Talk:Light cone

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Maybe this page could be turned into a history of the ice cream cone. Here is a site to check for some data. [1] But probably the name of the article would be changed. Rednblu 04:33, 3 Sep 2003 (UTC)

Light cones don't have anything to do with ice cream; it's a general relativity thing. Whoever wrote the current version of the article was just making stuff up. —Paul A 04:39, 3 Sep 2003 (UTC)

Exactly. I am reviewing my relativistic mechanics notes to see if I could assemble a rough draft for us to improve. Rednblu 13:36, 3 Sep 2003 (UTC)

I've made a few changes to this article. I want to make major changes to it though. I have a version in my sandbox which people can edit.---Mpatel (talk) 14:50, August 28, 2005 (UTC)

The biggest concern is to have another article on absolute future and absolute past, which are global concepts which should be contrasted with the light cone, which is defined at the level of tangent spaces.---CH (talk) 23:30, 14 September 2005 (UTC)Reply

Thanks. _{(Unsigned comment by 84.92.147.230 6 January 2006}

I agree with this comment. I'm trying to decipher some science fiction novels (1 2) that require thorough familiarity with the "light cone" concept to fully understand their plots. This article really isn't helping. 4.89.240.163 00:24, 26 February 2006 (UTC)Reply

Well, I'm no physicist - but I think I have a layman's interpretation for you. Someone please correct if I am wrong. Imagine that you are floating in empty space and that you have just set off a very bright explosion (some kind of super firework). The light from this explosion will take time to propagate throughout the universe. Another person floating 1 light year away will not see your explosion for one year. However, a person only one-half light year away will only need to wait for one-half year. A person only one-quarter light year away waits only one-quarter year, and so on and so forth. You can see how the gradual and symmetric nature of time and distance forms a 4 dimensional cone (which actually looks to us like an expanding sphere) - this is the future light cone, because these other floating observers will see your explosion in the future.

Now the past light cone might work this way. Imagine that you are again floating around in your empty space with your explosive device and you want to set it off. There is just one problem, you can't set your explosion off until you see someone else's explosion first (perhaps this is a game - things can get boring in empty space). So you wait and watch the empty skies until you see a bright explosion. Hooray! Now you can set off your own explosion. But a question, how far away was that explosion you just saw? If it was 1 light year away, then that means that guy set his explosion off 1 year ago. You had to wait an entire year before you saw it and could set off your explosion. But if you were closer you would have waited a shorter time. Imagine if his explosion was set off 1 million light years away, you would be sitting there pretty bored with your firework for a very long time!

You can imagine someone moving away from you at the speed of light. This person will never see your explosion, and thus can never be within your future light cone. Conversely, an old explosion set off somewhere very distant will been seen by you long after you saw an earlier explosion and already set off your own device. You might not even see this ancient explosion until long after the game has become boring and everyone within 100 light years doesn't play anymore. This ancient explosion long missed it's chance to effect the game, and thus is outside of your past light cone.

In these examples the explosions and reactions represent spacetime events and the causal influence they exert upon each other.

I hope Einstein is not turning in his grave at my simplistic interpretation.63.192.52.26 22:49, 4 January 2007 (UTC)Reply

I think the simplest way to put it is that Einstein posited that Light is an absolute speed and nothing can go faster then light. Since nothing can go faster then light anything we DO (or rather, any EVENT) cannot effect anything outside of your light cone (the speed of light times the time which has passed since the event). So an explosion a million light years away will not effect us immediately, only once our position (time AND space) has been enveloped by the light cone can we be effected by the light from the explosion). Everything works at light speed or LESS then light speed, so any effect on one thing from another must reside within this cone of space and time. Gelsamel 17:13, 27 August 2007 (UTC)Reply

Yeah, you guys have it all pretty much right. What you're missing is the special relativity implications. What you're saying also applies to ripples in a pond, where the angle of the ripple-cone is the speed of the ripples on the surface. Meanwhile, a motor boat comes by, faster than the speed of ripples, and breaks all these rules about who can't see what. This is where they came up with the Ether idea, they thought it was like the waves they were familiar with.

I'm adding a new intro section. OK, there. Should make more sense. OsamaBinLogin (talk) 18:27, 6 June 2009 (UTC)Reply

The volume of the light cone should be mentioned as (1/3)*pi*r^2*h whereas the one-third is relational to clopen conditions. —Preceding unsigned comment added by 24.22.111.99 (talk) 04:21, August 29, 2007 (UTC)

Not relevant is it? h=t, time passed, by the way, but the metric is odd, being r^2-(ct)^2, so does this volume make sense? 51kwad (talk) 11:35, 27 January 2011 (UTC)Reply

Would you please comment on the new "light cone" image we have prepared. The intention was to imply the correct scale of space-time relative to the observer. Interstellar light-years vs years scales seemed appropriate. Thoughts? Dhatfield (talk) 11:55, 27 June 2008 (UTC)Reply

Great Image! really gives you a better idea of what is going on. OsamaBinLogin (talk) 18:03, 6 June 2009 (UTC)Reply

I think the "hypersurface of the present" is likely to be confusing - there is no notion of simultaneity in relativity, so such a surface does not exist. I in fact stumbled upon this image in trying to describe to a friend the nonexistence of simultaneity and ended up pointing to the surface and saying "that doesn't exist." -Sean r lynch (talk) 21:23, 30 December 2010 (UTC)Reply

Assume A is an event within the past cone of B, and B is within the past cone of C. Is it true that A will always be within the cone of C? This would seem to be consistent with the idea that no information can travel faster than light, even through an intermediary. It also fits the simplified 2D block diagram. But since spacetime is curved and multi-dimensional, could B ever "see" an event A that C cannot see? Or is it true that (assuming I am observer C) I can see everything that B can see? A note in the article with your answer would be great. 72.208.56.42 (talk) 13:20, 7 October 2008 (UTC)Reply

OK you have to decide whether A is an event, or an object? An event exists at only one point in time. An object traces out a path along the time dimension. So, at event C (time of event C), it seems that object A exists, ... somewhere in space, at that time. Let's say that it's at a place different from C. (This is long after the event A happened, so there's time to move.) Could be anywhere (within the future cone of event A). We'll call this event 'Ac', object A at time of event C. At that time, objects A and C are outside of each other's past and future cones - they are right next to each other.

As time goes on, your past cone keeps on getting wider. That's the set of all events you can see, cuz there's been enough time gone by for the light to get to you. So, after some time (the time it takes for light to get between Ac and C), both objects A and C will be able to see events Ac and C.

spacetime is not necessarily curved, only in the presence of gravity (some mass or energy to generate gravity) or acceleration. Plain old special relativity spacetime is all straight lines (although often at strange angles to each other). OsamaBinLogin (talk) 19:36, 6 June 2009 (UTC)Reply

Just hypothetical, but what would happen to time if one could travel faster than light? Hexadecachoron ^talk _contribs 14:39, 17 September 2009 (UTC)Reply

The math of the Twins Paradox implies everything travels at speed c through 4-dimensional Minkowski spacetime. The equation has the form of the Pythagorean theorem, in which the hypotenuse of the right triangle has "length" c and the other two sides of the right triangle are speed in 3-dimensional space and the rate of aging. In other words:

speed² + aging_rate² = c²

It implies that if an object's speed through 3-dimensional space exceeds c then the object ages at an imaginary rate (the square root of a negative number) from the perspective of observers in rest frames. This does NOT appear to me to be the same as traveling backward in time, but it's unclear what "aging at an imaginary rate" physically means. 71.178.129.228 (talk) 13:54, 9 July 2023 (UTC)Reply

I guess it would run backwards, being as it stands still at light speed. 11:24, 27 January 2011 (UTC) — Preceding unsigned comment added by 51kwad (talk • contribs)

The space-time interval is always zero between two events connected by a light speed path BUT the slant edge of cone which is GREATER than the speed of light "c" represents the true motion of a pulse [world line ] in both space and time, therefore, isn’t time dilating even in a stationary light clock due to extra distance covered by a pulse in its world-line? 68.147.52.247 (talk) 02:53, 21 December 2011 (UTC)Eclectic Eccentric Khattak #1Reply

Sorry, but somehow I don't understand the diagram.

If time moves forward from bottom to top, where does the "Past Light Cone" come from? If the Flash of light has only been fired at the center of the visualized coordinate system, it only came into existence right there and thus cannot exist in the past.

Can someone clarify? — Preceding unsigned comment added by 80.187.102.21 (talk) 16:08, 2 December 2012 (UTC)Reply

The past light cone is the set of all possible light paths that could reach you at that moment, whether or not any actual light shines along that path. Similarly, the future light cone is the set of all possible light paths that originate from you at that moment, whether or not you are actually shining in any particular direction. Anyways, the past light cone can certainly be full of light: for example, you are in a lit room, in every direction you look some light is coming to you. --174.118.1.24 (talk) 03:49, 18 February 2013 (UTC)Reply

but the concept is easier to visualize with the number of spatial dimensions reduced from three to two.[citation needed]

Erm, OK, really? Do you really need a citation to "prove" that it's easier to visualize anything if it has less dimensions? Especially if you're talking about 4 dimensions which is not visualizable at all in 3D space, much less in 2D space (computer monitor)... — Preceding unsigned comment added by 80.187.102.21 (talk) 20:42, 2 December 2012 (UTC)Reply

I agree with the comment above. I'm a biochemist and even I can see that by symmetry, if something is a circle in 2-space, it is a sphere in 3-space. It's also clear that it would be extremely difficult to visualize the cone as as sum of overlapping spheres along the time dimension (or something similar), so it's a lot easier to reduce space to 2 dimensions and know that the third spatial dimension is symmetrical to the other two. Danja91 (talk) 19:02, 6 January 2013 (UTC)Reply

My judgement is that this is someone taking the mickey, so I will delete it. If anyone has an argument as to how it becomes harder to visualize, they can post it here. Mujokan (talk) 15:20, 3 February 2013 (UTC)Reply

Trivial. Fully agree with the removal. - DVdm (talk) 17:40, 4 February 2013 (UTC)Reply

The light cone is described in this article as the trajectory of a flash of light. This description is problematic. First, the past cone represents light converging to a point. The whole celestial sphere of star light provides the image of incoming photons from different directions. The "flash" image is wrong for the past cone. Further, in spacetime, light does not move, it just persists in rays between emission and absorption: the facility of isotropic lines from complex geometry is used to fit light between temporal and spatial separations. — Rgdboer (talk) 21:50, 6 September 2016 (UTC)Reply

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It doesn't look to me that light Cone slope 45° in the picture. — Preceding unsigned comment added by 101.161.162.162 (talk) 10:51, 21 May 2017 (UTC)Reply

I made a little change to the text where it talks about the light cone angle. This should clarify things, and explain why you don't see a 45° angle in the picture. It does look like bisection, right? - DVdm (talk) 11:46, 21 May 2017 (UTC)Reply

The Hubble volume is the mapping of the future light cone.

(Some slight deviation is that the Hubble volume might include few objects more/extra, which are superluminal not with reference to their locus (locus, a non relativistic small position in spacetime), but with reference to the central observer/present vertex/center of the causal sphere. That deviation is most of the times deemed nonimportant in cosmology.)

Hubble volume ≅ future light cone

The above is very important. Include it analytically. — Preceding unsigned comment added by 2a02:587:4110:d3cb:214b:c286:7fd5:63e (talk • contribs) 07:10, 11 July 2020 (UTC)Reply

Please sign all your talk page messages with four tildes (~~~~) — See Help:Using talk pages. Thanks.

Needs a reliable source. See wp:reliable sources. - DVdm (talk) 08:20, 12 July 2020 (UTC)Reply

Which is bad. Peter.A.Schneider (talk) 15:47, 21 July 2024 (UTC)Reply