Using Light as a absolute frame of reference?

[SlowThinker]

So whether theory is correct or not ? depends on the observation

Yes, there are many theories, logical, nice and simple, such as the existence of absolute time. Sadly, this theory does not agree with observation, so it is wrong. Physics works like that. Unlike math, it studies the behavior of the real universe.

and one more doubt why can't we say that space time around a charge should also be curved , we can say space around the electron will be a curvature such that it will attract positive charge and repel negative charge , the curvature changes depending on the what is placed there.

We use the idea of spacetime curvature in GR exactly because it does NOT depend on what is placed there. Otherwise it would make no sense. There were some attempts to view electricity as a 5th dimension but AFAIK not very successful.

 

[Nugatory]

h

Yes i understood the equivalence principle , but how we know that Einstein theory should be the only theory of gravity? and what is the drawback in Newtoninan theory of gravity ?

I said that you will find good explanations if you google for "equivalence principle path of light". This is not because the problem has anything to do with gravity or the difference between Einstein's and Newton's theories of gravity - it doesn't. It is because the first step in applying the equivalence principle is to work out what happens inside an accelerating room according to classical physics, and that is exactly the question you asked.

 

[FrancisClinton]

hI said that you will find good explanations if you google for "equivalence principle path of light". This is not because the problem has anything to do with gravity or the difference between Einstein's and Newton's theories of gravity - it doesn't. It is because the first step in applying the equivalence principle is to work out what happens inside an accelerating room according to classical physics, and that is exactly the question you asked.

I found an good explanation for the equivalence principle and i have understood it clearly.
According to General relativity , faster object curve less when compared to slower objects when moving around the heavy object , since space and time are single entity, time will run slow for the faster object so they will curve slower. Am i right ?

 

[PeterDonis]

I found an good explanation for the equivalence principle and i have understood it clearly.
According to General relativity , faster object curve less when compared to slower objects when moving around the heavy object , since space and time are single entity, time will run slow for the faster object so they will curve slower. Am i right ?

Whether this is right or wrong as a description of what GR says in general about objects moving near a massive body (it looks too vague to me to decide whether it's right or wrong), it has nothing to do with the equivalence principle. The EP asserts the local equivalence of an accelerating frame in flat spacetime with a frame at rest in a gravitational field in curved spacetime. What you are talking about--the "shape" of orbits of objects around a massive body--is not local.

 

[Herman Trivilino]

There were some attempts to view electricity as a 5th dimension but AFAIK not very successful.

Kaluza-Klein. Endorsed by Einstein. As I understand it, that theory isn't useful because it's classical. That is, it ignores quantum theory. Something that is acceptable in a theory of gravity but not in a theory of electromagnetism because the latter is so much stronger than the former. A nonquantum theory of electromagnetism can't account for how an atom behaves, for example.

 

[FrancisClinton]

Whether this is right or wrong as a description of what GR says in general about objects moving near a massive body (it looks too vague to me to decide whether it's right or wrong), it has nothing to do with the equivalence principle. The EP asserts the local equivalence of an accelerating frame in flat spacetime with a frame at rest in a gravitational field in curved spacetime. What you are talking about--the "shape" of orbits of objects around a massive body--is not local.

Then what is the correct explanation for this "faster object curve less when compared to slower objects when moving around the heavy object " .

 

[PeterDonis]

what is the correct explanation for this "faster object curve less when compared to slower objects when moving around the heavy object " .

I assume you are referring to a scenario like this: we are in the vicinity of a massive body, say a neutron star so the effects of its gravity are more easily measurable, and we fire two objects past the massive body: an ordinary projectile, moving at, say, 1/10 the speed of light, and a photon. The projectile's path will curve more than the photon's--i.e., if we measure the change in angle of the two paths, with respect to some fixed spatial reference point, we will find that the angle change is larger for the projectile than for the photon.

The first thing to note is that the "curvature" of these paths is spatial curvature. These paths are not curved in spacetime; they are geodesics, i.e., the straightest possible paths that there can be in spacetime for those objects. The reason they look curved spatially is that spacetime itself is curved.

The reason the photon's path curves less, spatially, is that it is moving faster, so it spends less time close to the massive body, so there is less time for the massive body to bend its spatial path. That's the simplest heuristic explanation. But this explanation does not rely on the equivalence principle; it can't, because it is looking at the entire paths, not just small segments of them that lie in a single local inertial frame.

 

[Robert Miller]

It is my understanding that the speed of c has slowed since it was first measured 300 years ago even accounting for the level of accuracy and extra lenses in early telescopes.
c has been accepted to have a reference speed when (c*h) multiplied by Planck's constant which has an inverse ratio to c.

 

[Herman Trivilino]

It is my understanding that the speed of c has slowed since it was first measured 300 years ago even accounting for the level of accuracy and extra lenses in early telescopes.

I hadn't heard that. And I know for a fact that it's not true.

c has been accepted to have a reference speed when (c*h) multiplied by Planck's constant which has an inverse ratio to c.

The speed of light is now used to define the standard of length for two reasons

1. It's the most precise way the metrologists can devise to do it.
2. The metrologists were compelled to find a way more precise than the previous way because of the demands of science and industry.

hc is not a ratio inverse to c.

h/c is a ratio, as indeed is any ratio with c in the denominator, a ratio that's inversely proportional to c.

These are simply examples of arithmetic reasoning and have no relevant physical consequence.

 

[PeterDonis]

It is my understanding that the speed of c has slowed since it was first measured 300 years ago even accounting for the level of accuracy and extra lenses in early telescopes.

Do you have a reference?

 

[Robert Miller]

I hadn't heard that. And I know for a fact that it's not true.
The speed of light is now used to define the standard of length for two reasons

1. It's the most precise way the metrologists can devise to do it.
2. The metrologists were compelled to find a way more precise than the previous way because of the demands of science and industry.

hc is not a ratio inverse to c.

h/c is a ratio, as indeed is any ratio with c in the denominator, a ratio that's inversely proportional to c.

These are simply examples of arithmetic reasoning and have no relevant physical consequence.

I can make a simple math expression error if you can make a simple verb tense error, or was it a comma error?
In science how does anybody know something for a fact when 300 years ago it was a fact that light speed was infinite, or to know the Universe was, and always had been a Steady State Universe before 1927?
How does the "Zero Point Energy" theory effect the "Big Bang" theory?

I grant you, I don't have a PhD.in mathematics, and this is on the hairy edge of modern cosmological research. That the theory is not as well developed as others that have been around much longer. Some of my friends view physics from a philosophical point of view as much as or more than a mathematical reasoning.

 

[PeterDonis]

How does the "Zero Point Energy" theory effect the "Big Bang" theory?

It depends on what you mean by "Zero Point Energy" theory. Can you elaborate?

 

[Nugatory]

I grant you, I don't have a PhD.in mathematics, and this is on the hairy edge of modern cosmological research. That the theory is not as well developed as others that have been around much longer.

This might be a good time to remind everyone of the PhysicsForums rule requiring that all theories discussed here must be supported by a reference to publication in an accepted peer-reviewed journal.

 

[Robert Miller]

It depends on what you mean by "Zero Point Energy" theory. Can you elaborate?

As I've heard ZPE described is random EM waves in a vacuum at 0'K. The amount of energy per cm^3.

 

[Herman Trivilino]

In science how does anybody know something for a fact when 300 years ago it was a fact that light speed was infinite, or to know the Universe was, and always had been a Steady State Universe before 1927?

I'm familiar with the 300-year history of measurements of the speed of light. I know for a fact that those measurements do not show evidence of a reduction in that speed during that 300-year period. This is not a statement about a scientific fact, it's a statement about a historical fact.

Over 300 years ago Galileo's measurements led him to conclude that if the speed of light is not infinite, it's so fast that his measuring techniques cannot detect it. That's not a measurement of an infinite speed (as if such a thing were even possible) and it's not a conclusion that it's an infinite speed, either. Roemer measured it over 300 years ago and demonstrated then that it's finite.

 

[PeterDonis]

As I've heard ZPE described is random EM waves in a vacuum at 0'K. The amount of energy per cm^3.

Quantum field theory does predict that quantum fluctuations will lead to a nonzero energy when the field is in its vacuum state. However, since the vacuum is the state with lowest possible energy, there is no way to extract energy from it. (Some people have claimed that this is not actually true, and have used the term "Zero Point Energy" to describe devices that purportedly extract energy from the vacuum. None of these claims have held up under independent testing.) So for most purposes, since the "zero" point of energy is arbitrary (only energy differences between states matter), we can just call the energy of the vacuum zero.

In cosmology, however, we see evidence for the nonzero vacuum energy that quantum field theory predicts; cosmologists call this "dark energy" and its effect is to cause the expansion of the universe to accelerate. We can measure this energy density by measuring the acceleration of the expansion, and the result is that this energy density is very, very small; if we convert it to a mass density (by dividing the energy density by $c^2$), it is about 29 orders of magnitude smaller than the density of water. One of the major open questions in physics is how to reconcile this measurement with the theoretical prediction from quantum field theory, which, at least as far as we know how to extract such a prediction at all, predicts a value about 120 orders of magnitude larger than what we observe.

A good discussion is here:

http://math.ucr.edu/home/baez/vacuum.html

 

[PeterDonis]

In science how does anybody know something for a fact when 300 years ago it was a fact that light speed was infinite, or to know the Universe was, and always had been a Steady State Universe before 1927?

If you're asking how we know that things like the speed of light haven't changed over time, that's actually not the right way to think about it. The speed of light, since it has units, is really a unit conversion factor, not a fundamental constant; the most closely related fundamental constant is the dimensionless coupling constant for the electromagnetic force, which is called $\alpha$ and has a value close to 1/137. There have been plenty of tests made, looking at light from very distant objects (which was therefore emitted a very long time ago), that strongly indicate that $\alpha$ has not changed perceptibly over the lifetime of the universe.

A good discussion is here:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/constants.html

But the above is not the claim you made earlier. You claimed that the speed of light had changed since 300 years ago. You have already been asked to provide a reference for this claim. Can you?

 

[Herman Trivilino]

Do you see how your reasoning is self-contradictory?

In science how does anybody know something for a fact when 300 years ago it was a fact that light speed was infinite, or to know the Universe was, and always had been a Steady State Universe before 1927?

It is my understanding that the speed of c has slowed since it was first measured 300 years ago even accounting for the level of accuracy and extra lenses in early telescopes.

You can't have it both ways. The scientific process either produces knowledge or it doesn't. On the one hand you use scientific knowledge to draw a conclusion and then at the same time claim that science can't be used to draw valid conclusions.

 

[FrancisClinton]

The reason the photon's path curves less, spatially, is that it is moving faster, so it spends less time close to the massive body, so there is less time for the massive body to bend its spatial path.

Whats the mechanism behind this , how the massive body knows the velocity of the object and bends the spacetime around it accordingly ?

The first thing to note is that the "curvature" of these paths is spatial curvature. These paths are not curved in spacetime; they are geodesics, i.e., the straightest possible paths that there can be in spacetime for those objects. The reason they look curved spatially is that spacetime itself is curved

so only space around the massive object is curved not spacetime , can you explain why this is ? , i can't able to understand your above explanation

 

[PeterDonis]

how the massive body knows the velocity of the object and bends the spacetime around it accordingly ?

It doesn't. The massive body bends spacetime, period; that curved spacetime geometry is invariant, it doesn't depend on which objects are moving through it. Each object just responds to the geometry of spacetime where it is located. The bending of the spatial path of the object is the overall result of the object following the local geometry along its entire path. The reason the spatial bending is different for objects with different velocities is that those objects are following different paths through the same spacetime geometry.

so only space around the massive object is curved not spacetime

No, that's not correct; I said the paths were curved in space (more precisely, their projections onto a spacelike surface of constant time are curved), but not in spacetime. In spacetime, the paths are straight (more precisely, they are geodesics)--but spacetime itself is curved around the massive body.

 

[FrancisClinton]

Each object just responds to the geometry of spacetime where it is located. The bending of the spatial path of the object is the overall result of the object following the local geometry along its entire path. The reason the spatial bending is different for objects with different velocities is that those objects are following different paths through the same spacetime geometry.

can you explain this with an example , the word " responds" you used in the explanation seemed to be more abstract.

 

[PeterDonis]

the word " responds" you used in the explanation seemed to be more abstract.

"Responds" just means that the local spacetime geometry at a given point determines the geodesic trajectory that the object will follow, given the object's velocity at that point.

 

[FrancisClinton]

okay i understood it , but does Einstein got all this idea from the thought experiment "Equivalence principle "?

 

[PeterDonis]

does Einstein got all this idea from the thought experiment "Equivalence principle "?

No. He only got a small piece of it. To get the rest took him about eight years (from 1907, when the basic insight that underlies the EP occurred to him--"when a person falls freely, he will not feel his own weight"--to the final form of the Einstein Field Equation in 1915).

 

[FrancisClinton]

No. He only got a small piece of it. To get the rest took him about eight years (from 1907, when the basic insight that underlies the EP occurred to him--"when a person falls freely, he will not feel his own weight"--to the final form of the Einstein Field Equation in 1915).

Yes he worked 8 years for the general relativity and had more struggles to prove it experimentally. I saw it in a documentary about einstein.

 

[Herman Trivilino]

Yes he worked 8 years for the general relativity and had more struggles to prove it experimentally. I saw it in a documentary about einstein.

Well, the struggles to prove it were not Einstein's. Those were efforts by others. Although Einstein did watch closely. And there are cases where he actively sought out experimentalists because he was keenly interested in their experimental techniques and results.