difference between free fall and zero gravity?

[Paulhoff]

a) We can't tell the difference.

There is a difference as I have said before.

Paul

 

[Bodhi Dharma Zen]

Philosophically, isn't there a difference between these two statements?

a) We can't tell the difference.
b) There is no difference.

Guess thats a much better way to frase what Im intrigued about.

 

[Ziggurat]

Philosophically, isn't there a difference between these two statements?

a) We can't tell the difference.
b) There is no difference.

Let me refine your first category a little bit to emphasize the point:

a) It is not possible to tell the difference.
b) There is no difference.

 

[Bodhi Dharma Zen]

Let me refine your first category a little bit to emphasize the point:

a) It is not possible to tell the difference.
b) There is no difference.

Nice.

Now, tidal forces have come in to the equation, but still they need some external means (in order to have referential information) in order to provide some information regarding our situation.

But, if no external references can be stablished, I find it very interesting that we cant tell the difference. From a purely naive point of view, what surprises me is that in a free fall one is being pulled down by a so called force. Why does the experience is so similar to being floating in a "non gravity" surrounded space?

 

[Ziggurat]

Now, tidal forces have come in to the equation, but still they need some external means (in order to have referential information) in order to provide some information regarding our situation.

You don't need any "external" information to measure tidal forces. What you do need, however, is nonlocal information.

 

[Bodhi Dharma Zen]

You don't need any "external" information to measure tidal forces. What you do need, however, is nonlocal information.

What do you mean by nonlocal? You need something external to have a reference. Right? Without that reference you simply cant know. What is weird about this is that it makes gravity relative?!?

Dog Im confused.

 

[Ziggurat]

What do you mean by nonlocal?

The experiment must have some minimum spacial extent to detect tidal forces. That extent need not be larger than your seal box, rocket, falling elevator, or whatever, but sensitivity to the effect is dependent upon the size of the experiment performed. Pick some nonzero sensitivity limit (since you'll always have one, even if it's really tiny) and it's possible to make the effect "disappear" by making your experiment smaller. This is not true for special relativity: all the laws and effects of special relativity work on a local scale (meaning you can shrink any special relativity experiment down as much as you want and everything looks the same).

 

[Ocelot]

Yes where will be, it is called red shift at first then has the speed of the space ship gets close to the speed of light all light will start to shift to the front of the space ship. That is why Einstein had the thought experiment done in a close elevator.

Paul

I'll have to have a think about that - I didn't take the General Relativity option as part of my physics degree but I still think you're wrong. Special relativity was compulsory but GR was all differential calculus adnv ery very dry. however I thought I had a basic handle on it. I don't see why if the universe was falling ain a gravitaional filed and we were staying still in our rocket ship this would be any different to a stationary universe with us accelerating. As the universe got faster and faster relative to us there would indeed be redshift (ahead/above) and blueshift (behind/below) in both scenarios but please xplain what you mean by light collecting at the front of the ship?

I might have to dig out a textbook or two.

 

[Ocelot]

Let me dispense with tidal forces.

In the thought experiment I outlined, the first scenario was set in a uniform gravitational field imparting an acceleration due to gravity of 10 metres per second.

In the real world there is no such thing as a uniform gravitational field. Point sources generate an attraction related to r squared. Therefore the far side of an object experiences slightly less attraction than the near side. The difference between these two forces is the tidal force If you were being attracted to an and infinitely long pole then the attraction would be related to r. here there would be a slightly smaller tidal force. If you were being attracted to an infinite plane of matter the attraction would be a constant. As such there would be no tidal force. No matter you distance from this plane the attraction would be the same.

There are no infinite planes of matter in the real world. However gravitational fields are large and when analysing a small enough portion of one they approximate to a uniform gravitational field. We assume for most purposes on the earth that we are in a uniform gravitational field.

By measuring tidal forces you may be able to detect that the gravitational field that you were in was not uniform. This much is true. However by determining that the apparent gravitational field strength at the top of your capsule was as equal as you could measure to the apparent gravitational field strength at the bottom of your capsule you have not ruled out the possibility that you are in an apparently uniform gravitational field.

Even if your measurements were beyond all conceivable accuracy you could still not tell the difference between and accelerating and accelerating being in a perfectly uniform gravitational field such as one created by an infinite plane of matter except by assumption that such a perfectly uniform gravitational field cannot exist.

 

[Ocelot]

Philosophically, isn't there a difference between these two statements?

a) We can't tell the difference. (edit. - It is not possible to tell the difference)
b) There is no difference.

Seems to me that the first is a matter of perception, and the other is a matter of reality and Truth, with a capital, T, whatever that means.

Does this mean that reality is defined by our perceptions? And if so, isn't that awfully close to the woo described in The Secret and What the Bleep Do We Know? ?

It depends whose philosophy you've addopted. If you say that existance is that which you experience (Phenomenology) Then there clearly is no difference between the two statements. If that's too subjective for you and you insist that there must be an objective truth then there may be a difference between the two statements (though of course if b is true then a should follow)

 

[Paulhoff]

I'll have to have a think about that - I didn't take the General Relativity option as part of my physics degree but I still think you're wrong. Special relativity was compulsory but GR was all differential calculus adnv ery very dry. however I thought I had a basic handle on it. I don't see why if the universe was falling ain a gravitaional filed and we were staying still in our rocket ship this would be any different to a stationary universe with us accelerating. As the universe got faster and faster relative to us there would indeed be redshift (ahead/above) and blueshift (behind/below) in both scenarios but please xplain what you mean by light collecting at the front of the ship?

I might have to dig out a textbook or two.

As you get nearer to the speed of light which will happen because to keep 1g you have to keep accelerating. Light from behind you will have shifted way into the infrared and longer wavelengths and the only light you will see will be ahead of you. But even if it didn’t, acceleration at low speed works OK, but as you speed up time will slow down as you know and mass will increase and the length of the ship and everything else will shorten in the direction of motion. It is not completely like being affected by gravity in which you do not keep going faster when on the surface on the planet.

Paul

 

[Ziggurat]

As you get nearer to the speed of light which will happen because to keep 1g you have to keep accelerating. Light from behind you will have shifted way into the infrared and longer wavelengths and the only light you will see will be ahead of you. But even if it didn’t, acceleration at low speed works OK, but as you speed up time will slow down as you know and mass will increase and the length of the ship and everything else will shorten in the direction of motion. It is not completely like being affected by gravity in which you do not keep going faster when on the surface on the planet.

That's irrelevant, because if everything around you were falling in a uniform gravitational field, your free-falling surroundings WOULD keep going faster and faster (and experience the continuing red-shift, length contraction, and everything else). So it would still look the same.

 

[Ocelot]

That's irrelevant, because if everything around you were falling in a uniform gravitational field, your free-falling surroundings WOULD keep going faster and faster (and experience the continuing red-shift, length contraction, and everything else). So it would still look the same.

That's what I thought. Glad to have someone agree.

 

[Paulhoff]

That's irrelevant, because if everything around you were falling in a uniform gravitational field, your free-falling surroundings WOULD keep going faster and faster (and experience the continuing red-shift, length contraction, and everything else). So it would still look the same.

So, has you look at the moon, it is getting redder, and farther away, I think not. Once again the thought experiment done by Einstein was in an elevator that had no windows.

Paul

 

[drkitten]

In the real world there is no such thing as a uniform gravitational field.

There are no infinite planes of matter in the real world.

Even if your measurements were beyond all conceivable accuracy you could still not tell the difference between and accelerating and accelerating being in a perfectly uniform gravitational field such as one created by an infinite plane of matter except by assumption that such a perfectly uniform gravitational field cannot exist.

Yes, but since that assumption is a valid one -- as you yourself have just stated, explicitly -- it seems acceptable to make it.

 

[Ziggurat]

So, has you look at the moon, it is getting redder, and farther away, I think not

The moon isn't IN a uniform gravitational field. It's in orbit in a (roughly) spherically symmetric gravitational field. Orbits aren't possible in a uniform gravitational field.

 

[Paulhoff]

The moon isn't IN a uniform gravitational field. It's in orbit in a (roughly) spherically symmetric gravitational field. Orbits aren't possible in a uniform gravitational field.

Geeee, we are talking about increasing speed to simulate gravity, remember the word is simulate. I am saying that it is not in reality equal to gravity.

Paul

 

[Ziggurat]

Geeee, we are talking about increasing speed to simulate gravity, remember the word is simulate. I am saying that it is not in reality equal to gravity.

It's equal to uniform gravity. That we do not encounter uniform gravity on anything beyond a local scale in the universe does not discount that equivalence.

 

[Ocelot]

Geeee, we are talking about increasing speed to simulate gravity, remember the word is simulate. I am saying that it is not in reality equal to gravity.

Paul

From Wikipedia

In the physics of relativity, the equivalence principle is applied to several related concepts dealing with gravitation and the uniformity of physical measurements in different frames of reference. They are related to the Copernican idea that the laws of physics should be the same everywhere in the universe, to the equivalence of gravitational and inertial mass, and also to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.

Bolding mine.

This suggests that gravity is in reality the same as the pseudo force experienced by an observer in an accelerated frame of reference.

 

[Paulhoff]

It's equal to uniform gravity. That we do not encounter uniform gravity on anything beyond a local scale in the universe does not discount that equivalence.

You win, if you don't understand that is not my problem.

Paul