse band in the night sky. The shape and structure of the Milky Way was deduced from multi-wavelength observations of hot stars and globular star clusters. Radio and infrared wavelength observations were particularly significant, since they can penetrate much farther through the dust toward the Galactic centre, as opposed to optical wavelengths.

Voyager-1 is the only human made device which has gone farthest from earth, roughly around 141 AU ( 1AU = earth-sun distance). It took Voyager-1 around 43 years from the time of launch from the earth on 5th September 1977, to reach there.

Nearest known blackhole (A0620-00) is around 165 million AU away from the earth. Our current technology is limited by speed and onload fuel, given the enormous distance the device has to travel before it reaches the blackhole. If in future we invent a technology such that the device can travel close to the speed of light (~ 300,000 km/s) with minimum fuel, then we should be able to reach the blackhole within human lifespan. Even if we reach near a blackhole, we have to account for the survival aspect of a device given the extreme conditions near the blackhole.

As we know that sun is revolving around the Milky Way center (where Sgr A* is located) which is at 8 kpc (1.6 billion AU) from the sun. According to Chandrasekhar’s theory of dynamical friction, it will take a few billion billion years for a sun-like star to merge into Sgr A* Black hole. In contrast, the lifespan of sun-like stars is much shorter, only around 10 billion years. Therefore, the Sun will reach the end of its life cycle long before that.

The Milky Way and Andromeda galaxies belong to our local group of galaxies. A local group is a pocket of space where the gravitational attraction between matter is strong enough to overcome the cosmological expansion of space due to dark energy. So a few dwarf galaxies and the three big ones i.e., Milky Way, Andromeda and Triangulum galaxies will merge/collide into each other while the rest of the space expands away from us. This is true for every local group in our universe.

In our solar system planet we know that life can not be supported with given atmospheric conditions. While we have discovered thousands of planets which are revolving around other sun-like stars in our galaxy Milky Way. As these planets are quite far from us, detecting the atmospheric conditions and life expectancy is a tremendously difficult task. Therefore there is no clear picture about aliens. World’s best minds are going to debate about the existence of life in the universe this year.

Reference: Link to a page.

When the mass of a star exceeds a certain limit it collapses to become a black hole. A black hole defined by an event horizon, from beyond which nothing (not even light) can come out. Past the event horizon the matter keeps on collapsing till it reaches a point till the density goes to infinity. It is not clear whether the laws of physics still hold at this point. According to the standard model (Big Bang), at the creation of space-time, the density and curvature were also thought to be infinite, this is known as the initial singularity.

There is no evidence so far, but theoretically, they can exist. Since we can’t communicate with them (as these ‘parallel universes’ are causally disconnected) there is no way of finding it out.

Yes. According to the equivalence principle, acceleration is indistinguishable from gravity. So if we rotate a spacecraft, the astronaut will feel the centrifugal force which in effect is equivalent to gravity.

Solving the restricted 3 body problem of bodies on a plane, Lagrange showed that the solution is an equation with five roots. The points L1, L2 and L3 lie on the line through the centre of the 2 bodies. The points L4 and L5 form equilateral triangles with the 2 other bodies.

A body at L4/L5 is acted upon by four forces i.e. the gravitational force of the 2 bodies, centrifugal force and the pseudo Coriolis force. If the object leaves L4/L5, the coriolis force curves its trajectory around the L4/L5 and the object will stay bound if the mass of the 2 bodies have a mass ratio greater than 24.96. Trojan asteroids are interesting examples of natural objects found at L4/L5.

Reference: Link to the animation: https://demonstrations.wolfram.com/OrbitsAroundTheLagrangePointL4/

This is called Hawking radiation which is negligible for astrophysical objects. The luminosity goes as 1/M2and it is significant for an asteroid mass black hole with a radius of 1 fermi. This process might have been important in the early universe.

M-theory has got 11 dimensions and the basic objects are D-branes (a membrane in D dimensions). In the early Universe, at high energies such objects would have existed for a short time. It is believed that all fundamental interactions are unified in 11 dimensions. Sometimes time is also considered as the 12th dimension. The collision of branes in higher dimensions is often thought as a possible explanation for the big bang.

Dark matter cannot be of negative mass as it is thought to have helped in galaxy formation. Dark matter has all the properties that are exhibited by ‘ordinary’ matter, except that dark matter particles do not interact with light. On the other hand, dark energy has a negative energy density that leads to repulsive gravity and negative pressure.

Moon’s tidal forces do affect the voyages in the sea causing high tides. But all other planets are too far away to have any significant effect on earth. The tidal forces fall as 1/r^3.

Milky way and Andromeda are gravitationally bound, and approaching each other at 300 km/s. This is purely a local effect. The expansion of the universe operates at large scales of megaparsecs. After 20 billion years everything else would have moved apart and we will see only the local group in an accelerated universe.

The estimate is about 10^79. Assuming a density of Baryonic matter 3x10^-31 gm/cc from observation.

There is no direct evidence so far. There is indirect evidence of these intermediate mass black holes.

Reference: Link to a paper

Yes. Some events in LIGO are thought to be such collisions. The neutron star can be tidally broken by the BH.

M33 doesn’t seem to have. In general all large galaxies have large supermassive BH in their core. There is no compelling reason why all of them should have. There seems to be some connection between the BH mass and the bulge mass of the galaxy.

M-theory can be proved either in very large accelerators or the early universe. Currently there are no such testable predictions that can unambiguously prove these theories. The only cases where their effects manifest are at very high energies.

Within relativity, space itself can expand faster than light (which supposedly happened near the beginning of the universe, during the so-called inflation era) but information cannot travel faster than light.

There is no unambiguous scientific evidence for UFOs.

Blackhole is a region of space-time where the gravity is so high that the escape velocity is greater than the speed of light. This is the perspective from Newtonian physics. From general relativistic consideration, around the black hole the space-time is completely warped that matter/light curves completely around. The boundary of the region from which there is no escape is called the event horizon.

The outside observer loses all information about the collapsing matter beyond the horizon of the BH, mainly because of the infinite redshift and time dilation of signals emitted at the horizon. For the comoving observer (the one getting sucked into the BH) nothing changes. For a solar mass black hole at the horizon, the collapsing matter would be heated to 10¹³ K (which is what the comoving observer would measure) and this would continue to get higher as the collapse proceeds beyond the horizon.

Reference: Link to a paper .