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Question on Spacetime
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If there is no such thing as distance then why does it take light from some sources longer to reach us than others?
Distance is just a concept, it doesn't have any existence in and of itself.
If one measures the distance between two objects, it involves measuring and imaginary line, between two imaginary (and arbitrary) points.
If one measures the length of something, then it is the object that has [some] intrinsic existence, and not the length.0 -
mangaroosh wrote: »The question was, to show how things change with resepect to time.
You could take the derivative of the length of the essay you wrote, or the limit of the reading time required...
For every moment of time (t) ∃ ε > 0 :
∀ ε > 0 ∃ δ> 0 : if 0 < |x–t| < δ, then |f(t)–L| < ε.
Just use this piece of magic to convince yourself that, no matter how small an amount of time you want to allow to occur, there will always be a moment there to accompany it.
Test it!
:pac::P0 -
mangaroosh wrote: »Distance is just a concept, it doesn't have any existence in and of itself.
If one measures the distance between two objects, it involves measuring and imaginary line, between two imaginary (and arbitrary) points.
If one measures the length of something, then it is the object that has [some] intrinsic existence, and not the length.
What a wonderful view of the universe you have. I'm just glad that it is not shared my mankind or we would not be communicating via the internet at the moment. We would be still stuck in the dark ages i.e. we would have no understanding of how anything works as we would not the tools necissary to do so.
Basically if we take you point of view we get rid of science. So I can stick with Newton, Einstein, Pastuer and Gauss, to name a few. Who have given us so rich an understanding of the complexity of life, or you with your barren (scientifically) outlook.0 -
mangaroosh wrote: »I haven't read Barbour's book yet (still waiting for the sister to send it to me).
The time parameter emerges simply from our capacity to remember, and from our capacity to imagine.
The present moment is all that ever exists. All of human existence takes place in the present, even if there is a different present for everyone, this still holds true i.e. all the events in your life take place in your present, and all the events in my life take place in my present, so the present (even if it is personal) is all that exists for anyone.
It is the present that is ever changing. Events will occur and pass, always in the present. It is the fact that we can remember events that have occured [and passed by] that leads to the concept of "the past".
The issue is that "the past" does not actually exist, it is simply a memory of what used to be the present, and therefore exists only as a thought or memory in a persons mind.
The concept of the future is based on our ability to imagine that change will continue to occur, so that what we experience as the present will change, and will be confined to the memory (non-existence).
We imagine that the present will change, never completely accurately, although we may be able to make predictions as to how it will manifest itself, at least in part (never in totality).
Of course, when it comes "time" to guage our predictions against reality, this will occur by comparing the present to what we predicted.
The act of making the prediction will reside only in the memory, while the predictions (if on paper perhaps) will always only exist in the present. The comparison of our predictions to the manifest events, will take place in the present.
It is because of our ability to remember the present (after it changes and ceases to exist) and our capacity to imagine the present (when it will change and come into existence), that we get the concept of a "time line" or the "river of time".
If one can picture time as a road, along which one walks, where the only part of the road that exists is the part under ones feet. A person may take a step forward on the road, and the part in front of them will come into existence, while the piece they just stepped off from will fall away into non-existence.
You are still implicitly referencing time. The "re" in the word remember, for example, implies time; "used to be" implies time etc. You have been arguing that time is artificial, and that all these implicit references are actually referencing something else, something more fundamental. But when I ask you what, you regress back to using words that imply time. Ultimately, it boils down to "How would you formulate the evolution of a system without time, and with only one "moment" in existence?"
So the conversation has progressed insofar as we now both accept that things change with respect to time (even if you believe time is artificial). What I then said is It is no good to assert that time is artificial unless you can explicitly show how the artificial time parameter emerges from more fundamental physical variables. What you have done here is reasserted your claim, but have not followed through on the consequences of that assertion. What does it mean, for example, to say one moment "falls away into non-existence"? What is it we are "remembering"?The question was, to show how things change with respect to time.
I have shown you how change with respect to time is classically defined.
Maybe your question should be rephrased.
I should probably also note that, in quantum mechanics, dynamical variables are an implicit function of time, but that will just confuse the issue further.If we leave the discussion on the universal present for now, we can say that all that exists for anyone is the present, even if it is a personal/relative present.
This would mean that there is no universal reality, rather an individual reality, but the same arguments will hold, on an individual basis.
A collection of individual realities is a bit excessive don't you think? When these relative perspectives are framed in 4 dimensional space, we get a set of consistent, invariant descriptions of reality that are the same for everyone, as well as a consistent causal structure. See, if you suppose spacetime is physical, then you explain a great range of phenomena. If you suppose time is artificial, you must ultimately suppose a bunch of unexplained convoluted physical mechanisms this artificial construct is describing.As for the predictions of GR, the issue isn't their accuracy, or their usefulness, rather the basic assumption upon which they are based, and the possible self-contained system that seems to support the assumption through experiments.
More clearly, while the predictions of GR may indeed be well supported, if there is a self-contained system within the experiments, then the assumption will neve be falsified, due to this self-contained system.
This self-contained system is, that time exists, and that a clock measures time.
I don't mean to sound rude but it's becoming clear that your lack of familiarity with the subject is really slowing down the conversation. GR is not a self-contained system, for even if clocks must be used to measure time, the predictions are still falsifiable. The world line of a photon, for example, is described by the null-cone structure of spacetime in GR. This means GR accurately predicts the bending of light in a gravitational field. If our assertion that time is a dimension of spacetime is incorrect, then why do photons behave precisely as if time is a dimension of spacetime? Is it coincidence? What about all the other predictions regarding the behaviour of cosmological entities? Are they all coincidences too?Examination of a clock shows that what is being measured is not something called time, rather a physical phenomenon. That is the measurement device measures the change in the physical phenomenon.
The contention is, that the physical phenomenon changes with regard to time, however, there doesn't appear to be a logical reason as to why this is true.
Indeed to simply state this, without showing how, or why it is true, is a non-sequitur.
Just to reiterate. If time is a dimension of spacetime, then we would expect very specific cosmological phenomena. These predicted phenomena have been observed, so we are confident in the assumption that time is a dimension of spacetime. There is no non-sequitur or circular reasoning.The issue of information only being able to travel at the speed of light, should then surely apply to the question of the lightning hitting the rods. Where the man in the train is travelling away from the rods, it means that the information has a longer distance to travel. As one rod will be closer to him than the other, the information from that rod will reach him before the other rod, making it appear that the lightning hit one rod before the other, when the reality of the event, may indeed have been different.
A man in the same inertial reference frame as the rods might still have one rod closer to him than the other. In his case, even though one rod is closer to him than the other, he would still "agree with the rods" and conclude that lightning struck both rods at the same time by considering the distance the information had to travel. A mean in the train, even after considering the distance the info had to travel, would not agree.The laws of physics won't change in either reference frame, so on that basis there would be no preferred reference frame, but depending on the question that is asked, one reference frame may be more suitable to answering the question than the other.
If by "more suitable" you mean the maths is a little easier to work out than sure. But if by more suitable you mean "better" or "truer" then not at all.With regard to the example of the star burning itself out, then the reference frame of the star is more suitable for answering the question "has the star burned itself out".
The issue however, would not be that the star was still burning (if we assume in this case that the star has burned itself out), rather that the information that the star sent out, before it burned out, is only reaching you now, while it would have already reached me on its way to you.
I would of course have to be walking towards the star, or rather the information from the star.
I said, in my previous post, that simultaneity is not due to the finite speed of the information. Rather it is evident from the fact that this speed is the same for everyone. Consider two people in the same inertial reference frame (IRF) as the star. If one person is further away from the star then the info will reach them later, but they still both agree on the time that the star burnt out, as they both consider the distance the light had to travel. But if they are in different IRFs, then even after they account for distance, they will not agree on when the star burnt out."Time does not exist" is not an assumption, rather a logical conclusion, based on the examination of the assumptions, upon which times existence is based.
Logic is a set of inference rules to be applied to assumptions. You cannot have a logical conclusion without assumptions. You are assuming that, because time is in some sense implicit, it is not physical.What we end up with, if time does not exist, is a fallacious argument for the eixstence of time. It has no bearing on the truth of the statement, or the reality of the situation.
Well GR is obviously tentative insofar as we could be wrong. But that doesn't mean we can't have confidence in it until then.0 -
sponsoredwalk wrote: »You could take the derivative of the length of the essay you wrote, or the limit of the reading time required...
For every moment of time (t) ∃ ε > 0 :
∀ ε > 0 ∃ δ> 0 : if 0 < |x–t| < δ, then |f(t)–L| < ε.
Just use this piece of magic to convince yourself that, no matter how small an amount of time you want to allow to occur, there will always be a moment there to accompany it.
Test it!
:pac::P
You are correct in one sense. The moment that will always be there to acompany "it", will be the present moment, which is the only moment in time whichever exists.0 -
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I don't mean to sound rude but it's becoming clear that your lack of familiarity with the subject is really slowing down the conversation. GR is not a self-contained system, for even if clocks must be used to measure time, the predictions are still falsifiable. The world line of a photon, for example, is described by the null-cone structure of spacetime in GR. This means GR accurately predicts the bending of light in a gravitational field. If our assertion that time is a dimension of spacetime is incorrect, then why do photons behave precisely as if time is a dimension of spacetime? Is it coincidence? What about all the other predictions regarding the behaviour of cosmological entities? Are they all coincidences too?
Firstly, my apologies, I do understand that it may perhaps be a bit frustrating trying to discuss this concept with a lay person. Your patience is appreciated, even if it isn't entirely obvious from the posting.
I have singled this point out, because it may allow us to narrow the focus of the discussion, reducing the need for a deeper familiarity with General Relativity (plus I lost my other response, but will reply in full later).
To clarify one thing. The contention is not that GR is a self-contained system, rather that it uses a self-contained system.
If GR experiments support the assumption that Time exists, then in those experiments it must be possible to observe time, and measure the effects various phenomena have on it - otherwise they could not be said to support the assumption.
The question is, how is time observed in those experiments? Or what is used to represent time in those experiments?
The answer to this question could negate the need for a strong familiarity with GR and its experiments.0 -
Mangaroosh, you seem very interested in this topic (and rightly so) and you are asking questions that are going beyond the standard questions people ask.
I am 99% certain that nobodies answer on this forum will satisfy you, I think the only way you'll actually get a deep understanding of the material is to study it for yourself.
I think you should watch the videos on this site, http://www.youtube.com/results?search_query=leonard+susskind+general+relativity&search_type=&aq=0&oq=leonard+susskind+g , to see what general relativity is actually like, what the world line of a photon is represented as etc...
This course utilizes the bare minimum of the math required to properly understand GR's technical details. Though I doubt it'll answer all of your questions I bet it will knock out the kinks in your thoughts about GR and cement any credible thoughts u may have for/against GR.
Contrary to what people say, I simply refuse to believe you can understand GR deeply unless you know the math behind it. You can get the general idea etc... but I've read so many laymans books on it & each one says things a little differently. It's only understandable to be confused about this subject, as I was, when you're basing all your knowledge of the subject on second hand condensation of the material...
I don't claim the course will be GR in it's complete form, nor do I claim your questions are invalid or anything I am just advising you to go for it & try learning about the subject yourself so that you can answer your own curiousity as you seem eager to find out what this theory is really saying.0 -
sponsoredwalk wrote: »Mangaroosh, you seem very interested in this topic (and rightly so) and you are asking questions that are going beyond the standard questions people ask.
I am 99% certain that nobodies answer on this forum will satisfy you, I think the only way you'll actually get a deep understanding of the material is to study it for yourself.
I think you should watch the videos on this site, http://www.youtube.com/results?search_query=leonard+susskind+general+relativity&search_type=&aq=0&oq=leonard+susskind+g , to see what general relativity is actually like, what the world line of a photon is represented as etc...
This course utilizes the bare minimum of the math required to properly understand GR's technical details. Though I doubt it'll answer all of your questions I bet it will knock out the kinks in your thoughts about GR and cement any credible thoughts u may have for/against GR.
Contrary to what people say, I simply refuse to believe you can understand GR deeply unless you know the math behind it. You can get the general idea etc... but I've read so many laymans books on it & each one says things a little differently. It's only understandable to be confused about this subject, as I was, when you're basing all your knowledge of the subject on second hand condensation of the material...
I don't claim the course will be GR in it's complete form, nor do I claim your questions are invalid or anything I am just advising you to go for it & try learning about the subject yourself so that you can answer your own curiousity as you seem eager to find out what this theory is really saying.
Thanks a million for the response. I will most definitely check out the link. I may be back with a few questions.
Just one question though, and hopefully you may be able to answer it, because it is, I think, a relatively basic question.
In GR experiments, what is used to represent time, or used to detect the effects various phenomena have one time?
Is it a clock that is used, or is there some other means of detecting/measuring time.
p.s. This is a genuine question0 -
mangaroosh wrote: »Thanks a million for the response. I will most definitely check out the link. I may be back with a few questions.
Just one question though, and hopefully you may be able to answer it, because it is, I think, a relatively basic question.
In GR experiments, what is used to represent time, or used to detect the effects various phenomena have one time?
Is it a clock that is used, or is there some other means of detecting/measuring time.
p.s. This is a genuine question
Many experiments do not use clocks at all. Gravitational lensing phenomena like the einstein cross would be examples of these. Or the orbits of planets.
Other experiments use periodic systems to measure time, but phenomena like time dilation are found to be independent of the specific type of system used.
Other experiments implicitly involve special relativity with no clocks. Particle physics experiments would be examples of these.The question is, how is time observed in those experiments? Or what is used to represent time in those experiments?
Time is not "observed", but rather inferred.0 -
sponsoredwalk wrote: »Mangaroosh, you seem very interested in this topic (and rightly so) and you are asking questions that are going beyond the standard questions people ask.
I am 99% certain that nobodies answer on this forum will satisfy you, I think the only way you'll actually get a deep understanding of the material is to study it for yourself.
I think you should watch the videos on this site, http://www.youtube.com/results?search_query=leonard+susskind+general+relativity&search_type=&aq=0&oq=leonard+susskind+g , to see what general relativity is actually like, what the world line of a photon is represented as etc...
This course utilizes the bare minimum of the math required to properly understand GR's technical details. Though I doubt it'll answer all of your questions I bet it will knock out the kinks in your thoughts about GR and cement any credible thoughts u may have for/against GR.
Contrary to what people say, I simply refuse to believe you can understand GR deeply unless you know the math behind it. You can get the general idea etc... but I've read so many laymans books on it & each one says things a little differently. It's only understandable to be confused about this subject, as I was, when you're basing all your knowledge of the subject on second hand condensation of the material...
I don't claim the course will be GR in it's complete form, nor do I claim your questions are invalid or anything I am just advising you to go for it & try learning about the subject yourself so that you can answer your own curiousity as you seem eager to find out what this theory is really saying.
Just starting the Susskind lectures now, cheers for the link by the way. Unfortunately I have fairly limited internet access where I am, so can't watch youtube vids, so it'll take me a while to get through 'em.
Just wondering if anyone has watched the Lee Smolin - The Problem of Time in Quantum Gravity and Cosmology lectures.
My math wouldn't be of the required level so I was unable to follow it, but was just wondering if anyone knew what came out of the lectures. I don't think Smolin backs up the notion of time being non-existent, but I was just interested to see what came out of the lectures.0 -
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mangaroosh wrote: »Just starting the Susskind lectures now, cheers for the link by the way. Unfortunately I have fairly limited internet access where I am, so can't watch youtube vids, so it'll take me a while to get through 'em.
Just wondering if anyone has watched the Lee Smolin - The Problem of Time in Quantum Gravity and Cosmology lectures.
My math wouldn't be of the required level so I was unable to follow it, but was just wondering if anyone knew what came out of the lectures. I don't think Smolin backs up the notion of time being non-existent, but I was just interested to see what came out of the lectures.
I haven't watched the lectures but I have read his book. I'll try and watch some of the lectures and see what they're about.0 -
Interesting article on the issue:
http://www.universetoday.com/2010/06/12/astronomy-without-a-telescope-is-time-real/0 -
I think that if time didn't exist then everything would have to be instantaneous.
Mostly because without time then motion couldn't take any time.Implying that any motion would have to be instantaneous.(Very weird saying that.)
Which would mean that any rest would have to be at rest permanently because it would take an infinite amount of energy to accelerate to that speed.
Which in itself would prove that time exists.0 -
Is this thread half philosophy and half physics? it seems a bit muddy ....
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I think that the Universe is simply a three-dimensional object made up of a substance (space) that is subject to constant change (expansion).
For some reason, space is under constant stress and in order to achieve equilibrium it must increase the volume of the universe to infinity (which cannot happen).
But it takes a long time not to happen.
Time, like distance or weight, is not an intrinsic quality of the Universe; it's a measuring system and like metres and kilograms help us to asses, identify and label certain characteristics of physical processes.
And all system of measurement are relative in that they refer to what are essentially arbitary units. The metre, kilogram or second give us different types of yardsticks and it has been shown the they can all be combined iby physics in calculations concerning mass, distance and speed and enable us to mathematically model the world around us.
So, we live in a 3-D world but then we noticed something odd; if you drop a plate and it breaks then collect the pieces together and drop them again... no matter how many times you do this, the plate gets more broken each time; it never returns to its original, organised form.
This 'tendency' to become more broken seems to be an intrinsic quality of the Universe and perhaps it is the only one. Moreover, if plates weren't picked up and dropped then eventually they still end up as dust.
What is it that causes things to decay? Change. Constant change is what. At a sub-atomic level in the plate there are systems of energy that interact with each other and the energy distribution is gradually converting some of the mass of the plate into photons and free electrons, molecular bonds are broken and the plate falls apart.
Now we find that it is necessary to add concepts such as rate of change into our intellectual toolkit. Understanding rate of change gives rise to predictive theories which have shaped the world around us today.
Also, we noticed that heating and cooling can have an effect on how quickly something changes but, and this is the useful but, the speed at which something changes can tell us something about the temperature of that something.
But how to measure 'rate of change'? Well, it involves counting events but that just gets you a number. In order for that number to mean anything it has to relate to another quantity. There are obvious problems with cycles per kilogram and cycles per metre isn't much better but if there were another reference that could represent 'quickness'...
The second is as real as a kilogram or metre and all are simply standardised references by way of which we can relate events.
When we measure rate of change what we are doing is acually comparing two rates of change. One is a reference, the clock, and the other is being measured. You could say that the 'time' of one clock is compared to the 'time' of another. And the reference clock becomes a tape-measure of sorts.
However, if the Universe has a wrist-watch I bet it shows a different time to yours.
Time as a concept allows us to view the Universe in terms of its dynamics; it allows us to predict outcomes. But there is something about the actual Universe too; there does seem to be a kind of 'progressive' nature to the it, it is constantly changing and at a certain rate; why not call it 'time'?
Any physical point can be plotted in the Universe using 3-D representation; height, length and breadth allow us to define any position in space but there is a fourth dimension, in and out - contraction/expansion. That fourth dimension represents the direction of change, which in an expanding Universe would be along the 'out-axis'.
Now this direction of change allows us to plot not only present position but also likely past or likely future positions too. This fourth axis allows us to measure rate of change in a manner that emulates the way the Universe does it; change happens in that direction so physics should model it.
But why not call it 'time'? It may be, and I tend to hold this view, that there is no actual thing called time but a process that behaves as a direction of change, hot to cold, is very similar in quality to the direction of distance; 'distant future' actually means 'as the consequence of a great deal of change' and time labels that direction in a way that relates to its behaviour. Why not call it time?
I do see a bit of a problem though; by accepting the space/time view presently accepted by physics as a complete description we may miss some other fundamental truth.
My instincts tell me that time is the effect caused by expansion and that expansion is governed by heat distribution; space expands faster in some areas than in others. The presence of energy and mass has an effect too.
But expansion: if space interacts with itself to cause expansion then that implies that there is a certain amount of energy contained in any given volume of space. This energy is ultimately what drives all the mechanisms of the Universe.
The volume of the Universe in increasing over time; it is bigger now as you read this than it was now as I wrote it. And on average, this rate of change is constant; rate of change in the Universe may be quantizable. If it is then the scale of time could relate to a highly accurate universal clock against which all things can be measured.
But it is the notion of time passing that I have a problem with. If rate of change in the volume of the Universe is related to time then the fourth-dimension is related to the size of the Universe; the time axis extends from the big bang, the beginning of time, all the way to the end of time which would be exactly the same if it was recalibrated to represent the size of the Universe.
Maybe time doesn't exist but expansion does? Maybe the time arrow doesn't point to the future of the Universe but to the edge of the Universe? It may be the case that rather than observing the passage of time, we are simply experiencing the stretching of 'now', the Universe as it it.
Perhaps the t-axis is actually the radius of the Universe, getting longer as it expands; a real direction with real distance just like the other three dimensions. Would that change anything?
All the calculations would still work but by thinking of the t-axis in terms of time may obfuscate the much bigger picture.
Since time and expansion are so closely linked and alike in nature, it may be that GR and QM have inadvertently accounted for a phenomenon that hasn't been investigated on account of it.
I kind of see what Roosh is trying to say and I would answer that in my opinion, the thing that is measured by clocks is the radius of the Universe.
The Universe may not be comfortable with 'now' but it cannot escape it; eternity.
In fact, you could say that the Universe exists due to its tendency to not be 'now'.:)0 -
I just wanted to add something about the way that the three dimensions of space differ from the fourth dimension. Although theoretically all four axes could extend to infinity there are only degrees of freedom in the three dimensions of space. The fourth allows no degrees of freedom; the Universe expands regardless.
Perhaps this could account for things like entropy and the direction of time?0 -
himnextdoor: The reason presentism, the notion that there is a unique 3D space of events defined as the present, is rejected is because it cannot account for simultaneity.0
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himnextdoor: The reason presentism, the notion that there is a unique 3D space of events defined as the present, is rejected is because it cannot account for simultaneity.
I think hysteresis can account for that; a cumulative knock-on effect that increases over distance. A bit like space-expansion.
But I have a problem with length-contraction. Do you seriously believe that a ladder that is falling lengthways into a black hole experiences length-contraction in any frame of reference?
I think that this is crucial. A ladder that is accelerating due to gravity will feel a larger force at the front than it would from the rear. Information cannot be instantly transferred and so the ladder must 'stretch', not contract.
Conversely, if the ladder is accelerated from the rear then it would contract. Matter accelerated by an explosion suffers length contraction while matter accelerated through gravity experiences stretching.
How about the clock that is falling into a black hole; does it tick slower and slower until it stops, frozen in time? Will it never be ripped apart and converted into photons by the black hole?
I reckon that any observer in any frame of reference would see the clock/ladder/spaceship accelerate (red-shift), stretch in the direction of gravity and reach incandescence long before it was red-shifted out of viewability and in a short space of time.
And can I ask you this; suppose that there is a star in our galaxy located twenty light-years away that goes supernova. It immediately releases a burst of gamma-radiation that will arrive at earth in twenty years time and will melt the entire surface of the planet. Twenty years later, equipment on earth registers the supernova, science gets excited then the earth melts. In what inertial frame would an observer be confused as to the sequence of events? In what frame of reference would the earth melt before the star went supernova? By observing the earth melting you are observing the effect of the supernova.
If the earth is in the light-cone of the supernova then how can any observer be in the earth's ligh-cone and not the supernova's?0 -
himnextdoor wrote: »I think hysteresis can account for that; a cumulative knock-on effect that increases over distance. A bit like space-expansion.
How?But I have a problem with length-contraction. Do you seriously believe that a ladder that is falling lengthways into a black hole experiences length-contraction in any frame of reference?
I think that this is crucial. A ladder that is accelerating due to gravity will feel a larger force at the front than it would from the rear. Information cannot be instantly transferred and so the ladder must 'stretch', not contract.
Conversely, if the ladder is accelerated from the rear then it would contract. Matter accelerated by an explosion suffers length contraction while matter accelerated through gravity experiences stretching.
A ladder accelerated by a uniform gravitational field would contract. A ladder falling into a black hole would experience "spaghettification".How about the clock that is falling into a black hole; does it tick slower and slower until it stops, frozen in time? Will it never be ripped apart and converted into photons by the black hole?
I reckon that any observer in any frame of reference would see the clock/ladder/spaceship accelerate (red-shift), stretch in the direction of gravity and reach incandescence long before it was red-shifted out of viewability and in a short space of time.
It depends on your frame of reference. From our perspective, the watch would never fall into the black hole.And can I ask you this; suppose that there is a star in our galaxy located twenty light-years away that goes supernova. It immediately releases a burst of gamma-radiation that will arrive at earth in twenty years time and will melt the entire surface of the planet. Twenty years later, equipment on earth registers the supernova, science gets excited then the earth melts. In what inertial frame would an observer be confused as to the sequence of events? In what frame of reference would the earth melt before the star went supernova? By observing the earth melting you are observing the effect of the supernova.
None. Causality is preserved. It is only events which are not causally linked that can have different ordering in different frames.If the earth is in the light-cone of the supernova then how can any observer be in the earth's ligh-cone and not the supernova's?
The earth (specifically, the event of the earth melting) is in the light-cone of the supernova. All observers, wherever they are and however they are moving, agree with this.0 -
A ladder accelerated by a uniform gravitational field would contract. A ladder falling into a black hole would experience "spaghettification".
What if the black hole was spinning?
Or what if the ladder was tumbling into the blackhole?
And just another question - not really related - if you were at the dead centre of the earth, how much gravity would you experience?0 -
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What if the black hole was spinning?
Or what if the ladder was tumbling into the blackhole?
Nothing too exotic would happen as far as I can tell. The ladder would be subject to tidal forces if it was tumbling, which would dissipate energy as heat (the classic example of this process in our solar system is Io, Jupiter's moon).
The same goes for a spinning black hole. The only bit of exotic physics I know of is the Kerr metric associated with spinning black holes is sometimes used to study solutions that include wormhole structures.And just another question - not really related - if you were at the dead centre of the earth, how much gravity would you experience?
No matter where you are, if you are in free-fall, you don't experience Earth's gravity. The weightlessness experienced by astronauts is not because they are far from earth. It is because they are freely falling.
As for your motion, if you were at the dead centre of the earth (and assuming the earth had perfect radial symmetry) you would not be moving at all.0 -
How?
I'm not sure what you mean by 'How?'.
I'm referring to the fact that the input always lags the output. You could regard the path travelled by a photon as a finite number of devices that accept the photon at one side and then pass it on to the the next device on the other, all arranged at equal distances along the path of the photon. In this way we can see how space introduces latency to photon travel, each device introducing a little more delay. If space were incompressible, the light would travel instantaneously; it would be like a rigid bar in all directions; energy applied to one point would appear at all points.
And the perceived latency is affected by motion; if an event detector is travelling away from the source then it will appear that fewer events take place and vice versa.
To use a clockwork analogy, imagine a system of large cogs arranged side by side in a line and are interlocked. When energy is applied to the first cog, it has to move a finite distance before it meshes properly such that it can overcome the inertia of the next one. Same when the second meshes with the third and so on. All these 'finite distances' add up to a period of transmission time that has been introduced by, and is related to the length of, the communication system itself and the cog furthest away turns last. This is what I mean by hysteresis; each cog will assign a different time for the application of force and will do so in the order of distance from the energy source or, if you will, event.
Two events could be considered as the application of energy at each end of the line, two motors that are switched on at the same time. Only the central cog will receive energy from both ends at the same time. If the motors are turned on at different times it will be a different cog that will receive energy from both ends at the same time and it might consider itself the central cog in a line that is longer than the actual line of cogs.
In other words, no individual cog could give a reliable and conclusive
eyewitness account as to simultaneity. But a survey of the cogs might.
Is that what you meant?A ladder accelerated by a uniform gravitational field would contract. A ladder falling into a black hole would experience "spaghettification".
What's the difference between 'a uniform gravitational field' and the gravity of a black hole?It depends on your frame of reference. From our perspective, the watch would never fall into the black hole.
I'd love to know why you think that.
Why would something falling into a black hole accelerate and then 'stop' at some notional edge? Surely at best we could lose sight of the watch as it becomes so red-shifted as to make detectable only at infinitely long wavelengths. If you were travelling away from the watch then this would happen sooner than if you were travelling toward the watch which could also allow you to see the watch destroyed. Just before you are.
For a stationary observer, I would expect the object to become darker and darker as it gets further and further until it becomes unseeable.
Surely a watch stopping at the event horizon has implications for entropy?None. Causality is preserved. It is only events which are not causally linked that can have different ordering in different frames.
Thank goodness for that.
The earth (specifically, the event of the earth melting) is in the light-cone of the supernova. All observers, wherever they are and however they are moving, agree with this.
And that. :cool:0 -
As for your motion, if you were at the dead centre of the earth (and assuming the earth had perfect radial symmetry) you would not be moving at all.
And if you were at the dead centre of a black hole, what happen to you? Would you experience any gravity at all, would it cancel out?
Newtons gravitation law gives the force as a relationship between two masses, and the distance between them.
But in objects - say something as large as a planet - where is the maximum net gravity, if gravitational forces in opposite directions cancel each other?
Does pressure decrease as you approach the centre of a planet?
Say you had a planet, made completely of fluid, where would the pressure peak?0 -
And if you were at the dead centre of a black hole, what happen to you? Would you experience any gravity at all, would it cancel out?
Newtons gravitation law gives the force as a relationship between two masses, and the distance between them.
But in objects - say something as large as a planet - where is the maximum net gravity, if gravitational forces in opposite directions cancel each other?
Does pressure decrease as you approach the centre of a planet?
Say you had a planet, made completely of fluid, where would the pressure peak?
Well yes, you would experience gavity at the centre of anything - gravity is what would keep you there.
If you were above the planet you would weigh less than you would at the surface where you weigh less that if you were at the centre. (I think)
And the value of 'g' would increase toward the centre too. The entire weight of the world is pressing on the centre which implies high density and pressure; high temperature. I do believe that gravity is what keeps the core at a very high temperature. A golfball at the centre of the earth would experience a pressure of around 5.954 x 10^24 pounds per square inch. That could generate enough heat to power a volcano.
Since you would accelerate at an increasing rate all the way to the centre and from that point would begin to decelerate then it would seem that the accelerating force increases all the way to the centre.0 -
himnextdoor wrote: »Well yes, you would experience gavity at the centre of anything - gravity is what would keep you there.
I had never really thought about this until a few months ago, reading Bill Bryson's a brief history of everything. Imagine if there were a cavity at the centre of the earth. If you were there, according to Bryson, all the gravity of the earth cancels out. You're now in whatever free fall the earth is in, you're neither falling towards the earth or away from it. As you're at the centre.
I wasn't happy with Bryson's explanation.
Newton's equation gives the force for two objects at a distance from each other. It's just two objects. You don't have to consider what angle they're at from each other - and since they're at a distance, all you have to consider is the effect of each's net gravity.
Now, if you make it a little more complicated ........let's have three objects in a line: A - B - C..........If A and C are identical. And B is in the dead centre between the two, the A and C's gravitational force on B will cancel out.
The oceans experience less of the earth's gravity when the moon is directly above them. Under the moon, water is lighter, so the oceans can expand. Everywhere there is a sea, you can see this happen everyday.
If you're in a cavity within a planet, anywhere, the gravitation you experience will be the net gravity from all directions.If you were above the planet you would weigh less than you would at the surface where you weigh less that if you were at the centre. (I think)
Yes, the force is inversely proportional to the square of the distance between two masses. The further away you go, the weaker the gravity is.
To escape the earths gravity, your escape velocity must be Mass.EscapeV = Mass.GravityatSurface......The further you get away, the lighter your rocket will be, the less power you need to avoid falling back to earth.And the value of 'g' would increase toward the centre too.
Would it? I think in Newton's law, the distance is considered the distance from the centre of the masses. If you're on the surface of the planet, the distance for the sake of the equation is considered the distance between you and the centre of the planet. However, if you're in a cavity within the planet, you have mass exerting a gravitational pull on you from all directions.
And this is where I'm curious - mass will fall towards the direction of the net gravitational force. The net force also determines its' weight (in terms of force). Where in a planet is the gravity at its maximum? Is it midway through the planet - if Bryson is right, it can't be at the dead centre. And for the same reason, the maximum gravity can not be just off centre.The entire weight of the world is pressing on the centre which implies high density and pressure; high temperature.
Is it pressing on the centre?I do believe that gravity is what keeps the core at a very high temperature. A golfball at the centre of the earth would experience a pressure of around 5.954 x 10^24 pounds per square inch. That could generate enough heat to power a volcano.
That would imply the hottest part of the earth is the core. The core is solid. Around the core is liquid.
If the net gravity, is less near the centre, then the pressure would also have to be lower. Unless there is another force dragging it in.Since you would accelerate at an increasing rate all the way to the centre and from that point would begin to decelerate then it would seem that the accelerating force increases all the way to the centre.
You couldn't be accelerating at an increasing rate. The more mass you put behind you, the more that mass pulls on you and decelerates you.0 -
And if you were at the dead centre of a black hole, what happen to you? Would you experience any gravity at all, would it cancel out?
Newtons gravitation law gives the force as a relationship between two masses, and the distance between them.
But in objects - say something as large as a planet - where is the maximum net gravity, if gravitational forces in opposite directions cancel each other?
Does pressure decrease as you approach the centre of a planet?
Say you had a planet, made completely of fluid, where would the pressure peak?
A planet made of fluid would have a pressure peak at the centre. Even though the net force of gravity at the centre is zero, the surrounding mass is being pushed towards it. It is this mass that is responsible for the pressure.
Being at the centre of a black hole is hard to describe, as the singularity has no classical (or even quantum) description known to us.0 -
himnextdoor wrote: »I'm not sure what you mean by 'How?'.
I'm referring to the fact that the input always lags the output. You could regard the path travelled by a photon as a finite number of devices that accept the photon at one side and then pass it on to the the next device on the other, all arranged at equal distances along the path of the photon. In this way we can see how space introduces latency to photon travel, each device introducing a little more delay. If space were incompressible, the light would travel instantaneously; it would be like a rigid bar in all directions; energy applied to one point would appear at all points.
And the perceived latency is affected by motion; if an event detector is travelling away from the source then it will appear that fewer events take place and vice versa.
To use a clockwork analogy, imagine a system of large cogs arranged side by side in a line and are interlocked. When energy is applied to the first cog, it has to move a finite distance before it meshes properly such that it can overcome the inertia of the next one. Same when the second meshes with the third and so on. All these 'finite distances' add up to a period of transmission time that has been introduced by, and is related to the length of, the communication system itself and the cog furthest away turns last. This is what I mean by hysteresis; each cog will assign a different time for the application of force and will do so in the order of distance from the energy source or, if you will, event.
Two events could be considered as the application of energy at each end of the line, two motors that are switched on at the same time. Only the central cog will receive energy from both ends at the same time. If the motors are turned on at different times it will be a different cog that will receive energy from both ends at the same time and it might consider itself the central cog in a line that is longer than the actual line of cogs.
What you are talking about is the "world picture" of an observer. I.e. An observer sees two motors start simultaneously, but concludes that they started at different times since they are at different distances, and hence the information has been travelling for different amounts of time. Information propagates through space-time at a finite speed. This is already incorporated into relativity.
The simultaneity issue is different. It persists even after the propagation of information is taken into account. If two observers are at the same location, they can disagree about the simultaneity of events.In other words, no individual cog could give a reliable and conclusive
eyewitness account as to simultaneity. But a survey of the cogs might.
Is that what you meant?
A survey of all observers removes the very notion of simultaneity. I.e Relativity says any account of space by itself is relative, and any account of time by itself is relative, and only the combined picture of space and time gives a physical, invariant dscription.What's the difference between 'a uniform gravitational field' and the gravity of a black hole?
There is no curvature in a uniform gravitational field.
Weyl Curvature
Ricci CurvatureI'd love to know why you think that.
Why would something falling into a black hole accelerate and then 'stop' at some notional edge? Surely at best we could lose sight of the watch as it becomes so red-shifted as to make detectable only at infinitely long wavelengths. If you were travelling away from the watch then this would happen sooner than if you were travelling toward the watch which could also allow you to see the watch destroyed. Just before you are.
Gravitational time dilation "slows" the descent of the object. More rigorously, we say it takes an infinite amount of co-ordinate time for the body to fall into the black hole. Remember that time dilation is not just a mechanical affect. It is an expression of the very geometry of events, so "falling" is as equally affected as "ticking".For a stationary observer, I would expect the object to become darker and darker as it gets further and further until it becomes unseeable.
Yes, this happens. Light becomes so red-shifted (exponentially so as the faller approaches the event horizon) that the falling body becomes invisible.Surely a watch stopping at the event horizon has implications for entropy?
Yep.
http://en.wikipedia.org/wiki/Black_hole_thermodynamics0 -
A planet made of fluid would have a pressure peak at the centre. Even though the net force of gravity at the centre is zero, the surrounding mass is being pushed towards it. It is this mass that is responsible for the pressure.
This what I thought before - but when I start thinking about it. If the net gravity at the centre is zero. I wish I hadn't forgotten all my maths - even my basic maths, I could model this - I need a little practice. If the net gravity at the centre is zero - then there must be somewhere else, where the net gravity is at its' maximum. I'm guessing this is at mid way point of the radius.
Actually, I realise this becomes harder to model. I would expect the maximum pressure at the gravity maximum. Water, at the bottom of the ocean, is at its densest, and highest pressure. It's not simply there because it's there, it's there because gravity took it there. It the earth was made of liquid (and not spinning) I'm guessing, beyond the gravity maximum, as far as the water is concerned the world turns upside and it gets less dense as it approaches the centre - even allowing for a gas cavity at the centre.
I am I gone crazy?
If the gravity maximum, cannot be at the centre - why would the pressure maximum be there?
I know it's assumed that the pressure is at it's maximum at the centre of the sun - is it?Being at the centre of a black hole is hard to describe, as the singularity has no classical (or even quantum) description known to us.
There is an assumption, that black holes start life as recognisable mass - collapsing stars etc.
If they're spherical - which they are - if the singularity happens at a critical point where - say a mass collapsing to its' Schwarschild radius, I'm wondering, if there are gravity maxima inside the mass, could they reach their critical gravity/density, before the outer radius - and you could have a singularity within a singularity.0 -
I had never really thought about this until a few months ago, reading Bill Bryson's a brief history of everything. Imagine if there were a cavity at the centre of the earth. If you were there, according to Bryson, all the gravity of the earth cancels out. You're now in whatever free fall the earth is in, you're neither falling towards the earth or away from it. As you're at the centre.
I wasn't happy with Bryson's explanation.
Such a cavity would need to be strongly reinforced to prevent collapse.
Gravity doesn't 'cancel out' any more than a million laser-beams focussed to a point cancel out. The energy is real but in all directions and this creates a 'crushing' force; the centre is maintained by this force causing is to resist motion relative to the surface of the planet.
Bill is right about one thing though; if you were at the centre of the earth, you would be in whatever free-fall the earth was in but... where on earth could you be in order not to be in free-fall with the earth?Newton's equation gives the force for two objects at a distance from each other. It's just two objects. You don't have to consider what angle they're at from each other - and since they're at a distance, all you have to consider is the effect of each's net gravity.
Now, if you make it a little more complicated ........let's have three objects in a line: A - B - C..........If A and C are identical. And B is in the dead centre between the two, the A and C's gravitational force on B will cancel out.
No cancellation as such; A and C accelerate B equally in opposite directions alright but the stresses caused by gravity are still acting on B which will be stretched in both directions. I the gravity of A and C were very large but equal then B could be ripped apart. Not exactly cancellation.The oceans experience less of the earth's gravity when the moon is directly above them. Under the moon, water is lighter, so the oceans can expand. Everywhere there is a sea, you can see this happen everyday.
Again, no. The oceans simply experience the moon's gravity in addition to the earth's.If you're in a cavity within a planet, anywhere, the gravitation you experience will be the net gravity from all directions.
That's true anywhere but that 'net' gravity will act in one particular direction.Would it? I think in Newton's law, the distance is considered the distance from the centre of the masses. If you're on the surface of the planet, the distance for the sake of the equation is considered the distance between you and the centre of the planet. However, if you're in a cavity within the planet, you have mass exerting a gravitational pull on you from all directions.
And this is where I'm curious - mass will fall towards the direction of the net gravitational force. The net force also determines its' weight (in terms of force). Where in a planet is the gravity at its maximum? Is it midway through the planet - if Bryson is right, it can't be at the dead centre. And for the same reason, the maximum gravity can not be just off centre.
If you're in a cavity below the surface of the earth then whatever it is you are stood on becomes the surface and your equation would need to be modified to reflect the new distance between you and the centre of mass.
You seem to suggest that Bill Bryson thinks that the centre of mass is outside the centre of the planet; that would mean that the centre of mass is actually a spherical shell that encloses the central region of planets. This in turn would mean that not only does gravity pull away from the surface, it pulls away from the centre too. It would mean the points of highest density would be away from the centre and therefore density would decrease between the centre of gravity and the true centre of the planet.
Rather than thinking of the centre of mass in terms of balanced forces, think of it in terms of being the bottom.
The centre of mass is the point where acceleration due to gravity is equal in all directions; this point cannot be away from the centre.
Let me ask you this; if the maximum force of gravity isn't dead centre (mass-wise) then how would fusion occur in stars? How could planets form?Is it pressing on the centre?
Yes, or rather, toward the centre. If you were able to fall through the centre of the earth, your momentum would take you a certain distance beyond the cantre and you would fall toward the centre again, oscillating, or bouncing, about a mean until you settled at the centre. (Not somewhere away from the centre.)That would imply the hottest part of the earth is the core. The core is solid. Around the core is liquid.
Which is the hottest part of a kettle in the process of boiling water, the water or the heating element?If the net gravity, is less near the centre, then the pressure would also have to be lower. Unless there is another force dragging it in.
It is not 'less' at the centre it is the sum of the forces from all directions which is larger.You couldn't be accelerating at an increasing rate. The more mass you put behind you, the more that mass pulls on you and decelerates you.
I suppose that is sort of right in a way.
If you start at the edge of a gravitational field in freefall then your rate of acceleration will increase over time. It would a long time before you reached reached the viscinity of earth and achieved an acceleration of 9.8 m/s/s.
As you fall below the surface of the planet, the rate of increase slows down as less mass exerts a pull on you and I would argue that the local mass behind (above?) you does not exert a decelerating force at all and if it did then the increasing density of the local mass in front of (below?) you would negate the effect.
It is not until you pass the centre until you begin to decelerate because the majority of the mass is then behind you after that point.0 -
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That would imply the hottest part of the earth is the core. The core is solid. Around the core is liquid.
The hottest part of the Earth is the inner core (about 5500 C, similar to the surface of the sun). It is solid because of the enormous pressure acting on it, not because it is cooler than the outer core.0
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