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No, it happened 26,000 years ago in our time frame as well



The problem is that "26,000 years ago" depends on perspective. For example, if space aliens were on a rocket that took off thousands of years ago but traveled at nearly the speed of light, they might say now (where "now" is approximately equal to our "now") that this actually happened 21,000 years ago.

In other words, you can say "26,000 years ago from Earth's perspective", but it isn't the definitive answer for everything in this galaxy, and it definitely isn't the same answer for the rest of the universe.


I mean you’re kind of mixing up two concepts. Distances and the passage of time do depend on frame of reference. But that’s not the same thing as saying something we observe today happened today in our “time frame”. In our frame of reference, this event happened 26,000 light years away 26,000 years ago.


To add to the other comments that are attempting to correct you (although the downvotes seem unwarranted): the misconception you are presenting is one of the most common ones I see in students when teaching relativity. The event did happen long time ago and the fact that time and space are relative have nothing to do with the fact that it takes us some time to see/hear that an event has happened.


> that time and space are relative have nothing to do with the fact that it takes us some time to see/hear that an event has happened

I am struggling to guess exactly what you mean by that. Could you clarify? Is the first part of the quote (up to "have") just a statement about coordinate charts? If not, and it helps with the clarification, I know how SO+(1,3) (and more particularly the one-parameter SO(1,1) boost subalgebra of O(1,3)) relates to the gauge symmetry of massless |helicity| >= 1 particles and flat spacetime, and I'm pretty sure you do too.


Yeah, sorry, trying to be brief made my statement rather unclear.

Here is the conversation I frequently have in intro physics if relativity is included (it is this particular very common misconception I was trying to point out):

student: "So, time is relative because light(information) has a finite speed. Therefore, if a star at distance 1kly explodes 1k years ago, for us it happens today?"

me: "No, it is a bit more subtle. What you have said is true both for sound and light, but we do not talk about 'sound' theory of relativity, so there must be something more to light. Time is relative because light(information) has a fixed finite speed in all inertial reference frames. The star exploding and we observing the star explode are two different events. The time between these two events is measurable, it is 1k years in our reference frame for instance." (and then I go into how the space-time-interval is the thing that is constant, how these two events are always in the same order, how only in the reference frame of the photon they happen at the same time, how in another reference frame it might be 1year or 1M years, etc).


Depending on how "intro" the conversation is, I'd start "events first" and explain how one can describe a set of events differently. I'd do it with a couple of transparencies, or a sheet of paper (for events and intervals) and one or more transparencies (for systems of coordinates). With your example, draw a Minkowski spacetime diagram with the spatial origin always at Earth, the events of light, neutrinos, and charged particles arriving at the Earth at different times (0 for the light, later for the heavier matter), and the exploded star with the simplification that all the particle occupy exactly same point in spacetime somewhere near the stellar explosion (this is literally a coincidence, in Einsteinian language). Draw in the intervals connecting that point with the points representing the terrestrial detections.

Then start with simply rescaling the coordinates on the timelike axis. Nothing changes but the labels. The same thing if we simply rescale the spacelike axis. We can measure in parsecs, lightyears, lightseconds, metres, and so on; on the time axis we can use seconds or fortnights or kiloyears and the but for the coordinate labels the diagram does not change. Then demonstrate a translation by sliding the timelike axis down so that the timelike origin is last week, last year, next year, and so on; demonstrate a spacelike translation by sliding around the origin on the spacelike axis. Again, invariance is manifest. We can do both types of translation at once, and put the origin on the exploding star instead of on the Earth. All we're changing is the labels associated with the events (and the labels for points along the intervals).

One can also show that a rotation of the coordinates also only changes labels, not relative locations -- this is where a pair of transparencies, or one paper one transparent layer works well. It might be handy to have the rescalings handy on transparencies so you can show that you can freely mix coordinate rescalings and translations and rotations, and the (literal!) underlying picture remains invariant in the face of all of these.

This opens up at least four interesting types of discussion: (1) if we calculate out the light's interval it's 0 (thus "null"). Any segment of the null path from exploded star to detector is also itself lightlike between the two ends of the segment, so dS^2 is still 0. What gives? (Answer: using a set of coordinates lets us deal in coordinate-based distances; and for extra credit we could talk about arbitrary parametrization, affine-parameter style); (2) why are the slopes of the different explosion products different, or why is the interval timelike for the non-light material?; (3) what do boosts do to coordinates?; (4) coordinates representing a moving observer with the origin always fixed on itself leave the underlying picture invariant but suggests kinematics and dynamics; and (5) a contrast between a "conventional" observer that remains at spatial zero but which "evolves" along the timelike axis compared to the same observer against a different set of coordinates.

For extra credit, explicitly distinguish between types of Lagrangian and Eulerian observers that differ only in their choices of systems of coordinates as in (4) and (5), even as the underlying picture of events remains identical for all observers.

This leads us into a block universe picture, where we have (classical or quantum) fields filling the whole of spacetime -- completely everywhere -- with arbitrary sets of coordinates imposed on portions of the spacetime used to describe field-values as (a) "objects" like worldtubes and instantaneous snapshots giving us something like a classical extended object; (b) dynamical laws that in a block universe picture simply predict the field-values (or "objects") at at a coordinate-location in spacetime given some set of field-values at another location in spacetime [1]; (c) the "conventional" splitting of the block universe into 3+1 spacetime, but showing that the split is as arbitrary as any choice of system of coordinates because we can e.g. rotate a set of Cartesian coordinates differently for relatively boosted "conventional" observers.

- --

[1] Essentially, this is pulling in the idea that initial values need not be in the relative past for a spacetime-filling IVP solution to obtain! Indeed, undermine the idea that we are evolving initial values in time, rather than extending some set of values outwards from a kernel. Any thought of time evolution can be cured by remembering that one should as a matter of course at least briefly consider all sorts of messed up coordinate charts on top of the underlying block picture. One is simply preferring to work with one static chart for purely personal reasons (which may include the ease with which an intended audience may follow the path to a set of results).

> 1year or 1M years, etc.

Here I would like to believe that "M" means "Martian". Or perhaps "Mercury". :D But those would correspond with a coordinate rescaling rather than a boost.


Carlos Rovelli has an excellent book called The Order of Time which helps a lot with grokking this. There is no universal clock. Time is relative.


I don't get how that works. 26,000 years ago it would have been FAR outside of our "light cone". In other words it was completely beyond anything that affected our world. Only recently that changed.

So doesn't it make sense that it only became part of reality yesterday ? And therefore to say that it happened yesterday. The famous supernova of 1572 AD also very much didn't happen anywhere close to 1572 AD.


So let's say you travel at the speed of light towards that event. When you arrive, 26,000 years later, would you then say the event happened 26,000 or 52,000 years ago? If the former, have you just travelled 26,000 years into the past, or did time stand still for 26,000 years from your perspective? I'd argue your clock has still moved forward 26,000 years.

If we would take a snapshot of the Universe VM spacetime at the current moment at the start of the event, the "now" in time would have to correspond to the current state for every other location if you want it to be useful and not only intuitive.


From my understanding, if you travel at light speed, space is compacted to very tiny distances and time stops. If you take the math of relativity to its extreme, time for you has stopped and your trip took no time. It actually takes no time because there is actually no distance between you and the black hole.

But this is where paradoxes come in because the black hole, stationary from in its frame of reference, would still experience 26,000 years and not see you arrive until then. From my understanding, general relativity solves this paradox with de-acceleration causing the one who was traveling at the speed of light seeing the rest of the universe's clocks' speed up.

So technically, your clock would have not moved 26,000 years forward, but other's would have.


> But this is where paradoxes come in because the black hole, stationary from in its frame of reference, would still experience 26,000 years and not see you arrive until then.

Special relativity solves this fine (there is no paradox in it for that particular thought experiment), no need to involve general relativity. There is nothing paradoxical about the observer near the center of the galaxy not seeing you until you are near, because you are traveling at speed almost as great as the speed of the information carrier (the light you emit if we are talking about space, or the sound you make if we are talking about something on Earth). So you actually do not even need relativity to explain the "they do not see us coming". You do need relativity to explain the fact that to you all of this took only a blink of an eye.


My bad I meant to talk about the fact that there is no universal reference frame, so in your frame you would believe you aren't moving, but the black hole is moving towards you. In in a scenario where you are traveling at .99c, from your frame of reference you are not traveling at all and the black hole is traveling to you, so you see the black hole experiencing incredibly slow time passage. However, that is not what the black hole sees. Hence a paradox.


Not a physicist, but afaik you are right. If you go with the speed of light (or close enough), your travel time goes to zero. It is common misconception that to “reach the stars” you have to spend a couple of lives. In fact, you can get there before you finish your sandwich.

This makes me think of lone sad CMB photons, who accidentally stop by hitting an atom and cannot even recognize a tiny bit of the universe they remembered since. A spark moment, and they lost everything to billions of years.


Does that also mean they would get hit by all the light energy that hit them on the way on the moment of "arriving" in that collision ?

Or is it just the first collision ever that stops them and they just happen to miss every last particle in outer space for 13 billion years perhaps.




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