why can we see stars?

This might be a really inane question, but if the universe is EXPANDING at the speed of light, and the ‘light’ from distant luminous objects is travelling TOWARDS my retina at the speed of light, how come they don’t cancel each other out?? How come we can detect changes in stars then? Sorry to be a nuff-nuff, but I thought someone might be able to enlighten my ignorant self. (I knew I should have done maths and physics at high-school, but I was more interested in boys and disco’s at the time!!)

The stars that you can see are all within the Milky
Way, our local galaxy, and so are relatively close
to us. But there are even distant galaxies are not
receding from us at the speed of light.

There are two mistaken ideas in your post.

First, the universe is not expanding at the speed of light. The expansion is fast, and some parts of the universe are receding at a significant fraction of the speed of light, but most stars aren’t moving away at anything close to that speed.

Second, the speed of light is a constant under all conditions. Therefore, it isn’t possible to ‘catch up’ to a photon, no matter how fast you’re moving. So you can’t ‘cancel out’ the light travelling toward you. The expansion of the universe shifts the wavelength of light, but doesn’t affect the speed.

I covered those quickly, so please ask any questions you may have. I’m sure I didn’t make it as clear as it could be.

So, how fast (in kph or mph) IS the Hubble constant then?? Please don’t give me some sort of esotetic equation…I am of a very retarded scientific intellect (and I don’t mean that in any sort of derogratory way either.)

IANAP,AISDRN (I am not a physicist, and I’m slightly drunk right now)

The speed of light is aprox. 186,282 miles/hour in a vacuum…

Saltire is correct in saying that the universe is NOT expanding at the speed of light. The portions of it that are the oldest, in my limited understanding of modern astrophysics, are (according to current theory) receeding from us at a large speed, but nowhere near the speed of light! Therefore we can see the light of stars that was emitted thousands or more years ago…

One small nitpick, however: the speed of light is NOT constant under all conditions! Light slows in water, for example (which accounts for the refraction of light we experience when we reach into a pond to retrieve a dropped coin… you’ll miss the first time!).

Good question kambuckta!!:smiley: Welcome!

[sup]Someone will be along shortly to demonstrate how and why I am wrong, and will point out that I have my head inserted in an uncomfortable place… but that’s all part of the game!![/sup]

The Hubble constant is roughly 75 kilometers per second per megaparsec. That’s equal to 23 km/s per million light year. 23 km/s is about 50,000 mph.
M31, the galaxy in Andromeda, is about 2.9 million light years away, so the expansion of the universe causes it to move away from us at 66.7 km/sec (2.9X23).

To supplement Squink’s answer - Huble’s Constant is not expressed in units of speed. The universe is expanding, which means that things that are farther away from us are moving away faster. Imagine you draw lots of dots on a piece of rubber cord, and you stretch the cord. Say the ends are initially 10 inches away from each other and moving away from each other at 1 inch per second. If you instead look at two dots 1 inch apart, they are only moving away from each other at 1/10 inch per second. The expansion speed, then, is 1/10 inch per second per inch.

Well…I hate to nitpick a nitpick, but the speed of light is constant under all conditions,it just appears not to be. The spped of light we know if the speed in a vacuum. the speed of light in water is the time it takes for water to absorb and re-emitt the light,which is slower than the speed of light. When the light travels between the water molecules,it’s still going at the speed of light.

Ah, the dangers of posting slightly drunk! :slight_smile:

Of course, I haven’t slept more than 5 hours in the last 2 days, so I could be even worse, but isn’t that suppossed to be 186,282 miles per second?


Or, to be exact, the speed of light is 299,792,458 meters per second.

The Andromeda galaxy is part of our local group and is therefore gravitationally bound to the Milky Way and the other galaxies in our group. It’s motion is not significantly affected by the expansion of the universe. It is in fact moving somewhat towards us. (There was a report published a few months ago where researchers claimed that it will actually collide with the Milky Way in the far future. Not all believe this.) You have to cite a galaxy outside our local cluster before the Hubble constant can be (approximately) applied.

BTW: The furthest objects we have detected so far are (seeming to) move away from us at over 90% the speed of light. This causes a massive red shift (which someone should have brought up by now in response to the original post).
Personally, I would rate 90% of the speed of light as not “nowhere near the speed of light” (Astroboy14).