Questions about Time, Distance, and Relativity

[Mudhammam]

No doubt this is all going to come out very clumsily, so please bear with me. I'm currently reading Brian Greene's mind-blowing but pleasantly accessible classic, The Elegant Universe. I was reading the section "Motion Through Spacetime" to my girlfriend in which Greene makes the point that a photon emitted at the big bang is the same age as it was "then." My girlfriend, recalling the recent Cosmos series, asked how it is that light can be used to measure the age of the Universe if the speed at which it travels eliminates any passage of time (in reference to the point-of-view of the photon). My muddleheaded answer to her clearly failed to clarify the reasoning for this. That's my first question. But furthermore, it created an additional question-mark for myself. When we speak of the passage of time and distance, per the theory of relativity, are we only speaking in terms of reference to ourselves? That is, is it correct to say that for us as observers, on "our clocks," the Universe is such and such an age, but that on other reference points, it could be different? Is the same true for distances? In other words, does the relativity principle eliminate what may otherwise have seemed to be the sure falsification of (classically conceived) contradictory statements since the truth about an event can only be measured from a particular vantage point, and that among a plurality of positions moving through spacetime each is entitled to a correct but different interpretation of said event, relative to themselves? Does it falsify "objectivity" in regards to the age of the Universe, as that seems to only be true per our level of description, given our state of motion, but not necessarily from a person, say, hypothetically standing on the edge of a black hole's event horizon?

[Exian]

I'm too dumb for this. By the time I get to the end of one of your sentences, I feel I've extended my mental tape measure too far, and it collapses to the floor with a metallic *plinck*. Lol
I won't quit trying though. I'll comprehend this eventually.

[Surgenator]

how it is that light can be used to measure the age of the Universe if the speed at which it travels eliminates any passage of time (in reference to the point-of-view of the photon).

Only the photon experience no passage of time. From the photons perspective, it has traveled 0 distance at 0 velocity for 0 time. To any other non-speed-of-light observer, the photon travels a non-zero distance at velocity c for a non-zero time. This sounds very wierd because traveling at the maximum speed limit of the universe is like doing math with infinities (and techically you are). It can be done, but it's really hard to have any intuitive sense about what is going on.

I still have problems conceptualizing relativity's predictions. However, you can't argue with results.

When we speak of the passage of time and distance, per the theory of relativity, are we only speaking in terms of reference to ourselves? That is, is it correct to say that for us as observers, on "our clocks," the Universe is such and such an age, but that on other reference points, it could be different? Is the same true for distances?

Yes, the same is true of distance. A common special relativity problem ask's what is the distance between Earth and alpha centauri if you were traveling at 0.9c. The answer is not ~4 light years but ~1.8 light years. This is why you shouldn't think of time and space being two different things. They are interdependent and should be considered as one thing, spacetime.

In other words, does the relativity principle eliminate what may otherwise have seemed to be the sure falsification of (classically conceived) contradictory statements since the truth about an event can only be measured from a particular vantage point, and that among a plurality of positions moving through spacetime each is entitled to a correct but different interpretation of said event, relative to themselves?

Not necessarily. You can still have events that no matter what reference frame you look at, the order of the two events would not change.

Does it falsify "objectivity" in regards to the age of the Universe, as that seems to only be true per our level of description, given our state of motion, but not necessarily from a person, say, hypothetically standing on the edge of a black hole's event horizon?

No.

[vorlon13]

A thought experiment:

run a string from here to the Andromeda galaxy (2 million light years m/l)

put a knot in the string every mile.

fly along side the string at .999whateverC relative to us stationary along side the string. To you, on the spacecraft, counting knots as they go by for an hour, you will note relative to YOU, that you're traveling at an extremely large multiple of the speed of light, and going "WTF?, I thought faster than light was impossible !!!"


If you get to the Andromeda galaxy in 20 years (thanks to time dilation), your speed you measured was 100,000 times the speed of light. To us stuck at home, however, it really did take you 2 million years to get there.

[Mudhammam]

Only the photon experience no passage of time. From the photons perspective, it has traveled 0 distance at 0 velocity for 0 time. To any other non-speed-of-light observer, the photon travels a non-zero distance at velocity c for a non-zero time. This sounds very wierd because traveling at the maximum speed limit of the universe is like doing math with infinities (and techically you are). It can be done, but it's really hard to have any intuitive sense about what is going on.

I still have problems conceptualizing relativity's predictions. However, you can't argue with results.

Yeah, it's quite confusing... Greene writes, "Einstein proclaimed that all objects in the universe are always traveling through spacetime at one fixed speed--that of light... If an object does move through space, however, this means that some of the previous motion through time must be diverted... We now see that time slows down when an object moves relative to us because this diverts some its motion through time into motion through space. The speed of an object through space is thus merely a reflection of how much its motion through time is diverted... the maximum speed through space occurs if all of an object's motion through time is diverted to motion through space. This occurs when all of its previous light-speed motion through time is diverted to light-speed motion through space."

Can you elucidate that?

Yes, the same is true of distance. A common special relativity problem ask's what is the distance between Earth and alpha centauri if you were traveling at 0.9c. The answer is not ~4 light years but ~1.8 light years. This is why you shouldn't think of time and space being two different things. They are interdependent and should be considered as one thing, spacetime.

Ah, that makes perfect sense (duh!).

Not necessarily. You can still have events that no matter what reference frame you look at, the order of the two events would not change.

Does it falsify "objectivity" in regards to the age of the Universe, as that seems to only be true per our level of description, given our state of motion, but not necessarily from a person, say, hypothetically standing on the edge of a black hole's event horizon?

No.

How so? I'm still a bit confused here, particularly on the last point... I'm sure this is a very amateurish question, but if time is not absolute as once conceived, how can an absolute age of the Universe be given? Because the speed of light is absolute? But wouldn't that be... zero? As a hypothetical aside, could spacetime ever be warped to such a degree that it "turns in on itself" and inverts causality?

[Napoléon]

---

A thought experiment:

run a string from here to the Andromeda galaxy (2 million light years m/l)

put a knot in the string every mile.

fly along side the string at .999whateverC relative to us stationary along side the string. To you, on the spacecraft, counting knots as they go by for an hour, you will note relative to YOU, that you're traveling at an extremely large multiple of the speed of light, and going "WTF?, I thought faster than light was impossible !!!"


If you get to the Andromeda galaxy in 20 years (thanks to time dilation), your speed you measured was 100,000 times the speed of light. To us stuck at home, however, it really did take you 2 million years to get there.

---

Le me:

[Alex K]

I'd love to respond in great detail, but I'm travelling, so it's going to be criminally short.
One important factoid: while in special relativity the passage of time can only be specified for a given observer ssystem, the real world is somewhat different: the CMB gives us an absolute rest frame at each point. This allows absolute definitions of the passage of time, e.g. for the age of the universe.

A light beam has indeed no proper time passage in its own frame of reference system.

[TheRealJoeFish]

Yeah, it's quite confusing... Greene writes, "Einstein proclaimed that all objects in the universe are always traveling through spacetime at one fixed speed--that of light... If an object does move through space, however, this means that some of the previous motion through time must be diverted... We now see that time slows down when an object moves relative to us because this diverts some its motion through time into motion through space. The speed of an object through space is thus merely a reflection of how much its motion through time is diverted... the maximum speed through space occurs if all of an object's motion through time is diverted to motion through space. This occurs when all of its previous light-speed motion through time is diverted to light-speed motion through space."

Can you elucidate that?

A good read of Flatland might help. How about this: let's say you're in a car going 100 mph, driving east to west. There's someone south of you, and he sees you silhouetted on the horizon. You turn your car 45 degrees to the right, so you're now going northwest. He can only see the car's east-west movement, do to him it now looks like you're only going about 71 mph (100*sqrt(2)/2).

When we're not moving, we're going (speed of light) through time. When we move, it's like we're still going the same speed, but we've turned 45 degrees (or, in reality, like, a billionth of a degree) off of the straight past-to-future line and into the set of dimensions we see.

That's probably a really inexpert way of thinking about it, but that's how I always sort of conceptualized it. Someone please tell me if I'm way off. And I will note that the particular paragraph you quoted, for me, was the one paragraph in that book that I have always remembered and cherished.

[Surgenator]

Only the photon experience no passage of time. From the photons perspective, it has traveled 0 distance at 0 velocity for 0 time. To any other non-speed-of-light observer, the photon travels a non-zero distance at velocity c for a non-zero time. This sounds very wierd because traveling at the maximum speed limit of the universe is like doing math with infinities (and techically you are). It can be done, but it's really hard to have any intuitive sense about what is going on.

I still have problems conceptualizing relativity's predictions. However, you can't argue with results.

Yeah, it's quite confusing... Greene writes, "Einstein proclaimed that all objects in the universe are always traveling through spacetime at one fixed speed--that of light... If an object does move through space, however, this means that some of the previous motion through time must be diverted... We now see that time slows down when an object moves relative to us because this diverts some its motion through time into motion through space. The speed of an object through space is thus merely a reflection of how much its motion through time is diverted... the maximum speed through space occurs if all of an object's motion through time is diverted to motion through space. This occurs when all of its previous light-speed motion through time is diverted to light-speed motion through space."

Can you elucidate that?

This is the first time I heard about we all moving at the speed of light. It does seem strange, and I would need more time to think about it. I did found a good link that discusses this specific topic. From my quick read of it, that we transverse the 4 dimensions at the speed of light. However, the amount we spend in the time dimension is what determines how much time we experience. Light, for example, doesn't spend any amount in the time dimension and hense transverse the space dimensions at the maximum speed.

Not necessarily. You can still have events that no matter what reference frame you look at, the order of the two events would not change.
No.

How so? I'm still a bit confused here, particularly on the last point... I'm sure this is a very amateurish question, but if time is not absolute as once conceived, how can an absolute age of the Universe be given? Because the speed of light is absolute? But wouldn't that be... zero? As a hypothetical aside, could spacetime ever be warped to such a degree that it "turns in on itself" and inverts causality?

This is not an easy nor amateurish question. My head is running in circles trying to come up with good answer. I know it has to do with time-like intervals and setting it up the events correctly.

Here is one possible explanation, but [b]I don't know if it's right.[\b] At the big bang, all of space was concentrated to a single point. Now, we can observe 13.8 billion years has past since then. So how far did we move in 3D space relative the big bang, 0. Yes zero because all the space was concectrated to a single point and expanded to what it is now. At any location you're standing now is all mapped back to a single point (this is what they mean by the big bang happened everywhere). So when you do the spacetime interval, you will always get 13.8 billion years has passed. This is independent of how fast your moving since every space lcoation is mapped back to a single point.

Maybe Alex has better luck. He sounds confident.

[Cyberman]

I'd love to respond in great detail, but I'm travelling, so it's going to be criminally short.

That's because you're moving. To we observers, it's much longer.