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ghostgirl wrote:Take my eyes (or possibly my mind, if we want to more accurately describe the point at which "I" become aware of the phenomena) as being the receptacle of an incoming beam of light, travelling towards me from an external source - so that my eyes/mind becomes the destination, as it were, - and let's say that this beam of light takes 5 minutes to reach me from it's source. Now let's imagine another beam of light reaching me from a source that's a much greater distance away, and coming from the opposite direction. And then let's add yet more beams of light, all coming from every direction imaginable, and all reaching Me from some random distance away, but of course all reach me simultaneously. Does this scenario imply that my eyes/mind - i.e. the destination - is/are 'the present', and that any physical distance outwards from my eyes/mind, is to be considered to be at least some time in the past, effectively making Me the centre of the Universe, or, rather, I should say, the most recent Time in the Universe?


Yes. In reality, everybody is the centre of the universe. It's often said that there is no centre of the universe, but that's actually a bit misleading because there is a centre of the universe, and it's everywhere.

Really, though, all those events happen at different points in the past and, when they reach you, it's only 'now' (the present) for you. More importantly, for other observers at the other ends of those light beams, time is moving at completely different rates than it is for you. Also, you've hit upon the major intuitive feature of spacetime that foxed us all until Einstein, namely simultaneity. It doesn't really exist, except from the perspective of a single observer, and relativity teaches us, above all other things, that there is no preferred reference frame. That's what the 'relative' means in relativity. It actually harks back to Galileo, but that digression would take us a bit off topic for the moment. Maybe I'll come back to it.

Anyhoo, Brian Greene uses a nice little analogy to deal with this, namely a loaf of bread. Your 'now' is simply a feature of the angle at which you cut the bread. You might think of time as a series of slices on the loaf of spacetime thus:

Image

Where the slices represent everything that's happening 'now' from your perspective. The length of the loaf represents time, while the width of the loaf represents space. However, what's really happening is that you experience time more like this:

Image

so that events in the past appear on your 'now' slice. If you extend that image so that the width is large, you can see that the separation between events gets even larger. It's much like a pair of scissors. On scissors with short blades, the tips of the blades will be reasonably close together for any degree of opening at the pivot. For scissors with much longer blades, the tips will be much further apart for the same degree of opening at the pivot. So, the further away something is, the further back in time it is, because of the angle of slice on the spacetime loaf.

Now we can bring motion into the picture. All motion warps spacetime. If you are in motion, time slows down, because you are using up some of your time motion for motion through space. Similarly, distances through space are contracted, and it is these two features that allow you to always measure the same speed for light, regardless of your motion. Time and space for you will seem to be exactly the same, and your measuring devices will measure the same passage of time and the same distances, but an external observer will see your distances (and your measuring devices) shorten in the direction of travel.

Going back to the loaf, let's imagine that you are separated from another observer by a huge distance, say 10 billion light years, but that you both agree on what time it is. In other words, you are sharing 'now' and agree on it. If you stand up and walk away from the other observer at 10 miles an hour, you will suddenly be observing him at 150 years before the now you agreed on. Similarly, if you stand up and walk toward him at the same speed, you will be observing 150 years into his future.

And another thing...

Let's say that my eyes are currently observing a photograph of a lit candle framed against a distant star-filled night sky, - a photograph is generally assumed to capture a single instant in time, - but what is it a single instant of? The picture captures light entering the lens of the camera, but the light entering the lens from the lit candle arrives from a very short distance away, whereas the light from the distant stars behind the candle has travelled a much greater, not to mention very varied, - distance. What does this say about the developed 'image', - has it indeed 'captured an instant in time'? Or is that notion inherently inaccurate, - a lie, in fact, - since the stars I can clearly see in the picture are no longer in the same position relative to the lit candle that the image implies?


Exactly (almost). What the photograph actually captures is the 'now' of a single observer, and what it observes is always in the past of the moment when the photons were captured. Indeed, you've touched once again on something deeply counter-intuitive, until you grasp it, that is. How big is the observable universe? Well, it's thought to be about 156 billion light years across. But how can that be, if we can only see 13.7 billion light years in any direction? Well, the answer is that the most distant objects we can see are no longer where they were when their light left to get to us, because the universe is expanding.

In your example, pretty much everything we can see with the naked eye, or indeed with a camera (without very long exposures) is in our own galaxy, which is gravitationally bound, and thus not subject to cosmic expansion (yet). However, those stars in the picture are not where they were, and in some cases, they aren't there at all. Take Betelgeuse, for example. That's the bright red star on the shoulder of Orion. It's a red giant. Extremely massive stars burn up their fuel very quickly, (stars that burn twice as bright live half as long, and you have burned so very brightly, Roy). Betelgeuse is headed for a catastrophic collapse sometime very soon. Actually, it's entirely probable that it's already gone supernova, but its light takes about 642 years to reach us, so we won't know about it yet.

My question, then, becomes, "What is 'an instant' in time?" Is there is even such a thing as an instant in time? Is a stationary photon, which unfortunately blinks out of existence the very 'instant' it becomes still, nevertheless the best description of an instant that we have? If we cannot explain an instant, then what is the sum of 1 instant + 1 instant? Can we add instants together at all? Is 'motion' considered to be the sum of many discrete instants linearly strung together?


That's one of the deep questions of physics, and we simply don't know. The smallest time we can reasonably work with is the Planck second, or 5.39x10^-44 seconds. Whether or not time is divisible beyond that, or indeed if it's a continuum, is still very much an active area of research. If M-Theory is correct, that's as small a division as you can have, because it means that time is quantised at that scale.

Is a beam of light really 'in motion' at all? Does it have a speed?


The beam isn't in motion, but the constituent photons certainly are, and at the speed c.

Or is it a stream of discrete instants reaching out through space-time, not moving at all, yet removed from Me by ever greater distances? What is the difference between 'distance' and 'time'?


There is no difference, because space and time can't really be separated in this way. This is the key feature of Special Relativity, and it was the nail in the coffin of Newtonian Mechanics. Newton thought that space and time were separate entities, and that time was fixed. Einstein showed us that they are simply features of a single entity, spacetime. It is only our perception that treats them differently, but they are actually the same. The easiest way I've found to get my head around it is to think of time as the dimension through which space moves, but the same is true in reverse, in that the spatial dimensions are what time moves through.

It should be pointed out that it's entirely possible that there is more than one time dimension, albeit curled up to somewhere around the Planck scale.



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Meant to post this for you:

http://documentaryheaven.com/do-you-kno ... ime-it-is/

Brilliant stuff, and will give an insight into just what a difficult question it is.



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If anyone wants to know the real, true story of Albert Einstein then watch this film.

A complete piss take but love it. :rofl:




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Hack Wrote:
Really, though, all those events happen at different points in the past and, when they reach you, it's only 'now' (the present) for you. More importantly, for other observers at the other ends of those light beams, time is moving at completely different rates than it is for you. Also, you've hit upon the major intuitive feature of spacetime that foxed us all until Einstein, namely simultaneity. It doesn't really exist, except from the perspective of a single observer, and relativity teaches us, above all other things, that there is no preferred reference frame. That's what the 'relative' means in relativity. It actually harks back to Galileo, but that digression would take us a bit off topic for the moment. Maybe I'll come back to it.


But couldn't that be interpreted as THERE IS NO SUCH THING AS TIME, there is only DIFFERENT PLACE? What I mean is, when I look off into Space, I can't see where a star is NOW, just as someone near that star can't see where I am NOW, - simply because NOW, for Me, can only exist where I physically am. But if there was a whole string of Me's stretching from here (my 'here' that is) all the way to the star, it would, for Me, be NOW all the way to it.


Hack Wrote:
Exactly (almost). What the photograph actually captures is the 'now' of a single observer, and what it observes is always in the past of the moment when the photons were captured. Indeed, you've touched once again on something deeply counter-intuitive, until you grasp it, that is. How big is the observable universe? Well, it's thought to be about 156 billion light years across. But how can that be, if we can only see 13.7 billion light years in any direction? Well, the answer is that the most distant objects we can see are no longer where they were when their light left to get to us, because the universe is expanding.


The Doppler Shift would seem to indicate that this is so, but in the world of physics are there any legitimate arguments against an expanding Universe? How is the Red Shift of a distant star affected by the fact that I am moving too, and perhaps not at the same speed as the star? Am I moving at a constant speed, or am I accelerating, - or perhaps de-ccelerating?


Hack Wrote:
There is no difference, because space and time can't really be separated in this way. This is the key feature of Special Relativity, and it was the nail in the coffin of Newtonian Mechanics. Newton thought that space and time were separate entities, and that time was fixed. Einstein showed us that they are simply features of a single entity, spacetime. It is only our perception that treats them differently, but they are actually the same. The easiest way I've found to get my head around it is to think of time as the dimension through which space moves, but the same is true in reverse, in that the spatial dimensions are what time moves through.


So is Time, or rather an Instant in Time, really just a set of empty co-ordinates in Space? I say a set of co-ordinates because I don't mean an Elementary Particle, since even an Elementary Particle (if such a thing exists) would be bigger than an an empty set of co-ordinates (i.e. even an Elementary Particle, as soon as it comes into existence would have to be at least 3 dimensional, and therefore would require it's own multiple sets of measurements/co-ordinates, if that makes any sense?


Hack Wrote:
The beam isn't in motion, but the constituent photons certainly are, and at the speed c.


What I meant was, is a photon definitely in motion, or could it only appear to be because during every 'step' of it's motion, it's not, it's actually a new stationary photon? (Oh god it's really hard to put into words what I'm thinking here?!?)

P.S. Thanks. Oh and I'll watch the "DO YOU KNOW WHAT TIME IT IS?" vid as soon as I, uh, get time. :groan:

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ghostgirl wrote:But couldn't that be interpreted as THERE IS NO SUCH THING AS TIME, there is only DIFFERENT PLACE? What I mean is, when I look off into Space, I can't see where a star is NOW, just as someone near that star can't see where I am NOW, - simply because NOW, for Me, can only exist where I physically am. But if there was a whole string of Me's stretching from here (my 'here' that is) all the way to the star, it would, for Me, be NOW all the way to it.


Well, there are certainly people who think that time is an illusion, but I'm not one of them. There is at least one physicist in the Brian Cox vid who thinks so. In reality, though, you're still treating time and space as separate, and it's been experimentally validated that that's the wrong way to think about it. In any event, if there is no such thing as time, there's no such thing as NOW.

The Doppler Shift would seem to indicate that this is so, but in the world of physics are there any legitimate arguments against an expanding Universe?


I haven't come across any. I've come across arguments, but none have been valid, and all have been arguing from a position of dogmatic ignorance.

How is the Red Shift of a distant star affected by the fact that I am moving too, and perhaps not at the same speed as the star?


It's shifted further, toward the red if you're moving away, toward the blue if you're moving toward, although by the tiniest amount at the speeds you can manage. There are quite a few things that can cause red shift (not strictly Doppler, as that's specific to sound, although the principle is precisely the same), such as motion, gravity, etc, but all are accounted for in observation of the expansion of the cosmos.

Am I moving at a constant speed, or am I accelerating, - or perhaps de-ccelerating?


A couple of points. Firstly, the third is redundant. There's no need for the word 'decelerating', and it's only entered the language through the vernacular. Slowing down is actually accelerating. In physics, acceleration is defined as any change in velocity. Velocity is a vector quantity, which means that it contains a speed component and a direction, as opposed to a scalar quantity. So, any change in velocity, including turning, speeding up and slowing down, is acceleration.

Special Relativity deals with observers in constant motion, your first case, and General Relativity deals with accelerated motion (that's why it deals with gravity, because being immersed in a gravitational field is precisely equivalent to acceleration; that was the core breakthrough in formulating General Relativity, in fact).

In short, it doesn't actually matter, because the theory deals with all cases. It's why GPS satellites have relativistic correction, and why it works. Time runs faster at the altitude that GPS satellites orbit by about 45 microseconds a day. It also moves slower for the satellites by about 7 microseconds a day due to their orbital velocity. So GPS satellites correct at a rate of 39 microseconds a day. That doesn't sound like much, but it will put you off course by about 10 metres per day if GPS weren't thus corrected.

So is Time, or rather an Instant in Time, really just a set of empty co-ordinates in Space? I say a set of co-ordinates because I don't mean an Elementary Particle, since even an Elementary Particle (if such a thing exists) would be bigger than an an empty set of co-ordinates (i.e. even an Elementary Particle, as soon as it comes into existence would have to be at least 3 dimensional, and therefore would require it's own multiple sets of measurements/co-ordinates, if that makes any sense?


Not really, because this is still treating time and space as separate, when really they are facets of the same thing.

What I meant was, is a photon definitely in motion, or could it only appear to be because during every 'step' of it's motion, it's not, it's actually a new stationary photon? (Oh god it's really hard to put into words what I'm thinking here?!?)


No, it's definitely in motion, because that's where its energy comes from. Photons have zero rest mast so, via E=mc2, they have no intrinsic energy. All their energy is essentially kinetic in nature. Indeed, we can actually measure individual photons, and it was this for which Einstein, in a paper unrelated to Relativity, won his Nobel prize.

P.S. Thanks. Oh and I'll watch the "DO YOU KNOW WHAT TIME IT IS?" vid as soon as I, uh, get time. :groan:


Excellent. I should also recommend a book for you; Why Does E=mc2 (and why should we care) by Brian Cox and Jeff Forshaw. It's a really accessible book about Special Relativity, and requires no mathematics more complicated than Pythagoras' Theorem.



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Ok thanks for that, I'm babysitting a 2 year old right now, but, shock, horror, I have more questions coming later!! Oh and I have "Why Does E=mc2", I picked it up at the airport and read it on holiday last year. :D

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hackenslash wrote:Meant to post this for you:

http://documentaryheaven.com/do-you-kno ... ime-it-is/

Brilliant stuff, and will give an insight into just what a difficult question it is.

Not having much luck getting this vid to stream, it's taken over an hour to do 8 mins, but will try again later. Maybe my broadband's just slow today.

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Ok, gotcha, and just watched, and... ...OMG, Free-Will... ...ISSUES!!! :hmmm:

Atomic clocks

So the atoms can only jump to particular orbits, yes? Do they jump instantly, then, because othewise it seems that the have to - however momentarily - exit in in-between orbits?


Regarding the deep space '3D' photo taken with a telescope, in the video... So normally, if I close one eye and look at the night sky with the other I'm getting the same result as that deep space photograph, right? - i.e. a 3D picture of the Universe. But I was born with binary vision for a reason, I've always been told that this was in order that I could have depth perception. Okay, that was a statement not a question, mainly because there's something I want to ask, I just don't exactly know how to phrase it... :confused:

There is one other question I have, regarding Dr Fay Dowker's "Granular Time", but sadly I haven't the time to formulate it right now.


Unrelated question...

Why are nearly all of the heavenly bodies out there in Space spherical, well, spherical-ish?

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ghostgirl wrote:Ok, gotcha, and just watched, and... ...OMG, Free-Will... ...ISSUES!!! :hmmm:


Indeed, and if you think that has implications for free will, you should have a shufty at the following:



Atomic clocks

So the atoms can only jump to particular orbits, yes? Do they jump instantly, then, because othewise it seems that the have to - however momentarily - exit in in-between orbits?


They move between orbits. It should be noted that it's the electrons that jump, not the atoms.

Regarding the deep space '3D' photo taken with a telescope, in the video... So normally, if I close one eye and look at the night sky with the other I'm getting the same result as that deep space photograph, right? - i.e. a 3D picture of the Universe. But I was born with binary vision for a reason, I've always been told that this was in order that I could have depth perception. Okay, that was a statement not a question, mainly because there's something I want to ask, I just don't exactly know how to phrase it... :confused:


I think I see what you're getting at. You're wondering how distance is determined in deep space.

Well, that's a long history, and it starts with your binocular vision. Your binocular vision allows you to perceive depth by an unconscious process involving trigonometry. Essentially, your eyes form the base of a triangle. As with any trigonometric function, if you know the length of the base of a triangle, along with the angles of the sides, you can work out how far away the tip of the triangle is. Similarly, we can work out the distance to astronomical objects by measuring the angles to them at opposing points in Earth's orbit around the sun and, knowing the length of the base line between those points, the angles to the point of interest give us the means to calculate the distance to the object itself. This method has limits, mostly rooted in the error margins, which increase with distance.

That's not the only way, of course. The best method is by using the starlight itself. If you know the composition and mass of a star, you know how brightly it should burn. If you have, say, a 100 watt light bulb, you know how much light it puts out, to a high degree of accuracy (in principle, anyway; in practice, the wattage of light bulbs is pretty variable, though to within a small margin for a given wattage), and thus you can work out precisely how far away it is by how much light you perceive (remember that post on photons in a three-dimensional universe and the inverse-square law). So, all we need to do is to find something we can use as a 'standard candle'. Luckily, we have such a beastie, although it takes the laying of a little groundwork.

The life cycles of stars are reasonably well-understood now, to the extent that we have a very good idea of how old the sun is, and indeed how long it's got left (it's about halfway through a 10-billion year life cycle). For a star the mass of ours, the end of its life will be pretty unremarkable. It will swell into a red giant, encompassing the orbits of Mercury, Venus, and Earth, and probably a fair way out to the orbit of Mars. Then, when it collapses, it will collapse into a white dwarf. A white dwarf is a remnant of a star in which fusion has ceased. It's held up against its own gravity by electron degeneracy pressure, a function of the Pauli Exclusion Principle which, in the simplest terms possible, prohibits fermions (matter particles) from occupying the same space. There is an absolute limit to the mass of a white dwarf, the Chandrasekhar Limit, above which a star will not degenerate into a white dwarf but will undergo further collapse into a neutron star. This is about 1.6 solar masses, if memory serves. There is also a limit to the mass of a neutron star, beyond which a star collapses into a quark star (hypothetical) and then a further limit beyond which a star will collapse into a black hole, but I digress. The interesting one for our purposes is the Chandrasekhar Limit, because it is this that is going to provide our 'standard candle'.

Most star systems are binary. That's to say that, in the majority of cases, two stars coexist about a common point of orbit (the barycentre). In the case of a white dwarf, this raises some interesting possibilities. One of these is that the white dwarf can actually accrete mass from its partner star. When this occurs (and it's pretty common), what happens is that the mass of the white dwarf is slowly pushed over the Chandrasekhar Limit. The result is a supernova whose absolute luminosity, a function of its mass, remember, is extremely predictable. We can now tell, by the apparent luminosity of the supernova, precisely how far away it is.

There is yet a third way of measuring distance to extra-galactic objects, and it was elucidated by the man whose name is carried by the bit of kit that took the image in question, namely Edwin Hubble. He was the man, of course, who realised that, contrary to popular belief, the universe was much larger than our own galaxy (the etymology of that word is, in itself, quite interesting), and that it was expanding. He did this by observing extra-galactic objects and recording the light from them. He realised that multiple observations of given objects showed that the light coming from them was gradually being shifted toward the red end of the colour spectrum. This is the famous 'red-shift', akin to the Doppler effect, in which light-waves are stretched out so that their wavelength increases. What he also measured is that the further these extra-galactic objects are, the faster they are receding from us, which means that we can tell by the amount of red-shift, to a very high degree of accuracy given the scale, how far away something is.

The furthest image in the Hubble Ultra-Deep Field is about 13.2 billion lightyears away, meaning that its light has been travelling toward us since only about 500,000 years after the Big Bang.

I should note, at this point, that that's pretty close to the absolute limit for observation because, prior to about 300,000 years after the Big Bang, the universe was opaque, so we can only go back about 200,000 years further than that before light becomes useless for observation. It's entirely probable that we won't see anything much older than we've seen (although we may be able to achieve better resolution) using anything rooted in electromagnetism, in any of the frequency spectrum, not just because of the opacity, but also because there were no stars to observe. The dim red lights in the Hubble image may well be the oldest thing we can observe.

This may change, not least because we're starting to build devices that don't rely on electromagnetism. The LIGO observatory (Laser Interferometer Gravitational-Wave Observatory), for example, designed to look for gravitational waves using interferometry (the same principle that Michelson and Morley used in their aether experiments), is essentially an observatory using gravity. Although not suitable for observing deep space in its current form, this technology could well provide the means to observe the universe using gravity. Another mission, LISA (Laser Interferometer Space Antenna) will, if ever launched, give us a space-borne gravitational observatory. It's unclear how this technology might be used at this time, but if it can be used suitably, it may give us observational data all the way back to the bang itself, because everything is transparent to gravity.

There is one other question I have, regarding Dr Fay Dowker's "Granular Time", but sadly I haven't the time to formulate it right now.


Not familiar with the name, but I'll do some research while you get your question together.


Unrelated question...

Why are nearly all of the heavenly bodies out there in Space spherical, well, spherical-ish?


Gravity.

I would say more, but that's pretty much the nub of the matter. Gravity operates in all directions, and pulls all matter toward a common centre. As for the second part, namely why they tend to be 'spherical-ish', that's a function of the conservation of angular momentum. It's what makes the Earth an oblate spheroid as opposed to a sphere. What is actually happening is that the rotation of the disc from which the solar system accreted was rotating. Conservation of angular momentum dictates that, during accretion, this rotation is transferred to the constituents, meaning that they rotate. As gravity drives accretion, conservation of angular momentum drives rotation. This rotation is, in itself, momentum, and it exerts a centrifugal (away from centre) force, while gravity exerts a centripetal (toward centre) force. This opposition of forces is what causes rotating bodies (including black holes) to be oblate.

Edit: working out how the Youtube tags work...



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Thank you!!!!!

Right I'm on 12 hour shifts for the next 4 days, so I don't have time to respond to this, and the other thread, immediately, but I will be coming back to discuss all your answers as soon as I get a chance.


Just one thing tho, regarding this:

Hack Wrote:
GG Wrote:
There is one other question I have, regarding Dr Fay Dowker's "Granular Time", but sadly I haven't the time to formulate it right now.



Not familiar with the name, but I'll do some research while you get your question together.


Dr Fay Fowler is the woman who's studying a sort of granular time proposition near the end of the "What Time Is It?" video, she's only in it during the last 10 mins or so.


Ok bye for now and thanks again!! :wave:

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My pleasure. No really it is. I can waffle about this shit forever.

Went back and watched the last 10 minutes of the vid. I'd forgotten about that portion. It basically comes back to what we were discussing earlier. If M-Theory is correct, it places a lower limit on the granularity of spacetime.

The short answer, though, is that we don't know.



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hackenslash wrote:My pleasure. No really it is. I can waffle about this shit forever.

Went back and watched the last 10 minutes of the vid. I'd forgotten about that portion. It basically comes back to what we were discussing earlier. If M-Theory is correct, it places a lower limit on the granularity of spacetime.

The short answer, though, is that we don't know.



:) Me too, I love this stuff! Right then, will get back to this tonight. Ah, LATE tonight most probably. :wave:

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Hey buddy, long couple of days and just too whacked to get my brain into gear at the moment, I do have lots of thoughts that need put into words, but right now my brain seems to be a bit on the mushy side.

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No worries. :D



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Dr. Hackenslash, here is something that has bothered me about Science, and that I've been meaning to look into, but never got around to it, and well here you are :wave:

It's this: Science has determined there are 4 fundamental forces in the Universe, one of which is Gravity. The other three forces are fairly well-understood and the bosons that carry them have been identified. But Einstein theorized that gravity is not really a force per se, but the apparent effect on the inertial properties of a mass that is travelling through a region of spacetime that has been "curved" by another mass.

Right?

So why are TOE guys trying to incorporate gravity into their theories in terms of being either a wave (gravity waves) or particle (the graviton)? Is this simply a contrivance made for mathematical purposes, for convenience and simplicity's sake? What I mean is, is the force of gravity expressed mathematically in terms of a particle (or wave) so that the issue can be approached mathematically within the context of both GR and QM, within which particles and wave behavior is already mathematically robust and well-understood?

Am I way off on this?



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It's been a year and no answer. Must I wait until the Heat Death of the Universe? :confused:



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Science determined that the earth was flat at one point in time. how do we know there are only 4 forces?

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smeggypants wrote:how do we know there are only 4 forces?


I guess my answer would be that so far they've only detected 4 of them :wave:



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dougal wrote:
smeggypants wrote:how do we know there are only 4 forces?


I guess my answer would be that so far they've only detected 4 of them :wave:



IOW we don't know how many there are :)

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