# Do tidal forces mean the Equivalence Principle is BS?

• ItsDaveDude
In summary, the equivalence principle is a consequence of the geometrical nature of GR, and any theory of gravity where passive gravitational mass is equal to inertial mass can be formulated geometrically.
ItsDaveDude
From another thread this question came up:

I am simply saying that there is no test you can do in a closed room that will distinguish between uniform acceleration and gravity. That is the very essence of EP.

What about tidal forces? Although they could be incredibly small (and I don't think anyone denies they exist) you could conceive of a test in a closed room that detects tidal forces (if it is gravity) and when it doesn't detect them it is uniform acceleration. Isn't this an effect that makes it misleading to say gravity and uniform acceleration are equivalent, because tidal forces are a detectable force that exist in one case (gravity) but not in the other (uniform acceleration), therefore making them not equivalent effects?

To me, this means uniform acceleration and gravity are not equivalent then, and the equivalence principle is just some happenstance of physics that happens to work if we don't look too closely, but in reality these are totally different physical laws/effects from different fundamental processes, and we shouldn't be going on about how they are they same effect or equivalent. Is this a fair statement? Is the equivalence principle just a convenience/artifice for thinking about physics but doesn't hold in reality if you want to think about physics fundamentally?

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The equivalence principle is fine, you just misunderstood it.

DaleSpam said:
The equivalence principle is fine, you just misunderstood it.

Why do you say that

The EP applies on distance scales sufficient enough so that tidal forces do not become apparent.

WannabeNewton said:
The EP applies on distance scales sufficient enough so that tidal forces do not become apparent.

But they still must exist, which makes it an artifice or convenience, and not an accurate representation of fundamental physics, my original post is asking if the statements I made originally are fair statements regarding the EP. Are they?

No, they are not fair, they are based on a misunderstanding of what the EP says. It never claims to apply to tidal effects, so objections regarding inaccuracies in the presence of tidal effects are irrelevant. There is nothing "misleading" or "BS" about the EP, you just misunderstood.

DaleSpam said:
No, they are not fair, they are based on a misunderstanding of what the EP says. It never claims to apply to tidal effects, so objections regarding inaccuracies in the presence of tidal effects are irrelevant. There is nothing "misleading" or "BS" about the EP, you just misunderstood.

So, forgetting whatever the EP principle says, and just speaking on the fundamental reality of physics, it is true that gravity and uniform acceleration are not really equivalent in reality, and aren't the same force or come from an identical fundamental physical process then. If something is wrong or currently unknowable in this statement please elaborate it to me.

ItsDaveDude said:
just speaking on the fundamental reality of physics, it is true that gravity and uniform acceleration are not really equivalent in reality
That is way too many "reality"s for my comfort, but yes, gravity and uniform acceleration are not the same thing in general, and the equivalence principle never claims that they are.

Although the EP was historically important during Einstein's development of GR it is not a postulate or anything similar. The EP is, in fact, a consequence of the geometrical nature of GR. In any geometrical theory of gravity passive gravitational mass is the same as inertial mass, and any theory of gravity where passive gravitational mass is equal to inertial mass can be formulated geometrically. This is the core of the EP.

DaleSpam said:
That is way too many "reality"s for my comfort

So you don't subscribe to the "many worlds" theory then?

No, I just don't like the words "reality" or "really".

DaleSpam said:
No, I just don't like the worse "reality" or "really".

Yeah, I was running out of ways to ask the original question so you might eventually actually respond to it and not have another non-responsive post from you.

But I did appreciate the answer once you gave it on the 3rd try.

If you ask a belligerent and misinformed question then you should expect non responsive answers while people react reasonably to the bad question.

DaleSpam said:
If you ask a belligerent and misinformed question then you should expect non responsive answers while people react reasonably to the bad question.

I may have asked an ignorant question but not a belligerent one. Belligerent? Really? Only if when someone asks the validity of a physical concept you take it as a personal offensive attack. And what "people" reacted? I agree you reacted I suppose, with a curt "you misunderstand physics" blow off, but that was just you buddy.

I think calling it "BS" is a little unwarranted.

Matterwave said:
I think calling it "BS" is a little unwarranted.

All of a sudden I feel like Matt Damon's character in Good Will Hunting.

You can find a good discussion of when the equivalence principle fails in section 24.7 of http://www.pma.caltech.edu/Courses/ph136/yr2006/text.html

ItsDaveDude said:
Belligerent? Really?
Yes. Words like "BS" and "misleading" are belligerent, particularly when you put "BS" in the title of the thread.

The equivalence principle is usually stated locally and that's where it's valid. It's not BS. The constancy of the speed of light is also only valid for a local observer and nobody says it's BS.

If you were a zero-dimensional person you wouldn't be able to tell the difference between gravity and your acceleration.

DaleSpam said:
Yes. Words like "BS" and "misleading" are belligerent, particularly when you put "BS" in the title of the thread.

If the word "misleading" is belligerent, then so is the word "belligerent" itself.

Dear OP,

What is the Equivalence Principle?

kmarinas86 said:
If the word "misleading" is belligerent, then so is the word "belligerent" itself.
This is BS and is misleading.

Do you not feel that this is a belligerent response? (btw, I do agree with you also, considering it in context in post 12)

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DaleSpam said:
kmarinas86 said:
If the word "misleading" is belligerent, then so is the word "belligerent" itself.

This is BS and is misleading.

Do you not feel that this is a belligerent response? (btw, I do agree with you also, considering it in context in post 12)

All criticisms of another's manners are ironic in a certain way. Think about all those people who always make comments about other people's behaviors... are they the nicest people in the world you have ever met? I hope not. That was the essence of my statement's point.

Sure. That doesn't mean that such criticisms are incorrect or inappropriate.

Well Equivalence Principle means that two objects of different mass fall at same rate and also that acceleration in non-inertial reference frame has the same gravitational properties as an object in an inertial reference frame. Tidal forces are basically the squeezing and squashing of objects near a gravitational field..Why? because of the vectors properties of the field.

Ikoro said:
Well Equivalence Principle means that two objects of different mass fall at same rate and also that acceleration in non-inertial reference frame has the same gravitational properties as an object in an inertial reference frame. Tidal forces are basically the squeezing and squashing of objects near a gravitational field..Why? because of the vectors properties of the field.

A black hole of one solar mass will definitely not fall on Earth at the same rate as a pile of bricks.

In other words, this "Equivalence Principle" (wrong definition by the way) is a nice approximation of reality. I'm sure there are better ones that will be discovered, and I don't care who discovers it.

kmarinas86 said:
A black hole of one solar mass will definitely not fall on Earth at the same rate as a pile of bricks.

In other words, this "Equivalence Principle" (wrong definition by the way) is a nice approximation of reality. I'm sure there are better ones that will be discovered, and I don't care who discovers it.

When you compare two masses of equal caliber obviously it won't fall at the same rate. All test particles fall the same in a gravitational field and the stated principle restricts itself to test particles.

Thnks wannebeNewton for clarifying that for him

Ikoro said:
Equivalence Principle means ... and also that acceleration in non-inertial reference frame has the same gravitational properties as an object in an inertial reference frame.

I'm sorry, wat?

Dickfore said:
I'm sorry, wat?

in simple terms, inertial reference frame means no gravitational field.
non inertial reference frame means gravitational field.
..derived from the term inertia-which is the tendency of an object to remain in uniform motion..blah blah blah.
So equivalence says that two objects of different masses fall at the same rate like a feather and an iron..so far u discount air resistance. .
Now, the second part is an addition from einsteins work to the equivalence principle, which is that an object accelerating not with constant velocity in a null gravitation field will experience the same gravitational effects as when an object is falling in a gravitational field..thats the simplest way i can put it...

who extended einstein's work? you?

Dickfore said:
who extended einstein's work? you?
Am done helping...go read, if you have a problem.

Ikoro said:
Am done helping...go read, if you have a problem.

Maybe you should learn that not all questions are asked in need of 'help'. The question was intended at the OP who apparently bailed out his own thread.

No, tidal forces do not say anything about EP. Tidal forces are the result of different parts of a body traveling in two different direction, while each part is doing exactly what EP said it should do.

Consider two masses on each side of the world. They will both inertially accelerate toward the Earth in opposite directions, just as EP demands. Now if you move them only a few kilometers apart they will still inertially accelerate toward the center of the Earth, meaning they will keep getting closer together the closer they get to the center of the Earth. Now, even if you put them right next to each other, as though they were one object, they are still far enough apart that inertial motion still tries to press them closer together as they approach the center of Earth. Hence their internal pressure must push them back apart in opposition to EP. This force can break up meteors, but not because of any problem with EP, but because EP is trying to require the parts of the meteor to come closer together when they cannot.

Saying this invalidates EP is like saying a rocket motor invalidates inertial forces, or inertial motion is invalidated by two meteors hitting each other. It is just not so.

ItsDaveDude said:
The EP about inertial vs. gravitational mass. The example about an uniform gravitational field compared to an accelerated frame, is correct and applies in reality. Just because most gravitational fields in nature are not uniform, doesn't mean that statements about uniform gravitational fields are wrong.

ItsDaveDude said:
doesn't hold in reality if you want to think about physics fundamentally?
Reality and fundamental principles are different things. Reality is always more complex than the idealized example meant to explain the application of a certain fundamental principle.

WannabeNewton said:
When you compare two masses of equal caliber obviously it won't fall at the same rate. All test particles fall the same in a gravitational field and the stated principle restricts itself to test particles.

This "restriction" is what I and the OP have a problem with. I'd rather deal with what logicians call "universals".

It seems evident that the Equivalence Principle sits somewhere between a "singular existential statement" and a "universal statement". This is not settling for those who see their "unlearning" of previous teachings as an obstruction against their ability to learn. This is bad pedagogy in my opinion.

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