Do tidal forces mean the Equivalence Principle is BS?

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SUMMARY

The discussion centers on the Equivalence Principle (EP) in physics, specifically addressing the implications of tidal forces on its validity. Participants argue that while the EP suggests uniform acceleration and gravity are indistinguishable in a closed room, tidal forces present a detectable difference that challenges this equivalence. The consensus is that the EP does not claim to apply in scenarios where tidal effects are significant, thus reinforcing its validity as a local approximation rather than a universal truth. The conversation highlights misunderstandings surrounding the EP and emphasizes its role in the geometrical framework of General Relativity.

PREREQUISITES
  • Understanding of the Equivalence Principle in General Relativity
  • Familiarity with tidal forces and their implications in gravitational fields
  • Knowledge of inertial and non-inertial reference frames
  • Basic concepts of gravitational mass versus inertial mass
NEXT STEPS
  • Study the implications of tidal forces in General Relativity
  • Explore the geometrical nature of gravity as presented in Einstein's theories
  • Investigate the limitations of the Equivalence Principle in non-uniform gravitational fields
  • Review the historical context and development of the Equivalence Principle in physics
USEFUL FOR

Physicists, students of General Relativity, and anyone interested in the foundational concepts of gravity and acceleration in modern physics.

ItsDaveDude
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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? :smile:
 
  • #10
No, I just don't like the words "reality" or "really".
 
  • #11
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.
 
  • #12
If you ask a belligerent and misinformed question then you should expect non responsive answers while people react reasonably to the bad question.
 
  • #13
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.

Belligerent question? About physics? Dang!
 
  • #14
I think calling it "BS" is a little unwarranted.
 
  • #15
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.
 
  • #16
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
 
  • #17
ItsDaveDude said:
Belligerent? Really?
Yes. Words like "BS" and "misleading" are belligerent, particularly when you put "BS" in the title of the thread.
 
  • #18
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.
 
  • #19
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.
 
  • #20
Dear OP,

What is the Equivalence Principle?
 
  • #21
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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  • #22
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.
 
  • #23
Sure. That doesn't mean that such criticisms are incorrect or inappropriate.
 
  • #24
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.
 
  • #25
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.
 
  • #26
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.
 
  • #27
Thnks wannebeNewton for clarifying that for him
 
  • #28
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?
 
  • #29
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...
 
  • #30
who extended einstein's work? you?
 

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