INTERSTELLAR TRAVEL, FERMI PARADOX

Read book chapter 13.

INTERSTELLAR TRAVEL: Dreams and realities...

Challenges for manned space flight we discussed in class:

- extra development cost
- safety
- additional weight of missions
- need to return
- time limit to space travel, while unmanned missions can keep going as long as there is no failure or loss of power.

Why is space travel expensive, even "local" (close to Earth and in solar system)?

- Every object launched from Earth needs to get a speed that is of order the escape speed from Earth. That figure is 11 km/s. To put something in close Earth orbit (like the space shuttle, which orbits at about 200-300 miles above Earth) requires a speed of about 7 km/s. In class, we compared these speeds with typical every day speeds.

Clicker question: When we launch a rocket from Earth at the escape speed of 11 km/s, and we stop the rocket engines, what will its speed be when it gets far away from the solar system without encountering other objects in its path?

a. 11 km /s

b. somewhere between 11 and 0 km/s

c. 0 km/s

d. It will be slow down, turn around and be its way back into the solar system.

Typical speeds in real life, expressed in meters/second:

walking: 1.5 m/s

car at high way speed: 30 to 40 m/s

airplane at cruising speed: 220 m/s

The amount of energy required to accelerate something from zero velocity to a velocity v scales with the velocity squared and with the mass of the object. This is called "energy of motion" or "kinetic energy". This ignores the additional energy required to accelerate a craft out of the gravitational force field of the Earth (and then later the Sun). For example, half of the energy required to get to the Moon is necessary to reach orbital speed around the Earth first.

So, per unit mass, it takes (11,000/220) squared = 2500 times more energy to launch something in orbit than have it go up in a commercial airplane. This does not include any considerations of forces acting to slow down the object once it is being accelerated (e.g. friction by air and the pull of gravity). This energy requirement explains why in any launch from Earth, most of the space craft is taken up by rockets and fuel, not scientific or human payload.

If we want to leave the solar system or travel from Earth to other planets, the gravitational pull from the Sun becomes important too. To leave the solar system from Earth's location requires a speed of 42 km/s. Since we are already going at 30 km/s around the Sun, we need to gain additional speed to make it to 42 km/s.

So, if we launch a space craft from Earth in a direction that takes it out of the solar system and it attains just the velocity of escape from the solar system by the time most of the fuel is gone, what will happen?

First, the craft experiences no friction in space, so it is not being slowed down by friction. It is being slowed down by the gravitational pull of the Sun. If it moves at just escape speed, it will continue to slow down and never go fast by the time it reaches large distances from the Sun. So, instead we accelerate it further on its way out of the solar system, using the gravitational "assist" from other planets. By cleverly using the knowledge of orbits and dynamics in the solar system, we can actually increase the speed of space craft greatly by having them "swing by" other planets. Thus, we can increase the speed from that required to leave Earth to higher speeds and send the craft on its way.

Still, there are enormous hurdles to get far at the typical speeds that can be reached. For example, going at 100 km/s if we could reach that, it would still take about 12,000 years to reach the closest star to us. We would not need to power the space craft much with rockets, but it would require energy to send signals back to us. And those signals would take longer and longer to reach us, and get fainter and fainter as the space craft gets further away. Once far away from the Sun, the craft cannot be powered by solar energy, so it must have its own internal power source; most likely, with current technology, this would be a nuclear reactor, using radioactive elements to generate energy.

A manned mission out of the Solar System is clearly completely beyond our technological capabilities. Will it always be?

The space craft has to be self-sufficient, it has to provide food, medical care, energy, life support systems etc to a large crew; if it were to travel for generations in terms of human life times, it would have to be very large and take along a lot of people. With conventional fuels and thinking we will not get very far. Several exotic ideas have been proposed in the past, e.g. using nuclear bombs behind the space craft to accelerate it to high speed. Other ideas include using the hydrogen in space as a source of fuel (nuclear fusion!) by having the space ship collect the hydrogen gas as it moves at high speed through the space between stars. The density of the hydrogen is very low and you would need to move fast and have a very large funnel to collect the hydrogen to make this work...

NEED FOR MORE EXTREME IDEAS: WILL EINSTEIN'S THEORIES OF RELATIVITY HELP?

Einstein developed the two theories of relativity that supercede Newton's laws of motion and gravity in important ways.

I. Year: 1905. SPECIAL RELATIVITY: The study of motion and time in reference frames that are not accelerating (That means e.g. that gravity is NOT included).

II.Year: 1915. GENERAL RELAVITY: The extension of special relativity to include accelerating frames and gravity. This would also include then the large-scale evolution of the universe and ultimately provide the theory to understand the expanding universe.

Why were the new theories needed? Maxwell's theory of electricity and magnetism (which includes the explanation for light) did not work in a Newtonian concept of space and absolute time.

In relative, space and time are linked into a four-dimensional "space-time" and time is not absolute. That is, it is NOT the same at all locations. Time, velocity, length and other quantities are all RELATIVE concepts that depend on how you move and where you are.

Key item: Einstein postulated that the speed of light is the same for every observer in any frame of reference moving at constant speed or at rest. This leads to extremely strange effects when traveling at speeds approaching the speed of light: the time will slow down compared to an observer at rest, and the distances will be foreshortened. These peculiar effects are called time dilation and length contraction. In General Relativity, it is shown that time also depends on Gravity: clocks run slower in strong gravitational fields.

How fast could we travel? Einstein's theory of special relativity sets a hard limit on the highest possible speed; nothing with mass can reach the speed of light. If we keep accelerating an object by giving it more energy it will still not exceed the speed of light, instead it will act as if its mass is increasing and since the energy of motion goes as product of mass and velocity squared, the gain in velocity will be less and less as we approach the speed of light. The relativistic effects become most noticeable only at speeds close to the speed of light, so in principle we could travel at e.g. 25% speed of light without too much non-Newtonian effects.

Time dilation, however, is one important advantage if you could travel at close to the speed of light, you can criss cross the Milky Way within one human life time.

The implications of Einstein's theories are:

• clocks slow down inside fast moving space craft compared to Earth (this has been measured). At the extreme limit of the speed of light, no time passes. In other words, for a light beam traveling through space there is no change in time if a clock could come along (which it can't, since it has mass!).

• astronauts could in fact reach the nearest stars in their life time if we could reach speeds close to the speed of light. Many generations might have passed on Earth but not in the space craft.  The challenges would be enormous to manage such "expeditions", you would not find your relatives if you could ever come back on Earth.

We discussed: the twin paradox. More on the twin paradox than you ever wanted to know.

Some review clicker questions.

1.. The "energy of motion" or kinetic energy is proportional to:

a. velocity.

b. mass squared.

c. velocity squared.

d. momentum.

2. What is the twin paradox?

a. That both twins keep the same age even though their clocks run differently.

b. That the twin on Earth has aged less than the twin in the fast-moving space craft.

c. That the twin in the space craft has aged less than the twin remaining on Earth.

d. That both twins could argue they should have aged less given that speeds are relative.

3. What is the solution to the twin paradox?

a. Einstein's relativity is only a theoretical construct, real life is more complicated.

b. The twins are not both in an inertial frame at all times so the situation is not symmetric.

c. Clocks run at different rates while moving but the time difference disappears once you bring them back together again at rest to they indicate the same time again.

d. It is still a paradox that is conundrum to scientists.

Are there better ways to travel?

Nuclear energy fueled craft would avoid the huge mass required in fossil fuels that power current space flight. E.g. Example of current research on nuclear-fusion powered space flight. It is quite possible this will eventually work, but we should also remember that we have not yet been able to get energy generation from nuclear fusion on earth, so it is a bit of question whether we could build it for a space craft. However, the ideas are different and the very idea that we need to generate exhaust to drive the space craft may actually make it easier to control the nuclear fusion chain.

Can we beat Einstein's limit to speed?

Here is an interesting link with discussion of real and movie space travel options. The author, NASA's Marc Millis, identifies three things that are needed to make inter-stellar space travel a reality:

1. The ability to exceed the speed of light

2. The mean to propel a vehicle without propellant. Propellant is the stuff shooting out the rocket engine that pushes the space craft forward. This is not necessarily the same as "gasoline" for a car: the gasoline is burned in the car's engine. The car has the friction against the road to push it forward. The only way to push a space-craft in vacuum forward, is by shooting large quantities of gas out of the exhaust. Hence the enormous mass of the fuel tanks required. We do burn it, but it still has to come out of the exhaust to have an effect (Newton's third law!).

3. The means to power such a space craft. Even for space crafts that don't require propellant, the energy to boost it to high velocities still is enormous. What energy source would we have in interstellar space?

See here for many more details: Warp speed?

And here is a more recent article on that: More recent news on the warp drive idea.

But, so far there is nothing known within physics today that contradicts Einstein's results. The speculation of being able to travel by short cuts through "worm holes" associated with black holes, or other exotic ways so far is still mostly speculation, be it in some cases based on our best understanding of the laws of physics. If the Universe is as we understand it today, 4-dimensional (3 space and 1 time coordinate) such short cuts are unlikely. However, there is much we don't know about the Universe. It may be possible that there are "hidden" dimensions that allow for short cuts and make travel possible. If we have ever been visited by aliens, they probably would have figured out to take advantage of such shortcuts.

In summary, where do we stand on space travel?

A. We have "mastered" space travel using conventional fuel sources. This will get us to Moon and with robotic missions throughout the solar system. It is already challenging to get manned missions to Mars with such a system.

B. Technologies are under development for other fuel sources. These include nuclear fusion powered space-craft. They will get us faster through the solar system but still do not easily enable travel to nearest stars, certainly not with manned missions.

C. We may learn more about the structure and nature of space-time to eventually discover shortcuts or other ways to "by-pass" Einstein's limit on speed or the energy it takes. But there is no guarantee that this will be possible.

Here is an excellent primer on black holes and the effects that happen there. Note that gravity slows down the light speed too and hence time goes slower near a black hole. Actually, you can think of the "slowing down of light" more like a distortion in space-time so that the light has to travel larger distances near the massive object than if the object were not there. The space time is also distorted, such that there is a gravitational Doppler shift to the wavelength of the light.
One way to see the effect of gravity on light is through gravitational lenses, which we discussed before:
Hubble picture of lensing galaxy.
Explore here: Black holes, gravity's relentless pull
Make sure that you explore several topics in the black hole web page, including:

• travel to a black hole

• seeing what happens to a clock that is dropped into a black hole

• seeing how the orbits are different around a black hole compared to Kepler's elliptical orbits

• how black holes form

• how we know black holes exist

LAST SECTIONS

THE FERMI PARADOX AND CONTACT

Read book chapters: rest of chapter 13

The Fermi Paradox.

Who was Enrico Fermi?  Famous 20th century physicist, working on elementary particle physics, discoverer of the "Fermi-exclusion principle", involved with the Los Alamos Manhattan project in the 2nd world war.

The Fermi paradox: Where is everyone?

If life is common and present on many other planets in the Milky Way, then surely some civilization would be far more advanced than we are today, and they should have been able to colonize our Galaxy, even if interstellar travel proceeded at slow pace. They could have hopped from star to star and colonized large sections of the Milky Way in millions of years, even travelling well below the speed of light (see book).

Even if the civilizations themselves could not travel, they would have been able to design self-replicating robots that could have traveled across the Milky Way. Where are they?

Motives for colonization:

- if our history on Earth is any indication...
- avoid extinction: send out groups of an advanced species to settle new colonies. Not everyone has to go!
- threats of star ending its life or "impending" impacts.

Non-motives for colonization:

- Cannot solve population growth problem on Earth.
- Conquest?
- Bring natural resources back to Earth. Seems too expensive.


Possible explanations for the Fermi paradox:

1. We are in fact the most advanced civilization. In this case, SETI will not find a signal for a long time...

Seems unlikely: our most rapid advance has happened in a relatively short period (see "cosmic calendar argument" in the book) and our solar system was formed 8 billion years after the Milky Way began to form so there are many stars and presumably planets out there that preceded us by many eons. While the very early MW might have lacked heavier elements, successive generations of stars would have created them and so there would be ample opportunity for life to develop elsewhere.

Also: everything we have learned since Copernicus has shown us that Earth is not the special place it was once thought to be. We are not the center of the solar system, there are billions of stars like the Sun, we are not near the center of the Milky Way, and there are billions of galaxies like ours.

Proponents of this idea: "Rare Earth Hypothesis". Many conditions may have conspired to make Earth more special than we think.

2.  Civilizations are not interested or able to colonize other solar systems.
     a. Technological challenges?
     b. sociological considerations?
     c. self-destruction

3. They exist but don't reveal themselves to us.

Why would we be interesting to them? The "Zoo hypothesis" (or "wildlife refuge").
The "sentinel" hypothesis. They are monitoring our progress and are waiting for us to advance more.

What is the most popular explanation in your eyes?

Or have we been visited?

Roswell
"Alien abductions"

Clicker questions, let's hear your opinions. Just 3 choices for each question:

a. I agree b. I do not agree c. I do not yet have an opinion.

1. We are the only advanced civilization.

2. There are other advanced civilizations but interstellar distances are too large.

3. There is nothing of interest for them here to come visit us.

4. There likely have been other civilizations but they are not around at the same time we are given that civilizations evolve and may go extinct.

5. They are already here and have been here, we just don't know it or accept it.

6. They are monitoring our technological advance and will contact us in due time.

BACK TO EARTH: LET'S PLANT OUR FEET IN SOLID SOIL AND MAKE A BALANCE OF WHAT WE DO AND DON'T KNOW AND WHAT SITUATION IS FROM SCIENTIFIC PERSPECTIVE

Scientific results thus far:

THE GOOD:
Billions and billions:

• Sun is a normal star of which there are many in the Milky Way.

• Stars like the sun can shine for billions of years due to the nuclear fusion in their centers. Our solar system is 4.5 billion years old, less than the age of the universe and the age of the Milky Way (12-14 billion years).

• The Milky Way is a normal spiral galaxy of which there are hundreds of billions in the Universe.

• The Earth is one of 4 terrestrial planets in the solar system. The other planets are like us in size and composition, but lack water (either liquid or all together) and have very different atmospheres.

• Planets around other stars are being found in large numbers. We cannot yet find planets of Earth mass easily but so far this is a technological limitation. There is no reason to claim that planets like Earth don't exist elsewhere. In fact, there are probably (you guessed it) billions and billions of them...

• Our solar system is not located in any special place in the Milky Way.

• Our Milky Way Galaxy is not located in any special place in the Universe.

• The Earth and its inhabitants are made out of atoms that are equally common elsewhere in the universe.

• Living beings are made out of a subset of those atoms.

• There is strong evidence that early life on Earth was primitive and that complexity of species increased over time due to evolution. Biological evolution is the result of mutations in genetic material upon reproduction, with natural selection as the "guiding mechanism" to push evolution in particular directions depending on the local and global conditions.

• There is overwhelming evidence for biological evolution occurring on earth, including evolution of species, mass extinction of species, increases in intelligence among some species with time,  advances in use of tools and technological discoveries.

All of this taken together, in my mind, makes the existence of life elsewhere a likely prospect.

THE (as of now still) BAD:
The uncertain or unknown:

• Science argues for the spontaneous generation of life in some primitive form on Earth, but we have not been able to replicate this in the laboratory.

• We have not yet found evidence of life on other places in the solar system. The likeliest places to find such life may be on Mars and on some of the major moons near the giant planets. Our guiding principle for establishing likely presence of life is the need for liquid water to exist. This establishes the "habitable zone" around other stars and planets.

• The Earth's moon seems rather special and its formation a fluke of nature. How essential has it been for development of advanced life on Earth?

THE UGLY....?
The perhaps impossible:

• The physical laws that we have discovered place severe limits on our ability to travel to other stars in reasonable amounts of time from the perspective of earthlings. The speed of light in vacuum is faster than any speed an object that has mass can attain. If we could travel at close to the speed of light, time dilation would allow the astronauts to reach the other stars, but we have no way of generating the amounts of energy required to do this in a spaceship. We have no scientific evidence that these same limitations would not apply to other civilizations.

• The jump from current technology in space travel to traveling through the Milky Way to other stars is an enormous one. It will require completely different technology than "conventional rocket" engines. It will require self-sustaining space ships in areas far from direct energy sources. It is a jump that is very much larger than the one from cars to airplanes, or airplanes to current space flight capabilities.

• We can in principle communicate with other civilizations through electro-magnetic waves which travel at the speed of light. It seems plausible that radio waves are the most likely wavelength range of electromagnetic waves to use for such communication. We are searching for signals of artificial nature that may be coming to us from other civilizations. We have not found any such signals but have only started looking recently and over a limited range in wavelength and search space. Given the weakening of signals with distance traveled and the enormous distances between stars, absence of signals will perhaps never imply evidence of absence of alien civilizations.

We don't have scientific evidence to support:

• Past or present visits by aliens

• Descent from aliens

• Monitoring by aliens

• Advice from aliens

• Materials from aliens

• Contacts from aliens

While absence of evidence is not evidence of absence, we should be guided by scientific principles to make conclusions, and with no evidence or theoretical supporting arguments, at this point absence of evidence at least implies no scientific case for support of the mysterious. Sorry, Captain Kirk.

All in all though, the universe is a big and miraculous place. Predictions about what we can and cannot do in the future have always been wrong. We should therefore not be guided by pessimism, but by the search for the unknown. It is only in this way that increases in our knowledge and technological improvements have come about. Enjoy the journey.