# Could we argue that if we were to look at a distance of 1000 lightyears into space, we may NOT look exactly 1000 years back in time?

**Passio71**- 6 Answers

Ah, I see what you're specifically asking now. I was thinking in terms of special relativity, not general, my mistake. Gravity does not affect the speed of light itself, but it will alter the path light takes through space. Gravity causes dilation of both space and time. So the speed of light won't change, but the path it has to travel is now longer, due to gravity stretching out the space it has to pass through. So this would cause light to take a longer time to reach us, but not because it's being slowed down, but because the path it's taking is being stretched out.

So light will take a longer time to reach us if there are objects with mass in the way, only because the distance it has to travel is being made longer. This time delay though is only on the order of a few seconds or minutes at most, even when looking at really massive objects. So this isn't of great concern when looking at distances of several thousand light years.

The best way to understand this is to visualize the extreme case of black holes. Black holes in terms of general relativity are points of space-time that have been infinitely stretched out. The amount of space time that has been infinitely stretched is the black hole's event horizon. If light follows this infinitely stretched path it will never go any where else, since it would have to follow an infinite amount of space to get anywhere.

Imagine space as a flat sheet and objects with mass cause stretching in this sheet. Black holes are where mass has stretched that sheet an infinite amount, creating an infinite well nothing can get out of. So something really massive like the black hole at the center of our galaxy would cause a significant time delay due to space stretching, but we don't look at that black hole. We instead observe the stars orbiting it and determine its position that way.

I hope this cleared up any confusion you had on this topic. I misread your initial question, and tried to explain everything in terms of special relativity, when I should have been incorporating general relativity.

Ah, but keep in mind that when we are observing distant objects we are looking at the light emitted from them. Einstein came up with his whole theory of relativity when scientists realized that the speed of light is a fundamental constant, being true in all reference frames.

So even if if we are looking at a point of intense gravity, the light we see from it is still moving at c (c is the speed of light constant) and will reach us at the same time as the light from another equidistant point.

You are right to an extent though. More massive objects will have an effect on the light near them. Since gravity causes time dilation, and the speed of light cant change, that means something else must change to keep that speed a constant.

This is where the concept of gravitational redshifting occurs. In stronger gravitational fields the frequency of the light around it must decrease, since the period (the time between oscillations) is increasing. The product of the two must equal c.

So as gravity gets stronger visible light lowers its frequency, or "shifts" to more red frequencies, which is why it's called redshifting. This effect though only becomes really apparent when looking at really, really massive objects, like galaxies. So no matter where in the universe we point our telescopes, we can can rest safe in the knowledge that the light we see has been moving at c for all its life and will never change that speed.

Oops, noticed I made an error in my previous answer. When I said c was the product of the frequency of the light and its period, I should have said it was the product of the frequency and its wavelength. Since the frequency is simply the inverse of the period, multiplying the two together would get you 1, which is ridiculous.

But the period does get larger in stronger gravitational fields, and since the frequency is simply the inverse of the period, it must get smaller, and since c is equal to the product of the frequency and the wavelength, and is a constant, as the frequency gets shorter, the wavelength must get longer, causing the redshift. Sorry about my previous mistake.

Aren't we actually looking at an illusion. C is constant, correct. Once the light gets to planet Earth though, the object that we see isn't necessarily 1000 years older than us if its time moves much slower or much faster because of very high or low gravity over there.

This is somewhat difficult.

Although you are technically correct, in aspects of usefulness, the difference would be a few seconds at best, so in relative terms of 3 seconds versus 1000 years, there is little difference.

...We would be still be looking back a thousand years into the past, but we would see them moving more slowly than us. Only the relative time frame would be different.

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