One way speed of light.

[olavisjo]

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Many experiments claiming to test the one way speed of light have been performed, but on closer analysis they have been shown to measure the two way speed of light.

Is it possible to measure the one way speed of light?

[Divine Insight]

That's an interesting theory you have there, but I don't see what it has to do with GR?

Perhaps, you can show me some of your equations? Do you use any existing ones to support your theory?

Thanks in advance.

I'm not proposing any new theory.

What I described to you is GR.

GR states that the existence of matter warps the spacetime around it.

Notice the word "spacetime".

It's not just space that is being warped, but time itself is also being warped. This may be difficult to intuit, but mathematically time is a measurable variable, and the rate of change of time various from point to point geometrically in a gravitational field (i.e. a warping of spacetime around a physical object)

What I'm saying (and what Einstein is saying in his General Relativity) is that the forces we perceive and call gravity are due to the fact that we reside in a warped spacetime field that his equations describe perfectly.

It's not due to the surface of the Earth accelerating outward.

The surface of the Earth is stationary relative to us, and relative to the center of the Earth, just as intuitively perceive it to be.

Einstein's equations are in no way suggesting that the surface of the Earth is accelerating outward. At least not with respect to our bodies, or with respect to the center of the Earth.

Now if you want to claim that the Earth's surface is somehow accelerating outward (or upward) with respect to the warped spacetime in which it resides, then technically you'd have a point. But that's a rather misleading point that isn't really all that intuitive.

To realize that the Earth's surface is actually stationary in a warped spacetime field seems to be to be far more intuitive with respect to what Einstein is saying.

But as far as equations go, I'm not proposing anything new or different. I'm just offering a perspective on understanding Einstein's GR just as it is.

No modifications required.

Can you produce an equation that actually describes the Earth's surface as accelerating outward (or upward)?

Einstein's equations describe a warped spacetime caused by the presence of the mass of the Earth itself. They don't describe an accelerating Earth's surface. That would imply that the Earth is becoming increasingly larger at an accelerated rate.

But that's not what's happening.

So I'm not proposing any "new theory", I'm just offer a better way to intuitively understand Einstein's theory just as it is. This idea isn't unique to me. Anyone who understands GR should be able to see that what I'm saying here is correct.

The spacetime in the vicinity of the Earth is warped. And that is what causes the effect that we perceive as Gravity.

The surface of the Earth isn't moving at all relative to the center of the Earth. It's basically stationary for all intents and purposes, save for the tidal effects of the moon. And the tidal effects of the moon move the surface of the Earth up and down periodically just like the tides. That has no perceptible affect on how we feel gravity.

[Divine Insight]

If you would like an equation and a graphical representation of the equation consider the following:

This is Einstein's Field Equation right?



This is just a graphic of the equation that can be found anywhere on the Internet.

Now if you plug in all the values for the situation of planet Earth you end up with a graphical description of the spacetime around the planet as shown below. (remember this graphic includes time, it's not just representing space being warp, but time is being warped too.)



What is this equation describing? An Earth that is rapidly expanding? No, it's not.

It's describing the curvature of spacetime caused by the presence of planet Earth.

Of course this graphic represents a warped 2-D sheet, but it's enough to get the general idea of what the equation is describing.

The actual equation is a 4x4 tensor matrix that would be extremely difficult, if not impossible to graph physically. 3-dimensions of space, and 1-dimension of time.

It's not describing anything about the surface of the planet expanding outward in an accelerated fashion.

I'm just trying to be helpful and as clear as I can.

The metaphor of gravity being caused because the Earth's surface accelerating upward toward us is a wrong impression. That's not what Einstein's equation is describing.

Einstein's equation is describing a warped fabric or "field" of spacetime.

In fact this is why it is called a "Field" equation.

It's just an equation of a 4-dimensional geometric field. That's what the equation is describing.

[A Troubled Man]

I'm not proposing any new theory.

What I described to you is GR.

Sorry, but what you proposed was not a description of GR.

GR states that the existence of matter warps the spacetime around it.

Notice the word "spacetime".

Yes, notice the word, "matter" and the fact that it is warping the spacetime around it. That would be the Earth in our example, and what is relative to us is the surface of the Earth.

It's not just space that is being warped, but time itself is also being warped.

Yes, I understand that, it's called gravitational time dilation. The further away you move from an object, the less the gravity, proportionally to the distance.

What I'm saying (and what Einstein is saying in his General Relativity) is that the forces we perceive and call gravity are due to the fact that we reside in a warped spacetime field that his equations describe perfectly.

It's not due to the surface of the Earth accelerating outward.

The point of that analogy is the fact that the gravity we experience is indistinguishable from acceleration, hence the affect of the Earths surface on us as free falling objects is that of acceleration.

The surface of the Earth is stationary relative to us, and relative to the center of the Earth, just as intuitively perceive it to be.

Einstein's equations are in no way suggesting that the surface of the Earth is accelerating outward. At least not with respect to our bodies, or with respect to the center of the Earth.

Yes, that is exactly what those equations suggest, accurately, too.

Now if you want to claim that the Earth's surface is somehow accelerating outward (or upward) with respect to the warped spacetime in which it resides, then technically you'd have a point. But that's a rather misleading point that isn't really all that intuitive.

Yet, it is completely understood by those who understand GR.

To realize that the Earth's surface is actually stationary in a warped spacetime field seems to be to be far more intuitive with respect to what Einstein is saying.

The matter you stand on is indeed stationary, however the effect of gravity from that matter is acceleration; ie, gravity. And, the more matter, the more gravity.

But as far as equations go, I'm not proposing anything new or different. I'm just offering a perspective on understanding Einstein's GR just as it is.

No modifications required.

Sorry, but your description above isn't a correct perspective on understanding GR.

Can you produce an equation that actually describes the Earth's surface as accelerating outward (or upward)?

That is exactly what Einsteins equation describes.

Einstein's equations describe a warped spacetime caused by the presence of the mass of the Earth itself. They don't describe an accelerating Earth's surface. That would imply that the Earth is becoming increasingly larger at an accelerated rate.

The surface of the Earth isn't moving at all relative to the center of the Earth. It's basically stationary for all intents and purposes, save for the tidal effects of the moon. And the tidal effects of the moon move the surface of the Earth up and down periodically just like the tides. That has no perceptible affect on how we feel gravity.

No, you're not getting it. The matter itself does move up towards us, the earth does not perpetually expand or get larger. It is entirely the effect of one of the properties of matter, that of gravity. It is acting upon us in the very same way acceleration would act upon us, GR states these are indistinguishable from one another.

Anyone who understands GR should be able to see that what I'm saying here is correct.

Sorry, but it wasn't correct.

[A Troubled Man]

We are in a state of free fall, and the Earth's surface is fixed, and thus we are actually exerting a downward push on the surface of the Earth due to our state of free fall. So in a sense, we are exerting a force on the Earth rather than the other way around.

That makes no sense. What force exactly are we exerting on the Earth?

Free fall means that there are no external forces acting upon us, none. That's why astronauts float in their spaceships when in orbit. Your example would have them moving towards the side of ship pointing at the earth.

Although, obviously the static surface of the earth is necessarily exerting an equal and opposite force against us.

No, it isn't, Einsteins equations show GR is NOT a force acting upon us. GR shows, as you stated earlier, that the Earth warps the spacetime around it.

But this actually stems from the fact that it is indeed a static surface that is preventing our continued free fall.

That doesn't preclude or change the effects of the object upon us as they continue to act on the spacetime around us while on the surface, which is acceleration.

So the surface of the Earth is not "accelerating up toward us". On the contrary, it's not moving at all. We're actually accelerating down toward it and this is what causes the effect of gravity.

That would be a variation of the experiment conducted on the moon from the astronauts of Apollo 15 when the dropped a hammer and a feather. Your logic would have the hammer always hitting the ground before the feather.

What causes our acceleration? That's the real question. And the answer in General Relativity is that this is caused because the time field in which we reside is not homogeneous.

Sorry, but time has absolutely no effect on anything. The effects we observe in nature are measured with time, but they are effects caused by the laws of nature, not time.

Time is passing by slower at our feet than it is for our heads. It is this non-uniform time field that causes our bodies to accelerate downward.

Sorry, but the difference in time dilation between our feet and heads is so infinitesimally small, it is hardly even measurable.

GPS satellite clocks are 20,000 km away and only tick faster by 45 microseconds a day compared to a surface clock.

So the surface of the Earth is basically in the same predicament we're in. It's trying to accelerate to the center of the Earth too, but it is being prevented from doing this by the atoms that make up the solid ball of the Earth itself.

Sorry, but nowhere in GR is that explanation evident.

In other words, the true explanation of gravity is that we exist in a non-uniform time field. It's technically incorrect to say that the Earth's surface is accelerating up toward us. That would imply that the Earth is becoming constantly larger at an accelerating rate which simply isn't true.

That's not what's happening. That's a misleading description that can be extremely confusing if a person tried to make sense of that scenario intuitively. The only way that would make sense intuitively is if the Earth was indeed rapidly expanding to become larger and larger at an accelerated pace, which isn't what's happening.

And yet, the very example used was that of an observer in an elevator, who would not be able to determine whether or not his elevator was on the surface stationary, or was it under acceleration in space. The observer would measure both to be identical.

Again, the Earth is not physically expanding, although if it helps to imagine such a thing, that is the same effect gravity would produce.

What's really happening is that time is passing by more slowly for our feet than for our heads. That may be difficult to intuit as well, but it's a far more accurate description of what's actually going on.

The funny thing is that this means that your head is actually older than your feet. By a very miniscule amount to be sure. But that's all it takes to create gravity.

Yet, nowhere in GR is that explanation evident.

I hope I've helped you understand the flaws in your argument.

[A Troubled Man]

What is this equation describing? An Earth that is rapidly expanding? No, it's not.

That is not what I was saying. Gravity has exactly the same effect as acceleration, hence the effect on anything at the surface of the earth is that of acceleration. True, the earth is not expanding.

The actual equation is a 4x4 tensor matrix that would be extremely difficult, if not impossible to graph physically. 3-dimensions of space, and 1-dimension of time.

And, if you notice, it is within that tensor we find time, along with the other dimensions, being affected by gravity. We do NOT see time anywhere in the matrix or formula showing it to have an effect on anything else. None.

[Divine Insight]

Sorry, but what you proposed was not a description of GR.

What I explained is precisely what GR is saying mathematically.

What you had originally proposed is simply wrong.

You originally said:

GR works from reference frames under acceleration and basically demonstrates that acceleration and gravity are indistinguishable from each other. Since that is the case, we can see that gravity is not a "pulling force" and that we are all literally in free fall and it is the earths surface that is accelerating up towards us, given that gravity and acceleration are indistinguishable.

That is an incorrect picture and only serves to misguide people. That's not what is physically happening. It's also not what Einstein's field equation is describing.

The point of that analogy is the fact that the gravity we experience is indistinguishable from acceleration, hence the affect of the Earths surface on us as free falling objects is that of acceleration.

You need to be very careful here. You're not being consistent in your above statement.

Yes, the experience (or resulting effect) of gravity is indistinguishable from acceleration. That I agree with.

But then you say, "hence the affect of the Earths surface on us as free falling objects is that of acceleration."

No it's not that of acceleration.

It's an effect that is indistinguishable from acceleration.

Just because the effect is indistinguishable doesn't mean that the cause of the effect is identical. It's not.

In acceleration the cause of the effect is due to a change in motion.

In gravity the cause of the effect is due to a warping of spacetime.

The effects are indistinguishable, but the causes are quite different.

One is caused by an objecting changing motion through a uniform field of spacetime. The other is caused by spacetime itself actually being warped (i.e. a non-uniform field)

The surface of the Earth is stationary relative to us, and relative to the center of the Earth, just as intuitively perceive it to be.

Einstein's equations are in no way suggesting that the surface of the Earth is accelerating outward. At least not with respect to our bodies, or with respect to the center of the Earth.

Yes, that is exactly what those equations suggest, accurately, too.

Well, now it's your turn to produce an equation and graphic that shows that.

The field equation and graphic representation that I contributed do not describe the Earth's surface accelerating outward. They describe a warped spacetime fabric around the planet of the Earth.

Anyone who understands GR should be able to see that what I'm saying here is correct.

Sorry, but it wasn't correct.

I'm absolutely certain that what I've described to you is precisely what Einstein's gravitational field equations are indeed describing. I've even provided for you a graphical representation of the equation to help you see it.

If you want to dismiss what I'm saying that's fine with me. But if you think that you're going to tell me anything about General Relativity, you're sadly mistaken.

Gravity is not caused by the Earth's surface accelerating outward. That's not what General Relativity is saying. Does it produce an equivalent effect in terms of results? Yes. But that's a totally different matter altogether. It's still wrong to tell people that gravity is caused by the Earth's surface accelerating outward toward us. That's not what General Relativity is saying.

I was merely trying to help you gain a better picture of what General Relativity is actually saying. But if you're going to be hostile to my help, forget it.

We'll just have to agree to disagree at this point. My apologies for offering an explanation. Had I known you were going to be non-receptive to it I wouldn't have bothered.

[A Troubled Man]

That's not what is physically happening.

It's also not what Einstein's field equation is describing.

Yes, the acceleration; gravity is physically happening and is precisely what the field equations describe. It is the entire crux of the "Equivalence Principle" that states being at rest in a gravitational field and a state of acceleration are "physically" identical.

The point of that analogy is the fact that the gravity we experience is indistinguishable from acceleration, hence the affect of the Earths surface on us as free falling objects is that of acceleration.

You need to be very careful here. You're not being consistent in your above statement.[/quote]

Yes, I am.

Yes, the experience (or resulting effect) of gravity is indistinguishable from acceleration. That I agree with.

Then, we do agree.

But then you say, "hence the affect of the Earths surface on us as free falling objects is that of acceleration."

No it's not that of acceleration.

It's an effect that is indistinguishable from acceleration.

Just because the effect is indistinguishable doesn't mean that the cause of the effect is identical. It's not.

In acceleration the cause of the effect is due to a change in motion.

In gravity the cause of the effect is due to a warping of spacetime.

The effects are indistinguishable, but the causes are quite different.

The causes are irrelevant if the effects are indistinguishable, because it is the effects we are talking about.

One is caused by an objecting changing motion through a uniform field of spacetime. The other is caused by spacetime itself actually being warped (i.e. a non-uniform field)

I really don't see your point here.

Well, now it's your turn to produce an equation and graphic that shows that.

Graphics don't show what equations show. The equation is Einsteins, that's what it shows.

I'm absolutely certain that what I've described to you is precisely what Einstein's gravitational field equations are indeed describing. I've even provided for you a graphical representation of the equation to help you see it.

Sorry, but that graphic does not support your view that it is the person who is exerting a force upon the earth.

If you want to dismiss what I'm saying that's fine with me.

It is YOU who is dismissing me with some oddball theory about how we exert a force on the earth because of gravitational time dilation between our feet and heads. Complete nonsense.

It's still wrong to tell people that gravity is caused by the Earth's surface accelerating outward toward us. That's not what General Relativity is saying.

Ah, I see you're still not understanding what I'm saying. I never said gravity is causing the earths surface to accelerate out towards us, I am simply parroting the equivalence principle in that gravity is indistinguishable from acceleration, which is the effect we experience from the earths surface.

We'll just have to agree to disagree at this point. My apologies for offering an explanation. Had I known you were going to be non-receptive to it I wouldn't have bothered.

Your understanding is false. Einstein used the gedanken of an elevator at rest on the ground and one out in space. He formulated the principle that the person standing in the elevator at rest on the ground experiences gravity identical to a person in an elevator in space under 1G of constant acceleration.

Hence, the effect of gravity on the ground is indistinguishable from the acceleration of the elevator in space.

[Divine Insight]

It is YOU who is dismissing me with some oddball theory about how we exert a force on the earth because of gravitational time dilation between our feet and heads. Complete nonsense.

It's General Relativity.

[Bust Nak]

It's not an easy thing to measure the speed of light on earth simply because of the requirements of the experiment, a very long distance between two points with only the vacuum of space between them.

But the OP was talking about one way speed and not just the speed of light. The one way speed would require the sender and reciever to be synchronized. And since time is relative, synchronisation is a matter of deciding whose clock we use and as such it is a matter of convention and cannot be measured in principle.

Then, there is the problem of signalling each point as to when photons are released to travel to the other point. Sure, we have the technology to do such an experiment, but the costs would be ridiculous. Since we have a very good understanding of electromagnetic radiation already, such an expensive experiment would be drastic overkill and would serve no real purpose to validate.

Surely the signal would travel as fast as the light we are trying to measure, and arrive at the same time as the photons arrive at the detector.

[A Troubled Man]

It is YOU who is dismissing me with some oddball theory about how we exert a force on the earth because of gravitational time dilation between our feet and heads. Complete nonsense.

It's General Relativity.

Sorry, far from it. There is nothing in GR explaining gravity to be the result of time dilation between the head and feet, or that a person in free fall is the one putting down ward forces on the earths surface. Pure nonsense.

I've already showed you through Einstein's own gedanken experiment that the surface of the earth is accelerating up towards us just like the observer in the elevator. It's called the Equivalence Principle. If you actually claim to know GR, you would know this.