The C is the speed of photon ( light ) ?

[harrylin]

[..] coordinate velocities[..] do not correspond in any way to observations. [..].

??!

But that is again coordinate speed. You can do the same thing in relativity. [..]

Exactly, and coordinate speed - which is what is actually measured - is just as useful there. Again: do you really think that coordinate speed in mechanics is useless, so that rocket science and GPS are useless too?!

 

[bcrowell]

??!

Exactly, and coordinate speed - which is what is actually measured - is just as useful there. Again: do you really think that coordinate speed in mechanics is useless, so that rocket science and GPS are useless too?!

No, coordinate speed is never measured. It can't be measured, because it has a different value if you describe the problem in different coordinates. For example, the redshift of a photon from a distant galaxy is what is measured. The coordinate velocity of the distant galaxy is not measured; it isn't even defined unless I first choose coordinates.

The distinction in Newtonian mechanics is unimportant, since Newtonian mechanics has an unambiguously defined way of talking about the velocity of object A relative to distant object B. GR doesn't.

I don't claim that coordinate-dependent quantities are completely useless. They are often useful, and I said so in #29, where I made the analogy with the phase of an electron's wavefunction, which is useful but not measurable. GPS uses coordinates called earth-centered inertial (ECI). They are useful. It's necessary for GPS to choose some coordinates. However, the choice is arbitrary, and therefore the coordinates don't correspond to measurable quantities.

 

[DrGreg]

Exactly, and coordinate speed - which is what is actually measured - is just as useful there. Again: do you really think that coordinate speed in mechanics is useless, so that rocket science and GPS are useless too?!

As a general rule, in curved spacetime coordinate values are not directly measured; you usually need to apply some conversion factor, except at those events where the components of the metric are equal to those of the Minkowski metric. That's what the metric is for, to convert coordinate values into "physically meaningful" quantities such as proper time and proper length.

I don't think anyone has claimed that coordinate speed is useless; the expressions used were "not physically meaningful" and "of no physical interest". My previous paragraph supports those views.

The only speed you can measure directly is between two particles whose worldlines intersect at a common event. When particles are separated you run into the old "parallel transport" problem and you have to agree on an artificial (i.e. coordinate-dependent) convention on how to parallel-transport one vector to the other. Having such a convention can be extremely useful but nevertheless the resulting coordinate velocity isn't "physically meaningful".

 

[bcrowell]

Not to beat a dead horse, but here's another point to help try to explain what DrGreg and I have been saying in the last couple of posts. Let's take a specific galaxy, UDFy-38135539, which is one of the most distant ever observed. You can find information about it by googling. Its redshift is 8.55. What is its coordinate speed relative to us? The answer is that its coordinate speed can be anything I like. I can choose coordinates in which a galaxy moving with the Hubble flow has all its coordinates constant except for t; in such coordinates, both UDFy-38135539 and our own galaxy have coordinate speeds that are nearly zero. I can choose other coordinates such that our own coordinate speed is still zero, but UDFy-38135539's is some fraction of c. I can also make it equal to c, or greater than c.

 

[dailydc]

Actually photons always travel at c (whether they are in a vacuum or not) even though light, in an imperfect vacuum, may travel slightly slower than c.

When a photon hits an atom, it may be absorbed by the atom, then, after a short delay, another photon is emitted. Delays like this mean that the average speed of light can be less than c even though each individual photon travels at exactly c.

The above explanation is over-simplified. For a more accurate explanation see the FAQ https://www.physicsforums.com/showthread.php?t=511177

Thank you for good replying .
Can i really trust to the Article of Sir ZapperZ ?
As a science it has to be invoked ?
It was a good information for me .

 

[bcrowell]

Can i really trust to the Article of Sir ZapperZ ?

That one was written before I was here, but anyway our current process for posting a FAQ entry is that it goes through a process of review by the mentors, and it doesn't get posted unless it reflects their consensus. Of course, the best thing is to be able to evaluate for yourself whether you trust a particular argument, rather than trying to figure out whether the source is authoritative.

 

[atyy]

So the feature of space causes Photons can not easily move at "c" in vacuum and decrease their speed . is that true ?

In some sense this is true. For example, the paper first describing http://en.wikipedia.org/wiki/Shapiro_delay" says "Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased ..."

However, the "speed of light over a large distance" and a "gravitational potential" are not always meaningful concepts, and are a somewhat looser way of speaking.

Instead, the more general idea is that the *local* speed of light is always the same. That idea completely determines the paths of light rays in curved spacetime. An equivalent way of stating this is "light rays follow null geodesics".

 

[harrylin]

In some sense this is true. For example, the paper first describing http://en.wikipedia.org/wiki/Shapiro_delay" says "Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased ..."
[..]
Instead, the more general idea is that the *local* speed of light is always the same. That idea completely determines the paths of light rays in curved spacetime. An equivalent way of stating this is "light rays follow null geodesics".

A precision here, as "local" can stand for different things: with "the local speed of light is always the same" you surely mean, not the local speed of light along its path as determined with a single reference standard for length and time (Shapiro, GPS, etc), but the local speed of light as measured with multiple *local reference systems* along its trajectory.


Harald

 

[harrylin]

[..] The only speed you can measure directly is between two particles whose worldlines intersect at a common event. [..]

No, coordinate speed is never measured. It can't be measured, because it has a different value if you describe the problem in different coordinates. For example, the redshift of a photon from a distant galaxy is what is measured. The coordinate velocity of the distant galaxy is not measured; it isn't even defined unless I first choose coordinates.[...]

As this is rather off-topic, here's my last comment. Measuring is based on a chosen measurement system with measurement standards to which one relates. Again: GPS definitely uses coordinate speed; and I guess that most people would agree that what GPS does, is "measuring".

I don't claim that coordinate-dependent quantities are completely useless. [..]

Good that that misunderstanding has been settled. Harald

 

[dailydc]

Actually photons always travel at c (whether they are in a vacuum or not) even though light, in an imperfect vacuum, may travel slightly slower than c.

When a photon hits an atom, it may be absorbed by the atom, then, after a short delay, another photon is emitted. Delays like this mean that the average speed of light can be less than c even though each individual photon travels at exactly c.

The above explanation is over-simplified. For a more accurate explanation see the FAQ https://www.physicsforums.com/showthread.php?t=511177

Hello again

Everyone can explain why speed of photon decrease in a gravitational field or Bose–Einstein condensate ?

We Know bosons aren't atom

 

[harrylin]

Hello again

Everyone can explain why speed of photon decrease in a gravitational field or Bose–Einstein condensate ?
We Know bosons aren't atom

"A Bose–Einstein condensate (BEC) is a state of matter of a dilute gas"
- http://en.wikipedia.org/wiki/Bose–Einstein_condensate


Harald

 

[dailydc]

"A Bose–Einstein condensate (BEC) is a state of matter of a dilute gas"
- http://en.wikipedia.org/wiki/Bose–Einstein_condensate


Harald

Yes , You're right .
I must see english wikipedia
Since english is not my first or seconds language , i always see our wikipedia language
And in that page was no reference to this case . sorry

But

Can you explain why speed of photon decrease in a gravitational field ?

 

[Dale]

Can you explain why speed of photon decrease in a gravitational field ?

It doesn't decrease in any inertial frame, but with tidal gravity present there are no global inertial frames.

 

[harrylin]

Yes , You're right .
I must see english wikipedia
[..]
But
Can you explain why speed of photon decrease in a gravitational field ?

Wikipedia isn't very reliable but usually it's a good place to start.

About the "why" we can only form hypotheses, and different people tend to give different answers to "why" questions - but see my post #11.

In other words, the speed of light (as well as many other things) is determined by the local state of the space through which it propagates. Thus Einstein came to admit: "the fact that "empty space" in its physical relation is neither homogeneous nor isotropic, [...] has, I think, finally disposed of the view that space is physically empty."
- http://en.wikisource.org/wiki/Ether_and_the_Theory_of_Relativity

 

[dailydc]

It doesn't decrease in any inertial frame, but with tidal gravity present there are no global inertial frames.

So what's shapiro effect ?

 

[Dale]

So what's shapiro effect ?

I think you mean, how is it consistent with my comment. The Shapiro effect does not occur in an inertial frame, it only occurs where there is tidal gravity and therefore no global inertial frames.

If you really just want to know what it is: http://tinyurl.com/3t5y4gj

 

[harrylin]

So what's shapiro effect ?

I answered your question already (before you asked it) in my post #11

 

[Imager]

From the FAQ: Do Photons Move Slower in a Solid Medium?

So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.


I am confused about this statement from the FAQ mentioned earlier, first it says "the lattice does not absorb this photon" then is says "and it is re-emitted". How can it NOT be absorbed and still be emitted?

Bob

 

[pervect]

Given that if you perform a local experiment the speed of light is always "c" , regardless of gravity, whether you're deep in a gravity well or far outside it, what do people mean when they say "the speed of light varies"?

What they mean is that in some particular coordinate system, the rate of change of the position coordinate with respect to the time coordinate isn't always the same.

This really has more to do with the behavior of coordinates than any property of light itself.

It's a bit like saying that naval vessels "move faster" when they are near the north pole, because the rate of change of the lattitude with respect to time is greater.

 

[ghwellsjr]

It's a bit like saying that naval vessels "move faster" when they are near the north pole, because the rate of change of the lattitude with respect to time is greater.

Don't you mean longitude?

 

[pervect]

Don't you mean longitude?

Yes - good catch...

 

[harrylin]

Given that if you perform a local experiment the speed of light is always "c" , regardless of gravity, whether you're deep in a gravity well or far outside it, what do people mean when they say "the speed of light varies"?

What they mean is that in some particular kind of coordinate system, the rate of change of the position coordinate with respect to the time coordinate isn't always the same.

For one, I don't mean that when I say that.
Instead, I mean that for *any* "universal" coordinate system (that uses single, non-local standards for length and time) the speed of light isn't *everywhere* the same. Only for a particular coordinate system that is kept very small, or one that has "elastic units", the speed of light is "always c".

This really has more to do with the behavior of coordinates than any property of light itself.
It's a bit like saying that naval vessels "move faster" when they are near the north pole, because the rate of change of the [longitude] with respect to time is greater.

The Earth is a physical object to which those coordinates relate. Thus, do you mean that:
1. there is a physical equivalent to the Earth, but which we cannot see?
or
2. that it's an artifact of using a certain kind of coordinate system?
- if 1.; what is it?
- if 2.; then Einstein's light bending calculation was based on a mere artifact. Then how do you explain the bending of light?

Harald

 

[WannabeNewton]

You seem to keep confusing coordinate systems and what is physically happening. Yes $\frac{\mathrm{d} \phi }{\mathrm{d} y}$ for a photon uses a specific coordinate system but it can be done in any arbitrary coordinate system and so the coordinate value is ambiguous even though physically light does bend. People use the coordinate system that is the easiest to work with and that allows one to define classical parameters. You could use a convoluted coordinate system for the schwarzschild metric that results in some crazy value for $\frac{\mathrm{d} \phi }{\mathrm{d} y}$ for the photon but it is easier to make sense of certain parameters when the coordinate system itself is logical, like the impact parameter for the photon which is $b = L/E$ which would be easily definable in the usual coordinate system for the schwarzschild metric but not so in some other coordinate system. But the physical action of light bending is still happening, we just use a certain system to quantify it. Also, for an arbitrary manifold it is not always possible to construct an atlas with a single, universal coordinate chart. In general, a maximal atlas on a manifold will contain multiple smoothly sewn coordinate charts.

 

[harrylin]

[..] But the physical action of light bending is still happening, we just use a certain system to quantify it. [..]

Perhaps you did not see my post #57? Thus my same question no.2 for you: If you think that Einstein's calculation is based on an artifact from choosing a certain coordinate system which gives a misleading result, then there should be no speed gradient and therefore there can be no light bending based on the wave model of light that he used for GRT. Thus how do you explain the effect? Clearly you do not deny that it occurs.