How do we see "old" galaxies?

[Nano]

In the theory of general relativity both energy and matter bends the geometry of spacetime which causes gravity, thus if the vacuum energy of empty space is negative, it would bend spacetime in the opposite directions as positive energy or mass, which would be like a gravitational repulsion.

 

Ok. But in what kind of situations does energy, say photons, bend spacetime? Considering that they lack a resting mass?

[michel123456]

The values are small enough to be far below our observable limit with current technology, but I think there would also be problems with finding reference points and couping with equipment being affected by the phenomen under measurement.

 

 

Exactly.

 

 

 

In the theory of general relativity both energy and matter bends the geometry of spacetime which causes gravity, thus if the vacuum energy of empty space is negative, it would bend spacetime in the opposite directions as positive energy or mass, which would be like a gravitational repulsion.

 

I made once the supposition that gravitational repulsion=negative gravity=product of mass traveling backward in time.

[Spyman]

Ok. But in what kind of situations does energy, say photons, bend spacetime? Considering that they lack a resting mass?

There is no special situations when gravity acts as opposed to normal situations when gravity is passive. Earths gravity don't suddenly turn on or off. Just like gravity caused by matter, gravity caused by energy is always present. All photons in every cind of situations does always bend spacetime.

 

Photons don't have rest mass but they have momentum, they do carry energy and according to Einstein's theory, mass and energy are related by E=mc² and it is predicted that energy will have an equivalent gravitational effect.

 

General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1915. It is the current description of gravitation in modern physics. It unifies special relativity and Newton's law of universal gravitation, and describes gravity as a geometric property of space and time, or spacetime. In particular, the curvature of spacetime is directly related to the four-momentum (mass-energy and linear momentum) of whatever matter and radiation are present.

http://en.wikipedia.org/wiki/General_relativity

 

As I understand GR, if all the matter Earth consists of was converted to radiation, trapped but evently spread out inside a huge spherical mirror with the same radius as the surface we have today, like a giant inside out disco ball, then we would be able to walk around as normal on its outside.

[Anura]

Anything and everything that you see is in the past. No matter how close or distant.

[michel123456]

Anything and everything that you see is in the past. No matter how close or distant.

 

Funny how the same discussion arises again & again.

 

Yes you are right: everything around us is in the past.

 

The past has a position in space: outside all around the observer.

 

Question: does the present has also a position in space?

 

Answer: yes. Here, at the point of space where the observer is.

 

Q: does the future has also a position in space?

 

A: yes. "inside the observer".

 

Nothing more logical.

[vordhosbn]

Can you give a real world example of the length between point P (present) and point F (future)?

[michel123456]

No. A distance like that would be negative and as such does not belong to the "real world".