Observe all the ages of the universe

[juan avellaneda]

well i have this question , maybe fool. Hope not misunderstood
if we can observe all the ages of the universe , then why we cannot see twice the same galaxie in two different ages. Or maybe we cannot see it but can see its gravitationals efects over other galaxie in the past??

 

[ahrkron]

We don't see any object at two different times because nothing travels faster than light. Whenever an object moves, its light shows the full trayectory it followed.

In order to produce an image from a different place that reached us in parallel with its original image (without considering gravitational lensing), an object would need to move past its own light, and start transmitting again from its new location. A space-time diagram will probably help you understand it.

 

[Phobos]

Imagine the galaxy emitting light toward the Earth. Every instant, more light is sent toward us. Like a long train of light. We can only see the light that reaches us "now" (one point in that train). We can't see the light that has passed us or that has not yet reached us. Telescopes don't reach out to capture light...they only capture more of the light already here.

So, we can only see that galaxy one instant at a time.

Because the speed of light is finite, it takes time to reach us. Therefore, the image we see is old (an image of the past).

The further away we look, the older the images are. This is why we can see different ages of the universe. But we can't see the same object at different ages. We can see similar objects (like other galaxies that are basically the same shape or size) at different ages so we can then get idea of how things changed over time.

 

[Phobos]

oh, and welcome to Physics Forums!