Why We Can Never Build The Time Machine

It has been a long, unfulfilled dream of humankind to obtain control over the passage of time. One cannot help but fantasize about bending time backwards, pause its eternal flow and dodge the inevitable death if technologically possible.

Clearly, time is a captivating phenomenon. That is why, a common theme in all science fiction is time travel. However, is building the time machine so trivial as depicted in the movies, like Back to the future? Is it theoretically as well as practically allowed?

What we gather about time travel from fiction is that it is either going back to a bygone era or jumping forward in the future. Time travel is a trip not in space which has three dimensions, but it is a journey in the fourth time dimension, the one we do not understand fully.

Theory of relativity

In non-relativistic physics, time was absolute, independent of the observer and same throughout the universe. This was proposed by English scientist Sir Isaac Newton when he thought that time progressed at consistent pace for everyone everywhere.

But in relativity, as theorized by German physicist Albert Einstein, time is no longer an absolute concept.

Firstly, time is treated like an alternate dimension to spatial dimensions of length, width and height. In other words, time is a new corridor to pass through.

Secondly, time is not the same for everyone everywhere as Newton had assumed. Time slows down as you travel faster, for example.

In fact, when subatomic particles are accelerated to nearly the speed of light, their lifetimes expand dramatically. They would usually decay faster, but when moving at relativistic speed inside the particle accelerator, they experience time more slowly (relative to other particles) and live longer.

Furthermore, from general theory of relativity, an upgraded version of special relativity, it is known that time passes more slowly for objects in strong gravitational fields, than for those objects which stay far from such fields.

As a result, if there were twin brothers and one of the twins orbited a black hole while the other around the earth, can you guess which of the twins would be older?

Coming back to the question: Is time travel as shown in the movies like Looper or Back to the future practical? Many scientists agree that the idea of time travel at the push of a button is not possible as it would violate the law of causality.

The paradox

Time's flow is like a river as it speeds up, meanders and slows down. Time can also have whirlpools and fork into two or more rivers, says American physicist and futurist Michio Kaku.

As soon as the button of the time machine is pressed, it may be possible for one to go backwards in a parallel world. Therefore, deprived of the opportunity to change the turn of events in the reality one came from. A new reality would be built from scratch, avoiding the grandfather paradox.

This idea of time travel was proposed by British physicist David Deutsch who used the terminology of multiple universes to solve the widely debated grandfather paradox.

The paradox comes from the idea that if a person travels to a time before their grandfather had children, and kills him, it would make their own birth impossible.

Deutschian time travel solves the paradox only theoretically. The time traveler emerges in an alternate universe, but very similar to his own. Can such universes pop in and out of existence merely on the whim of the time traveler? The idea sounds good on paper but its practical possibility is highly doubtful.

What is possible

As per most scientists and engineers, time travel is impossible as you have seen in the movies. The late English astrophysicist Stephen Hawking once joked: "I have experimental evidence that time travel is not possible." Hawking hosted a party for time travelers in 2010, but no one came.

Yet, we have observed that time slows down and does not always run at the same pace everywhere, which can help to travel forward in time, at least, relative to another. As shown in the realistic movies like Interstellar (2014).

Also, when we observe the universe, we are looking back in time. Our own Sun’s light, for instance, takes about 8 minutes to reach on earth. We see the Sun the way it was 8 minutes ago; so if the Sun disappeared this instant, we wouldn't know.

You will be surprised to know that NASA's James Webb telescope can study light that was emitted by the most ancient galaxies 10 billion years ago. This means, we can peek at the birth of the universe, more or less, if we design an even larger, better telescope.

Summing up

Time travel at the push of a button is out of the question. To construct such a machine violates not only the laws of physics but also common sense. It is much like building a perpetual motion machine, a hypothetical machine that can do work infinitely without an external energy source.

Lastly, reiterating that time travel is far more impossible technically than it is theoretically. There may still be undiscovered physics that allows construction of a time machine. It might be possible but would involve vast amounts of energy and money.

4 Unsolved Mysteries About The Higgs Boson

On July 4, 2012 the Higgs Boson particle was discovered at the Large Hadron Collider that is operated by CERN, the European organization for nuclear research. It took 60 years to first detect the elusive particle and there is still a lot to learn about it, scientists say.

CERN closed the largest particle accelerator for maintenance work that was extended due to delays caused by the pandemic. In 2022, scientists celebrated the 10th anniversary of Higgs Boson discovery. They now hope to uncover more as LHC has gotten back in action after 3-year hiatus.

1. Is the Higgs connected to dark matter?

Since dark matter makes up about 30% of the universe's mass and considering Higgs boson's relation to mass, scientists want to find if the two are connected somehow. They may explore, for example, whether or not the Higgs boson particle decays into a dark matter particle.

As of current understanding, scientists know that the Higgs boson particle can decay into boson, fermion and muon. The goal is to see which other kind of mysterious particle the Higgs boson particle can decay into. So far no unusual particles have been detected in collider experiments.

2. Does the Higgs boson interact with itself?

Matter particles (such as electron) move through the Higgs field and acquire their characteristic mass. More interaction means more the mass attained by the particle. Scientists hope to run experiments to find if the Higgs boson particle interacts with itself as predicted by the standard model.

This is the main question about the Higgs particle right now, say scientists working at the council for nuclear research. According to the standard model, when the Higgs particle self-interacts, it would create pairs or triplets of Higgs bosons, that are yet to be detected in the experiments.

3. Are there other Higgs particles?

The Higgs boson is an excitation of the all-pervading Higgs field that helps other particles pass through it and acquire mass. For this reason, it was nicknamed the God particle by the media, although some scientists refer to it as the Goddamn particle as it took so long and multi-billion dollars to find it.

The particle which was found in 2012 has zero spin and no electric charge. Theories alternate to the standard model predict the presence of more than one kind of God particle. Detection of additional Higgs particles in the collider experiments would mean that there must be new physics out there.

4. How does the Higgs interact with matter?

One thing scientists know for sure is that the more massive a given particle, the greater its interaction with the Higgs field must be. The nuances of this are yet to be understood even though the measurements thus far match the predictions of the standard model, the precision of these measurements isn’t great enough.

Models other than the standard model propose the existence of one kind of Higgs particle that interacts only with heavy particles and another that interacts with only lighter particles. Similar exciting challenges in particle physics await scientists working at the large hadron collider.

5 Richard Feynman Quotes on Quantum Mechanics

American physicist Richard Feynman won a Nobel Prize for physics in 1965 for his work in quantum electrodynamics. Feynman was a man who always jumped into an adventure: as an artist, a story-teller and an everyday joker whose life was a combination of his intelligence, curiosity and uncertainty.

Despite making fundamental contribution to the field of quantum mechanics, Feynman was often perplexed by its complexity. Feynman said, We don't know what an atom looks like but we can calculate its behavior. It is like a computer trying to calculate how fast a car is going without being able to picture the car.

Following are five quotes by Richard Feynman that reflect his views on quantum physics. As students, we may derive one or two equations, solve few problems and be done with it. But it is a great insight to look back as to how a previous generation of physicists grappled with the bizarreness of quantum mechanics.

1. Personal struggle: I always have had a great deal of difficulty in understanding the world view that quantum mechanics represents. Because I'm an old enough man that I haven't got to the point that this stuff is obvious to me, okay? I still get nervous with it. And therefore, some of the younger students, you know how it always is, every new idea, it takes a generation or two until it becomes obvious that there's no real problem. It has not yet become obvious to me that there is no real problem.

2. Nature is absurd: What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school. It is my task to convince you not to turn away because you don't understand it. You see my physics students don't understand it... That is because I don't understand it. Nobody does. Quantum mechanics describes nature as absurd from the point of view of common sense. And yet it fully agrees with experiment. So I hope you can accept nature as She is - absurd.

3. Relativity VS quantum mechanics: There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe there ever was such a time. There might have been a time when only one man did, (Einstein) because he was the only guy who caught on, before he wrote his paper. But after people read the paper a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I think I can safely say that nobody understands quantum mechanics.

The difficulty really is psychological and exists in the perpetual torment that results from your saying to yourself, "But how can it be like that?" which is a reflection of uncontrolled but utterly vain desire to see it in terms of something familiar. But nature is not classical, dammit, the imagination of nature is far greater than the imagination of man.

4. The mystery of atom: It always bothers me that, according to the laws as we understand them today, it takes a computing machine an infinite number of logical operations to figure out what goes on in no matter how tiny a region of space, and no matter how tiny a region of time.

How can all that be going on in that tiny space? Why should it take an infinite amount of logic to figure out what one tiny piece of space/time is going to do? So I have often made the hypotheses that ultimately physics will not require a mathematical statement, that in the end the machinery will be revealed, and the laws will turn out to be simple.

5. On nature of reality: Does this then mean that my observations become real only when I observe an observer observing something as it happens? This is a horrible viewpoint. Do you seriously entertain the idea that without the observer there is no reality? Which observer? Any observer? Is a fly an observer? Is a star an observer? Was there no reality in the universe before life began? Or are you the observer? Then there is no reality to the world after you are dead? I know a number of otherwise respectable physicists who have bought life insurance.

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5 Major Differences Between Sgr A* and M87*

Take a close look at the two black hole images and you can ascertain a few differences by your own. Notice the speed of accretion disk, which is a gas-like flow around the black hole. Or the size of dark spots in the center that give you a faint idea of the black hole event horizon.

The latest image by event horizon telescope is that of Sagittarius A*, a black hole in the center of our Milky Way galaxy. That this is a supermassive black hole was first recognized by physicists Reinhard Genzel and Andrea Ghez, for which they won the Nobel Prize in 2020.

Event horizon telescope or EHT is a worldwide network of radio telescopes that took the first ever picture of a black hole in 2019. It was that of M87*, an enormous supermassive black hole in the heart of Messier 87 galaxy in the constellation Virgo.

While the two black hole pictures look almost similar, for the laws of physics that govern their behavior are the same, the new image is more exciting than before. For one, it is located in our neighborhood; and second, it was way too difficult to catch a glimpse of.

1. Schwarzschild radius: It is the size of the sphere from which even light will fail to escape. For supermassive M87* this is 18 billion km, four times the radius of our solar system! For Sgr A*, Schwarzschild radius is only 12 million km.

2. Relative size: Our black hole is 31 times wider than the Sun, as shown in the figure below. Whereas the black hole in Messier 87 is 27,000 times wider than the Sun. If Sgr A* was the size of a doughnut, then M87* would be the size of a football stadium.

3. Distance: Our neighborhood black hole Sagittarius A* is obviously closer, located 25,000 light years away from the earth. Whereas M87* is 55 million light years away! So if it took 1 hour to get to Sgr A*, then it would take 91 days to reach M87*.

Despite being closer, observing Sgr A* was more challenging than expected. Scientists had to look through the galactic plane and filter out the noise from intermediate stars and dust clouds in their data, collected across continents.

4. Mass: M87* is 6 billion times more massive than the Sun whereas Sgr A* weighs 4 million Suns. Thus, our black hole Sgr A* is 1500 lighter in comparison.

5. Speed: Around the black hole is a bright ring of materials that swirl at great velocities. The material disk of M87* rotates over a course of many days at roughly 1,000 km/s, while it takes only a few minutes for material to move around Sagittarius A* because it is much smaller.

Why is the picture of our black hole kind of blurry? One of the reasons is that we don't have a direct view of the object while sitting on one of the arms of the galaxy and secondly, its accretion disk is spinning very fast compared to M87* so it's like taking a picture of a toddler who cannot stand still.

Climate Scientists Win Nobel Prize In Physics

In 2020, mathematician Roger Penrose was bestowed upon the most prestigious honor in science along with Andrea Ghez, who became only the fourth woman laureate in physics and Reinhard Genzel of the Max Planck Institute, for furthering our understanding of the black holes.

This year, the Nobel Prize foundation has again elected three joint winners. One half of the Nobel prize to climate scientists Syukuro Manabe of U.S.A and Klaus Hasselmann of Germany and the other half to Italian physicist Georgio Parisi.

We have all read about the global warming in our school textbooks, that humans are influencing the climate and the earth's temperature by burning fossil fuels. But how did the scientific community arrive at that conclusion in the first place?

The answer is, works of notable scientists like Syukuro Manabe, who is a senior meteorologist at the Princeton University, have helped establish humanity's increasing role in much of everything that is gone wrong with this planet.

Starting in the 1960s, Manabe pioneered the use of computers to simulate climate change. He demonstrated in 1970 that increase in the amount of carbon dioxide levels will rise global temperatures by 0.57°C by 2000. He was spot on as the earth had warmed by 0.54°C.

Klaus Hasselmann, leading oceanographer in Germany and the then director of the Max-Planck-Institute of Meteorology, also arrived at the same conclusion. He showed that despite short term weather fluctuations, climate models are reliable in long term.

Their studies further revealed that the global temperature is projected to increase by an additional 2°C – 3°C during the 21st century. So, we may take climate change lightly today but in the future its dangers will be observable in day to day life as the scientists have warned.

The third winner is Geogio Parisi whose research areas include statistical mechanics and complex systems. He has developed a mathematical model in order to understand complex systems such as the earth's global climate, the human brain and ultimately the entire universe.

Did you know that total 115 Nobel prizes in physics have been awarded since 1901? The winners this year include some of the oldest awardees. Manabe and Hasselmann are 90 and 89 respectively, while Parisi is relatively younger at 73 years.

Their recognition by the Nobel Prize committee shows that our knowledge about the climate change is built upon strong scientific foundations. Thus, no matter how much the politicians, the industrialists or the others deny climate change, it is happening at every moment.

After the announcement, Giorgio Parisi said in relation to climate change: “It is very urgent that we take strong decisions and move at a very strong pace. It is clear for future generations that we have to act now to tackle the climate change."

5 Applications of Dimensional Analysis

The concept of dimensional analysis was introduced by French mathematician Joseph Fourier in 1822. It is a useful method in physics and engineering to identify the relationships between various physical quantities by analyzing their base quantities.

There are seven base or fundamental quantities of primary importance in physics. The rest of all the physical quantities are derived, that is, written in terms of the following base quantities:

1. Mass (M)
2. Length (L)
3. Time (T)
4. Temperature (K)
5. Electric current (A)
6. Amount of substance (Mol)
7. Luminous intensity (Cd)

Others are derived quantities, for example: Speed is distance upon time, where distance is a type of length. So, the dimension of speed in terms of base quantities is [L][T]^-1

1. Find dimension of unknown quantity

Example: To increase the temperature of a substance by ΔT, heat required is given by Q=mSΔT where m is mass and S is specific heat capacity of the substance. We can find the dimension of S by doing simple algebra.

S=Q/mΔT

S=[M L² T^-2]/[M][K]

S=[L² T^-2 K^-1]

Since heat (Q) is a type of energy we have used the dimension of energy [M L² T^-2]. Knowing the dimensional dependence is useful while performing experiment in laboratory.

2. Find unit of physical quantity

Example: The dimension of force is [M L T^-2]. Therefore, the unit of force in the standard MKS system is kg.m.s^-2 corresponding to the base quantities used. Likewise, the unit of force in the French CGS system is g.cm.s^-2

3. To check correctness of formula

In an equation, the left hand side should match the dimensions on the right hand side. That's because, you can't have apples on one side and oranges on the other. This is useful in multiple choice questions as it helps in eliminating the wrong options.

Example: Is force F=mv²/r correct? where m is mass, v is velocity and r is radius. To find out, we start by writing the dimensions on both sides.

[M L T^-2] = [M][LT^-1]²/[L]

[M L T^-2] = [M][L²T^-2][L^-1]

∴ [M L T^-2] = [M][L][T^-2]

The formula is correct. In fact, it is the centripetal force if you might recall, which acts upon a body directed towards a fixed center, thus keeping it in orbit.

4. To roughly derive formula

Example: Suppose that the time period of a simple pendulum is dependent on mass of the bob, length of the string and gravitational acceleration due to earth.

Let's assume that the time period is proportional to some powers of mass, length and gravity: t∝m^al^bg^c

To find the formula of time period, we must obtain values for a, b and c. We start by writing the dimensions on both sides.

[M]⁰[L]⁰[T] = (constant)[M]^a[L]^b[LT^-2]^c

The dimension of constant is 1. So,

[M]⁰[L]⁰[T] = [M]^a[L]^b[LT^-2]^c

Comparing the exponents on both sides, we get:

a=0, b+c=0 and -2c=1

On solving we obtain values of a=0, b=1/2 and c=-1/2. Now, putting back in the original equation t∝m^al^bg^c  gives:

t ∝ m⁰l^1/2g^-1/2

t=(constant) l^1/2g^-1/2

Thus, time period of a simple pendulum is independent of mass of the bob. This is because, physically speaking, mass is both the cause of swinging motion and the resistance to swinging motion, so it cancels out.

5. To express in new base quantities

This type of questions can come up in competitive exams like the JEE-Main. Example: If pressure, velocity and time are taken as base quantities, then what would be the dimension of force?

Let's start by assuming powers

Force = P^av^bt^c

And write dimensions on both sides

[M L T^-2] = [ML^-1T^-2]^a[LT^-1]^b[T]^c

[M L T^-2] = [M]^a[L]^-a+b[T]^-2a-b+c

Comparing powers on both sides

a=1, -a+b=1 ⇒ b=2 and -2a-b+c=-2 ⇒ c=2

∴ Force = Pv²t²

How Antimatter Was Discovered By Carl Anderson

British physicist Paul Dirac showed in 1928 that every particle in the universe should have an antiparticle with the same mass as its twin, but with the opposite electrical charge. Across the pond, an American physicist would detect the first such particle, four years later.

Carl Anderson, inspired by the work of his Caltech classmate, Chung-Yao Chao, set up an experiment to investigate cosmic rays under the supervision of physicist Robert Millikan. In 1932, he won the Nobel Prize in physics at the age of 31, becoming one of the youngest recipients.

Discovering the positron was no easy feat but the mechanism he employed to do so was fairly simple and ingenious enough to overcome the limited budget. He found the mysterious particle almost by accident with the help of his own improved version of the cloud chamber.

A cloud chamber is a sealed box with water vapor. When a charged particle goes through it, the vapour is ionized and leaves behind a trail. Thus, the trajectory of the particle can be seen virtually. Carl used a mixture of water and alcohol to get clearer photographs.

Carl included a Lead plate in the middle to slow down the particles and surrounded the chamber with a large electromagnet, which caused the paths of ionizing particles to curve under the influence of magnetic field.

As can be seen in the picture, the radius of curvature of the track above the plate is smaller than that below. Thus, the particle entered from the bottom, hit the Lead plate and came to a halt above it due to loss of energy. This and the direction in which the path curved helped in identifying that the charge was positive.

That it was antielectron and not proton was determined by the observation that the upper track was much longer in length than predicted for proton. A proton would have come to rest in a much shorter distance, since it is heavier. The trajectory observed was that of a particle much much lighter than the proton.

So, that's how the first antimatter was found and Dirac was proven right within a matter of few years. Furthermore, antiproton and antineutron were discovered in 1955 and 1956 respectively. The first antiatom was produced by CERN in 1996.

Why antimatter is important? Because, studies related to antimatter will help in our understanding of the early universe. Also, Positron emission tomography or PET scan is used to detect early signs of cancer. Scientists hope that some day, antimatter may be used for the treatment of cancer. Who knows!?

Science is either physics or stamp collecting?

What is science? In one simple sentence, science is the study of nature. However, different sciences like: astronomy, chemistry, biology, geology, etc. have different approaches to do so. Thus, not all sciences are equal.

The quote 'All science is either physics or stamp collecting' by New Zealand physicist Ernest Rutherford perfectly reflects that inequality. According to him, physics is the king of sciences because it is fundamental to all other fields of study.

But why exactly did Rutherford think in that manner? Despite himself being the recipient of Nobel Prize in chemistry, what made him consider physics the most noble of sciences?

The answer lies in stamp collecting - a hobby in which people collect and classify stamps as objects of interest or value.

Stamps are available in many varieties - big and small, square and round, stamps with famous human faces, stamps with animals and birds, stamps commemorating anniversaries, etc.

 Rutherford's stamp 

Similarly, some branches of science, such as zoology or botany for example, are mostly concerned with collection and classification of species - animals and plants, respectively.

Although this would be dumbing down those sciences but that is more or less the purpose, isn't it? In other words, those sciences are not fundamental sciences and their scope is limited only to Earth.

Physics, according to Rutherford, is the only science that has an elaborate structure consisting of observation, experiment and mathematics. Physics captures our imagination from mysterious atoms to supermassive galaxies. It truly is the universal embodiment of the scientific method.

By this definition, the science which is closest to physics is astronomy. You observe and measure the effects of, say a black hole on its surroundings, with the help of a telescope and basic knowledge of mathematics. In this way, like engineering, astronomy is an application of physics and mathematics.

Chemistry is a unique science because it has the 2nd most direct impact on day to day life after physics. The objects we use, such as plastic, glass, steel, etc. are all obtained by chemical processes.

Our body is a chemical engine and the food we eat are organic molecules. But just like biology, there is a lot of nomenclature and classification rules in chemistry to deal with. Chemistry is also not universally the same, like on different planets, but the laws of physics governing those chemistries are the same.

Likewise, sciences like computer science and psychology are neither fundamental nor universal. They are narrowed specializations and are heavily dependent on logic, mathematics and observation.

All the sciences, however, must ultimately be experimental because that is how they progress. That is how the hypotheses are tested and verified and accepted. So it is worth pointing out that no amount of belief can make something true. Sciences keep evolving with time as new evidence is uncovered.

Finally, it is equally important to mention that the statement "all science is either physics or stamp collecting" had more truth to it back in Rutherford’s time than today.

As you know, for example: With Darwin's theory of evolution, biological sciences have too become observational rather than just being classification sciences.

So, over time, sciences evolve and become more and more physics-like. They are no longer merely observe and classify but start using mathematical models. Still, Rutherford's point is intact, physics will be the king of sciences.

Hawking's black hole theorem confirmed by gravitational waves

A black hole has often been portrayed as the ultimate villain in sci-fi movies due to its mysterious nature. From the death of a large-enough star it emerges with such a strong gravitational field that not even light can escape from within its grasp.

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However, in spite of its wildly mysterious behavior, the black hole obeys certain simple rules. One of those rules, first proposed in 1971 by English physicist Stephen Hawking, has been proven correct by the help of gravitational waves.

The area law, derived from Einstein's general relativity, states that it is impossible for a black hole to decrease in size, at least in the short term. Mathematically:

Recently, a team led by astrophysicist Maximiliano Isi from Massachusetts Institute of Technology studied the gravitational wave data released by the merger of two black holes.

Their calculations show that the total surface area of the resulting black hole is greater than the combined areas of the two smaller black holes. Therefore, Stephen Hawking was right.

However, while black holes cannot shrink according to Einstein's general relativity, they can do so as per the quantum mechanics.

Hawking worked that out too in 1974 – a concept known as Hawking radiation, which is predicted to emit because of strange quantum effects near the black hole's event horizon.

In his 1988 book A Brief History of Time, Hawking thus wrote: Black holes ain't so black. The release of these radiations would cause the black hole to shrink over longer time period and evaporate eventually.

Hence, theoretically speaking, both general relativity and quantum mechanics hold true. Maximiliano Isi said: "I am obsessed with these objects because of how paradoxical they are."

Now that the area law has been established for short to medium time frames, the researchers' next step would be to detect Hawking radiation by observing older black holes; no substantial evidence has been recorded so far.

Isi concludes: Black holes are those phenomena where gravity meets quantum mechanics, which makes them the perfect playgrounds for our understanding of reality.

Who discovered that we are made from star stuff?

Astronomer Carl Sagan popularized the phrase "We are made of star stuff" when he said: Nitrogen in our DNA, calcium in our teeth, iron in our blood and carbon in our food; were made in the interiors of collapsing stars.

However, most people wouldn't know the name of that scientist who actually found it out. It was German American physicist Hans Bethe (1906-2005) who wrote it in a paper titled "Energy Production in Stars" as early as in 1939.

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In 1930s, at the time when European scientists were debating quantum mechanics, Bethe migrated to United States and contemplated the stars. He thus became the first person to figure out that conversion of hydrogen into helium was the primary source of energy in a star.

The process is called nuclear fusion in which many nuclei combine together to make a larger one. It so happens that the resultant nucleus is smaller in mass than the sum of the ones that made it. So, by virtue of Einstein's equation E=mc², the mass is converted to energy.

When a star would eventually run out of hydrogen (its primary fuel) it would start converting helium into carbon, nitrogen, oxygen and so on, in order to keep itself hot.

However, those reactions themselves will halt at some point and the star would no longer be able to support itself against its own gravity and it will die in an explosion.

Therefore, it was proposed that most of the material that we're made from, came out of the dead stars which spewed out those chemical elements into the universe for further use. Hence, we are made of star stuff.

Bethe's groundbreaking paper not only helped in understanding the inner workings of the stars but also solved the age-old questions like: 'How do stars shine?' 'Where did the chemical elements come from?'

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He won the 1967 Nobel Prize in physics for this theory of stellar nucleosynthesis. Bethe would continue to do research on supernovae, neutron stars, black holes and other problems of astrophysics well into his late nineties.

Carl Sagan and Hans Bethe share the stage at Cornell

Now, Carl Sagan, who was earlier at Harvard University, joined Cornell in 1976 and became immediate colleagues with Hans Bethe who had been at Cornell since coming to America in 1935. While Bethe was a professor of physics, Sagan was a professor of Astronomy.

It was unfortunate that the general public still did not know about stellar nucleosynthesis despite Bethe discovering it some 40 years ago and winning the highest prize for it a decade ago. Carl Sagan changed this.

Their common interests in science and politics brought them even closer. Bethe was also a fan of Sagan's 1980 show Cosmos: A personal voyage. In one of the episodes, when Sagan said "We are made of star stuff", he immortalized Bethe's work in television history.