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Physics, Astronomy & Cosmology / Was the very early Universe massless?
« on: 12/12/2016 19:40:17 »
Richard Muller, “Now” (2016) says: “In the initial Big Bang, before the appearance of the Higgs, all particles were massless.”
1. Is this the generally accepted view?
2. What about mass that is not dependent on the Higgs, would that have been negated by the extreme temperature?
1. Is this the generally accepted view?
2. What about mass that is not dependent on the Higgs, would that have been negated by the extreme temperature?
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Physics, Astronomy & Cosmology / Do macro systems have wavefunctions?
« on: 08/12/2016 11:32:46 »
In Pop Sci literature one often meets references to the wavefunctions of macro systems. My understanding is that only an isolated system can be described by a wavefunction. Typically, macro bodies are not isolated systems. Decoherence will have taken place, irrespective of any human intervention. So it is not appropriate to talk of a wavefunction for any macro system that is not adequately protected from surrounding systems, or from internal interactions.
Am I on the right track?
Am I on the right track?
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Physics, Astronomy & Cosmology / Is information physical?
« on: 06/12/2016 11:29:46 »
One often meets the statement: "Information is physical." Is this correct, or is it the processing of information that is physical?
In other words, is it the processing of information that increases entropy?
In other words, is it the processing of information that increases entropy?
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Physics, Astronomy & Cosmology / Does what we see depend on how we look?
« on: 05/09/2016 21:46:34 »
I may well present three scenarios in this thread; it, sort of, depends on the responses.
In each scenario there is a hypothetical space craft, travelling at c and an astronaut on the craft with a flashlight which he shines from the back of the craft towards the front. There will also be one or two remote observers, with hypothetical instruments that can be turned on and off very quickly.
(1) The astronaut turns on his flashlight. In his RF the light travels at c to the front of the craft.
Observer A makes her observation as the astronaut turns on his flashlight. She sees no light because in her RF the craft and the light are travelling at the same speed.
In each scenario there is a hypothetical space craft, travelling at c and an astronaut on the craft with a flashlight which he shines from the back of the craft towards the front. There will also be one or two remote observers, with hypothetical instruments that can be turned on and off very quickly.
(1) The astronaut turns on his flashlight. In his RF the light travels at c to the front of the craft.
Observer A makes her observation as the astronaut turns on his flashlight. She sees no light because in her RF the craft and the light are travelling at the same speed.
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Physics, Astronomy & Cosmology / Do vectors militate against time reversal?
« on: 23/06/2016 23:07:25 »
This thought experiment involves a friction-free snooker table, in a vacuum. The cue-ball is alone on the table, placed centrally on the balk cushion.
At t=0 the cue strikes it, sending it straight up the table at constant velocity, v, with constant momentum, p.
At t=1 it crosses the brown spot.
At t=2 it crosses the blue spot.
At t=3 it arrives at the pink spot; at which point, time is reversed.
It crosses the blue and brown spots at t=2 and t=1, respectively.
What happens to v and p, both are vectors, so at t=2 and t=1, in reversed time, the ball is travelling towards the balk cushion, but the vectors point away from it?
At t=0 the cue strikes it, sending it straight up the table at constant velocity, v, with constant momentum, p.
At t=1 it crosses the brown spot.
At t=2 it crosses the blue spot.
At t=3 it arrives at the pink spot; at which point, time is reversed.
It crosses the blue and brown spots at t=2 and t=1, respectively.
What happens to v and p, both are vectors, so at t=2 and t=1, in reversed time, the ball is travelling towards the balk cushion, but the vectors point away from it?
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Physics, Astronomy & Cosmology / Does Sascha's math make sense?
« on: 04/06/2016 22:03:46 »
Sascha Vongehr says:
http://www.science20.com/alpha_meme/fundamental_nature_light-75861
The math looks fairly simple, but I think I must have become lost somewhere in trying to use it.
I'd be grateful is someone could de-fog me; idiot level, please.
http://www.science20.com/alpha_meme/fundamental_nature_light-75861
Quote
In fact, we would experience about one second of travel time between earth and moon, if we moved with a velocity v that equals light velocity divided by the square root of two: v=c/√2. At 90% light velocity, i.e. at v=9c/10, our travel time will be only a third of a second! At 99.9% of the speed of light, the travel time we would experience has reduced to a thirtieth of a second, or 33.3 milliseconds.
The math looks fairly simple, but I think I must have become lost somewhere in trying to use it.
I'd be grateful is someone could de-fog me; idiot level, please.
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Physics, Astronomy & Cosmology / Are biological clocks subject to time dilation?
« on: 30/05/2016 22:53:41 »
Sean Carroll asks the question: What would it be like if time passed more quickly, or more slowly? His answer is interesting.
This raises the question: what do we really know about biological clocks in terms of time dilation? We are told that the astronauts who have spent the most time in Earth orbit are minuscule fractions of a second younger than they would be if they had remained on Earth; but what does that actually mean?
People who suffer from any of the forms of progeria appear to age more quickly than the vast majority of people. Is this due to their biological clocks running at a different rate? Are their bodies "experiencing" time at a rate that is different from that experienced by their minds? If biological clocks can vary in this way, what evidence do we have to indicate that they will be influenced in the same way as mechanical or atomic clocks by time dilation?
Quote
“The crucial question there was: Compared to what? The idea that “time suddenly moves more quickly for everyone in the world” isn’t operationally meaningful; we measure time by synchronized repetition, and as long as clocks of all sorts (including biological clocks and clocks defined by subatomic processes) remain properly synchronized, there’s no way you could tell that the “rate of time” was in any way different. It’s only if some particular clock speeds up or slows down compared to other clocks that the concept makes any sense.”
This raises the question: what do we really know about biological clocks in terms of time dilation? We are told that the astronauts who have spent the most time in Earth orbit are minuscule fractions of a second younger than they would be if they had remained on Earth; but what does that actually mean?
People who suffer from any of the forms of progeria appear to age more quickly than the vast majority of people. Is this due to their biological clocks running at a different rate? Are their bodies "experiencing" time at a rate that is different from that experienced by their minds? If biological clocks can vary in this way, what evidence do we have to indicate that they will be influenced in the same way as mechanical or atomic clocks by time dilation?
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Physics, Astronomy & Cosmology / Is there proof that inflation had a beginning?
« on: 06/11/2015 21:22:23 »
http://discovermagazine.com/2013/september/13-starting-point
Guth, Vilenkin and Borde propose the following thought experiment: Imagine a universe filled with particles. As it steadily expands, the distance between particles grows. It follows that observers sprinkled throughout this expanding universe would be moving away from each other until, eventually, they occupied widely scattered regions of space. If you happened to be one of those observers, the farther an object was from you, the faster it would be moving away.
Now throw into the mix a space traveler moving through space at a fixed speed: He zooms past Earth at 100,000 kilometers per second. But when he reaches the next galaxy, which is moving away from us at, say, 20,000 kilometers per second, he will appear to be moving only 80,000 kilometers per second to observers there. As he continues on his outward journey, the space traveler’s speed will appear smaller and smaller to the observers he passes. Now we’ll run the movie backward. This time, the space traveler’s velocity will appear faster and faster at each successive galaxy.
If we assume inflation is eternal into the past — that it had no beginning — the space traveler will eventually reach and overtake the speed of light. A calculation by Borde, Guth and Vilenkin showed that this would happen in a finite amount of time. But according to the laws of relativity, it is impossible for any massive object to reach the speed of light, let alone exceed it. “This cannot happen,” says Vilenkin. “So when you follow this space traveler’s history back in time, you find that his history must come to an end.”
“The fact that the traveler’s journey backward in time hits an impasse means that there’s a problem, from a logical standpoint, with the assumption of an ever-expanding universe upon which this whole scenario is based. The universe, in other words, could not always have been expanding. Its expansion must have had a beginning, and inflation — a particularly explosive form of cosmic expansion — must have had a beginning, too. By this logic, our universe also had a beginning since it was spawned by an inflationary process that is eternal into the future but not the past.”
This works only if time reversal is invoked, and the galaxies are moving closer together, with their closing speed, relative to the galaxy at which the astronaut turned, decreasing. This would give the required effect, but is it fairytale physics?
Guth, Vilenkin and Borde propose the following thought experiment: Imagine a universe filled with particles. As it steadily expands, the distance between particles grows. It follows that observers sprinkled throughout this expanding universe would be moving away from each other until, eventually, they occupied widely scattered regions of space. If you happened to be one of those observers, the farther an object was from you, the faster it would be moving away.
Now throw into the mix a space traveler moving through space at a fixed speed: He zooms past Earth at 100,000 kilometers per second. But when he reaches the next galaxy, which is moving away from us at, say, 20,000 kilometers per second, he will appear to be moving only 80,000 kilometers per second to observers there. As he continues on his outward journey, the space traveler’s speed will appear smaller and smaller to the observers he passes. Now we’ll run the movie backward. This time, the space traveler’s velocity will appear faster and faster at each successive galaxy.
If we assume inflation is eternal into the past — that it had no beginning — the space traveler will eventually reach and overtake the speed of light. A calculation by Borde, Guth and Vilenkin showed that this would happen in a finite amount of time. But according to the laws of relativity, it is impossible for any massive object to reach the speed of light, let alone exceed it. “This cannot happen,” says Vilenkin. “So when you follow this space traveler’s history back in time, you find that his history must come to an end.”
“The fact that the traveler’s journey backward in time hits an impasse means that there’s a problem, from a logical standpoint, with the assumption of an ever-expanding universe upon which this whole scenario is based. The universe, in other words, could not always have been expanding. Its expansion must have had a beginning, and inflation — a particularly explosive form of cosmic expansion — must have had a beginning, too. By this logic, our universe also had a beginning since it was spawned by an inflationary process that is eternal into the future but not the past.”
This works only if time reversal is invoked, and the galaxies are moving closer together, with their closing speed, relative to the galaxy at which the astronaut turned, decreasing. This would give the required effect, but is it fairytale physics?
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Physics, Astronomy & Cosmology / Are there exceptions to the constancy of c?
« on: 30/10/2015 20:52:10 »
J Richard Gott uses a thought experiment involving an astronaut who uses a flashlight on a spacecraft travelling faster than c.
He says: “If an astronaut’s rocket were to travel by us at faster than the speed of light, a light beam he sent forward could never catch up with the front of his rocket. The light beam could never catch up because the front of the rocket would be moving faster and have a head start. Any athlete should know that catching another runner who is running faster and has a head start is impossible. The astronaut’s observations would be most peculiar: he would take out a flashlight and shine it towards the front of his rocket, but he would never see the beam of light arrive.”
The final sentence indicates that he is talking about the observation of the astronaut on board the craft; not an outside observer.
Surely, if the astronaut is on the rocket, the front of the rocket would be stationary in his frame of reference. At subluminal speed he would see the light beam move towards the front at c. Why would this be different at superluminal speed?
He says: “If an astronaut’s rocket were to travel by us at faster than the speed of light, a light beam he sent forward could never catch up with the front of his rocket. The light beam could never catch up because the front of the rocket would be moving faster and have a head start. Any athlete should know that catching another runner who is running faster and has a head start is impossible. The astronaut’s observations would be most peculiar: he would take out a flashlight and shine it towards the front of his rocket, but he would never see the beam of light arrive.”
The final sentence indicates that he is talking about the observation of the astronaut on board the craft; not an outside observer.
Surely, if the astronaut is on the rocket, the front of the rocket would be stationary in his frame of reference. At subluminal speed he would see the light beam move towards the front at c. Why would this be different at superluminal speed?
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Physics, Astronomy & Cosmology / Do photons and massive particles behave the same in double slit experiments?
« on: 25/10/2015 01:39:02 »
Photons passing individually through double slits produce an interference pattern. If behind the slits there are detectors that can determine which slit a photon passed through, the interference pattern vanishes. This occurs, even if the decision to turn on the detectors is not made until after the photon has passed through the slit.
Is this result the same with electrons, and other massive particles?
Is this result the same with electrons, and other massive particles?
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Physics, Astronomy & Cosmology / Are there different ways of looking at time dilation?
« on: 21/10/2015 03:41:26 »
I’m quite happy to accept the label “crackpot”, but my crackpottery does not extend to denying time dilation. However, I like to try to look at things from a range of perspectives.
Let’s establish two spacetime events and call them X and Y. We also have two people, call them Alice and Bob, (just to be different).
Alice and Bob are present at both X and Y, at which points time is synchronous for both, because they are both in the same reference frame.
Assume that between X and Y, Alice had taken a return space flight, at 0.8c, to a planet that is 10 light years from Earth. (Ignore acceleration). Bob has remained stationary on Earth.
In Bob’s reference frame (RF) there were 25 years between X and Y; while for Alice there were 15 years.
There are two ways of looking at this.
1. For two people who are in the same reference frames at X and Y, a different amount of time has passed between those two events.
2. For the two people, the same amount of time has passed, but Alice has been able to achieve/experience less in the course of that time because change has happened more slowly.
How might we distinguish? How does Alice know that only fifteen years have passed? We all have an innate sense of the passage of time, but we know that that is distinctly unreliable, so let’s discount it.
Alice’s clock shows the passage of only fifteen years. That’s much more reliable, but how can she be sure it’s not just that her clock was running slow?
It might be argued that every “accurate” clock runs slow, relative to a clock designated as stationary relative to some reference point, by exactly the same amount, at 0.8c, but this may tell us only that acceleration influences the rate of change in a consistent way.
How can we be sure that anything actually happens to time?
Let’s establish two spacetime events and call them X and Y. We also have two people, call them Alice and Bob, (just to be different).
Alice and Bob are present at both X and Y, at which points time is synchronous for both, because they are both in the same reference frame.
Assume that between X and Y, Alice had taken a return space flight, at 0.8c, to a planet that is 10 light years from Earth. (Ignore acceleration). Bob has remained stationary on Earth.
In Bob’s reference frame (RF) there were 25 years between X and Y; while for Alice there were 15 years.
There are two ways of looking at this.
1. For two people who are in the same reference frames at X and Y, a different amount of time has passed between those two events.
2. For the two people, the same amount of time has passed, but Alice has been able to achieve/experience less in the course of that time because change has happened more slowly.
How might we distinguish? How does Alice know that only fifteen years have passed? We all have an innate sense of the passage of time, but we know that that is distinctly unreliable, so let’s discount it.
Alice’s clock shows the passage of only fifteen years. That’s much more reliable, but how can she be sure it’s not just that her clock was running slow?
It might be argued that every “accurate” clock runs slow, relative to a clock designated as stationary relative to some reference point, by exactly the same amount, at 0.8c, but this may tell us only that acceleration influences the rate of change in a consistent way.
How can we be sure that anything actually happens to time?
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Physics, Astronomy & Cosmology / Does quantum mechanics (QM) require an absolute time frame?
« on: 19/10/2015 19:04:12 »
Somewhere, I no longer remember where, I read that QM requires a universal time that could be synchronized across the Universe.
Is this right? If so, why?
Is this right? If so, why?
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Physics, Astronomy & Cosmology / Do orbiting bodies accelerate?
« on: 01/09/2015 23:59:14 »
This is just a series of thoughts leading to a “conclusion”. I would appreciate comments/criticism, please.
1. Velocity is a vector which includes speed and direction.
2. Acceleration is change in velocity.
3. Change of speed with constant direction = acceleration.
4. Change of direction with constant speed = acceleration.
5. A body orbiting at constant speed is constantly accelerating.
6. Gravity is not a force that holds an orbiting body as though it were on a string.
7. Gravity alters the geometry of spacetime such that it becomes (or acts as though) curved.
8. The curve thus formed is a geodesic, and is defined as the most direct path from A to B in curved spacetime.
9. Thus, a geodesic is equivalent to a straight line in flat (non-curved) spacetime.
10. A body travelling at constant speed in a straight line in flat spacetime is not accelerating.
11. It should be reasonable to argue that a body following a geodesic at constant speed is not accelerating.
12. It should, therefore, be reasonable to conclude that an orbiting body is not accelerating.
1. Velocity is a vector which includes speed and direction.
2. Acceleration is change in velocity.
3. Change of speed with constant direction = acceleration.
4. Change of direction with constant speed = acceleration.
5. A body orbiting at constant speed is constantly accelerating.
6. Gravity is not a force that holds an orbiting body as though it were on a string.
7. Gravity alters the geometry of spacetime such that it becomes (or acts as though) curved.
8. The curve thus formed is a geodesic, and is defined as the most direct path from A to B in curved spacetime.
9. Thus, a geodesic is equivalent to a straight line in flat (non-curved) spacetime.
10. A body travelling at constant speed in a straight line in flat spacetime is not accelerating.
11. It should be reasonable to argue that a body following a geodesic at constant speed is not accelerating.
12. It should, therefore, be reasonable to conclude that an orbiting body is not accelerating.
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Physics, Astronomy & Cosmology / Could this mean a new force?
« on: 30/08/2015 17:21:36 »
https://uk.news.yahoo.com/subatomic-particles-appear-defy-standard-100950001.html#W4wF3Nr
This looks fascinating. Thoughts from the experts would be appreciated.
Is this the same lepton universality break that BaBar thought they had discovered?
This looks fascinating. Thoughts from the experts would be appreciated.
Is this the same lepton universality break that BaBar thought they had discovered?
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Physics, Astronomy & Cosmology / Does gravity get out of a black hole?
« on: 23/07/2015 22:28:54 »
Recently I came across three questions about black holes and gravity. Below are the questions and my initial attempts at answers.
1.) How does gravity get out of a black hole?
It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.
2.) Why is the speed of gravity restricted to the speed of light?
Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.
3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?
If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.
I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.
1.) How does gravity get out of a black hole?
It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.
2.) Why is the speed of gravity restricted to the speed of light?
Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.
3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?
If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.
I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.
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Physics, Astronomy & Cosmology / What happened to the age of the Universe thread?
« on: 15/04/2015 17:20:08 »
I looked for it to respond to Pete's post of yesterday, but it had vanished.
No reward offered, but it would be good to know what happened.
No reward offered, but it would be good to know what happened.
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Physics, Astronomy & Cosmology / How old is the Universe?
« on: 13/04/2015 03:36:40 »
How old is the Universe? That seems like an easy question to answer. It’s 13.7 billion years, or close to.
Where do we measure that time from? That’s another easy one: now.
Relativity tells us there is no universal now, all we can identify is the spacetime event that is “here and now”. So, what do we mean when we say that we measure back from now? Any reference frame that is in motion relative to ours will identify a different here and now. Every part of the Universe is in motion relative to every other part, and the faster that relative motion, the greater will be the discrepancy between the measurement of here and now with respect to each part.
We are assured that the more distant (from us) parts of the Universe are moving away from us at speeds faster than c. What can we say about the measurement of here and now in those areas? All these areas are parts of the Universe. If we can’t identify a “present” for them, how can we say how their “present” relates to the start of the Universe from their perspective?
We could assume that any measurement taken from any part of the Universe would show that the Big Bang occurred at 13.7 billion years before that local “present”, but what does that actually mean? Is our measurement of the age of the Universe just a feature of our particular location in spacetime?
Where do we measure that time from? That’s another easy one: now.
Relativity tells us there is no universal now, all we can identify is the spacetime event that is “here and now”. So, what do we mean when we say that we measure back from now? Any reference frame that is in motion relative to ours will identify a different here and now. Every part of the Universe is in motion relative to every other part, and the faster that relative motion, the greater will be the discrepancy between the measurement of here and now with respect to each part.
We are assured that the more distant (from us) parts of the Universe are moving away from us at speeds faster than c. What can we say about the measurement of here and now in those areas? All these areas are parts of the Universe. If we can’t identify a “present” for them, how can we say how their “present” relates to the start of the Universe from their perspective?
We could assume that any measurement taken from any part of the Universe would show that the Big Bang occurred at 13.7 billion years before that local “present”, but what does that actually mean? Is our measurement of the age of the Universe just a feature of our particular location in spacetime?
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Physics, Astronomy & Cosmology / How should one interpret spacetime diagrams?
« on: 18/03/2015 22:03:41 »
Question about Michael Huemer’s spacetime diagrams.
http://home.earthlink.net/~owl232/twinparadox.pdf
In Figure 2 the Earth is considered as stationary; its world line simply follows the vertical time axis.
Space Twin is considered to be in motion relative to the Earth. He moves to the right at 0.5c, then to the left at 0.5c.
In Figure 3 Space Twin is considered to be stationary. His world line starts by following the vertical time axis. The Earth is considered as being in motion relative to Space Twin, so it moves to the left at 0.5c.
The reason that Earth is considered to be moving at 0.5c is that that was the speed of the spacecraft in the first F of R.
At t = 8.66, Space Twin decides to change speed and direction.
Figure 3 then shows Space Twin as being in motion. If we are still in Space Twin’s F of R, why are we seeing him as being anything other than stationary?
Should Fig. 3 not show Earth as moving towards the space craft? If not - why not?
http://home.earthlink.net/~owl232/twinparadox.pdf
In Figure 2 the Earth is considered as stationary; its world line simply follows the vertical time axis.
Space Twin is considered to be in motion relative to the Earth. He moves to the right at 0.5c, then to the left at 0.5c.
In Figure 3 Space Twin is considered to be stationary. His world line starts by following the vertical time axis. The Earth is considered as being in motion relative to Space Twin, so it moves to the left at 0.5c.
The reason that Earth is considered to be moving at 0.5c is that that was the speed of the spacecraft in the first F of R.
At t = 8.66, Space Twin decides to change speed and direction.
Figure 3 then shows Space Twin as being in motion. If we are still in Space Twin’s F of R, why are we seeing him as being anything other than stationary?
Should Fig. 3 not show Earth as moving towards the space craft? If not - why not?
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Physics, Astronomy & Cosmology / Relative Simultaneity
« on: 14/02/2015 20:29:23 »
Extract from the Haifa Lectures:
"Consider the following ‘thought experiment’: A cat is crossing an intersection in the road. At the center of the intersection there is a manhole cover and at the curb there is a workman with his hand on a lever that would open the manhole cover. As soon as the cat steps toward the manhole cover, the workman simultaneously pulls the lever that opens it. The cat then falls below the street, never to reach the other side of the road. Suppose now that a helicopter is flying over the road, at a speed close to the speed of light! In looking down at the road, the pilot sees the cat crossing the road and the workman pulling the lever to open the manhole cover non-simultaneously, i.e. at a different time than when the cat reaches the manhole cover in the road. The pilot then expects the cat to reach the other side of the road. But instead he sees that the cat disappears midway across the road! He then asks himself: “why didn’t the cat get to the other side of the road?” He answers that it was because what he saw was influenced by the fact that he was in a moving frame of reference, relative to the cat and the road. To learn what really happened he applies the Lorentz transformation to put himself into the frame of reference of the cat and the manhole cover, independent of any outside observer! This is called the ‘proper’ frame of reference — it involves only the interacting things — the cat and the Earth that pulls it downwards. In this (proper) reference frame, he learns that the cat did not reach the other side of the road because the workman pulled the lever at the precise time when the cat stepped down toward it, and so it fell below the street before reaching the other side. Thus we see that the relativity of simultaneity in this theory is not physical; it is only descriptive regarding a viewing from the frame of reference of the observer. To say that relative simultaneity is a physical fact is to predict a paradox — that, in this example, the cat would reach the other side of the road and it would not reach the other side of the road!"
What Sachs seems to be saying here is that there is a “proper” frame of reference in which it is possible to identify what “really happened”, as distinct to what appeared, in another F of R, to have happened.
I was under the impression that one had to consider every F of R as having the same validity in terms of reality; isn’t that right?
"Consider the following ‘thought experiment’: A cat is crossing an intersection in the road. At the center of the intersection there is a manhole cover and at the curb there is a workman with his hand on a lever that would open the manhole cover. As soon as the cat steps toward the manhole cover, the workman simultaneously pulls the lever that opens it. The cat then falls below the street, never to reach the other side of the road. Suppose now that a helicopter is flying over the road, at a speed close to the speed of light! In looking down at the road, the pilot sees the cat crossing the road and the workman pulling the lever to open the manhole cover non-simultaneously, i.e. at a different time than when the cat reaches the manhole cover in the road. The pilot then expects the cat to reach the other side of the road. But instead he sees that the cat disappears midway across the road! He then asks himself: “why didn’t the cat get to the other side of the road?” He answers that it was because what he saw was influenced by the fact that he was in a moving frame of reference, relative to the cat and the road. To learn what really happened he applies the Lorentz transformation to put himself into the frame of reference of the cat and the manhole cover, independent of any outside observer! This is called the ‘proper’ frame of reference — it involves only the interacting things — the cat and the Earth that pulls it downwards. In this (proper) reference frame, he learns that the cat did not reach the other side of the road because the workman pulled the lever at the precise time when the cat stepped down toward it, and so it fell below the street before reaching the other side. Thus we see that the relativity of simultaneity in this theory is not physical; it is only descriptive regarding a viewing from the frame of reference of the observer. To say that relative simultaneity is a physical fact is to predict a paradox — that, in this example, the cat would reach the other side of the road and it would not reach the other side of the road!"
What Sachs seems to be saying here is that there is a “proper” frame of reference in which it is possible to identify what “really happened”, as distinct to what appeared, in another F of R, to have happened.
I was under the impression that one had to consider every F of R as having the same validity in terms of reality; isn’t that right?