difference between free fall and zero gravity?

[Paulhoff]

Two things that have not been mentioned, one time slows down in gravity and two the shortening of the dimension in the direction of gravity.

Paul

 

[Ocelot]

Here is where I would disagree. Even without really understanding why do I disagree. Acceleration is perceived/measured as changes of inertia, but its very different (from the physical point of view) if the cause of that acceleration is the gravity "force" of a large object or momentum gained from the inside of the vehicle/body by other means.

What was demonstrated?

OK I need to clarify some terminology here. Inertia is that property of an object which resists a change in velocity. It is proportional to its mass. Acceleration is perceived/measured as a change in velocity.

Having made that clarification lets examine what would happen if we were in a sealed capsule 5 metres wide and 25 metres tall. We are told that we are either stationary in a gravitational field imparting an acceleration due to free fall of 10 metres per second squared or we are in free space being accelerated at a rate of ten metres per second squared.

We design an experiment to determine which. I climb up to the top of the capsule and release a ball from by the left hand wall at a velocity of 1 metre per second.

We calculate what we should observe in the two situations.

If we were stationary in a gravitational field then the ball would be acted on by gravity. After one second it would have moved in a horizontal direction one metre from the wall and it would continue to move with a horizontal velocity of one metre per second. It would have accelerated in a vertical direction by 10 metres per second giving it a vertical velocity of 10 metres per second. It would have travelled 5 metres towards the floor.

I continue these calculations to get these figures

time, t (seconds)............. 0. 1. 2. 3. 4. 5
horizontal velocity, (m/s).... 1. 1. 1. 1. 1. 1
distance from wall, x (metres) 0. 1. 2. 3. 4. 5
vertical velocity (m/s)....... 0. 10 20 30 40 50
distance from ceiling, y...... 0. 5. 10 15 20 25

It looks like the ball will trace a parabolic arc through the capsule and land in the opposite corner.

Lets do the same calculations for what would happen if we were in the capsule in free space.

After one second the ball will still be moving horizontally at 1 metre per second however we will have accelerated relative to it by 10 m/s we will have risen by 5 m. We continue these calculations and end up predicting the same observations.

Darnnit that won't work we say and set to thinking again.

What about light we ask. If we're being accelerated through free space then in the 17 billionths of a second it takes for a photon to cross the capsule we'll have accelerated 167 billionths of a metre per second and the photon will hit the wall 83 nanometres from the ceiling. We're pretty sure that light is unaffected by gravity so if we had instruments sensitive enough we'd be able to tell the difference. The fact that we don't have such sensitive instruments is beside the point at least we know that there is a fundamental difference between the two situations.

don't be so sure says Einstein. I reckon there is no difference and therefore reverse your logic to say that light must therefore be affected by gravity. This worked quite well for me when I examined linear frames for special relativity, he says, lets see where it takes me when I allow acceleration. I'll call it general relativity.

And that's exactly what he did. The equations that dropped out of these assumptions made some quite startling predictions - time dilation, the warping of space. However lets stick with that first one: light is affected by gravity.

These day we all know that light has mass and is affected by gravity however before the General Theory of Relativity this was crazy talk.

So how was this tested. Clearly you'd need a huge mass and a ray or two of light that travelled a long distance after it was deflected. The 1910 solar eclipse offered such an opportunity. When the moon blocked the sun the light from the stars around the sun would be visible as if it were night. This star field could be photographed both in the presence of the eclipsed sun and later, at night. If Einstein were correct then the mass of the sun would deflect the light from these starts making them appear further from one another than normal.

Guess what - Einstein was right.

So what does this mean for us in our capsule. It means that even using sensitive instruments and photons we can't tell the difference between the two situations. It means in fact that there's no differnece between the two situations. If we are in free space rocket engines thrusting and we take an alternative approach and simply look out of the window even that will not tell us the difference between us accelerating and rest of the universe falling in a gravitational field that we are resisting with an equal and opposite force. It means that in reality there is no difference.

 

[Schneibster]

Worth mentioning that Sir Arthur Eddington was the astronomer who observed the eclipse, and he did so on the island of Principe, off the coast of West Africa. And I hate to say it, but it was 1919. Other than that, a very nice little exposition; easy to grasp, and well put together.

 

[JanisChambers]

Interesting stuff here. I remember hearing about how Gravity and Energy summing up to zero, I can't really remember the details, I'm actually hoping others know more about this.

 

[I less than three logic]

Interesting stuff here. I remember hearing about how Gravity and Energy summing up to zero, I can't really remember the details, I'm actually hoping others know more about this.

I think that was one of Hawking's ideas, that gravity is essentially negative or borrowed energy from the expansion of the Universe from the singularity, and that the energy extracted from the Big Bang and the negative energy of gravity summed to 0. This energy would eventually have to be paid back in a sense. As the Universe used up the energy causing it to expand, the expansion would slow to a stop. Then, the Universe would start to collapse back towards the singularity with time running in reverse.

I don't believe Hawking's still supports this idea, and I'm not really sure I have any of this correct. I've been up way longer than I should, and I'm pulling this from memory without checking anything.

 

[Endymion]

I must admit that I had trouble understanding Ocelot's post, so this may have already been explained in a more eloquent way, but a person reaching terminal velocity is not feeling gravity at all. The faster he falls, the more air pushes back at him until it is pushing as hard as gravity is pulling. Then the person would not simulate a zero g environment, he would actually be in a zero g environment. Falling at terminal velocity is the same as being suspended between the earth and the moon.(except for the landing)

 

[Endymion]

I think I have heard somewhere that gravity is possibly a byproduct of the electromagnetic force, and not a fundamental force in its own right. Does that make sense to anyone, or is it pseudoscience?

 

[Endymion]

...the vector of the forces on objects that are separated angularly with respect to the center of mass of the massive object are different. Each points toward the center of mass...

I thought the vectors are perpendicular to the radius (tangential), and caused centripetal force, not centrifugal force?

 

[Ziggurat]

I thought the vectors are perpendicular to the radius (tangential), and caused centripetal force, not centrifugal force?

That's not what he meant. Consider you and a friend standing side by side on a perfectly spherical earth. You and your friend both feel the pull of gravity, and feel it in a particular direction: namely, towards the center of the earth. But the thing is, since you two aren't standing in exactly the same spot, the direction to the center of the earth isn't exactly the same for the two of you: it's at an angle. If you were both falling towards the earth instead of standing on it, then as you both got pulled towards that single point, you'd also come closer together. This is the tidal "squeeze", and it's caused by the fact that the gravitational field is spherical.

 

[Jekyll]

I must admit that I had trouble understanding Ocelot's post, so this may have already been explained in a more eloquent way, but a person reaching terminal velocity is not feeling gravity at all. The faster he falls, the more air pushes back at him until it is pushing as hard as gravity is pulling. Then the person would not simulate a zero g environment, he would actually be in a zero g environment. Falling at terminal velocity is the same as being suspended between the earth and the moon.(except for the landing)

No. Terminal velocity means that the resistance of the air pushing up equals in force the pull of gravity downwards. In this situation you feel the same weight as if you were stationary in the gravitational field. What you're saying is equivalent to claiming that you don't weigh anything when you're on the ground because the ground takes all your weight.

It's dropping in free fall that is locally indistinguishable from zero g.

 

[Ziggurat]

These day we all know that light has mass and is affected by gravity however before the General Theory of Relativity this was crazy talk.

Light has a momentum 4-vector. Whether or not it has "mass" depends upon which definition you want to use. It has relativistic mass, but relativistic mass is redundant with energy, and is generally dropped from modern texts because it's a superfluous concept. "mass" without any qualifier is generally taken to mean invariant mass, and light has zero invariant mass.

 

[Paulhoff]

So what does this mean for us in our capsule. It means that even using sensitive instruments and photons we can't tell the difference between the two situations. It means in fact that there's no differnece between the two situations.

That is only if one does not look outside of the capsule. If one where to do that then the illusion will be lost.

Paul

 

[Ocelot]

Worth mentioning that Sir Arthur Eddington was the astronomer who observed the eclipse, and he did so on the island of Principe, off the coast of West Africa. And I hate to say it, but it was 1919. Other than that, a very nice little exposition; easy to grasp, and well put together.

Mea Culpa on the 1910/1919 mix up. Cock up on the history front old bean.

Incidentally later analysis of the data suggested that Eddington was subject to confirmation bias. If you correctly assess the possible error in measurements there was significant probablity of this positive result coming about by chance. More accurate measurements have subsequently confirmed General Relativity.

 

[Ocelot]

I must admit that I had trouble understanding Ocelot's post, so this may have already been explained in a more eloquent way, but a person reaching terminal velocity is not feeling gravity at all. The faster he falls, the more air pushes back at him until it is pushing as hard as gravity is pulling. Then the person would not simulate a zero g environment, he would actually be in a zero g environment. Falling at terminal velocity is the same as being suspended between the earth and the moon.(except for the landing)

Oh dear. We have a confusion of terms.

When a skydiver refers to free fall they are indeed talking about a terminal velocity. As the name suggests terminal velocity is the final velocity you reach after accelerating to a velocity where the force of wind resistance balances the force of gravity. This means that they are at a constant velocity. Acceleration being rate of change of velocity is then zero.

When a physicist says free fall they really mean free - free of wind resisitance - free of any other force. As such an object in the atmosphere is only in free fall for the short amount of time before wind resistance ceases to be negligable. In this free fall acceleration remains constant.

Of course nit pickers will say that as you apprach the source of the gravitational field it becomes stronger and of course they're right and we must allways rememeber to check any assumption that this effect will be negligable.

 

[Ocelot]

I think I have heard somewhere that gravity is possibly a byproduct of the electromagnetic force, and not a fundamental force in its own right. Does that make sense to anyone, or is it pseudoscience?

The key word there is possibly. I heard that gravity was possibly an effect mediated by angels.

Ever since Maxwells laws unifeid the Electrostatic Force with the Magnetic force the race has been on to unify the other forces of nature. Gravity, Weak Nuclear and Strong Nuclear. The standard Theory of Particle Physics Offers a Model that sucessfully predicts the actions of the Two Nuclear forces and the Electrostatic and Magnetic forces. However it does not explain gravity. Gravity is sucessfully modelled by General Relativity.

The Grand Unified Theory (GUT) or Unified Field Theory is sometimes desribed as the Holy Grail of Physics.

We don't have it (yet) but if we did it would sucessfully model all five forces.

Various Theoretical Physicists have attempted to build models that POSTdict the observations predicted by General Relativity and the Standard Model. The famous example of string theory has obtained moderate sucess in this regard however such theories fall short when it comes to producing testable predictions. There is no doubt that the theoretical modelling stage is an essential part of the scientific method but a hardcore Popperian would find much of the discipline to be unfalsifiable.

Whather the theroy you encountered falls into either this category or the less structured wooly thinking of junk science I cannot point to any empirical evidence to suggest that it is true.

 

[Ocelot]

That is only if one does not look outside of the capsule. If one where to do that then the illusion will be lost.

Paul

Ahem

If we are in free space, rocket engines thrusting, and we take an alternative approach and simply look out of the window, even that will not tell us the difference between us accelerating and rest of the universe falling in a gravitational field that we are resisting with an equal and opposite force. It means that in reality there is no difference.

 

[Cuddles]

I think I have heard somewhere that gravity is possibly a byproduct of the electromagnetic force, and not a fundamental force in its own right. Does that make sense to anyone, or is it pseudoscience?

I believe that was in a New Scientist article a while ago, although unfortunately I can't seem to find it. As far as I remember it was not an accepted theory and falls somewhere between speculation and complete junk.

On the other hand there are several theories that propose a link between gravity and electromagnetism. However, these just propose novel ways of converting, for example, gravity waves to light. They are still contorversial, but this is just because they have not been tested yet, rather than because they would significantly alter our understanding of the fundamental forces.
http://www.newscientist.com/article/mg17523611.800-can-earths-magnetic-field-sway-gravity.html
http://www.newscientist.com/article/mg18024204.900

 

[Paulhoff]

If we are in free space, rocket engines thrusting, and we take an alternative approach and simply look out of the window, even that will not tell us the difference between us accelerating and rest of the universe falling in a gravitational field that we are resisting with an equal and opposite force. It means that in reality there is no difference.

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

 

[Just thinking]

Hmmm ... would being at the center of a planetary body be the closest thing to Zero-G while not being in a state of Free-Fall? (I know -- you're in free-fall around the sun ... so put yourself at the center of a galaxy if you need to.) After all, unlike falling while in orbit, the net gravitational field is zero, where in low orbit it's essentially the same as on the surface -- just that you're falling with it.

 

[NobbyNobbs]

So what does this mean for us in our capsule. It means that even using sensitive instruments and photons we can't tell the difference between the two situations. It means in fact that there's no differnece between the two situations.

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

a) We can't 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? ?