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Brian Cox on The Sky At Night 700th

Pakmoto

By Pakmoto
in The Astro Lounge

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MikeWilson

MikeWilson

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Hi Pakmoto,

Light travels at a certain speed, like everything else. Except that the speed of light is constant (it doesn't change) and nothing can go faster than light.

He was saying that Andromeda is so far away that in the time it took the light to leave the suns (stars) there, to travel across the cold blackness of space and then to arrive at our planet where our eyes and cameras can detect them - the entire human race evolved on this planet.

A sobering thought.

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Pakmoto

Pakmoto

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Ah! thanks for that Mike, now why couldn't he have said that :D

I've watched that bit over and over again just to try and get a grip with what he was trying to say, but I think he got a little excited about it :rolleyes:)

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Hobbes

Hobbes

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Didn't they also talk about the theory that if you could reach the speed of light time would stop for you so that you could travel infinite distance in zero time however the time dilation would only apply to you not to the rest of space where time would be passing normally. So if we could travel at light speed we could cross the 2.5 million light years to the andromeda galaxy and it would feel as though no time had passed - we would not age at all but for everything else 2.5 million years would pass.

Rgds

Rob

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zcapp96

zcapp96

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He was actually talking about the effects of time dilation. As you approach the speed of light your perception of time slows down. For an object at the speed of light the time dilation effect is absolute and for that object time will in effect stop. Therefore it does not matter how far the light has come, as far as it is concerned the journey was instantaneous. To the rest of the universe time has passed however.

^^^ Hobbes beat me to it! ^^^^

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Axe

Axe

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just popping to andromeda for a research mission aged 45 ....just got back from andromeda aged 50 ''hey where'd everyone go? what year is it kind sir?''...''the date is now 2,500,002,010''...

welcome to the sky at nights 2,500,000,700th episode!...lets join our team on mars!

lol it would really mess your mind up

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Gra

Gra

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just popping to andromeda for a research mission aged 45 ....just got back from andromeda aged 50 ''hey where'd everyone go? what year is it kind sir?''...''the date is now 2,500,002,010''...

welcome to the sky at nights 2,500,000,700th episode!...lets join our team on mars!

lol it would really mess your mind up

Would that not be year 5,000,002,010 as it's a return journey :D

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Pakmoto

Pakmoto

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Wow, so it could be possible that half way through you journey to Andromeda, someone from Earth could pull you up and say.....

"what the hell are you doing in that space ship, it's so out dated! we use warp now"

:D

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zcapp96

zcapp96

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watched QI last night and tghey stated that if u put light through frozen sodium ? they slowed light to 38 miles on hr ......

The speed of light is actualy the speed of light in a vacumm so if it is passing through any medium it will be slower. You have to take anything that QI says with a pinch of salt, they often get things hopelessly wrong. They don't have a panel of experts picking over the questions, probably just a bunch of media studies interns looking through wikapedia! I remember Alan Davies getting a question 'wrong' for saying the earth only had one moon!

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zcapp96

zcapp96

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Not quite, one's own perception of time does not change one bit.

Sorry, should have added 'in comparison to an observer at rest in respect to you in the same frame of reference' but was just trying to get it across in simple laymans terms!

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Rover

Rover

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So thinking about it, Betelgeus (I have probably spelt that wrong) could have already have gone supernova but we are still waiting for the light to show it has and as we look at it now it is still showing us how it looked in the past?

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Jiggy 67

Jiggy 67

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So thinking about it, Betelgeus (I have probably spelt that wrong) could have already have gone supernova but we are still waiting for the light to show it has and as we look at it now it is still showing us how it looked in the past?

Correct, that what a light year is all about, the distance light travels in a year. If something is 1,000,000 light years away, then you are looking at it as it was 1,000,000 years ago

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Wattsy

Wattsy

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The speed of light is actualy the speed of light in a vacumm so if it is passing through any medium it will be slower. You have to take anything that QI says with a pinch of salt, they often get things hopelessly wrong. They don't have a panel of experts picking over the questions, probably just a bunch of media studies interns looking through wikapedia! I remember Alan Davies getting a question 'wrong' for saying the earth only had one moon!

that episode really bugged me

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JamesF

JamesF

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Sorry, should have added 'in comparison to an observer at rest in respect to you in the same frame of reference' but was just trying to get it across in simple laymans terms!

I think part of the problem with talking about time dilation etc. is that it just doesn't work very well in layman's terms. Partly perhaps because the notion of time as immutable is very deeply ingrained into our subconscious. For some reason it doesn't seem to be a big problem to think of speed as always being relative to something else, but treating time as something variable is more difficult to get to grips with.

I think it can be misleading to refer to "normal" time, too, as it can be interpreted as meaning there's a "more correct" version of time whereas in fact there's just "time as measured in our frame of reference" and "time as measured in someone else's frame of reference".

So, for instance, if you are on a spaceship travelling to Andromeda at a large fraction of lightspeed then for you time seems to pass just the same as it ever did. And so it does for me, left here on earth. However, from my point of view, your time appears to be passing very slowly relative to mine, and from your point of view, mine seems to be passing very quickly relative to yours. Let's hope I have that the right way around :D

What I struggled to to get my head around for a while was the suggestion from Why does E=mc^2? that if we could travel at a very large fraction of light speed. say 0.999999c, then it would be possible to travel to Andromeda in 50 years when it takes light, which is travelling faster, 2.5 million years. Then I realised that the authors meant that it would appear to take 50 years on the space ship. To an observer still here on earth it would appear to take 2.5 million / 0.999999 years for the space ship to arrive, so you'd not get there faster than light at all; you'd just get there faster than light appeared to make the trip in someone else's frame of reference.

James

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themos

themos

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your time appears to be passing very slowly relative to mine, and from your point of view, mine seems to be passing very quickly relative to yours

No! Moving clocks ALWAYS appear to go slower, never faster.

Then I realised that the authors meant that it would appear to take 50 years on the space ship.

Yes. In fact there's no lower limit to "travel time" imposed by relativity. Just be careful not to confuse which clock you are using: for "travel time" you look at the clock you are carrying with you.

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ismangil

ismangil

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You mean Professor Brian Edward Cox can slow light?

(Yeah, OK, I have looked at YouTube)

Allan

Have you not all realized that Prof Cox is actually a time traveller from the future, and his only hope of getting back is educating us on warping time-space continuum!

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