Generic physics questions from another thread

2355 Views 76 Replies 13 Participants Last post by Luchesi, 2 mo ago

mmsbls

Discussion Starter · 2 mo ago

In another thread, several questions arose surrounding the physics of time, reality, quantum mechanics, and other related issues. The questions were off-topic and require significant discussion, so rather than address them in the music thread, I thought I'd create this thread.

I do not question that the past was real, but I question that is real. I tend to think that all points in time exist, because it doesn't make sense to me that they are unequal in the way that only 1 exists and the others don't. But I think within each point of time they seem like a singularity. It doesn't make sense that it is the year 2023 and not 2003, so probably it is both the year 2023 and 2003 exist but encapsulated in different worlds or realities. But in the encapsulated world we live in it doesn't matter if other points in time or even other universes exist, unless they can interact with us. Only one point in time exists within and for the reality we live in. And within this reality the past is more like a memory, and we can not be 100% sure about it.

Physicists view spacetime as real. Many view all points in spacetime as having a reality similar to our "now". Most physicists would say that the only reality that we can meaningfully talk about are the regions of spacetime that did or could interact with us. I'm not sure exactly what is meant by "the past is more like a memory." In some cases we can be as sure about it as we are about the present.

Light and other particles from the sun takes roughly 8 minutes to reach earth. Any measurement we make of the sun represents a reality that is at least 8 minutes old and from the past. What we learn from those measurements relates to the past.

In the particle physics experiments I worked on, particles would collide and remnants of the collision would stream into our detector where we measured various quantities. From those measurements, we deduced very specific results about the collision that took place in the past. Our understanding of physics reality in large part comes from analysis of data representing past events.

Essentially all of evolutionary biology and cosmology comes from analysis of data that originated in the past.

If you are suggesting that we can know about the present data but require some theory to reconstruct the past, that's true, Still, I'm not sure there are physicists who have any serious doubts about most of those reconstructions.

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BachIsBest

· #21 · 2 mo ago

relm1 said:
I think time is a construct that helps explain causality (sequences of events), so we can understand them easier. It is clearly relative and doesn't exist without mass. Massless particles like photons have no concept of time. That is another way of saying they exist in all time simultaneously.

This is completely incorrect. Massless particles like photons don't have a rest frame so any statement about their "concepts" are borderline meaningless. Saying "time doesn't exist without mass" makes no sense. A universe with only massless particles would still time-evolve.

relm1 said:
It's not a reach to go from that to time isn't real. It's a property of reality rather than reality...a way of describing what we observe rather than what actually is true. The 2023 vs 2003 is nonsense because neither is real, they are manmade constructs like noon or Monday. We have months because of the lunar 30 day orbit. It's not exact its approximate. Similarly, all our measurements of time are guesses. 100 million years ago what we would call an hour (1/24 of the day cycle) would be different amount of time than it is today or a 100 million years from now.

Time is certainly real. 100 million years ago the hour hadn't been invented and 100 million years from now the hour, if humans are still around and using SI units, will have the same meaning. The modern SI hour is defined in terms of fundamental physical laws, not 1/24th of a day.

relm1 said:
BUT, at some point, there is an absolute time. At the speed of light, time doesn't exist and speed is distance over time. Since the speed of light is a constant, at some level, time is a constant too. This could be explained by saying, there is no such thing as time.

This is wrong. Again, making statements about observers travelling at the speed of light is almost certainly nonsense (since observers can't travel at the speed of light). The speed of light being constant has no bearing on whether or not time is a constant (spoiler alert: time is not constant).

Phil loves classical

· #22 · 2 mo ago

I'm interested in the results of the testing of this theory that the speed of light is variable. It's a neat idea..

uwaterloo.ca

Did light break its own speed limit? | Physics and Astronomy

Waterloo astrophysicist makes international news exploring the puzzle at the heart of the Big Bang. Niayesh Afshordi, along with João Magueijo of Imperial College London, have proposed a novel theory in which light traveled faster during the first critical moments after the Big Bang, explaining...

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Aries

· #23 · 2 mo ago

mmsbls said:
Physicists view spacetime as real. Many view all points in spacetime as having a reality similar to our "now". Most physicists would say that the only reality that we can meaningfully talk about are the regions of spacetime that did or could interact with us.

Which regions can not interact with us? Isn't it just a matter of time until every region can interact with us (restricted by the speed of light)?

mmsbls said:
In some cases we can be as sure about it as we are about the present.

We have history books, and a lot of imaginations about the past in our brains. We can imagine how Beethoven wrote the 9th symphony. But is it still real or just an imagination now?

An interessting thought about this: Beethoven composing the 9th is 200 years ago now, so the information, light and gravitation of Beethoven writing the 9th is now 200 lightyears away in the space. What if an alien civilisation 200 lightyears away constructed an giant mirror pointing towards the earth? After 200 more years the light of Beethoven writing the 9th would reach the earth again, and if we have the requiered technology, we will be able to actually see Beethoven writing the 9th again. So in some way Beethoven writing the 9th is still real in a sense. The information of everything that ever happened in the universe probably still flys through the universe at least as gravitation, and it could be theoretically maybe partially mirrowed to us. Maybe it is senseful to view all points in spacetime as real, because it everything can still somehow effect everything.

mmsbls said:
So the wave function will predict the probability of the decay of the Uranium atom leading to the death of Schrödinger's cat, but only in our actual world will we see whether the Uranium decayed or not.

So one present can result in different future measurements, but the wave function will be true for all spacetime. A complete description of one present cannot result from different pasts.

As long as something does not interact with the outside world it is just this probabillity distribution for the outside(and this probabillity distribution acts to different rules as the Double-slit experiment shows), but when it interacts with the outside, it takes a specific form.

But why can one present not result from different pasts? When you interact with Schrödlingers cat, which state was a probabillity distribution before, how can you know which exact probabillity distribution it was before? Aren't all probabillity distributions possible, that could have resulted in the actually state of the cat that it actually took when you interacted with it?

mmsbls said:
I've never heard of the inch rule.

Oh sorry I mean an inch ruler.

A meter is officially defined as a distance light passes in a specific time. But I think what people rather understand as a meter is what is the distance drawn on an inch ruler or yard stick. So measured by an unaccelerated yards stick the speed of light is variable, and you can accelerate to a higher speed than 299,792,458 m/s. I think this is important to know, because it means that we can reach a stars that is 4 light years away, theoretically in less than 4 years.

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Aries

· #24 · 2 mo ago

relm1 said:
I think time is a construct that helps explain causality (sequences of events), so we can understand them easier. It is clearly relative and doesn't exist without mass. Massless particles like photons have no concept of time. That is another way of saying they exist in all time simultaneously. It's not a reach to go from that to time isn't real. It's a property of reality rather than reality...a way of describing what we observe rather than what actually is true.

My body is subject to an aging process and you can relate it to the movement of the earth around the sun. When I was a kid I asked myself why I am a kid? I was logicall that I wil be adult at some point, but it didn't make sense that that I now experience the point in time when I am a kid and not an other point in time. What makes the current state of development of the world so special that it is experienced now instead of another state (point in time). Why is now now?

Kreisler jr. said:

" "eternalism" (roughly past, present, future are equally real, the present is only indexical, i.e. pointing to the moment of utterance) "

Why is the index where it is? Doesn't it make more sense that there are infinite indices?

Also I want to question: "Cogito, ergo sum."/"I think, therefore I am."

There is the experience of my thinking, but how do I know that "I" have this experience? It could be "indexical". It could be the world who has this experience, and it is just separated from other "experiences" like an index from another.

It does not make sense that I am I, as well as it doesn't make sense that now is now. It rather seems likely to me that points in time and points of matter are encapsulated for themselves, so that they just seem like a singularities for the universe that experiences them.

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mikeh375

· #25 · 2 mo ago

Aries said:
..............An interessting thought about this: Beethoven composing the 9th is 200 years ago now, so the information, light and gravitation of Beethoven writing the 9th is now 200 lightyears away in the space. What if an alien civilisation 200 lightyears away constructed an giant mirror pointing towards the earth? After 200 more years the light of Beethoven writing the 9th would reach the earth again, and if we have the requiered technology, we will be able to actually see Beethoven writing the 9th again. So in some way Beethoven writing the 9th is still real in a sense. The information of everything that ever happened in the universe probably still flys through the universe at least as gravitation, and it could be theoretically maybe partially mirrowed to us. Maybe it is senseful to view all points in spacetime as real, because it everything can still somehow effect everything.

Just think @Aries, you might be able to invent a way of talking to Ludwig. You could ask him to compose some new works to counter all the work of those nasty atonalists....

If I'm not mistaken, it's a major ask finding gravity waves from even some of the most explosive, energy releasing events in the whole Universe, let alone the tiny twists and turns of Beethoven's quill as he beamed his quavers - perhaps atonality is safe after all from his avenging CPT.

Aries

· #26 · 2 mo ago

mikeh375 said:
Just think @Aries, you might be able to invent a way of talking to Ludwig.

I think it is one-sided. The past can have an effect on us, but we can not have an effect on the past.

An werid thing about quantum mechainics is, that as long as particles don't interact with the outside world, they are in this wave form and act differently in the way that would be impossible without this wave form. But from the perspective of these particles the outside world is the one that doesn't interact with them, so the outside world has to be in this wave form that acts differently until there is an interaction. So the outside world would act in a different way, but that is not what is happening in our reality. So maybe something doesn't make sense here, or the reality is split all the time.

Bwv 1080

· #27 · 2 mo ago

mmsbls said:
I don't understand what you are saying here. Almost nothing we're learned about basic science is directly observable. Would any scientist confuse a model with data?

Nothing in the Schrödinger equation predicts multiple worlds in the same sense as the equation predicts the probability of finding a system in a particular state. Why would anyone make the jump to believing in this untestable and unknowable conclusion?

As to theoretical physics being more or less a mature science:

blogs.scientificamerican.com

Was I Wrong about “The End of Science”?

One of the coolest—and most stressful–moments of my career took place November 7, 1996, when I was a staff writer for Scientific American.

blogs.scientificamerican.com

Bwv 1080

· #28 · 2 mo ago

Aries said:
Which regions can not interact with us? Isn't it just a matter of time until every region can interact with us (restricted by the speed of light)?

We have history books, and a lot of imaginations about the past in our brains. We can imagine how Beethoven wrote the 9th symphony. But is it still real or just an imagination now?

An interessting thought about this: Beethoven composing the 9th is 200 years ago now, so the information, light and gravitation of Beethoven writing the 9th is now 200 lightyears away in the space. What if an alien civilisation 200 lightyears away constructed an giant mirror pointing towards the earth? After 200 more years the light of Beethoven writing the 9th would reach the earth again, and if we have the requiered technology, we will be able to actually see Beethoven writing the 9th again. So in some way Beethoven writing the 9th is still real in a sense. The information of everything that ever happened in the universe probably still flys through the universe at least as gravitation, and it could be theoretically maybe partially mirrowed to us. Maybe it is senseful to view all points in spacetime as real, because it everything can still somehow effect everything.

But that example is equivalent to aliens having hidden a video camera and humans discovering it 400 years later. The 200 LY distance does not make the past any more or less real

mmsbls

Discussion Starter · #29 · 2 mo ago

Bwv 1080 said:
Nothing in the Schrödinger equation predicts multiple worlds in the same sense as the equation predicts the probability of finding a system in a particular state. Why would anyone make the jump to believing in this untestable and unknowable conclusion?

I think some physicists feel that the hypothesis of the collapse of the wave function seems to arbitrarily exclude information that existed in the wave function. If an electron has a 50-50 chance of being spin up or spin down, what happens to the other "half" after the collapse? Interestingly, shortly after Hugh Everett wrote his thesis, very few physicists took the hypothesis seriously. My understanding is that a much greater percentage of theorists now take it seriously.

Physicist, Max Tegmark, thought of an experiment to test the many worlds hypothesis. He considered creating a gun that would fire based on the outcome of a quantum system that had an extremely high probability of one state and an extremely low probability of another state after one second. The gun fires when the high probability state occurs. You can think of this as similar to the Schrödinger's cat setup, but the probability of the gun firing is exceedingly high. He suggested sitting in front of the gun. After a few seconds, you would have a very, very high probability of being dead, but if the many worlds hypothesis were true, you would be alive in some universes. The you in those universes would realize that there was a very high probability that the many worlds hypothesis was correct.

As to theoretical physics being more or less a mature science:

John Horgan wrote something a bit surprising in his article. I understand that he may have a bias toward his original thesis in The End of Science, but in considering if there have been any discoveries that contradict his prophecy, he says, no but "the startling discovery in the late 1990s that the expansion of the cosmos is accelerating comes closest." In a few years, cosmological discoveries showed that all the basic particles we previously thought comprised our reality only make up roughly 4% of our universe. Further, the expansion suggests a 5th force or at least a huge modification of Einstein's equations. In my mind those discoveries changed particle physics enormously.

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Bwv 1080

· #30 · 2 mo ago

mmsbls said:
I think some physicists feel that the hypothesis of the collapse of the wave function seems to arbitrarily exclude information that existed in the wave function. If an electron has a 50-50 chance of being spin up or spin down, what happens to the other "half" after the collapse? Interestingly, shortly after Hugh Everett wrote his thesis, very few physicists took the hypothesis seriously. My understanding is that a much greater percentage of theorists now take it seriously.

Physicist, Max Tegmark, thought of an experiment to test the many worlds hypothesis. He considered creating a gun that would fire based on the outcome of a quantum system that had an extremely high probability of one state and an extremely low probability of another state after one second. The gun fires when the high probability state occurs. You can think of this as similar to the Schrödinger's cat setup, but the probability of the gun firing is exceedingly high. He suggested sitting in front of the gun. After a few seconds, you would have a very, very high probability of being dead, but if the many worlds hypothesis were true, you would be alive in some universes. The you in those universes would realize that there was a very high probability that the many worlds hypothesis was correct.

Does not make sense to me as the odds of you living in the universe where the p=10^-12 event happened are the same as the p=10^-12 event happening in the single universe

mmsbls

Discussion Starter · #31 · 2 mo ago

Bwv 1080 said:
Does not make sense to me as the odds of you living in the universe where the p=10^-12 event happened are the same as the p=10^-12 event happening in the single universe

Tegmark did not suggest the experiment expecting that anyone would actually carry it out. He did suggest that it would be a way of testing the Many Worlds hypothesis. The difference between the Many Worlds case and the single universe case would be that in the Many Worlds case you would be guaranteed to survive in at least one universe. In the single universe you would die and that would be that.

Bwv 1080

· #32 · 2 mo ago

mmsbls said:
Tegmark did not suggest the experiment expecting that anyone would actually carry it out. He did suggest that it would be a way of testing the Many Worlds hypothesis. The difference between the Many Worlds case and the single universe case would be that in the Many Worlds case you would be guaranteed to survive in at least one universe. In the single universe you would die and that would be that.

But you would never know either way because:

A) no information available about other universes

and perhaps more importantly

B) you are dead

mmsbls

Discussion Starter · #33 · 2 mo ago

Aries said:
Which regions can not interact with us? Isn't it just a matter of time until every region can interact with us (restricted by the speed of light)?

There are several theories that include regions of spacetime that could not interact with us, but some of those are a bit complicated. The simplest situation is the universe we know. Because our universe expansion is accelerating, there will be regions that will recede from us faster than the speed of light so we will not be able to interact with them. In fact, eventually all parts of our universe except for the local galaxies that are gravitationally bound to us will recede faster than the speed of light. The universe then will look rather bare compared to the one we see now.

But why can one present not result from different pasts? When you interact with Schrödlingers cat, which state was a probabillity distribution before, how can you know which exact probabillity distribution it was before? Aren't all probabillity distributions possible, that could have resulted in the actually state of the cat that it actually took when you interacted with it?

If I understand you, yes, there could be more than one wave function that could have led to the empirical state of the cat. Constructing wave functions is very difficult except for very simple systems so physicists use other methods to understand reality.

A meter is officially defined as a distance light passes in a specific time. But I think what people rather understand as a meter is what is the distance drawn on an inch ruler or yard stick. So measured by an unaccelerated yards stick the speed of light is variable, and you can accelerate to a higher speed than 299,792,458 m/s. I think this is important to know, because it means that we can reach a stars that is 4 light years away, theoretically in less than 4 years.

I'm not sure exactly what you mean here. The speed of light in a vacuum is constant, and objects with mass cannot ever reach or exceed that speed. We can not travel through spacetime a distance of 4 light years in less than 4 years.

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mmsbls

Discussion Starter · #34 · 2 mo ago

Bwv 1080 said:
But you would never know either way because:

A) no information available about other universes

and perhaps more importantly

B) you are dead

You would never know for certain, but the lower the probability of survival, the greater confidence the surviving you would have in the Many Worlds hypothesis.

Bwv 1080

· #35 · 2 mo ago

mmsbls said:
We can not travel through spacetime a distance of 4 light years in less than 4 years.

to be pedantic, you could but could travel 4 LY at 1G constant acceleration in about 3.6 years. It would of course take longer from the POV of an earthbound observer

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BachIsBest

· #36 · 2 mo ago

Bwv 1080 said:
to be pedantic, you could but could travel 4 LY at 1G constant acceleration in about 3.6 years. It would of course take longer from the POV of an earthbound observer

No. You can travel to a star four light years away in under four years, but that is because the length between yourself and the star is contracted, not because you travelled a distance greater than four light years in less than four years. If you were carrying a device to measure how far you travelled it would read significantly less than four light years when you arrived at the star four light years away.

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BachIsBest

· #37 · 2 mo ago

Bwv 1080 said:
Nothing in the Schrödinger equation predicts multiple worlds in the same sense as the equation predicts the probability of finding a system in a particular state. Why would anyone make the jump to believing in this untestable and unknowable conclusion?

As to theoretical physics being more or less a mature science:

Was I Wrong about “The End of Science”?

One of the coolest—and most stressful–moments of my career took place November 7, 1996, when I was a staff writer for Scientific American.

blogs.scientificamerican.com

The Schrodinger equation itself only gives us the wavefunction. The fact that the wavefunction gives probabilities is an additional axiom of the theory. The original (stated) motivation to introduce the many worlds hypothesis is that it, in principle, requires only the Schrodinger equation. Of course, this no longer seems to be the case, and I personally do not like the many worlds interpretation.

Just to be clear, there are vanishingly few (I would say none, but I have not read every paper in every prestigious physics journal) papers on the many worlds interpretation and other such "philosophies" being published in leading physics journals. Virtually every theoretical physics paper published in a prestigious journal will: explain the result of an experiment; predict or detail the result of a future experiment; propose a new experiment along with detailing what it tests; provide a deeper mathematical understanding of an existing theory or model; propose, with a significant amount of mathematical detail worked out, a new theory or model, along with a motivation as to why one would want to consider this new theory or model.

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Aries

· #38 · 2 mo ago

mmsbls said:
There are several theories that include regions of spacetime that could not interact with us, but some of those are a bit complicated. The simplest situation is the universe we know. Because our universe expansion is accelerating, there will be regions that will recede from us faster than the speed of light so we will not be able to interact with them.

As far as I know the expansion of the universe means a expansion of the coordinate system. So something that is distant to us but doesn't move relative to us, still gets more distant over time, because the coordinates system expand, so the same distant coordinate becomes more distant. I guess this is what makes receding with more than light speed possible. But that this expansion can happen with more than ligh speed is still surprising for me.

mmsbls said:
In fact, eventually all parts of our universe except for the local galaxies that are gravitationally bound to us will recede faster than the speed of light. The universe then will look rather bare compared to the one we see now.

But this is still a topic of research isn't it? We don't really know for sure the future of the universe now, or?

mmsbls said:
I'm not sure exactly what you mean here. The speed of light in a vacuum is constant, and objects with mass cannot ever reach or exceed that speed. We can not travel through spacetime a distance of 4 light years in less than 4 years.

The speed of light is defined as 299,792,458 metres per second. Between 1983 to 2019 a meter was defined as the length of the path travelled by light in a vacuum in
1/299,792,458 of a second.

So by these two definitions the speed of light is defined as the speed of light, what is a questionably useful information.

What makes more sense, and what is better understandable is to define a meter by the actual earth like 1 meter = 1/40,000,000 of the perimeter of the earth.

But the earth contracts if you accelerate. But that is like the same as if the speed of light gets faster meastued by the earth-perimeter if you accelerate. I think both describes the same, but I think it is useful to clarify that there is no minimum duration for the travel between actual distances on the earth or between neighbouring planets and stars.

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mmsbls

Discussion Starter · #39 · 2 mo ago

Aries said:
As far as I know the expansion of the universe means a expansion of the coordinate system. So something that is distant to us but doesn't move relative to us, still gets more distant over time, because the coordinates system expand, so the same distant coordinate becomes more distant. I guess this is what makes receding with more than light speed possible. But that this expansion can happen with more than ligh speed is still surprising for me.

The expansion of the universe can certainly be confusing. What actually expands is space itself. The constraint on faster than light travel refers to objects in space (particles, spaceships, galaxies, etc.). If there were no gravitational attraction, our own galaxy would expand, but the stars are gravitationally bound to the galaxy as a whole so the galaxy remains essentially the same.

But this is still a topic of research isn't it? We don't really know for sure the future of the universe now, or?

Until a few decades ago we knew the universe was expanding, but we thought the gravitational attraction of the universe's mass was slowing the expansion rate. Now we know that something is causing the expansion rate to increase. I believe cosmologists think that "something" has always been a repulsive pressure opposing the force of gravity, but now that "something" is stronger than the gravitational attraction. The general view is that the repulsive pressure will increase over time as it has in the past and continue to increase the rate of expansion.

The speed of light is defined as 299,792,458 metres per second. Between 1983 to 2019 a meter was defined as the length of the path travelled by light in a vacuum in
1/299,792,458 of a second.

So by these two definitions the speed of light is defined as the speed of light, what is a questionably useful information.

What makes more sense, and what is better understandable is to define a meter by the actual earth like 1 meter = 1/40,000,000 of the perimeter of the earth.

But the earth contracts if you accelerate. But that is like the same as if the speed of light gets faster meastued by the earth-perimeter if you accelerate. I think both describes the same, but I think it is useful to clarify that there is no minimum duration for the travel between actual distances on the earth or between neighbouring planets and stars.

The speed of light is always a constant in any reference frame. Everyone will always measure that speed as "c".[/QUOTE]

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relm1

· #40 · 2 mo ago

BachIsBest said:
This is completely incorrect. Massless particles like photons don't have a rest frame so any statement about their "concepts" are borderline meaningless. Saying "time doesn't exist without mass" makes no sense. A universe with only massless particles would still time-evolve.

Time is certainly real. 100 million years ago the hour hadn't been invented and 100 million years from now the hour, if humans are still around and using SI units, will have the same meaning. The modern SI hour is defined in terms of fundamental physical laws, not 1/24th of a day.

Take it up with them:

www.scientificamerican.com

Time’s Passage is Probably an Illusion

From the fixed past to the tangible present to the undecided future, it feels as though time flows inexorably on. But that is an illusion

www.scientificamerican.com

Time Might Not Exist, According to Physicists
https://www.nytimes.com/2004/01/01/opinion/the-time-we-thought-we-knew.html

BachIsBest said:
This is wrong. Again, making statements about observers travelling at the speed of light is almost certainly nonsense (since observers can't travel at the speed of light). The speed of light being constant has no bearing on whether or not time is a constant (spoiler alert: time is not constant).

I don't think you understood my point probably too angry that I disagreed with you. What I said is that speed of light is a constant and speed being distance over time is complicating that space and time are relative. I don't share your sense of absolute certainty but think it's fascinating/strange that speed can be absolute at the speed of light yet the elements of speed (space and time) are relative.

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