Relativity and Cosmology

The concept of space and the concept of time seem to be inextricably linked by their very natures.

For instance, if one did not experience time, all things would appear to be in the same place since all journeys would be instantaneous. I've wondered what kind of effect this would have on something like light, that, if relativity doesn't fail for massless particles at the speed of light, should not experience time.

Hi Roger, you wrote: "I've wondered what kind of effect this would have on something like light, that, if relativity doesn't fail for massless particles at the speed of light, should not experience time."

The usual time dilation and length contraction of relativity actually do fail for light. Those concepts only apply to massive particles, which according to relativity theory cannot achieve the speed of light.

One must also keep in mind that as an observer approaches the speed of light relative to some or other inertial frame, it is only as viewed by that inertial frame that the observer's time apparently approaches a stand-still. The same for length contraction. The observer does not notice any changes in time or lengths in her inertial frame. So, what does it say about photons?

Jorrie

Jorrie,

It's good to know that time and length dilation fails for massless particles. I guess that makes sense as their are finite wavelengths.

You make a very good point that I had not considered. Even if time dilation did apply to massless particles, in their reference frame they would still experience time.

To get back to my core point though, how could one determine distance without the existence of time? Is there a way? I can't think of any. I say this because I'm wondering if time is just a consequence of space. In relativity they contract the same way for massive particles.

Roger

Hi Roger,

You said "I say this because I'm wondering if time is just a consequence of space."

Since mass and energy are equivalent, if only makes sense that time and space are also somewhat equivalent, does it not?

It's inconceivable that distance (as an entity) can exist without time as a referee. So it seems that speed (scalar) cannot happen without distance which makes no sense without time. Thus light speed (a seemingly constant entity) must be dependent upon time also, which brings into play: distance.

So what do we do with light speed; 299,792,458 meters/second since it must be determined by V(scalar) = D/T? Seems to me that it comes down to SPACE! Space must be the independent variable??? In other words it's independent of distance, time and speed. Most importantly, it's independent of "distance". So if seems to me that we can't have distance without space, BUT we can have space without distance.

What bothers me in this is "what is light speed anyway?" other than a made up constant that makes our math make sense.

Just MHO,

-John

You Wrote "What bothers me in this is "what is light speed anyway?" other than a made up constant that makes our math make sense."

Actually the speed of light is one of the few things that makes a lot of sense. If you get a chance please take a look at this blog entry I wrote up for it a while back:

Speed of Light Derivation

Basically that derivation will tell you that


where ε₀ is the permittivity of free space and μ₀ is the permeability of free space.

Basically the equation tells you that the speed of light is determined by the ease that an electric field can penetrate space and the ease with with a magnetic field can penetrate space. Since light is an electromagnetic wave, that makes a lot of sense. Now if you were to ask me why ε₀ has the value it does, well, now I got nothing.

I agree regarding the time and space relationship.

One thing that has bothered me is, I know inflation theory does a good job explaining how things turned out, how the hell did it happen. Why did space just decide to start expanding and why are we so sure it stopped? What is the physics behind it. Does anyone know? Is the space being stretched or expanding?

Hi Roger,

Thanks for the reply. The link to the math is beyond my expertise but the point is well taken. At first I thought I could relate to your explanation regarding the "ease" of an electromagnetic wave penetrating "space" but it still leaves me in the dark (pun intended)

You said in your mentioned thread that "ε₀ is the Electric Permitivity and μ₀ is the Magnetic Permeability of free space. ε₀ is constant, but if it was larger, then the speed of light would be slower." Then, as you said "if you were to ask me why ε₀ has the value it does, well, now I got nothing."

So does the speed of light really make sense after all? Whether it does or not still doesn't (at least for me) resolve the whole idea of time. Dang it! Time! what the hell is it?

-John

Hi again, Roger. you wrote: "One thing that has bothered me is, I know inflation theory does a good job explaining how things turned out, how the hell did it happen. Why did space just decide to start expanding and why are we so sure it stopped?"

The 'how' is a long story of quantum fields and 'false vacuums' and to be sure, nobody knows. That it happened is reasonably well supported by evidence. BTW, spatial expansion did not stop, it is just the initial rapidly accelerating expansion that stopped (the exponential phase). Then there was a period of decreasing expansion rate, but today the expansion rate is apparently increasing again - the so-called 'dark energy' issue.

"Is the space being stretched or expanding?"

Since it is believed that spacetime was created in the BB, it seems reasonable that more spacetime is being created as the cosmos expands. The favored model today gives a preference to a 'just closed' spacetime, meaning that spacetime is finite, yet unbounded.

Maybe starting another thread will be better if we want to go into more details on this. Let's first try to come to grips with the speed of light...

Jorrie

This is something that I have thought about from the first time I heard about inflation in the early universe. If space is still being created, where is that occuring. Does it occur everwhere equally, or does it come out of specific locations. Furthermore, what happens to matter that happens to be sitting in space and suddenly there's more space. How can you say where you are if there is twice as much space as before?

I saw an astrophysicist on NOVA confidently say that space doesn't expand around matter. I have no idea how she would know that.

"Is the space being stretched or expanding?"

Just re-read your #10, my vote is for 'expanding', because 'being streched' has the connotation that something is pulling at the 'edges' from the outside. There isn't supposed to BE an 'outside' though.

And as it has been explained (somewhere) to me, it should be thought of expanding the way a ball of yeasty bread dough with raisins in it expands - everywhere at once, not just from the center out - so there is no 'where' to think of the universe as expanding from. The dough expands all about the raisins which then end up farther apart from each other. If we are standing on one of those raisins, the rest of them mostly move farther away relatively speaking, although a few might seem to approach more closely. This is supposed to represent reality. One must wonder, though...

Last time I was in a yeasty bread dough, it was 3:00 AM and the police officer didn't think me throwing raisins at him was very funny.

<seriously> that's a good analogy. We're just one raisin away from Alpha Centauri.

Jorrie has said (in other threads) that we (us) are (always) at the center of the BB and space is expanding away from us in all directions, in reply to other's questions concerning where is the source (beginning) of the BB. (If I said that wrong Jorrie, please slap my wrist). IOW no matter where one is in the cosmos, space is always expanding away from you. Thus, there's no way to describe where WE are relative to the BB.

Your bread dough analogy puts it into perspective very well. (Well said Enviroman!)

-John

The problem is that space doesn't end where "we" begin. Our bodies are mostly empty space (some more than others) If all space is expanding, so too should that space be expanding, at a very small rate granted, but all the same expanding.

I think you've touched the heart of the problem Roger. The LHC may get us a little closer to such answers, don't you think?

Maybe we're all just a bunch of sub-Higgs particles, ready to burst forth only to be annihilated by our evil anti-Higgs. (what's an innocent particle to do?)

-John

I'm excited about the LHC. Imagine if they find the Higgs particle. What a stunning success that would be for the standard model. It really would usher in a new era of science. Thank god the Europeans have picked up where we here in the US have dropped the ball.

Hi Roger, you wrote: "The problem is that space doesn't end where "we" begin. Our bodies are mostly empty space (some more than others) If all space is expanding, so too should that space be expanding, at a very small rate granted, but all the same expanding."

The standard view is that space only expands where there is no forces countering the expansion. Gravitationally bound structures do not get larger, or so the conventional wisdom holds. This may or may not be true, depending on our ability to measure the tiny expansions expected.

To show how tiny, at 70 km/s per 3.26 Mly, it boils down to an expansion rate of about 10^-18 m/s per meter distance, i.e., a piece of space 1m in diameter expands by ~10^-10m (10 nm) per year. Even if our bodies expanded at that rate, would we have noticed?

I believe that the nuclear and molecular forces prevent solids/fluids to expand. I'm not totally convinced about the ultra-weak force of gravity preventing medium scale expansion though. I touched upon that in this previous thread on cosmic expansion.

Jorrie

"...a piece of space 1m in diameter expands by ~10^-10m (10 nm) per year. Even if our bodies expanded at that rate, would we have noticed?"

What if we expanded by 10 times each year. Would we notice that? The world would be bigger, and we would be bigger, and all would be relative!

Jorrie,

You Wrote "it boils down to an expansion rate of about 10^-18 m/s per meter distance.....would we have noticed?"

Probably Not, though with attosecond lasers and all the other crazy stuff experimentalists are doing now, I can't say I'm certain we don't have some method to detect a change that small (though I admit its highly unlikely). That would be a cool thread, how to detect 10^-18 m/s spatial expansion.

You bring up a good point regarding nuclear forces resisting the spacial expansion. I guess I can see how there is an energy barrier and in high potentials such a barrier is too high to allow expansion. So spacial expansion only happens when nothing is around to disrupt it. It does go a long way in explaining the swiss cheese structure of the universe. The voids get voidier.

Hi again Roger.

"The voids get voidier." Precisely!

So, where does this bring us relative to the conventionality of relativity? Especially the conventionality of distance and time measurement and their dependence on the speed of light?

Jorrie

Jorrie,

You Wrote: "Especially the conventionality of distance and time measurement and their dependence on the speed of light?"

I wish I knew.

Not to switch topics on you but I have a question. As I understand it, the cosmological redshift occurs because of spatial expansion. One thing that's bothered me about it is the energy of light is dependent on its wavelength. A photon that is redshifted has less energy (E=hν), so where does the lost energy go? It seems like a violation of conservation of energy to me for a photon of light to lose energy simply because the space it occupied expanded.

I wonder how that compares to the amount of energy input needed to cause the expansion? That's the part that has always bugged me - how much energy is needed to expand the universe? And whence the source?

You Wrote: "....dough expands all about the raisins which then end up farther apart from each other"

Here's the problem, the "raisins" are mostly space themselves, why isn't that space expanding. This is the implicit assumption that drives me crazy (Your explanation is dead on for the standard explanation). If you look at an atom, it's mostly space with a tiny tiny nucleus. If you look at a nucleus, its mostly space with tiny tiny quarks. My point is if space is expanding everywhere at the same time, then why aren't we seeing that in atomic spectra as the electrons and the nucleus are pushed apart?

"...why isn't that space expanding. This is the implicit assumption that drives me crazy"

I have to agree. That really bugs me too.

Hi Roger, you asked: "... how could one determine distance without the existence of time? Is there a way? "

The meter was originally defined as 1/10,000,000 of the distance between the poles and the Equator, so it is certainly possible to determine distance without time. Other measures are the astronomical unit and the parsec, neither requiring time in their definitions.

However, the standard meter is today defined by using time and the speed of light, simply because it is accurate and convenient. It is nevertheless a convention...

Jorrie

Yes, but how were the parsec and AU defined?

Hi habib, you asked: "Yes, but how were the parsec and AU defined?"

Both can be be expressed in terms of time, but you do not need time to measure distances in those units. By recording the apparent position of a nearby star over a long time, you can find the peak angular shift relative to the distant stars, giving you its distance in parsecs. For the AU, you can determine how many times Earth orbits the Sun for one orbit of (say) Saturn and thus determine Saturn's mean distance from the Sun in AUs.

Jorrie

Not to be obtuse, but in your answer, you referred to "time" in both definition vis:

"By recording the apparent position of a nearby star over a long time" and "you can determine how many times Earth orbits the Sun..."

Once again, this seems to be measuring distances or relative position over a time interval as opposed to say a straight geometric exercise.

What am I missing?

Hi habib, I guess it's a matter of semantics and not "really" time.

I suppose I could have written: "By recording the apparent position of a nearby star until the pattern starts to repeat itself..."

and

"... count how many oppositions of Earth and Saturn it takes for Saturn to return to the same position against the distant stars..."

The idea is that time in years, seconds or whatever is not used in the calculations of those distances.

Jorrie

Jorrie;

I still see a direct connection between space and time here (which is likely why we use the term 'space-time continuum', eh?). True, a single event will have no referents from which either space or time could be absolutely derived, but an event occurs when <now> and where <here> instead of <then> and <there> so it seems there must be a duration implied that kept both the 'now' and the 'then' separate, and a distance implied that kept the 'here' and 'there' distinct.

Now, two events will have a measurable duration between them, and a measurable distance as well, but are those any more real than the implied versions? Perhaps more definable, but still they exist. I think (correct me if inaccurate, please) that space and time do have an inextricable linkage, I wish I could explain that better...

Hi Enviroman,

Please forgive the intrusion into your question to Jorrie, but I would like to interject MHO of a couple of things you said.

You said "a single event will have no referents from which either space or time could be absolutely derived,..."

The only single event I can conceive of is the BB which would be time zero, hence as you said, neither space nor time could exist. Everything since then, by definition, has a "then" and a "now" depending on where one is on the "time" continuum which also, by definition, implies a duration. All time, all duration, is based on time zero; the BB. Of course, there's still the possibility that the BB did not actually happen as we think it did. In that case, single events, reference points, may very well have to be reconsidered altogether.

Also, and I think Jorrie may well agree, space itself began with the BB (again, if the BB is a given). Therefore, like "then" and "now" "space" established itself as a reference point with the BB.

Sorry for the interruption,

-John

No worries, this was not a private message! I was thinking more in terms of a hypotetical single item or event, without regard for the 'real' universe (as we think we know it, that is). Yes, I think if BB happened, that would be where and when space and time respectively started, IF it happened as we imagine it did. Hmmm...

Hi EnviroMan. You wrote: "I think (correct me if inaccurate, please) that space and time do have an inextricable linkage, I wish I could explain that better..."

True. Everyone has a hard time explaining this fact. I liked this guy's angle on it:

"We have calculated Newton's law starting from a world with no space and no time." -- Carlo Rovelli, August 2006.

My view is that light (actually all EM waves) is the unifying agent for spacetime. It maybe that without light, there may have been no space and no time, hence the Biblical beginning: "Let there be light..."

Jorrie

And then there was a BIG bang, and there was light in the firmament. The "Intelligent Design" camp wonders how any of this could have come about w/o purpose. My take is that this is the only way it COULD have come about, or it would have been different. We can still trace the evidence back to 'shortly' after the BB, and perhaps we'll one day get even closer. But whether some deity figure wagged his beard or just an energy bubble suddenly burst, the real question is: did space/time START then, or was there dimension/duration into which it expanded?

Jorrie,

You gave the example that the meter was defined as 1/10,000,000 of the distance between the poles and the Equator. In other words your saying that a distance (meter) is defined as the smaller part of a another distance (poles to the equator). But how was the original distance measured?

I guess what I'm trying to say is I want to get at the fundamental question "what is distance" and what I believe is it's reciprocal question "what is time". Here's a thought experiment.

I guess the way I envisioned it was imagining what traveling would be like if there was no time. Lets say we were to go on a trip from Earth to Mars to Jupiter to Saturn. In the real world we would blast off from Earth, some time would pass and we'd reach Mars, then some more time would pass and we'd reach Jupiter, then some more time would pass and we'd reach saturn.

Now lets take that same trip from Earth to Mars to Jupiter to Saturn, except in this universe there is no time. There would be no trip right? Just some jumbled superposition of Earth, Mars, Jupiter, and Saturn.

So I guess I'm arguing that in the first trip, the trip with time, there is distance. In the second trip, the one without time, there is no discernable distance, just a jumbled mess.

Jorrie,

Something else to think about, when space contracts, time contracts as well.

Now, is that a consequence because the universe has a speed limit (distance/time), or does the universe have a speed limit because time and distance are linked fundamentally?

Or have I had one to many Coca Colas.

I'm leaning towards that last one.

Roger

Depends on what you mixed the Coca Cola with...but that would explain why you are leaning...

Now that WAS a stimulating thought, though. If the BB is/was for real, and space is expanding, and time is intimately linked to distance, then as space expands (is created) then time expands (is also created), which would explain how we get time, and possibly why time's arrow flies in one direction only. Thoughts?

It's a neat thought right? If space is expanding time moves forward, entropy increases (after all, you have more space to fill). If space contracts, time moves backwards and entropy decreases? I don't know.

Hi Roger,

You were asking Jorrie, excuse me for butting in. Your thoughts are interesting.

"...does the universe have a speed limit because time and distance are linked fundamentally?"

I think you answered this in your post 10 as well as it can be answered. Personally, I think this has nothing to do with space being linked to time. Just my opinion.

S

StandardsGuy,

You Wrote:"Personally, I think this has nothing to do with space being linked to time. Just my opinion."

Mine is just opinion as well. Actually, I probably wouldn't even go that far, I'd call it "light conjecture that is probably wrong". Still, it's fun to speculate.

"...how could one determine distance without the existence of time? Is there a way?"

How about with a tape measure? But then without time, maybe you couldn't roll it out.

Einstein said that time is the fourth dimension. If it really is another dimension of space, then it seems to me that the passage of time is caused by the expansion of space.

We have atomic clocks to measure time. So if we have one that looses 1 second in a billion years, how do we know that the size of a second didn't change over that time? I find it unlikely that the passage of time is linear.

S

Hi S,

You said "How about with a tape measure?"

Believe it or not that's a profound question! Just suppose we do have some object that we choose to call a "tape measure".

As you said, without time we couldn't roll it out, but it really goes beyond that. The very creation of the tape measure required someone, or something, to imprint a series of graduations upon the object which, of course, involves time (imprinting grad 1, then moving to grad 2, 3, etc.). But we're smarter than that aren't we? Of course! We simply use a parallel imprinter; imprint all graduations simultaneously- time problem solved! Well, not really. The imprinter had to be created all at once, which implied that all preceding processes occured instantly, and on and on... So what are we left with?

TIME!!!

-John

So the theory is saying that perception of simultaneous events depends on the relationship of the observers frame of reference to the frame of reference of the event?

This sounds similar to a puzzle you posed a while back.

Hi habib. A single event obviously has no unique frame of reference, but two events do. Mac has (in another thread) coined the term "proper frame" for an inertial frame in which the two events are either simultaneous or co-located. If they are simultaneous in their proper frame, they will not be simultaneous in any frame that is moving relative to the proper frame.

Jorrie

Sir if A single event obviously has no unique frame of reference,but a difference no chaos a motion the proper frame will still proper and the reference frame become a theory.Also the frame will still unique into is travel modified but Time will change as an strategic point of .....as a reference of...as a point moving on a Triangle.Sir.phil

Sorry Phil, I couldn't figure out what you were trying to say...

Jorrie

Please Mr Jorrie,ever say sorry to me.I have the privilege to be able discussing some idea with you.This is lot for me.I am not able NOW to use the accurate and the convention,,,,,convention source off communication.But I am working for the last 4 years on abstract also strong MATH,will back me or just correct me.I will clarified Sir...now I have to study ,this will also clarified many THOUGHT for my own self.I have test ,3 test for the council CANADA next Friday on,Fcc,Bcc,electron beam,squeeze Time.Mr Jorrie it is a Privilege.phil

My brain hurts.

How can one measure time without reference to events, event being defined, for my purpose, as the change in realtionship between two reference points.If the reference points change, there must be motion.If there is motion, there must be a certain velocity.If there is a velocity, a measurement using time must be involved.Even the Cesium clock requires the counting of events,and by this total, a time interval is determined and accepted for future reference.Hence, there is no time without motion, and no way to measure motion without time, and round and round we go.At best, we can make only an arbitrary judgement on anything involving space or time.

This judgment allow us to make some sense of the Gordian Knot of reality.

(The parallels between Gordian Knot and string theory are eerie, IMHO)

To paraphrase Einstein:I wonder if curiosity has it's own reason for existence?

Hi Guest,

See this thread.

Messy? Yes. And no.

It SEEMS messy, because it requires that we alter the way in which we view things, the movement of things and the timing of the movement of things; + view, movement and timing of energy, also.

It is not messy, because we CAN conceptualize it, in order to more fully understand what is ACTUALLY going on.

HOWEVER! It's important to remember that ALL of these concepts are merely REPRESENTATIONAL of the "actual" processes which are occuring.

It started with Newton. His definitions were absolute, for the concepts which he was defining. However, we must always remember that his definitions also take considerable mathematical computation and complex engineering, in order to take advantage of the concepts or to estimate what temperatures, pressures, etc. are being dealt with.

As with all information, it is merely a tool, to assist us in understanding something which may have otherwise been out of our grasp.

tomfranpat

Hi Jorrie,

Messy indeed. Looks like you have made a good summary of your discussions with Mac. "...there is no absolute simultaneity" This being the case, I think you and Mac will never agree.

S

Speaking of voids getting "voidier", check this out:

http://science.howstuffworks.com/hole-in-universe.htm

A void 1 billion light years across.

This may be off topic (you decide).

http://mb-soft.com/public2/twinspar.html

The point I want to make here is that when you have a paradox, either the theory is wrong, or it is being presented wrong. The latter is apparently the case in the twins paradox.

Hi S, haven't heard of this guy before, but his writing certainly seems 'cranky' to me.

Quote: "The consequences of this are enormous. When this is all carefully and thoroughly analyzed, an ENTIRE trip (acceleration, cruising and deceleration to the initial inertial rest frame) results in a very different conclusion than if only the SR portion of a trip is considered. In that second case, a Twins Paradox story actually seems reasonable, where they would meet being many years different in age. However, a correct explanation of such a real trip must necessarily include periods of apparent rapid aging (during GR acceleration) as well as the very well known period of slowed aging (during SR cruising). An entire trip then necessarily involves (as seen from the initial location, i.e., Earth) first a perceived rapid aging during acceleration, then the well-known perceived slowed aging during the SR constant velocity part of the trip, and finally another perceived rapid aging during deceleration..."

This is all utter hogwash. Acceleration per se has no effect on atomic (or biological) clocks (perhaps on aging due to increased wear and tear, but not temporally!) I have described the rationale here and also in the eBook under 'Twin Paradox'.

Jorrie

Jorrie,

I replied to this yesterday, but it apparently didn't take, so I try again. I didn't mean to upset you; this wasn't intended to be confrontational. Carl seems to get a bit 'wordy', and you quoted a wordy section. At least he comes to the same conclusions as you: the twins paradox is not a paradox.

"Acceleration per se has no effect on atomic (or biological) clocks..."

You know better that this as shown here:

http://en.wikipedia.org/wiki/Gravitational_time_dilation

I can only assume that you were 'torqued' at me when you said this.

-S

Hi S. You wrote: "I didn't mean to upset you; this wasn't intended to be confrontational. "

No problem, I did not notice confrontational tones, just strong opinions.

" "Acceleration per se has no effect on atomic (or biological) clocks..."

You know better that this as shown here:

http://en.wikipedia.org/wiki/Gravitational_time_dilation"

You are making the same mistake as many others, probably by equating acceleration to gravity. The equivalence principle is often wrongly interpreted as saying such nonsense. It simply says that you will have a hard time detecting the difference inside a tiny, isolated lab. Once you can look out the window, or your lab is large enough, you can obviously detect the differences in many ways.

Time is influenced by gravitational potential, not the acceleration caused by the gradient of the potential. It is true that Einstein used accelerated frames to originally illustrate gravitational time dilation, but only in the sense to show that the front clock of an accelerated rocket ticks slower than the rear clock, qualitatively. Quantitatively, the effects are totally different and essentially have nothing to do with each other. (The front clock travels slower than the rear clock, due to Lorentz contraction, hence it suffers less velocity time dilation during the acceleration.)

Atomic clocks have been shown to run according the the average speed and not the acceleration experienced in centrifuges.

Regards,

Jorrie

Hi S, you wrote:

• "Wikipedia says "This is supported by General Relativity due to the equivalence principle that states all accelerated reference frames possess a gravitational field." Do you think the author of this is a 'crank'?"

I find no such statement or inference in the Wikpedia article that it references; what 'crank' said that? Maybe one must look at the context, but it sounds weirdo!

The referenced article takes us through Einstein's process of developing the equivalence principle and eventually general relativity. The young Einstein did make a few assumptions on the equivalence of inertial acceleration and gravitational acceleration under strict limiting conditions, e.g. a uniform gravitational field (which does not exist), a small laboratory (actually, infinitesimally small)...

If you read the section "Modern usage", you will find the true meanings of the various equivalence principles.

• "I don't think I can. Are you referring to an experiment?"

I guess if one looks scientifically, it boils down to experiment. Sitting static on Earth's surface vs. being accelerated near Earth is one observation that jumps to mind.

• "What is "gradient of the potential"?"

Newtonian gravitational potential is U = GM/r (essentially the potential energy per unit test mass). The gradient is the differential with respect to r, i.e., d/dr(U) = -GM/r^2, the gravitational acceleration of the test mass in the field.

• "'...the front clock of an accelerated rocket ticks slower than the rear clock"
• "My jaw is on the floor. I don't want to touch this one."

This is the one place where inertial acceleration and a gravitational field are more or less equivalent. Atomic clocks on mountains run slower than clocks at sea level, which is qualitatively (not quantitatively) the same as the nose and tail clocks in an accelerated rocket.

• "Are you saying that atomic clocks that are spinning (accelerated) do not show a slow down of time? If so I would very much like a link to that experiment."

Yep. It is very easy to verify if one has the equipment. Just change the radius and set the angular speed of the centrifuge to keep the tangential speed the same, while changing the acceleration for different runs on the same clock.

My source is Misner, Thorne, and Wheeler's Gravitation. Haven't got it with me, so I can't quote a paragraph or page now. There seems to be a scarcity of web links to such experiments. I'll look around and let you know if I find something.

Hope we are making progress.

Jorrie

Hi Jorrie,

"I find no such statement or inference in the Wikipedia article that it references; what 'crank' said that? Maybe one must look at the context, but it sounds weirdo!"

I haven't been able to retrace my steps and find the quote, but it's in Wikipedia somewhere. Under http://en.wikipedia.org/wiki/Equivalence_principle you find:

"we [...] assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system." (Einstein 1907)

Boldness is mine. I don't see how you can not agree with this and still not say that Einstein was wrong. You can't have it both ways. I didn't see anything under Modern usage that settled our differences.

I have in my writings a reference to Hermann Bondi's book Relativity and Common Sense. He has an example of a space traveler who travels for 20 years at 1G, then turns around and travels back at 1G for 20 years. He ages 40 years in the process but finds the earth 48,000 years older on his return (no mention of slowing down?). You obviously find Bondi to be a 'crank' (We use the term crack-pot here). What is your take on this, would the earth age 40 years in your interpretaion? Out of curiosity, what percent of physicists do you think would side with Bondi, and what percent with your interpretation?

"Hope we are making progress."

Not so far. I have heard stories similar to the above since I was a boy, so it's well ingrained. I thought I had finally learned that it is the acceleration and not the speed that makes the difference, but now?

Regards

S

p.s. I can be persuaded with enough info and lots of chocolate.

Hi S. You wrote, quoting from the Wikipedia article:

• " "we [...] assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system." (Einstein 1907)
• Boldness is mine. I don't see how you can not agree with this and still not say that Einstein was wrong. You can't have it both ways. "

This particular quote has led so many people up the garden path that I think we must put it straight right here, on CR4, once and for all.

First, the proper context: It is snipped from Einstein's 1907 paper, English translation called: "On the relativity principle and the conclusions drawn from it", which was about special relativity. I'll quote some of Einstein's paper more fully:

• "So far we have applied the principle of relativity, i.e., the assumption that the physical laws are independent of the state of motion of the reference system, only to nonaccelerated reference systems. Is it conceivable that the principle of relativity also applies to systems that are accelerated relative to each other?
• "While this is not the place for a detailed discussion of this question, it will occur to anybody who has been following the applications of the principle of relativity. Therefore I will not refrain from taking a stand on this question here."

At the time, Einstein just started to think about including gravity into his theory of special relativity. He did not know how to do it yet and he made a number of mistakes en route. The quote under question was not a mistake, but if read fully and in context, an early attempt to show the progress that he has made. He considers two systems ƒ1 and ƒ2, the former moving with acceleration ƒa and the latter being at rest in a (hypothetical) homogeneous gravitational field, exerting a force equivalent to the acceleration. He wrote:

• "As far as we know, the physical laws with respect to ƒ1 do not differ from those with respect to ƒ2 ; this is based on the fact that all bodies are equally accelerated in the gravitational field. At our present state of experience we have thus no reason to assume that the systems ƒ1 and ƒ2 differ from each other in any respect, and in the discussion that follows, we shall therefore assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system."

What Einstein did not know in 1907, was that space is curved. He only realized that in 1912 and he had to restate some of his 1907 assumptions, amongst others that his equivalence principle only holds in an infinitesimal piece of spacetime. Many people seem to have missed this and wrongly cling to the incomplete quotation, without realizing the context. Gravity and acceleration are not generally equivalent. His: "... in the discussion that follows, we shall therefore assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system." was applicable to the specific point that he tried to make about acceleration, not a generalization.^[1]

You also wrote:

• "I have in my writings a reference to Hermann Bondi's book Relativity and Common Sense. He has an example of a space traveler who travels for 20 years at 1G, then turns around and travels back at 1G for 20 years. He ages 40 years in the process but finds the earth 48,000 years older on his return (no mention of slowing down?). You obviously find Bondi to be a 'crank' (We use the term crack-pot here)." (Emphasis mine)

Not very nice of you, S! I've written on similar, long 1g voyages in my eBook and on my website, with the proper science explained, i.e., that it is the resulting velocity time dilation and not the acceleration that causes the time differences. I've also said this often on this Forum.

I'll rather not comment on the rest of that paragraph of yours, but I trust that I've given you some info to chew on...

Regards,

Jorrie

[1] Much of this is discussed by Abraham Pais in 'Subtle is the Lord ...', (1982) Oxford University Press.

Hi Jorrie,

I certainly seem to have "ruffled your feathers", so to speak. I assure you it was not intentional. I am not noted for tact, but seldom rile one on purpose. I don't have Bondi's book in my possession, so I couldn't re-read to see if I got the wrong meaning. I've made a note of the book you mentioned.

I thought we might have discussed this issue before, so I searched yesterday, but could find no such discussion. Perhaps you could enlighten me on the best way to do a search of this kind. To avoid this situation in the future I intend to start a document to keep track of relativity facts and research. Any links that you could give me along those lines would be appreciated.

I'll make comments that I may have on your e-book directly in the relevant places.

I still await the centrifuge experiment. Any luck finding it?

regards,

S

Hi S, no problem. I enjoy robust debate!

A direct web link to the centrifuge experiment still eludes me - I found the following ones that shed some light on the subject, though:

http://www.physicsforums.com/showthread.php?t=72406
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970701a2.html
http://van.physics.uiuc.edu/qa/listing.php?id=1360

Regards,

Jorrie

Hi Jorrie,

"I enjoy robust debate!"

I hadn't though of it as a debate, only a discussion. I'm here to learn, but also to teach if possible. I have learned from this discussion. Thanks for the links, they are very applicable, especially the first one. I have a much greater understanding of where you are coming from. Now if I can get you to understand where I am coming from. Your link #1 has the ammunition I was looking for, but supports your position as well. Jeff273 says:

"The answer to that quesiton is that you can either use a gravitational time dilation formula (if you use a rotating frame of reference), or you can use a velocity-dependent gamma formula (if you use a non-rotating frame of reference). The former approach requires GR, the later approach requires only SR. The result from either approach is the same - the clock that's not accelerating ticks faster than the clock that is accelerating, and the magnitude of the effect can be given by 1/sqrt(1-(w*r)^2/c^2).

The formulas that apply are either

t/tau = 1/sqrt(1-v^2/c^2) (the SR approach)
t/tau = 1/sqrt(1-2*U/c^2), a weak-field GR approach where U = -Phi is the negative of the Newtonian potential energy.

Either approach alone is correct - applying a "double whammy" would give the wrong answer (there is no approach where it makes sense to have both a velocity red-shift and a gravitational one)."

So it seems that one can use SR (your choice) or GR (my choice), but not both at once. The "Theory of Relativity" was not complete with SR, so Einstein went on the complete with GR. I know, you going to object, but to me the GR approach makes the most sense. Here's the reason: You have 2 clocks, one on the bench, and one on the centrifuge, say 5 feet apart (using the center of the centrifuge). As the centrifuge rotates, the clocks are say 4 feet apart at the closest and 6 feet apart at the farthest. The average velocity between the two clocks is zero. This is the velocity that counts, so SR effects cancel, leaving only the acceleration to make a difference.

regards

S

p.s This experiment may not have been done!

"p.s This experiment may not have been done!"

I'd just about be willing to bet it HASN'T been done. But at what speed would that centrifuge need to turn to have an observable effect? Would it not need to be a pretty significant fraction of 'c'?

Hi EnviroMan,

Atomic clocks have been put on high mountains and found to run faster, and that's less than 1G change. I don't know how long they were there before the readings were taken. The atomic clock would have to be battery operatable or you would have to use slip rings to power it up from the line. I also don't know how many G's they will take and still operate correctly. in Jorrie's third link, the questioner talks about a centifuge with half a million G's, but I don't believe there's such a thing.

S

Hi Jorrie,

"This particular quote has led so many people up the garden path that I think we must put it straight right here, on CR4, once and for all."

I don't see that happening. In Wikipedia in the Introduction to general relativity under the subtopic 'Gravity and Acceleration': "Conversely, any effect observed in an accelerated reference frame should also be observed in a gravitational field of corresponding strength"

In Einsteins book Relativity (authorised translation by Robert W. Lawson, copyrighted 1961 by the Estate of Albert Einstein) on page 82: "Thus on our circular disk, or, to make the case more general, in every gravitational field, a clock will go more quickly or less quickly, according to the position in which the clock is situated (at rest)."

more comments coming up

regards,

S

Correction: it's on page 81, but their's a similar comment on page 82.

Icing on the cake:

From George Gamow's book Gravity (2002) on pages 120-122 he was talking about a thought experiment of an accelerated rocket ship with light coming in a window and being bent by the acceleration: "Thus concluded Einstein, if the principle of equivalence is a general principle of physics, light rays from distant stars should be bent as they pass close to the surface of the sun on the way to a terrestrial observer. His conclusion was brilliantly confirmed in the eclipse of 1919 when a British astronomical expedition to Africa observed the displacement of the apparent positions of stars in the neighborhood of the eclipsed sun. Thus the equivalence of the gravitational field and the accelerated systems became an indisputable fact of physics." (emphasis mine)

Hi S.

We are doing some traveling around the Easter Weekend and I have only GPRS speed Internet - barely possible to comment at the moment. Should be back home this Friday and will comment fully then.

Briefly, what Gamow said here is correct in the historical path of Einstein's development of GR. Einstein himself later realized that this is only half the effect of gravity on light, while it is the full effect of acceleration on light. It is only in the weak field approximation that the two yield more or less the same result. Clifford Will describes the difference succulently in his Was Einstein Right?

Laters,

Jorrie

Hi S, more comment, as promised.

"In Wikipedia in the Introduction to general relativity under the subtopic 'Gravity and Acceleration': "Conversely, any effect observed in an accelerated reference frame should also be observed in a gravitational field of corresponding strength".

True from an accelerated frame to gravitational field comparison (qualitatively, not quantitatively), but definitely not true the other way round. Not all tidal gravity characteristics are present in accelerated frames.

"Thus on our circular disk, or, to make the case more general, in every gravitational field, a clock will go more quickly or less quickly, according to the position in which the clock is situated (at rest)."

I still maintain that the time dilation on a rotating disk is velocity induced, not centripetal acceleration induced. The velocity time dilation in closed orbits around Earth has been measured very accurately and agrees* with the SR interpretation, so why would it be absent on a rotating disk?

Jorrie

* The GR effect of gravitational time dilation is also present, but is easy to separate from the velocity one.

Hi Jorrie,

"True from an accelerated frame to gravitational field comparison (qualitatively, not quantitatively), but definitely not true the other way round. Not all tidal gravity characteristics are present in accelerated frames."

This sounds interesting. Is your reference the same as in post 89?

"The velocity time dilation in closed orbits around Earth has been measured very accurately and agrees* with the SR interpretation, so why would it be absent on a rotating disk?"

The difference here is that in the above situation the center point of the rotation is the reference. In my example that center point has zero average velocity to a clock outside the circle.

"The GR effect of gravitational time dilation is also present, but is easy to separate from the velocity one."

How so, since they are mathematically equivalent according to Jeff237?

Hi S, you wrote: "This sounds interesting. Is your reference the same as in post 89?"

Yep, and from the same author's technical book:

Will C.M. 1981,1993. "Theory and Experiment in Gravitational Physics". Oxford University Press.

Or on the Internet:

http://relativity.livingreviews.org/Articles/lrr-2006-3/

"The difference here is that in the above situation the center point of the rotation is the reference. In my example that center point has zero average velocity to a clock outside the circle."

Compared to orbital speeds, the surface of Earth has practically zero average velocity. If the small surface velocity is taken into account, it makes negligible difference to the result. Orbits have indisputably demonstrated that the velocity and gravitational time dilation predictions of GR (which includes SR) are correct. The GPS system makes use of this fact to achieve the accuracies that it does.

Clocks on a centrifuge suffer a common gravitational time dilation (the same for all surface clocks) plus a velocity time dilation, based upon its position (radius), determining it's relative speed to the central clock.

"How so, since they are mathematically equivalent according to Jeff237?"

I don't know in what context Jeff237 said that, but gravitational time dilation and velocity time dilation are most definitely not mathematically equivalent! A reasonably mathematical discussion on it did occur before on this forum, in my Blog: http://cr4.globalspec.com/blogentry/3754/Orbiting-Clocks-Puzzle.

Full relativistic gravitational time dilation: dΤ² = (1-2GM/rc²) dt²

Full relativistic velocity time dilation:: dΤ² = (1-v²/c²) dt²

There is no general relationship between v² and 2GM/r, except in the special cases of escape velocity and circular orbital velocity.

Regards,

Jorrie

Hi Jorrie,

Thanks for the links and info. I must take a break now to do my income taxes. Perhaps talk later after that and after reading what the links have to say.

Regards,

S

You guys may not be too interested in alternatives (or variations) on GR, but in the cited thread (06.13.03, 01:04) on Physics Forum Yogi points out that the spatial inflow model of gravitation provides an explanation why centrifugal acceleration differs from gravitational acceleration with respect to time dilation:

http://www.physicsforums.com/archive/index.php/t-2435.html

Jon

That was a very interesting read - thank you!

It led me to some new ways of thinking about gravity and the universe, which will take some time to congeal. One of them involves the concept that mass has as a property of its being, the force we refer to as gravity. The more mass the more gravity. The more gravity the more curvature there is of the space-time continuum. The more mass the slower the curvature (radius) of its surface. Therefore:

More mass = more gravity = more curvature, and more mass = slower curvature. Don't have any clue yet what this means (if anything) but as it is a new idea to me, it will keep me occupied playing with it for awhile. Love it!

Hi EnviroMan, I agree, very interesting.

Just one point: what do you mean by "The more mass the slower the curvature (radius) of its surface."?

Curvature has magnitude and direction, but not quite a speed!

Jorrie

More an analogy than a measurement, Jorrie. I was thinking - short radius = quick turn, long radius = slower turn, as an automobile on a highway. Actually, though, now that I think about it, if a photon stream moves past a small mass with little gravity, it will be bent by that gravity only a little. The same photons moving past a large mass with accompanying greater gravity will deflect more - but around a larger radius. Will this have any net effect on the photons?

Hi EnviroMan.

You wrote: "if a photon stream moves past a small mass with little gravity, it will be bent by that gravity only a little. The same photons moving past a large mass with accompanying greater gravity will deflect more - but around a larger radius. Will this have any net effect on the photons?"

Correct, but the net effect is just the amount of bending of the photon's path. At distance of r =3GM/c^2 from the center of a black hole, a photon can be deflected enough to be in an orbit around the hole. Closer than that and the photon falls in.

Jorrie

Understood. Probably the most difficult part of this entire concept (at least for someone who deals with it on an irregular basis) is the fact that photons are not "things", but discrete amounts of energy. And even that term "discrete" seems to be almost incorrect. We do not speak of partial photons, or super-sized photons, they just are what they are, and are only that certain level of "energetic". And yet photons can be deflected by gravity almost as if they were bits of matter. This is probably why I find the subject interesting - the challenge of understanding.

Hi Jon.

This business of "infalling space" is a trick sometimes used when dealing with black holes; if I remember correctly it is called "free-fall coordinates". It gives for most part the same results as any other treatment of curved spacetime, but as Yogi said, there may be some inconsistencies.

Anyway, I haven't heard of the gravity/centrifugal acceleration aspect of free-fall coordinates before. Will take a deeper look, thanks.

Jorrie

Hi Jon,

The link was very interesting, however the thread you cited doesn't seem to exist. What do those numbers mean? They look like dates and times, but in weeks? What do you think of the 'spacial inflow' theory?

regards,

S

Hi StandardsGuy,

Are you saying that you were unable to read the thread? Or are you looking for links to the cited articles? The thread itself is dated 2003 and is from Physics Forum's archive.

I like the spacial inflow theory a lot. It seems to have gotten little attention, so it's hard to say at this point whether it's valid or not. But I lean in the direction of thinking it provides a superior physical description of what gravity does, than regular GR does. I also think it's better because it does not rely much, if at all, upon the mathematically abstract and physically unproven concept of spatial curvature (as distinguished from spacetime curvature).

Jon

Hi Jon,

I am saying that I cannot find (06.13.03, 01:04) as you posted in #95 from your link or by reading the "entire thread". I found nothing that explained any difference between gravity and acceleration on clocks.

S

Hi again S.

Apologies, I gave the info, but forgot about the' chocolate'.

Here's the ultimate thought to your: "I thought I had finally learned that it is the acceleration and not the speed that makes the difference, but now?"

Bondi's traveler does a 5 year 1g acceleration, a 10 year -1g acceleration and finally a 5 year 1g acceleration in order to 'stop' again at Earth's vicinity. These details are not important, just the fact that she and her clocks (atomic and biological) were subjected to 20 years of 1g acceleration, in her own frame of reference. Clocks and bodies on earth were also subjected to gravity that felt like 1g acceleration for the same time according to her clock. If acceleration was the cause of her time dilation, why would Bondi say that clocks on Earth aged 48,00 years while she was away?

Her speed!

Regards,

Jorrie

• "Are you saying that atomic clocks that are spinning (accelerated) do not show a slow down of time? If so I would very much like a link to that experiment."

"Yep. It is very easy to verify if one has the equipment. Just change the radius and set the angular speed of the centrifuge to keep the tangential speed the same, while changing the acceleration for different runs on the same clock"

This experiment sounds weirdo. It would take only one run if was long enough. Have one clock on the bench next to the centrifuge, Set both clocks at the beginning, and compare them after the run.

on to the next comment

S

Jorrie,

As you say, the measure of distance is based on a specified wavelength of light, presumably in a loosely defined reference frame, e.g. the local rest frame nowhere in particular on the earth's surface. Presumably the number of wavelengths is then used to construct a rod of fixed length, which is used "in the field" to take actual distance measurements. The length of that rod will vary in use only due to relativistic length contraction along the axis of movement. The person on the train who is making "two-way" measurements of the light reflections inside the train presumably does not "correct" for the rod's length contraction since from his perspective everything is happening within a single inertial frame. The length of the railroad car is contracted by the same proportion as the length of the rod. Which should result in a consistent measurement of the speed of light inside the car, regardless of the train's velocity. Doesn't that as a practical matter avoid the concern you expressed? I'm probably misunderstanding your point.

Jon

Hi Jon. You wrote: "The length of the railroad car is contracted by the same proportion as the length of the rod. Which should result in a consistent measurement of the speed of light inside the car, regardless of the train's velocity."

I suppose one should just say that inside the car, all time and distances are 'proper' and hence the speed of light is c. According to the platform observer, what you said sounds right.

My main point is that length contraction is a clock synchronization (or simultaneity) issue and hence it is conventional. Einstein's simultaneity convention seems to work the most consistently, but it is not the only possible convention. If one, after accelerating a frame, fail to resynchronize all clocks, you will get quite different results, which may be quite valid, given the "new" synchronization convention.

Jorrie

Jorrie,

I'm looking forward to your answer. After looking around a bit I'm suspecting that you're going to discuss the relative nature of simultaneity. If you do, can you use Minkowski diagrams and explain in very simple terms. I'm interested in understanding but I'm afraid I have a limited understanding of the Minkowski diagrams (which seem to be invaluable as a visualization tool).

Roger

Hi Jorrie,

Could you please explain more about the implications of measuring distance, and the speed of light, with unsynchronized clocks. From that perspective, could the speed of light reasonably be said to differ in different frames?

It seems to me that while synchronizing clocks is a useful exercise for many purposes, it may be equally useful to let clocks get more and more unsynchronized over time, and compare how length and speed of light would vary. You suggest that such an approach results in less "consistent" results. How so?

Jon

Hi Jon.

Sorry, been out of town for a while and I've missed this one.

The one-way speed of light obviously depends on how clocks are synchronized, with the Einstein method ensuring that it is always c. If one come up with a different method, the one way speed could be different in different inertial frames.

As an example, if you synchronize the nose and tail clocks of an inertial spaceship and then accelerate it relative to the original inertial frame, the clocks will go out of sync according to the Einstein method. If you fail to resynchronize the clocks after the acceleration, some physical measurements will become more complex, i.e., you will have to take into account the anisotropic propagation of light when both clocks are needed for some measurement.

Mostly it does not matter, because a single clock can be used for most measurements, e.g. radar distance measurements.

Jorrie