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Author | Topic: What is Time and Space | |||||||||||||||||||||||||||
GDR Member Posts: 6202 From: Sidney, BC, Canada Joined: Member Rating: 2.2 |
As a disclaimer to this post I would just like to note that I have no background in physics, but have read books by Hawking and Greene, and as they say, a little knowledge is a dangerous thing. Also, I have questions about how science has calculated time and distance, but I am not trying to make any point whatsoever in regards to my Christian faith. (I don’t find my faith and science in conflict at all.) I am just trying to understand better what it is I’m reading.
I find time as defined by relativity fascinating and confusing. For example we say that a particular star is 10 billion light years away. We are saying that the light from this star took 10 billion years to reach us travelling at the speed of light. If however we had a seat on the photon travelling here no time would have passed at all. If you have zero time, then no matter what the velocity is, you have zero distance. In other words from our perspective the universe is huge, but from the perspective of a photon we are back to a singularity with the Earth and the star being co-located. As I see it, as photons are always travelling at the speed of light, it is still existing at the exact same instant as when it came into being. Which view represents reality? Also, in Greene’s book, The Fabric of the Cosmos, he says that if someone on that star 10 billion light years away started walking towards me at 10 mph it would put him 150 years in the future from my perspective. I understand that the speed of light is constant relative to everything regardless of motion. However motion causes time to change, (as I understand Greene). As everything is growing further away from everything else then everything is in motion relative to everything else. As there is no standard reference point in space there isn’t a reference point for time either. We can’t say how fast we are moving through space, as there is nothing to reference our rate of motion to. Doesn’t this mean that the relative time between us and any other body is something of an unknown? If in the end, as time is relative, does a year have any real meaning; and if it doesn’t then neither does the term light year. It just seems to me with my extremely limited understanding; relativity makes it impossible to say that the universe is a particular size or age because we can only measure things from our perspective on space and time. If we were elsewhere in the universe with a different vector in time and space wouldn’t we come to entirely different conclusions? How can we say what perspective if any represents reality? Please keep any answers simple as the only math I’m prepared for is distance = velocity X time. This message has been edited by GDR, 07-28-2005 09:26 PM This message has been edited by GDR, 07-29-2005 06:35 AM This message has been edited by GDR, 07-29-2005 10:51 AM
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Thread moved here from the Proposed New Topics forum.
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 5.4 |
GDR writes: It just seems to me with my extremely limited understanding; relativity makes it impossible to say that the universe is a particular size or age because we can only measure things from our perspective on space and time. I'm sure there are better answers, but my short answer is that of course the universe can have no one size or age from all perspectives. That's what relativity tells us. The amount of time that has passed between events, and the amount of space that exists between two points, is relative to the observer's reference frame. There is no one right answer. But there may be a more meaningful question than the one you raise about how old the universe would appear if you were traveling on a photon. This oft-mentioned Einstein thought experiment has never proved helpful to me because it is never described, at least not in anything I've read, what Einstein thought he would see while traveling on the photon. It might be better to ask how old a universe an observer in a galaxy 10 billion light years away would see. Between ourselves and this observer space is expanding at a collosal rate, yet because of the high degree of uniformity in the structure of the known universe (that we assume for the moment applies to the entire universe) we know that this distant observer would also see a universe 13.7 billion years old. Another interesting question to ask is what an observer in a heavy gravtitational field would see. Let's say life somehow evolves on the surface of a neutron star, which has a very strong gravitational field, and let's assume this neutron star is relatively close to us within our own galaxy so that we know he sees pretty much the same thing we see when he peers up at the sky. How old a universe would this observer see? I don't think I can answer this question. I know that when we turn our telescopes on this observer we would see time passing by more slowly for him, because he's in a strong gravitational field. But I'm not sure what he would see when he looks at us or the rest of the universe. There's a couple people here who are very good at this type of question, Eta and Sylas. Hopefully they'll chime in, because I know more people than just us are interested in answers to questions like this. --Percy This message has been edited by Percy, 07-29-2005 09:10 AM
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NosyNed Member Posts: 9004 From: Canada Joined: |
Starting on page 233, Greene explains the uniformity of time.
quote:
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 5.4 |
I don't own Greene's book so I can't look it up, but by "notion of time" are you sure he means the same thing as "perception of time"? The excerpt isn't long enough to tell, but I would have guessed that he means that time has the same nature throughout the universe, and that we can figure out what the perception of time would be from another reference frame that is moving or accelerating relative to our own.
--Percy
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cavediver Member (Idle past 3673 days) Posts: 4129 From: UK Joined: |
I'll come back to this later if I've got time (it's my 10th wedding aniversary) but PLEASE PLEASE PLEASE
It's HAWKING !!! Hawkins was the lad in Treasure Island... [ed for sp] Which view represents reality? Both are equally valid views. What you think of as reality (distance and time) is not a well defined concept. Space-time is four-dimensional and real distances are distances between 4d "events" not 3d "places". For example, I clap my hands at 10pm GMT here in the UK, and you clap your hands at 12pm GMT in Sidney. There is a 4d "distance" between the two events. It is "time-like" distance because I could manage to get from me to you between the two events (in my private space shuttle). However, if the events are such that I couldn't get between them, even at the speed of light, then the distance is "space-like". If the events are arranged so that light could just race from one event to the other, then the distance is "light-like" or "null". And that null distance is indeed zero. To ask how far away something is, you have to specify in what 4d direction. Don't forget, a star is not a sphere sat in space, but a very long line stretching through space-time, as are we all (though not quite so long in our case!). The distance between the two lines depends totally on the direction you measure. The "minimum" distance is in a null-direction, and is always zero! But in 4d space-time what you are interested in not the shortest distance (which is always zero) but the furthest distance. The furthest distance is generally what we experience.
the only math I’m prepared for is distance = velocity X time Oh dear... This message has been edited by cavediver, 07-29-2005 09:51 AM
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cavediver Member (Idle past 3673 days) Posts: 4129 From: UK Joined: |
The amount of time that has passed between events, and the amount of space that exists between two points, is relative to the observer's reference frame. There is no one right answer.
This is only true for coordinate time and space, and these are not real concepts (as they depend upon your point of view). Real concepts do not depend upon a point of view. The proper time or proper distance along a chosen path between two events is invariant.
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 5.4 |
cavediver writes: This is only true for coordinate time and space, and these are not real concepts (as they depend upon your point of view). Real concepts do not depend upon a point of view. The proper time or proper distance along a chosen path between two events is invariant. Okay, now I'm really confused. You'll have to explain the theory of invariantivity to me. --Percy
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cavediver Member (Idle past 3673 days) Posts: 4129 From: UK Joined: |
When I get back... dashing out. Ready for some maths?
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GDR Member Posts: 6202 From: Sidney, BC, Canada Joined: Member Rating: 2.2 |
cavediver writes:
Honestly. I had noticed that myself and edited it between the time you posted and the time that I read your post. It's HAWKING !!! Hawkins was the lad in Treasure Island Thanks for the reply. I've got to think about it more before I can reply to it. This message has been edited by GDR, 07-29-2005 07:57 AM Everybody is entitled to my opinion.
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GDR Member Posts: 6202 From: Sidney, BC, Canada Joined: Member Rating: 2.2 |
Percy writes: I don't own Greene's book so I can't look it up, but by "notion of time" are you sure he means the same thing as "perception of time"? The excerpt isn't long enough to tell, but I would have guessed that he means that time has the same nature throughout the universe, and that we can figure out what the perception of time would be from another reference frame that is moving or accelerating relative to our own. Thanks Nosy. That helped. There was a lot to absorb in that book for a neophyte and I hadn't mentally connected the two sections. Percy he is replacing the pennies (that represent the galaxies) on the balloon with identical clocks. As the balloon expands the clocks maintain symmetry so that they maintain a consistent cosmic time. Our personal perception of time varies as we move around from place to place within that cosmic model. One question I have of that model though is this. Instead of a balloon we use the planet Earth. I have phenomenal eyesight that is bent by the gravity of Earth so that I can see right around the planet. From Canada I can look east and see cavediver in the UK but I can also look west over a greater distance and see him from the other side. (As carediver’s back look different from his front I wouldn't even realize that I am looking at the same person.)In fact if I look harder enough in any direction I can see my own back. This message has been edited by GDR, 07-29-2005 07:58 AM Everybody is entitled to my opinion.
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 5.4 |
GDR writes: Percy he is replacing the pennies (that represent the galaxies) on the balloon with identical clocks. As the balloon expands the clocks maintain symmetry so that they maintain a consistent cosmic time. Our personal perception of time varies as we move around from place to place within that cosmic model. Right. In other words, the expansion of space does not change the relationship of clocks that are not moving with respect to one another. In even more detail, consider two clocks, both stationary with respect to you, but one is in front of you and the other is 10 billion light years away. The distant clock will appear to be receding from you at great velocity because of the expansion of space, but it is really stationary with respect to you. Both clocks will keep perfect time, meaning each will click off seconds at precisely the same rate.
One question I have of that model though is this. Instead of a balloon we use the planet Earth. I have phenomenal eyesight that is bent by the gravity of Earth so that I can see right around the planet. From Canada I can look east and see cavediver in the UK but I can also look west over a greater distance and see him from the other side. (As carediver’s back look different from his front I wouldn't even realize that I am looking at the same person.)In fact if I look harder enough in any direction I can see my own back. Earth's gravity isn't strong enough to bend light around its circumference. For that you need to be at the event horizon of a black hole. --Percy
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GDR Member Posts: 6202 From: Sidney, BC, Canada Joined: Member Rating: 2.2 |
percy writes: Earth's gravity isn't strong enough to bend light around its circumference. For that you need to be at the event horizon of a black hole. I understand that but I'm trying for a metaphor for the universe and that was the best I could do. Hypothetically then, if the gravitational pull of the Earth bent my vision so that I could see beyond the horizon, (which I know doesn't happen in reality), and if I had strong enough vision, (which I don't), I could see my back. I've probably gotten off the OP with this line of thought however.
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cavediver Member (Idle past 3673 days) Posts: 4129 From: UK Joined: |
Earth's gravity isn't strong enough to bend light around its circumference. For that you need to be at the event horizon of a black hole. Not quite The photon orbit in Schwarzschild geometry is 50% further out than the event horizon. So you don't actually need a black hole to get the photon orbit, just a spherical mass M contained within r < 3M. It's so cool though. You build a tubular ring around the black hole at the photon orbit and once stood inside, it look straight! And you see lots of copies of yourself. If you build it between the photon orbit and the event horizon, it looks as if it is curving the wrong way! See, even the outside of black holes are fun
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cavediver Member (Idle past 3673 days) Posts: 4129 From: UK Joined: |
Hypothetically then, if the gravitational pull of the Earth bent my vision so that I could see beyond the horizon, (which I know doesn't happen in reality), and if I had strong enough vision, (which I don't), I could see my back. In a closed universe (as described by Greene) yes you could, except you normally don't have time to! The universe collapses before your sight gets all the way around. If you prop up the universe from collapsing with a cosmological constant ("dark energy") then it's possible as long as the expansion doesn't always outpace light itself. In the Einstein Static Universe (ESU), where the universe stays the same size, then it is perfectly feasible. You can consider other topologies too, like a torriodal universe. In that case you get to see multiple copies of yourself much more easily...
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