What are these gravitational waves, which researchers around the world are competing to discover? What is their importance? Why is the challenge of discovering them so great? We will try to answer these and other questions here
In August 2002, a large and strange-looking scientific facility was put into operation for the first time on an arid plain near Hanford, in the state of Washington in the United States. The facility includes two vacuum tubes, each 4 km long, which together form the shape of the letter Rish. The pipes, which have a diameter of 1.2 m, are covered with a concrete structure and are a home for a laser beam, which cuts them back and forth. The construction of the facility began in 1996, and thousands of man-years were invested in it.
The name of the project is LIGO, an acronym for "Laser Interferometer Gravitational-Wave Observatory", and it is designed to discover gravitational waves - waves whose existence is currently a hypothesis. Three detectors with a similar operating principle were built in other places in the world: VIRGO in Italy, which is operated in European cooperation; GEO in Germany, operated in German-English cooperation; and Tama in Japan.
What are gravitational waves?
When a stone is thrown into the pool, ripples spread through the water originating from the point of impact of the stone. Similarly, Einstein's theory of gravitation (also known as "general relativity") predicts that the movement of masses, for example stars in their orbits, will cause ripples in space-time itself, which will spread throughout the universe. Alternatively, if we liken the gravitational waves to sound waves, then stars in a cyclic orbit produce - as we will see later - a pure sound, like an astronomical sound (a sound, let's recall, is a metal fork that produces a continuous, precise and clean sound, and is used to direct a musical instrument).
To better understand the essence of gravitational waves, it is useful to go back to the past. In 1687, Newton was able to explain the movement of all the planets that were known at the time, using a single law, the "law of universal gravitation". According to this law, any two masses in the universe are attracted to each other with a characteristic (proportional) force inversely proportional to the square of the distance between them.
It is difficult to exaggerate the record of this achievement on the development of science. At the same time, in 1905 it became clear that Newton's law, which had survived unchanged for over two hundred years, was incomplete and that it needed correction.
In the same year, following discoveries in the fields of electricity and magnetism, the young Albert Einstein published his first and revolutionary theory of relativity (which was later called "reduced" or "special"), one of whose central principles is that there is no physical phenomenon that can propagate faster than an electromagnetic wave, That is, there is no speed faster than the speed of light. However, Newton's law of force does not consider the movement of the bodies at all, but only refers to the distance between them.
According to the Newtonian model, as soon as a mass changes its position, for example the Earth in its movement, the gravitational force it exerts on other bodies changes immediately throughout the universe, and this contradicts Einstein's principle of maximum speed.
Since the concept of "gravitational waves" is based on the concept of "wave", let us refresh our memory a little regarding the nature of waves. Many phenomena are identified as wave phenomena. One of the most famous of them is the waves of the sea (and waves in water in general). Sea waves, like any wave phenomenon, are characterized by a certain wavelength, amplitude and speed of advance.
The wavelength is the horizontal distance between two successive peaks of the wave, and the amplitude is the height of the peak above sea level, and it represents the strength of the wave. The speed of the wave does not usually depend on the amplitude or the length of the wave, but only on the type of wave and the medium. For the sake of completeness, we will mention that certain waves have another property - polarization.
The body that excites the waves is called a source. For example, drops falling from a dripping faucet into a sink full of water are a source of waves in the sink. The example of the source performing a cyclic movement is particularly interesting. In such an example it is clear that the cycle time of the waves will be the error of the source.
Sound and light are waves: sound is a wave of density changes in a medium, and light is an electromagnetic wave. The electromagnetic waves are particularly relevant when it comes to the gravitational waves. Like the Newtonian force of gravity, the first quantitative description of the electric force (Coulomb's law) referred only to the distance between two charged bodies and not to times and speeds. During the 19th century, a more advanced theory of the complex of electromagnetic phenomena was formulated, a theory which was sealed with the formulation of "Maxwell's equations" by James Clerk Maxwell.
The sources of the electromagnetic field are electrically charged bodies. According to this theory, the electric force thanks to the charge will change when the charge moves from its place, but the change is not instantaneous throughout the universe, but the "knowledge" of the change spreads at the speed of light in the form of an electromagnetic wave - a wave made of a vibrating electric and magnetic field. Such waves were a prediction of Maxwell's equations. Indeed, their existence was verified a few decades later, and thus the development of the technology of wireless transmission became possible.
The qualitative characteristics of the resolution of the contradiction between Newton's theory of gravitation and Einstein's (special) theory of relativity can be deduced by analogy from the electromagnetic theory that was known when special relativity was formulated. Since in the field of gravity it is masses - and not electric charges - that play the role of sources, it must have been clear to Einstein that just as moving electric charges create waves at the speed of light, so too masses in motion must create waves of a new type, "gravitational waves", which also move at the speed of light.
Einstein immediately understood that the theory of (special) relativity would entail a change in the theory of gravity, where the need for gravitational waves is just one demonstration of the gap between the theories, and it soon became clear that this was not a cosmetic change and the addition of corrections, but a deep and comprehensive change. The task was great even for Einstein's standards, and he struggled with it for about a decade until in 1915 he was able to complete the formulation of a Torah that combines the force of gravity together with the principles of the theory of relativity.
Einstein called this theory "general relativity", while the first theory of relativity was henceforth called "reduced" or "special". This new Torah was completely different from any physical Torah known up to that time. The challenging concept of the 4-dimensional space-time of the theory of limited relativity is replaced here by an even more surprising and abstract concept, that of a curved space-time, and the force of gravity itself "disappears" from the Torah and is replaced by a geometric curvature of the space-time. In this case, the mathematics was almost as challenging and sophisticated as the depth of the physical concepts, and this goes some way to explaining the difficulties that Einstein faced and held him back.
Einstein's new theory of gravity (general relativity) had several revolutionary consequences, the central ones being the prediction of the dynamic universe, of black holes, and not least - of gravitational waves propagating at the speed of light. This theory also makes it possible to describe in detail the physical properties of the gravitational wave. The fundamental feature of curved space-time is that distances between points are not rigid, and in particular they may change over time.
The effect of a passing gravity wave is such that the distance between two points (whose line connecting them is perpendicular to the direction of the wave's progress and which are not acted upon by external forces) changes rhythmically. More precisely, the maximum change in the distance between the points in the presence of a gravity wave is equal to the product of the wave amplitude by the distance between the points.
At this point, a question may arise: How was Newton's theory able to describe the movement of bodies in the solar system, and provide predictions that were successfully tested over about 200 years according to astronomical observations that developed and reached unprecedented accuracy, and all this despite its shortcomings?
The explanation for this is that in the solar system the typical velocities of massive bodies are much lower than the speed of light, so the influence of the theory of relativity is correspondingly smaller. For example, the speed of the earth in its orbit around the sun is about 30 km per second. This speed, which is indeed very high compared to everyday speeds (a car at a speed of 90 km/h travels 25 meters per second, i.e. it is a thousand times slower or more), is only one part out of ten thousand compared to the speed of light, which is about 300,000 km per second .
Accordingly, the gravitational waves emitted by massive bodies in the solar system are very weak (low power). However, later on we will describe energetic processes in the universe, such as collisions between black holes and the collapse of stars, in which movement occurs at speeds close to the speed of light, and therefore much stronger gravitational waves will be emitted and the loss of energy in the pens will be significant.
To summarize: Gravitational waves are created by masses in motion, and they advance at the speed of light, are characterized by wavelength and polarization, and are expressed as a rhythmic (periodic) distortion of space.
* Barak Kol is an associate professor at the Rakah Institute of Physics at the Hebrew University of Jerusalem. Prof. Cole deals with Einstein's theory of gravity and field theories to describe the elementary particles.
More of the topic in Hayadan:
18 תגובות
Prof. Barak Kol, thank you very much for the article. Here are some questions that I would appreciate if you could clarify: (a) Do gravitational waves interfere like light waves and scatter like them by reflection or refraction? And if so, how? (b) In the case of the collapse and merger of two black holes into one black hole: the mass difference released in this process becomes both the kinetic energy of the black hole and the energy of gravitational waves created in the process of collapse. These waves propagate in space-time and decay. What is the "friction" mechanism in space-time that allows the decay of the waves? Or in simple words what absorbs the energy of the gravitational waves?
With all due respect, this is an extremely impressive article
In addition:
A moving system that is not acted upon by an external force will eventually stop and come to rest just as Aristotle said. He just didn't know that the reason for this was a loss of energy from emitting gravitons.
If there is anything interesting about the gravitons, this is exactly the point that by discovering their emission from an inertial system we will know if the system is fixed in space or moving.
There is nothing to compare it to electromagnetic waves and not to two bodies surrounding each other in empty space since inertial systems do not radiate and do not rotate.
By the way, if gravitons are found and the reality of an absolute static axis system is proven, I suggest calling it an ether.
Hello Barak, I would also like to receive the second part in Word format, sorry but I didn't know about its existence.
I would also be happy if you continue to participate in the discussion. Several more articles in advanced fields of astrophysics have appeared since then.
Avi. (My address is at the bottom of every page on the website).
Michael and Shaul Shalom,
I was asked to address the qualitative questions that arose in the discussion that developed, and I will at least try to address the centrality of them.
First, I recommend reading the full article as it appeared in Galileo (in "Yaden" only the first chapter was published). For your convenience, this version can be found on my website, on the "Front of Science" page
http://www.phys.huji.ac.il/~barak_kol/Frontier/
GWavesGal.pdf
Gravitational waves are indeed similar to electromagnetic waves in many ways - they have a wavelength and polarization, they are transverse (and not longitudinal - the polarization in both cases is perpendicular to the direction of progress, which is the definition of a transverse wave), carry momentum and maintain friction (all according to Einstein's theory of gravity since they have not yet been observed in the experiment). Indeed, it is speculated that within the quantum theory of gravity the waves will be composed of quanta called "gravitons". However, it is clear that given that it is very difficult to discover the waves themselves, it is difficult to describe the discovery of the graviton in the foreseeable future.
Regarding the questions regarding Mach's principle and the absoluteness of space-time, I do not find that the entanglement in this issue justifies the effort. As long as it is a rotating even system, it is common to see the rotation as absolute (and not relative) and it is clear that gravitational waves will be emitted.
I hope I have contributed to the discussion,
Best regards,
Prof. Barak Kol
Rekh Institute of Physics
The Hebrew University
to ask:
I'm still waiting, but it turns out that the response I'm waiting for is not as fast as lightning (I assume that the request for a response requires a medium in which you can move and since this medium is human, the speeds are limited, but I haven't given up yet).
Barak Kol writes in the article (at the end): "Gravitational waves are created by masses that are in motion, and they advance at the speed of light, are characterized by wavelength and polarization, and are expressed as a rhythmic (periodic) distortion of space." So I allow myself to say that the answer to your first question is yes.
I don't know if the author is convinced that there are particles that carry the force of gravity. These are not required by the theory of relativity but indeed, in the spirit of quantum theory, such particles were predicted and called "gravitons".
We are deep in the realm of speculation here, discussing the properties of yet-to-be-discovered particles, but the expectation is, of course, that when they are discovered, quantum theory will apply to them as well (or perhaps it will already be worthy of the name "the unified theory" to which physicists are trying to strive).
The term "waves" is generally used to describe periodic phenomena. I don't know if the propagation of the gravitational effect of a change in the position of a body should be called a wave because people may think that this is a cyclical thing, but in the absence of another expression, I have no suggestion for correcting the wording of the section discussing two moving bodies.
Questions about two bodies in empty space are indeed questions that can decide between an absolute space and a space completely defined by the masses. Here, too, you can ask a much simpler question than the one you asked. If the space is not absolute - what needs to happen so that the bodies do not fall on each other due to the force of gravity? After all, in the absence of a background, there is no rotation, and if there is no rotation, it is not possible to create a centrifugal "force" that would oppose the force of gravity.
Unfortunately, we do not have "experimental universes" where we can check which of the options is correct.
You are talking about two bodies running away from each other (this is an even more problematic experiment than that of just two bodies in space because in order to "run away" from each other, you probably also need to mobilize the dark energy).
In any case, the answer is indeed derived from the question of the absoluteness of the space. If you accept Mach's principle in its most extreme form (as I think Einstein would have done too) there is no escaping the conclusion that both bodies will indeed produce gravitational waves.
A small addition for clarification:
Situation A: I have a metal coil and a magnet moves inside it. Is there a current? was created.
Situation B: I have a metal coil with two magnets or two metal coils 'standing' in relation to each other (ie the relative speed between them is zero), and in each one there is a magnet. Now one magnet moves relative to the coil around it and one magnet does not move relative to the coil around it. Only the moving magnet creates a current. Right? Let's say so.
Situation C: An object resting in space creates gravitational waves. Right? Yes.
Situation D: Two objects in space run away from each other. It is possible that they both move in relation to space-time and therefore both create waves, but it is possible that one 'stands still' (if in the case its velocity in relation to space-time is zero) and only the other moves relative to space-time and therefore only it creates the waves, as in situation b.
Easy summary: the title of the article is 'What are gravitational waves'. I, at least, still don't know the answer.
Bye.
To Mr. Gamish and Michael ('I waited, I waited, and who didn't come? Michael' (Leah Goldberg). So here is Shabbat from the heights of the returnees).
Nice, nice for the reference.
The following sentence appears in the article: "Sound and light are waves: sound is a wave of density changes in a medium, and light is an electromagnetic wave. The electromagnetic waves are particularly relevant when it comes to gravitational waves."
The distinction between latitudinal waves (which must pass through a medium but do not carry anything but vibrate the medium in itself like sound waves, water waves) and longitudinal waves (normal electromagnetic radiation that also passes through a vacuum) is one of the first diagnoses Einstein made. From the above sentence quoted from the article, I understand that the writer decided that gravity waves are longitudinal waves to which the concepts you spoke about and based on which you built your answer should be applied.
From this I conclude:
A. Gravitational waves have polarity, momentum, wavelength, etc; is it true?
B. From the things quoted, I understand that the author is convinced that there are particles (I once heard them called 'gravitations') that carry the gravitational waves just as photons carry electromagnetic radiation (hence the question of whether there is entanglement should be understood in the sense: should quantum mechanics also apply to gravitational waves).
B. What will be the nature of the gravitational waves that will be created when two equal bodies 'escape' from each other in space in opposite directions at a slow speed: will they both create gravitational waves (I am not discussing the speed of those gravitational waves - this is discussed by the theory of relativity - but rather the question of whether in this situation gravitational waves will even be created) . From here maybe an opening for discussion if space-time is absolute.
Bye.
to ask:
This is a response I prepared yesterday and delayed its publication because I hoped (as I still hope) that Barak Kol would give a more qualified and learned answer.
Since they have already started to answer you, I decided to publish the things even though I have not yet despaired of his response.
The author of the article did explain that waves are created as a result of motion. He also said that the most interesting case (and I would add - the most convenient to investigate) is that of cyclic motion (because we will probably never be able to reliably detect a momentary and one-time change).
He also compared the waves created by periodic motion to the electromagnetic waves created by the periodic motion of a charge.
Your question could, therefore, also have been asked in relation to electromagnetic waves.
The question of an absolute time space could also arise on the basis of more elementary phenomena: consider the centrifugal force. In relation to which system should it be measured (or alternatively - what is the system in relation to which persistence exists?).
This is a point that, as far as I know, has not been decided. The two competing approaches are the approach of an absolute space versus the approach of the space defined by all the masses in the universe (or at least those whose existence information could have reached us).
I am not an expert on the subject and I would be happy if it would be possible to receive Barak Kol's response to your questions, but as far as I understand you, there is definitely a possibility of such waves clashing.
According to the theory of relativity, since nothing can move at a speed exceeding the speed of light, the law of the addition of velocities cannot apply to gravitational waves and they obviously undergo exactly the same transformations that the speed of light undergoes.
Ask: Good question, but you could also ask it about electric charges and electromagnetic waves. The name of the solution is known - according to the theory of relativity, moving observers will see waves moving at the same speed (the speed of light), but at a different wavelength (Doppler shift). I assume that the same solution will take hold here as well. Again - it is the relative movement that determines. If the speed of propagation of gravitational waves is not absolute (like the speed of light), then you are right. It seems to me that according to the theory it is (I am not an expert), but of course there is no experimental observation.
^^
According to what is written towards the end, this is only the first article and there will be sequels.
Can not wait.
Roger McBride Allen coffin ring.
Recommended!
A science fiction book that tells about what happens when humanity mistakenly harnesses the power of gravity.
(which fortunately/unfortunately does not yet exist)
Two questions for the distinguished writer or anyone who knows how to answer:
A. If I understood correctly, gravitational waves are created only when objects are in motion and not when they are at rest in relation to space-time; That is, there is an implicit assumption here that space-time is 'standing', and it is possible to know what is in motion relative to it and what is not; Not only can you tell what's in motion and what's not, but you can also tell what's in stronger motion and what's less. I understand it right?
B. Can interference (constructive/destructive) also be found in relation to gravitational waves?
shimi1952@walla.co.il
By the way, I would love to hear from anyone who is interested in the subject by mail shimi1952@walla.co.il
To the question, what is the greatest challenge in the discovery of gravitational waves?...the author did not answer and I want to expand on the subject...these waves are waves that exist in the universe and with the help of a gravitational wave amplifier it is possible to create a counter wave which can be used to propel spaceships...a similar technology was used by the aliens who came to Earth in recent years...this Actually, the possibility of moving vast distances in the universe without fuel...:)...you are invited to watch the following video talking about Bob Lazar who is a physicist who worked in the secret Area 51 in the USA...:
Link to video
One more thing if you want to know more about gravitational waves and Bob Laser or about gravitational waves search for the following words on Google:
gravitational waves, bob lazar, gravity amplifier