Exploring Space Without Gravity: What Do the EFE Say?

In summary, the conversation discusses the concept of expansion of space and the existence of gravitational fields in different scenarios. The Einstein field equations are mentioned as a way to predict curvature in spacetime. The concept of a gravitational field is deemed not very useful in general relativity. The conversation ends with a suggestion to ask the original poster for clarification on what they are asking.
  • #1
zeromodz
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If space is expanding faster than light, wouldn't that mean that there are some parts of space where there exists no gravitational field because gravity travels at c also. What do the Einstein field equations say about space without a gravitational field?
 
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  • #2
zeromodz said:
If space is expanding faster than light, wouldn't that mean that there are some parts of space where there exists no gravitational field because gravity travels at c also.
No.

It is useful to be a little more exact about 'expansion of space'

The models we use to show 'expansion of space' assume two important things, the size of the universe is infinite and there is no empty space between matter.

zeromodz said:
What do the Einstein field equations say about space without a gravitational field?
When there is no gravitational field spacetime is a Minkowski spacetime as in special relativity. One could still 'tinker' it by introducing a non-zero cosmological constant.
 
  • #3
zeromodz said:
If space is expanding faster than light, wouldn't that mean that there are some parts of space where there exists no gravitational field because gravity travels at c also.

You seem to be imagining that the big bang was an explosion that occurred in a specific location in space. It wasn't. The big bang occurred throughout all of space.

zeromodz said:
What do the Einstein field equations say about space without a gravitational field?

The Einstein field equations predict curvature, not gravitational fields. The concept of a gravitational field is not very useful in general relativity, because by the equivalence principle the gravitational field is zero at any location for a free-falling observer.
 
  • #4
Passionflower said:
When there is no gravitational field spacetime is a Minkowski spacetime as in special relativity.
This is incorrect. For example, in an FRW solution, comoving observers everywhere throughout the universe measure gravitational fields that are zero. However, the spacetime is not Minkowski.

Note that the converse is also false. For example, an accelerating observer in Minkowski space detects a nonvanishing gravitational field.
 
  • #5
bcrowell said:
This is incorrect. For example, in an FRW solution, comoving observers everywhere throughout the universe measure gravitational fields that are zero. However, the spacetime is not Minkowski.

Note that the converse is also false. For example, an accelerating observer in Minkowski space detects a nonvanishing gravitational field.
Seems like you are mixing up coordinate effects.

A FLRW spacetime (without a cosmological constant) with no mass-energy becomes a Milne universe which is a Minkowski spacetime with a non-inertial coordinate system.
 
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  • #6
Passionflower said:
Seems like you are mixing up coordinate effects.

A FLRW spacetime (without a cosmological constant) with no mass-energy becomes a Milne universe which is a Minkowski spacetime with a non-inertial coordinate system.

In any FLRW spacetime, including those with nonvanishing matter, the gravitational field is zero as measured by any comoving observer. (Actually it vanishes for any free-falling observer, of which comoving observers are an example.) This follows by symmetry, since the spacetime is isotropic.
 
  • #7
bcrowell said:
In any FLRW spacetime, including those with nonvanishing matter, the gravitational field is zero as measured by any comoving observer. (Actually it vanishes for any free-falling observer, of which comoving observers are an example.) This follows by symmetry, since the spacetime is isotropic.
It seems we are talking about two different things here.

Do you disagree that a spacetime with a zero lambda and without mass and energy must be a Minkowski spacetime?

We should ask the poster of the topic what he is asking, I seriously doubt he is asking about measured gravitational fields, I think he is simply asking if there is no mass-energy in the universe what will happen. But of course I could be wrong.
 
  • #8
Passionflower said:
It seems we are talking about two different things here.
We're not just talking about two different things. You made a mistake in your #2, and I pointed it out.

Passionflower said:
We should ask the poster of the topic what he is asking[...]
I agree that it would be good to hear from the OP at this point.
 

FAQ: Exploring Space Without Gravity: What Do the EFE Say?

How do astronauts adapt to the lack of gravity in space?

Astronauts undergo extensive training and preparation before going into space. This includes exercises to strengthen their bodies and simulate the effects of microgravity. Once in space, they also have specialized equipment and techniques, such as bungee cords, to help them move around and perform tasks without gravity.

What are the effects of long-term exposure to microgravity on the human body?

Long-term exposure to microgravity can have negative effects on the human body, such as muscle and bone loss, changes in vision, and a weaker immune system. This is why astronauts have strict exercise and nutrition routines while in space to combat these effects.

How does microgravity affect objects and experiments in space?

In microgravity, objects and experiments behave differently than they do on Earth due to the lack of gravity. For example, liquids form spheres and flames burn in a more spherical shape. This can be both beneficial and challenging for conducting experiments in space.

Can spacecrafts function without gravity?

Yes, spacecrafts are designed to function without gravity. They use a combination of propulsion systems and the gravitational pull of celestial bodies to navigate and maintain their trajectory. However, microgravity can still affect certain systems on board, such as fluid dynamics and electronics.

How do scientists study the effects of microgravity on Earth?

Scientists use various methods to simulate microgravity on Earth, such as parabolic flights, drop towers, and centrifuges. These simulations can be used to study the effects of microgravity on physical objects, biological systems, and even human subjects before sending experiments and astronauts into space.

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