The Block Universe and The Flow of Time

[rajnz00]

“People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion.” Albert Einstein

First a shout out to my friends on this forum, specially to Dave. I hope you are all well and safe?

And now to the subject at hand.

I have never believed in the Block Universe, that time does not flow, or that the past still exists, or that the future exists for that matter, or that the future is pre-ordained, or that we do not have free will. If that’s what General Relativity says or implies, then it must be wrong in some fundamental assumption it makes.

In any case GR must be wrong at some fundamental level, just as Newtonian mechanics is, despite its remarkable accuracy in describing the Universe, because it is incompatible with Quantum Mechanics.

“But the world is a quantum one, and gelatinous spacetime is also an approximation.” The Order of Time, Carlo Rovelli

Carlo Rovelli is of the opinion that the flow of time is an illusion and that time, as we perceive and experience it, is an emergent property, an illusion based on a false image emerging from our unified experience.

“Reality is often very different than it seems. The Earth appears to be flat but is in fact spherical. The sun seems to revolve in the sky when it is really we who are spinning. Neither is the structure of time what it seems to be: it is different from this uniform, universal flowing. I discovered this, to my utter astonishment, in the physics books I read as a university student: time works quite differently from the way it seems to. In those same books I also discovered that we still don’t know how time actually works. The nature of time is perhaps the greatest remaining mystery.”

I think his analogy is a false one. The curvature of the Earth and the relative motion of the Sun can be easily seen and understood. This is in no way similar to the passing of Time. Nor does it mean that everything is an illusion. After all that the Earth is round is not an illusion.

“It is Entropy, not Energy, that Drives the World” The Order of Time, Carlo Rovelli

Most physicists seem to accept that the Second Law of Thermodynamics provides the arrow of time. It is the only Physical Law that provides directionality with time. However, no experiment has ever shown that change of entropy has any effect on time as measured by clocks, unlike experiments which have confirmed that both speed and gravity effect the duration of time. Time does not measure differently on Earth between day and night as the entropy changes.

“We human beings are an effect of this great history of the increase of entropy, held together by the memory that is enabled by these traces. Each one of us is a unified being because we reflect the world, because we have formed an image of a unified entity by interacting with our kind, and because it is a perspective on the world unified by memory. From this comes what we call the ‘flowing’ of time. This is what we are listening to when we listen to the passing of time.” The Order of Time, Carlo Rovelli

What? This doesn’t make any sense to me. It’s not because we have formed an image that we think we perceive the passing of time. We feel it in our every passing moment as we are born, grow old and die. We do not die before we are born. We have a history, so does our world and so does the Universe. We all have a past and a present and an evolving future. The present of the Universe, because it is so vast, is different in different areas, but together the universe has a history. Just like in our bodies our different organs may age differently, but age they do. Our organs are tied to our bodies and the galaxies to the Universe.

As Carlo Rovelli himself admits about the arguments in his book “Many parts of this story are solid, others plausible, others still are guesses hazarded in an attempt at understanding the whole.”

I think he has erred where he entered the plausible and guesses. It is also very plausible that he is wrong.

In his book of Confessions, St Augustine asks himself how we can be aware of duration – or even be capable of evaluating it – if we are always only in a present which is, by definition, instantaneous. How can we come to know so clearly about the past, about time, if we are always in the present? In the here and now, there is no past and no future. Where are they? Augustine concludes that they are within us:

“It is within my mind, then, that I measure time. I must not allow my mind to insist that time is something objective. When I measure time, I am measuring something in the present of my mind. Either this is time, or I have no idea what time is.” St Augustine

Obviously, the past exists in memory. But the future doesn’t. We cannot remember something that hasn’t happened and herein lies the fallacy of the Block Universe. If the future exists, just like the present, and, it is claimed, the past, then where is it? Do we descend to Astrology, a pseudoscience that has been thoroughly debunked?

“There is no special moment on Proxima b that corresponds to what constitutes the present here and now [on Earth?] …… In my opinion, it is the most astounding conclusion arrived at in the whole of contemporary physics.” The Order of Time, Carlo Rovelli

“If … events are causally connected, (i.e. the time between event A and event B is less than the distance between them divided by the speed of light), precedence order is preserved in all frames of reference.” – Wikipedia

Imagine a huge frictionless billiard table that stretches between the Earth and Proxima b. A frictionless billiard ball B1 is struck at great speed towards Proxima b, at time T1. This strikes ball B2 at time T2, which speeds on to Proxima b, and so on. Eventually a ball Bn arrives at time Tn on Proxima b. Since causality has been maintained throughout, Time Tn must be later than Time T1 in all frames of reference.

Imagine also some time in the past a ball B1’ was struck on Proxima b towards the Earth at time T1’ this speeds towards the Earth and strikes ball B2’ and so on, till at time Tn’ a ball Bn’ arrives on Earth. Time Tn’ must also be later than Time t1’ in all frames of reference.

Imagine also that Tn – T1 = Tn’ – T1’. Based on this, clocks on both the Earth an Proxima b can be synchronised at fractions of the interval Tn – T1 or Tn’ – T1’ and midway between Earth and Proxima we could have a Mid-Earth-Proxima Standard time and watches both on Earth and Proxima b could be synchronised with this standard time and thus moments on Earth would correspond to moments on Proxima b and vice-versa.

There may be a flaw in this reasoning, and maybe someone can point it out, but regardless of whether this is true or not, it has no bearing on the argument that the flow of time is real.

Does Time Really Flow? New Clues Come From a Century-Old Approach to Math.
Quanta Magazine

“The majority of physicists believe in the block-universe view, because it is predicted by general relativity, however, if somebody is called on to reflect a bit more deeply about what the block universe means, they start to question and waver on the implications.” Marina Cortês, Cosmologist, University of Lisbon.

Swiss physicist, Nicolas Gisin argues that time in general and the time we call the present are easily expressed in a century-old mathematical language called intuitionist mathematics, which rejects the existence of numbers with infinitely many digits. When intuitionist math is used to describe the evolution of physical systems, it makes clear, according to Gisin, that “time really passes and new information is created.” Moreover, with this formalism, the strict determinism implied by Einstein’s equations gives way to a quantum-like unpredictability.

The fallacy lies apparently in the assumptions of simple arithmetic. Numbers are a construct of the mind, they don’t really exist. More importantly, they cannot be endlessly divided. 1 + 1 may not be equal to exactly 2. They are a bit fuzzy. From this, apparently, it flows that time flows and the paradox of the block universe, determinism, the absence of free will, that physicists wanted us to believe, because of the conclusion of their equations, is false.

“I am a physicist who has my feet on the ground. Time passes; we all know that.” Nicolas Grisin

Yep. We sure do.

[BurtJordaan]

What? This doesn’t make any sense to me. It’s not because we have formed an image that we think we perceive the passing of time. We feel it in our every passing moment as we are born, grow old and die. We do not die before we are born. We have a history, so does our world and so does the Universe. We all have a past and a present and an evolving future. The present of the Universe, because it is so vast, is different in different areas, but together the universe has a history. Just like in our bodies our different organs may age differently, but age they do. Our organs are tied to our bodies and the galaxies to the Universe.

But isn't this exactly what Rovelli was saying?

[curiosity]

Space-time is indeed a strange phenomenon. IMO many of the problems we encounter when trying to gain a better understanding of it, stem from the way so many people try to think of space alone, or time alone, when Both Minkowski and Einstein told us, that one cannot exist independently of the other. The Minkowski description of space-time ended with the words "Only a kind of union of the two will preserve an independent reality.“ Whereas the Einstein description closed with the speed of light being a constant.

How space and time can combine in such a way that the speed of light is a constant, is a simple concept and without it being a constant, I seriously doubt our reality could even exist.
So... Why are the words of these gifted scientists practically ignored?

[rajnz00]

What? This doesn’t make any sense to me. It’s not because we have formed an image that we think we perceive the passing of time. We feel it in our every passing moment as we are born, grow old and die. We do not die before we are born. We have a history, so does our world and so does the Universe. We all have a past and a present and an evolving future. The present of the Universe, because it is so vast, is different in different areas, but together the universe has a history. Just like in our bodies our different organs may age differently, but age they do. Our organs are tied to our bodies and the galaxies to the Universe.

But isn't this exactly what Rovelli was saying?

I don’t think so. I think that Rovelli’s thesis is that Time does not flow, that it is an illusion. He makes many interesting observations, on his way to his conclusions.

“The World is Made of Events, not Things”

“Thinking of the world as a collection of events, of processes, is the way that allows us to better grasp, comprehend and describe it. It is the only way that is compatible with relativity. The world is not a collection of things, it is a collection of events.

The difference between things and events is that things persist in time; events have a limited duration. A stone is a prototypical ‘thing’: we can ask ourselves where it will be tomorrow. Conversely, a kiss is an ‘event’. It makes no sense to ask where the kiss will be tomorrow. The world is made up of networks of kisses, not of stones.”

Why is the world not made of Things And Events? Why is it one to the exclusion of the other? We see both existing at the same time. Events happen on Things. Kisses - Events, happen on Things - humans.

To my mind stones are important because they exist for a long time. Some asteroids have been around for longer than our solar system. Much longer than humans, so they provide a backdrop for comparison for human and even geological events. Kisses are events perpetuated by humans. Rocks also have events, they crash into other rocks, are hit by hammers. In the grand scheme of things these events are the same as human caused events, if less frequent.

[rajnz00]

Space-time is indeed a strange phenomenon. IMO many of the problems we encounter when trying to gain a better understanding of it, stem from the way so many people try to think of space alone, or time alone, when Both Minkowski and Einstein told us, that one cannot exist independently of the other. The Minkowski description of space-time ended with the words "Only a kind of union of the two will preserve an independent reality.“ Whereas the Einstein description closed with the speed of light being a constant.

How space and time can combine in such a way that the speed of light is a constant, is a simple concept and without it being a constant, I seriously doubt our reality could even exist.
So... Why are the words of these gifted scientists practically ignored?

I am sorry that I ignored the words of Minkowski, I didn’t however ignore the words of Einstein. If you note, I started my post with his quote. The quote is relevant to the concept of a Block Universe, one in which the Universe is a giant block of all the things that ever happen at any time and at any place. The one in which, the past, present and future all exist — and are equally real. This is the concept I am arguing against.

As you say that the concept that space and time are interlinked in a way that the speed of light is constant is a simple one. Time slows, even stands still and space shrinks.

But brilliant as Minkowski was and as Einstein was, we can disagree with some of the statements they made and the conclusions they reached. Einstein himself did not fully comprehend the implications of his theories and in fact argued against some of them.

I do not fully understand Minkowski’s words, "Only a kind of union of the two will preserve an independent reality.” Maybe you could explain.

We can have a sense of reality with Newton’s dynamics. Whizzing along near the speed of light is not a reality for us humans. And tell me, if there were some sentient “observers” whizzing past Earth at breakneck speeds, in different directions, each, for some strange reason, carrying extremely precise clocks, and each of them hell bent on “observing” all the events on Earth, would they “disagree” as to whether I was born before my father? Or that the battle of Waterloo happened before Julius Ceasar was stabbed to death in the Senate? And if they disagreed, would they exchange notes? Come to a fist fight even? How exactly would they do that? These are not realities to me.

[bangstrom]

I do not fully understand Minkowski’s words, "Only a kind of union of the two will preserve an independent reality.”

The union of space and time means there is an element of motion through time included in any motion through space. Motion through space is like moving through time zones where you need to set your clock back by one second for every 300,000 km of distance traveled.

There is no way two observers could disagree about the before and after of events unless one of the observers was moving backward in time. Two observers could find two remote events to be simultaneous for one but not for the other but, if they add one second for every 300,000 km of distance from the events at the time when they made their observations, their accounts should agree.

[davidm]

r And tell me, if there were some sentient “observers” whizzing past Earth at breakneck speeds, in different directions, each, for some strange reason, carrying extremely precise clocks, and each of them hell bent on “observing” all the events on Earth, would they “disagree” as to whether I was born before my father? Or that the battle of Waterloo happened before Julius Ceasar was stabbed to death in the Senate? And if they disagreed, would they exchange notes? Come to a fist fight even? How exactly would they do that? These are not realities to me.

They are not realities for relativity theory, either. The theory predicts that no observer will ever say Waterloo happened before Ceasar was stabbed, or that you were born before your father. You need to read up on the theory to see what it actually says.

[rajnz00]

I do not fully understand Minkowski’s words, "Only a kind of union of the two will preserve an independent reality.”

The union of space and time means there is an element of motion through time included in any motion through space. Motion through space is like moving through time zones where you need to set your clock back by one second for every 300,000 km of distance traveled.

There is no way two observers could disagree about the before and after of events unless one of the observers was moving backward in time. Two observers could find two remote events to be simultaneous for one but not for the other but, if they add one second for every 300,000 km of distance from the events at the time when they made their observations, their accounts should agree.

Well that's not so bad. Pretty neat actually. Thank you for that. Much better than my picture of observers whizzing up and down disagreeing with each other. Rather like standard times and eventually zones being established by the British Railways after the terrible confusion of each town having its own time.

Still leaves the question as to what is time, unanswered. Indian languages, based on Sanskrit, don't have articles in their sentences. Latin is the same I believe. So I used to have (Indian) guys ask me, in English, What is Time? (translating in their head from Hindi). They meant of course, what is the time. I always had a stock answer - Time is man's futile effort to limit the limitless, but if you want to know what is the time, it's 5.30 (or whatever the time was).

“If I look at where we have paradoxes and what problems we have, in the end they always boil down to this notion of time.” Renato Renner, Professor Theoretical Physics

Time in quantum mechanics is rigid, not bendy and intertwined with the dimensions of space as in relativity. Furthermore, measurements of quantum systems “make time in quantum mechanics irreversible, whereas otherwise the theory is completely reversible,” so time plays a role in this thing that we still don’t really understand.” Renner

Maybe there is a Universal Time in some vast Frame of Reference, that encompasses the whole Universe, where time really does tick along independently of any observer, but is an aggregate of all the matter in the Universe.

[rajnz00]

r And tell me, if there were some sentient “observers” whizzing past Earth at breakneck speeds, in different directions, each, for some strange reason, carrying extremely precise clocks, and each of them hell bent on “observing” all the events on Earth, would they “disagree” as to whether I was born before my father? Or that the battle of Waterloo happened before Julius Ceasar was stabbed to death in the Senate? And if they disagreed, would they exchange notes? Come to a fist fight even? How exactly would they do that? These are not realities to me.

They are not realities for relativity theory, either. The theory predicts that no observer will ever say Waterloo happened before Ceasar was stabbed, or that you were born before your father. You need to read up on the theory to see what it actually says.

What is the disagreement all about then? I always found that a bit confusing. The agreement was much better than the disagreement. Better to have universal agreement

[BurtJordaan]

Maybe there is a Universal Time in some vast Frame of Reference, that encompasses the whole Universe, where time really does tick along independently of any observer, but is an aggregate of all the matter in the Universe.

Yes, there is. And time was never dependent on any observer in relativity. It is just observed to progress differently by different observers, although always unidirectional by all.

Just like we work with an average length of the Earth-Sun-second here on Earth. And it oscillates a bit during the course of every day and it even changes over the long term, because the rotation rate of Earth slows down due to tidal effects. Once the average has changed by a full second, we adjust our atomic clocks by means of a leap-second to stay in line with Earth-Sun time.

Likewise, there is an average universal time coordinate that can be deduced from observations. It is simply called cosmological time and is based on the average time observed by hypothetical co-moving observers, i.e. those who measure the CMB as the same average temperature in all directions (isotropic). We say hypothetical, because such an observer or clock probably doesn't exist anywhere, so they are just "paper clocks'' for convenience.

The quirky part (if one wants to dissect it to the core) is that we express the age of stars and the cosmos in units of Earth time, which differs from true cosmic time. So, when Earth time has a positive leap second, the age of the universe has, as we conceive it, a negative leap second. But who cares? The universe probably just ignores us and goes on its merry way. And we don't work with seconds or even days on the cosmic scale - the best we can measure on those scales are +- a few million years at the very best.

[rajnz00]

Maybe there is a Universal Time in some vast Frame of Reference, that encompasses the whole Universe, where time really does tick along independently of any observer, but is an aggregate of all the matter in the Universe.

Yes, there is. ...

.... there is an average universal time coordinate that can be deduced from observations. It is simply called cosmological time and is based on the average time observed by hypothetical co-moving observers, i.e. those who measure the CMB as the same average temperature in all directions (isotropic). We say hypothetical, because such an observer or clock probably doesn't exist anywhere, so they are just "paper clocks'' for convenience.

How can Cosmological Time be deduced from observations, How can the average time be "observed" if the observers are hypothetical? Don't understand this.

If there is a Universal Time that encompasses the whole Universe, then it would be very close to Newton's Absolute Time.

"Absolute, true and mathematical time, of itself, and from its own nature flows equably without regard to anything external, and by another name is called duration: relative, apparent and common time, is some sensible and external (whether accurate or unequable) measure of duration by the means of motion, which is commonly used instead of true time ..." Isaac Newton

Unlike relative time, Newton believed absolute time was imperceptible and could only be understood mathematically. According to Newton, humans are only capable of perceiving relative time, which is a measurement of perceivable objects in motion (like the Moon or Sun). From these movements, we infer the passage of time. Wikipedia

"The real father of modern cosmology is the Russian physicist Alexander Friedman (1888-1925). He was the first ever to countenance (in 1922) a dynamic universe - a huge (and possibly infinite) system of galaxies all moving under their mutual gravity according to Einstein’s laws of General Relativity. No one before Friedman, not even Einstein, had dared to give up the picture of a static universe! The cosmological model that Friedman constructed is accepted to this day. It expands equally in all directions, having apparently started with a “Big Bang”. Each galaxy can consider itself to be at the center of the expansion. Friedman realized that a homogeneous evolving universe determines its own unique preferred time, nowadays known a cosmic time. Its moments are defined as global sets of events at which the smoothed-out universe is in the same state (same density of matter, same density of radiation, same recession rate of nearby galaxies, etc.). And the interval between moments is defined as the proper time on any standard clock that partakes of the smoothed-out motion pattern.

Cosmic time, so defined, allows us to assign in an observer-independent way - just as in Newton’s theory - a unique date AB (“after the Big Bang”) to events anywhere in the universe.

.... Thus globally, let us say in the realm of philosophy, time has preserved its Newtonian features. Only locally, where physicists must dirty their hands, has time become a little tricky. Here Einstein reigns. " Time from Newton to Einstein to Friedman, Wolfgang Rindler

[rajnz00]

"Newton, forgive me, you found the only way which, in your age, was just about possible for a man of highest thought and creative power." Albert Einstein, Autobiographical Notes

But it appears that if one goes away from the size of the galaxies and galactic clusters and takes the bigger picture, applying GR to the whole Universe, Newton's absolute time agrees with it.

The only possible difference being the explanation that it flows of and by itself but instead it flows because of all the mass in the universe.

".... in physics, one adopts the simpler hypothesis until forced by observations to adopt a more complicated one. "
"After exposing all the pitfalls of relativistic time at the level of “small” gravitating systems like globular clusters, or even galaxies and clusters of galaxies, General Relativity, when applied to the universe as a whole, brings us full-circle almost back to Newton’s absolute time and even Newton’s absolute space. The basic reason for this is the incredible and unexplained regularity of our universe. " Wolfgang Rindler

Here's a question if space-time deforms in the presence of mass, what is it? what is it made of? Is it much different from the concept of aether?

[BurtJordaan]

How can Cosmological Time be deduced from observations, How can the average time be "observed" if the observers are hypothetical? Don't understand this.

Rindler gave you the short answer in the excerpt that you quoted:
"And the interval between moments is defined as the proper time on any standard clock that partakes of the smoothed-out motion pattern."
There are no natural standard clocks that partook in this smoothed-out motion pattern, because the cosmos is not homogenic. Every clump of matter distorts the spacetime and influences the clocks. On top of that, we know of no galaxy that does not have what is called peculiar motion, through space, as denser areas attract the lesser dense areas. This further influences the time and hence the clocks that Rindler and others were talking about are hypothetical - "thought clocks", or "paper clocks" as I have mentioned.

If there is a Universal Time that encompasses the whole Universe, then it would be very close to Newton's Absolute Time.

Note the "very close". Not precise and over billions of years any slight deviation from cosmological time adds up and can be millions of years on the time of a typical life of a galaxy, star, whatever.

... Thus globally, let us say in the realm of philosophy, time has preserved its Newtonian features. Only locally, where physicists must dirty their hands, has time become a little tricky. Here Einstein reigns. [/i]" Time from Newton to Einstein to Friedman, Wolfgang Rindler

As said above, Einstein reigns on the global scale as well. I think Rindler here wrote for philosophers, not for physicists...

How do we measure the cosmic time? We can look at orbiting periods of very distant objects and the periods of quasar flashes, but ages we calculate indirectly from a lot of different observations and depend on Einstein's theory to determine a value.

We have however found that the averaged-out time is good enough for most practical purposes, because the observable universe is so vast that on the large scale, the lumpiness can be ignored as insignificant.

[BurtJordaan]

Here's a question if space-time deforms in the presence of mass, what is it? what is it made of? Is it much different from the concept of aether?

The big difference is that if you move through a Newtonian aether, it would not affect your space or time. In Einstein's spacetime, your space and time are influenced by movement through gravitational fields. One can go into long discussions of the mechanisms involved, but exactly what causes this is not clear, because we do not have that elusive quantum theory of gravity (yet?). And empty space is surely the field of quantum mechanics.

[bangstrom]

But it appears that if one goes away from the size of the galaxies and galactic clusters and takes the bigger picture, applying GR to the whole Universe, Newton's absolute time agrees with it.

The only possible difference being the explanation that it flows of and by itself but instead it flows because of all the mass in the universe.

Yes, this is the difference between Newton’s and Einstein’s concept of time. In GR, time flows slower as the local gravitational field increases and globally the rate of time is determined by the total mass and space-time curvature of the universe. The fastest possible rate of time should be found in the intergalactic spaces far from any massive object and this would be as Rindler said , “...almost back to Newton’s absolute time and even Newton’s absolute space.”

Here's a question if space-time deforms in the presence of mass, what is it? what is it made of? Is it much different from the concept of aether?

Our whole concept of space-time is derived from our observations of matter. If two objects move in unison, we say they are connected but, if they move independently, we say they are separate. Space-time is a body of assumptions we make about our observations of material objects and what may transpire between them. Space-time appears to be a some 'thing' real with wavelike properties.

In the case of entanglement, space-time appears to be a non-persistent phenomenon contrary to our observations of matter in the macro world. Widely separated particles can momentarily appear to act as if connected with no space-time between them.

The difference between Einstein’s space-time and the aether is that the aether can move photons about like leaves in the wind but space-time can not. Aether theory tells us we can detect absolute motion through space by observing photon drift but Relativity tells us motion is Galelian in that all motion is relative to some arbitrary point of reference.

For those who find the existence of photons a bit too weird, there is no difference between the two because you can't detect the drift of an imaginary particle.

[rajnz00]

Here's a question if space-time deforms in the presence of mass, what is it? what is it made of? Is it much different from the concept of aether?

Our whole concept of space-time is derived from our observations of matter. If two objects move in unison, we say they are connected but, if they move independently, we say they are separate. Space-time is a body of assumptions we make about our observations of material objects and what may transpire between them. Space-time appears to be a some 'thing' real with wavelike properties.

In the case of entanglement, space-time appears to be a non-persistent phenomenon contrary to our observations of matter in the macro world. Widely separated particles can momentarily appear to act as if connected with no space-time between them.

The concept of space-time does my head in. What is this 'thing' which is so strong (rigid?) that only the most powerful explosions in the Universe can cause barely the tiniest of ripples? Yet we seem to move effortlessly through it as though it doesn't exist.

Are we part of space-time, or is it the stage in which we play out our lives like the three-dimensional space in which we played out our lives, through time, in Newton's world?

[BurtJordaan]

The concept of space-time does my head in. What is this 'thing' which is so strong (rigid?) that only the most powerful explosions in the Universe can cause barely the tiniest of ripples? Yet we seem to move effortlessly through it as though it doesn't exist.

To preserve sanity, I generally look at spacetime as the gravitational field of the universe at large. In a sense, we are part of that gravitational field, because we have mass and mass influences the gravitational field.

And as you have said, it is incredibly stiff - that is why light can move so fast through it (light is not ripples in the gravitational field). The stiffness requires large masses that orbit around each other in order to make observable gravitational waves.

But all spacetime ripples are not small - near those block holes that collided 'recently', the ripples were massive. They fade away like all ripples as they spread out while traveling and are only tiny when they arrive here due to the (fortunate) large distance of the original event.

If you and I jump up and down, we do create the tiniest of gravitational waves in our immediate vicinity. This is because we accelerate up and down. If we would have traveled uniformly in free space, we would create no gravitational waves and hence there is no resistance from the overall gravitational field. But accelerate, and there is resistance. Inertia is also tied up in this view, but this gets rather complicated.

As I said, this is only my personal thinking in order to not lose my sanity, but it is surely not the full story.

[rajnz00]

The concept of space-time does my head in. What is this 'thing' which is so strong (rigid?) that only the most powerful explosions in the Universe can cause barely the tiniest of ripples? Yet we seem to move effortlessly through it as though it doesn't exist.

But all spacetime ripples are not small - near those block holes that collided 'recently', the ripples were massive. They fade away like all ripples as they spread out while traveling and are only tiny when they arrive here due to the (fortunate) large distance of the original event.

How far away would a black hole collision have to be to create a ripple/ wave about a metre high on Earth? How much damage would that do to the Earth? Would it merely make the Earth bob up and down by a metre? Or would it rip it apart? Would it rip the Solar system apart? Throw the planets out of orbit? Cause earthquakes on Earth?

A big explosion on Earth, like the A-bomb on Hiroshima caused a shock blast wave to level buildings in its surroundings. But that shock wave required a medium like air. An A-Bomb explosion in outer space, I presume, would not cause a shock wave? If a black hole collision causes a wave in space-time, then space-time must be a medium similar to air.

If so, in that way, is space-time similar to the concept of aether?

[TheVat]

The concept of space-time does my head in....

Welcome to the club! And welcome back. My physics is rusty right now, but I liked your question about how close we would have to be to a BH collision to notice GW effects at the scale of everyday life.

[BurtJordaan]

How far away would a black hole collision have to be to create a ripple/ wave about a metre high on Earth? How much damage would that do to the Earth?

A tough question to answer, but I have done such calcs some two decades ago, but am a bit lazy to do it now.

Here is the simplified equation that I have used (at the top of the screenshot).

Gravitational Waves excerpt

The only other information that one needs is that the amplitude of the waves will increase linearly as the event comes closer to us.

If you are really interested, read the pdf from where the screenshot comes from (attached).

[rajnz00]

The concept of space-time does my head in....

Welcome to the club! And welcome back.

Thanks

[rajnz00]

How far away would a black hole collision have to be to create a ripple/ wave about a metre high on Earth? How much damage would that do to the Earth?

A tough question to answer, but I have done such calcs some two decades ago, but am a bit lazy to do it now.

Here is the simplified equation that I have used (at the top of the screenshot).

GravWaveAmpl.png

The only other information that one needs is that the amplitude of the waves will increase linearly as the event comes closer to us.

If you are really interested, read the pdf from where the screenshot comes from (attached).

Those are actually 2 questions. The first how far would the explosion have to be to cause a wave of amplitude 1 m on Earth? and the second - how much damage would it do?

But that screenshot seems to answer the first question, along with your information that the amplitude will increase linearly as the event comes closer to us.

Correct me if I am wrong.

Assuming 2 coalescing/ colliding black holes 10 times the mass of the sun. At a distance of 10^25 m they cause an amplitude of 10^-21 m, and if the amplitude increases linearly with distance, if we require an amplitude of 1 m, then the distance should be 10^25 x 10^-21 m, which gives us a distance of only 10^4 m or the colliding black holes should be at a distance of only 10 kilometers to produce an amplitude of 1 m.

If this is true, then it tells us that space-time, or the "fabric" of space-time, is extraordinarily rigid.

Of course two colliding black holes at a distance of 10 km would obliterate the Earth with radiation, so we wouldn't have to worry about the effects of the gravity waves.

But here's another question: That excerpt says that it would squeeze 1 - 2 m bars by 10^-21 m, so is the squeezing (and stretching?) proportional to the size of the object? If this is so, then even a 10^-21 m wave would contract and expand the Earth 13 x 10^6 m by 13 x 10^-15 m or about a billionth of a millimeter.

Also if the Earth were to expand and contract by 1 m, then the colliding black holes would have to be 10 x 10^10 m away or around the center of the Milky way. Is that correct?

[BurtJordaan]

Also if the Earth were to expand and contract by 1 m, then the colliding black holes would have to be 10 x 10^10 m away or around the center of the Milky way. Is that correct?

Up to just before this, I think you have it correct. To get your one meter ripples, we have to scale the ripples up by about a factor 10¹⁴. So we have to scale the 10²⁵m distance down by that factor, meaning, to about 10¹¹m. That's quite a bit closer than the center of the Milky Way, which is about 10²⁰m (26,700 lyr) from us.

Or did I goof somewhere?

Anyway, any 1 meter stretch and squeeze of the Earth should do little harm. I think the Moon, the Sun and the rotation of Earth do a lot more than that.

Another question is if we would have survived. Colliding black holes emit a lot more harmful things, other than gravitational waves.

[rajnz00]

Also if the Earth were to expand and contract by 1 m, then the colliding black holes would have to be 10 x 10^10 m away or around the center of the Milky way. Is that correct?

Up to just before this, I think you have it correct. To get your one meter ripples, we have to scale the ripples up by about a factor 10¹⁴. So we have to scale the 10²⁵m distance down by that factor, meaning, to about 10¹¹m. That's quite a bit closer than the center of the Milky Way, which is about 10²⁰m (26,700 lyr) from us.

Or did I goof somewhere?

Anyway, any 1 meter stretch and squeeze of the Earth should do little harm. I think the Moon, the Sun and the rotation of Earth do a lot more than that.

Another question is if we would have survived. Colliding black holes emit a lot more harmful things, other than gravitational waves.

I also got that the collision would have to have been 10¹¹ m from us (10x10^10 = 10^11).

Where I stuffed up was in getting the distance to the centre of the galaxy.

I divided the diameter of the galaxy 10²¹ by two to get 5 x 10¹⁰, instead of 5 x10²⁰ and we are not at the outer edge of the milky way , so 10²⁰ should be about right, perhaps the distance should be a little more than that.

10¹¹ m should be just about the distance to the sun? As you say that explosion at that distance would do heaps more damage than the mere expansion or contraction might do.

If the moon and the sun keep squeezing and stretching the Earth, they should have some influence in earthquakes I should think. I wonder if there is a correlation between earthquakes and high tides.

[rajnz00]

Anyway, any 1 meter stretch and squeeze of the Earth should do little harm. I think the Moon, the Sun and the rotation of Earth do a lot more than that.

If the moon and the sun keep squeezing and stretching the Earth, they should have some influence in earthquakes I should think. I wonder if there is a correlation between earthquakes and high tides.

Digressing a bit, I did a search and found that there is a correlation between low tide and earthquakes in the mid-Atlantic ridge. They said the correlation was surprising because just like me, they thought the correlation should have been with high tide.

"How tides can trigger earthquakes
A new study reveals the inner workings of tidally triggered earthquakes, and finds that even the slightest stress can set off a tremor"
https://www.sciencedaily.com/releases/2 ... 091035.htm

[BurtJordaan]

Yes, that's interesting. I did overestimate the tidal expanding and contraction of Earth's radius a bit - it's less than half a meter on the "solid" surface of Earth, so the 1 m gravitational waves win out by a little bit.

But at the distance of the sun, the black holes would have swallowed Earth rather quickly, long before they even collided.

[rajnz00]

Zeno’s paradoxes and Space-time

The paradox of Achilles and the tortoise (the hare and the tortoise), the paradox of length. If the tortoise gets a head start on Achilles, Achilles can never overtake it because it will first have to cover half the lead, then half of that length, and so on for an infinite number of divisions. This was solved by modern calculus, as the sum of an infinite diminishing series is a finite number. However, the basic axiom that the paradox exposes is not solved by modern calculus.

To Zeno, who did not have the use of calculus, there was a logical answer that this paradox revealed – length, and hence space, is not infinitely divisible. If we could have a slow-motion camera that slows the motion of the runners down to any degree that we like, there would come a time when there would be a gap between one frame and the next and, no matter how fast our cameras were, they would never show any position in-between the two, not because the cameras were not fast enough, but simply because there was no position in between. In the micro-micro-microscopic world we don’t run like a cheetah, we hop like a kangaroo.

This is (one of) the fundamental mistakes of GR, in my view, which assumes space to be continuous.

Zeno’s arrow and the arrow of time and the paradox of time, Einstein’s error no.2 – for later

[rajnz00]

Zeno’s paradoxes and Space-time

The paradox of Achilles and the tortoise (the hare and the tortoise), the paradox of length. If the tortoise gets a head start on Achilles, Achilles can never overtake it because it will first have to cover half the lead, then half of that length, and so on for an infinite number of divisions. This was solved by modern calculus, as the sum of an infinite diminishing series is a finite number. However, the basic axiom that the paradox exposes is not solved by modern calculus.

To Zeno, who did not have the use of calculus, there was a logical answer that this paradox revealed – length, and hence space, is not infinitely divisible. If we could have a slow-motion camera that slows the motion of the runners down to any degree that we like, there would come a time when there would be a gap between one frame and the next and, no matter how fast our cameras were, they would never show any position in-between the two, not because the cameras were not fast enough, but simply because there was no position in between. In the micro-micro-microscopic world we don’t run like a cheetah, we hop like a kangaroo.

This is (one of) the fundamental mistakes of GR, in my view, which assumes space to be continuous.

Zeno’s arrow and the arrow of time and the paradox of time, Einstein’s error no.2 – for later

[bangstrom]

Zeno’s paradoxes and Space-time
This is (one of) the fundamental mistakes of GR, in my view, which assumes space to be continuous.

Space may be discontinuous but I don’t consider this to be a “mistake” of GR because GR does not deal with such fine details so the discreteness of space can safely be ignored. It does imply that GR may not work at the quantum level except when events are averaged out.

[Faradave]

If we could have a slow-motion camera that slows the motion of the runners down to any degree that we like, there would come a time when there would be a gap between one frame and the next and, no matter how fast our cameras were, they would never show any position in-between the two, not because the cameras were not fast enough, but simply because there was no position in between.

We needn't abandon continuous space. Rather understand that any detector (camera, eye, etc.) is comprised by discrete particles, which trade discrete energy quanta. Detector granularity thus satisfies the paradox, as you describe, even on the background of an ideal "continuum".

P.S. With regard to the block universe, spatial expansion suggests trapezoidal blocks, which tend to form arches, domes, even spheres. In the case of the cosmos, that might be 3-spheres.