# What is time??

Discussion in 'Science & Society' started by Shadow1, Feb 5, 2011.

1. ### arfa branecall me arfValued Senior Member

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@river: time always has been a measurement of the movement of something.

There is a problem with "the movement" of light, though. We can't see light "moving" through space, right? We can, however, measure the frequency of light, so if we use a fixed frequency as a reference we can also fix a distance spatially. There is nothing particularly difficult or weird to understand.

What is weird, is that light has no 'time distance', and objects with mass do; light "travels" through space but has no time dimension in spacetime, because photons all lie on the (observer's) lightcone. So any light you see, like from distant stars, has an apparently infinite velocity, or is 'Newtonian' (i.e. instantaneous).

3. ### Pincho PaxtonBannedBanned

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What's this then?..... In a vacuum, light travels 299792459 meters in one second.

Of course it has a time distance.

5. ### river

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agreed

true

can't see the beam , as in a flash light beam

light has infinite velocity ?

7. ### arfa branecall me arfValued Senior Member

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In flat spacetime, the worldlines of photons all lie on the future lightcone, and have zero Minkowskian 'distance', or zero degrees of rotation into the interior of the same lightcone. What you are referring to is measurement, relative to a predefined time interval: the second.
Did you skip over the word "apparently" in the phrase: "apparently infinite velocity"?

Can you tell by looking at a star how far away it is, or does the light appear to reach you "instantaneously"?

8. ### river

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I have been thinking this for several yrs now

I would like an experiment where light is blocked from one point in space , a star , light yr away , then released , unblocked

it would be interesting

9. ### arfa branecall me arfValued Senior Member

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Would it matter if the light was blocked at the star (somehow), or blocked at the point of observation? What would doing either tell you about the light when it was 'unblocked'?

That is, how would it be interesting? It's just that I can block light from distant or local objects, by closing my eyes, say. So are you interested in the difference between having your eyes open or closed? If you are, you might have missed out on some early childhood stuff.

10. ### river

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just at some point where it would take a yr to reach me , thats all

11. ### arfa branecall me arfValued Senior Member

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"That's all"? How would you locate this point?

12. ### river

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a star one light yr away from us

13. ### arfa branecall me arfValued Senior Member

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How do you measure the distance, is what I'm asking. How do you tell the distance to any star, how do astronomers do it?

14. ### river

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Proxima Centauri V645 Cen 4.2 11.05 (var.) 15.5 M5.5Vc

so our nearist star is 4.2 light yrs away

nevertheless block that light beam one light yr away from us

then see what happens

15. ### arfa branecall me arfValued Senior Member

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The very first thing that will need to happen, is locating the point 1 light year away between you and the star. How do you propose doing that? Where are you, relative to Proxima Centauri? Remember, the earth and the sun are moving through space.

Again, why would this be "interesting"? What do you think it will tell you, and about what?

16. ### river

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by a space object that is launched from Earth and is directed to a certain point in space that blocks the light from a star , one light yr away

it would tell me about the speed of light and about space

17. ### ReikuBannedBanned

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Nope, it don't. Arfa is right.

In psuedo-vector space of relativity, distance equals zero when velocity equals "c" because of length contraction.

$0 = \sqrt{(1 - \frac{v^2}{c^2})$ when $v=c$

So light actually does not take a year to travel a lightyear, it actually takes no time at all. It's birth is simultaneously it's death.

18. ### wlminexBannedBanned

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. . . if so . . . why does it take 4.2 years for light to 'reach' us (the observer) from Alpha Centauri . . . . and why do physicists say that the light presently reaching us from Alpha Centauri was initiated 4.2 years ago at Alpha Centauri? . . . if LF contraction reduces the distance traveled to zero
(0)? . . . just curious

19. ### river

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Mister

so my thinking is true ?

so that if I block light a light yr from here , unblocked I would see the light instantaneously

wow

20. ### arfa branecall me arfValued Senior Member

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Well, that's "almost" correct. But light does have a 'time-distance' from an external frame of reference.

What you mean to say is that light has no proper frame of reference, it can't be given a time coordinate in Minkowski spacetime because its worldline is congruent with the lightcone and can't be Lorentz-transformed (the transform is always 45[sup]o[/sup] = 45[sup]o[/sup], or 0 = 0, which is what you've really pointed out).

21. ### river

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we have come upon an interesting discussion here

22. ### ReikuBannedBanned

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Yes, moving in what is more collectively known as a null trajectory. But yes, that is true also, from a bradyon's point of view (we are bradyons, or tardyons, things that move sub-luminal speeds) experience light taking time to reach other places.

A unique way to solve this paradox is by saying a photon does not have a frame of reference.

23. ### OnlyMeValued Senior Member

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A distance in space does not become length contracted by virtue of an object moving through it.

The object may become lenth contracted in the direction of its velocity.

And an object/observer/clock may become time dilated by virtue of its velocity.

But unless you can define space as an object and then give it a velocity.......

Space does not become length contracted as any function of the velocity of objects moving through it.

Last edited: Jan 12, 2012