# What is Light cone: Definition and 40 Discussions

In special and general relativity, a light cone is the path that a flash of light, emanating from a single event (localized to a single point in space and a single moment in time) and traveling in all directions, would take through spacetime.

View More On Wikipedia.org
1. ### I Rindler Wedge: Timelike or Spacelike? Intuition & Light Cone

Intuitively, the Rindler wedge is timelike in Minkowski coordinates and an object crossing the horizon enters a spacelike region. This seems at odds with my understanding of the light cone where the 2 regions are reversed. I think this may be related to the signature of the metric but I'm not...
2. ### I Probability of finding a particle outside its light cone

Say we just created a particle (high probability of one-particle state), is the probability of a very far away detector getting triggered at the time of creation (probability of finding a particle outside of its light cone) zero according to QFT? Since we can detect particles and make...
3. ### I Light Cone in Rindler Coordinates: Visualization & Reasoning

Im trying to visualize what form the light cones take in Rindler coordinates. Below is my drawing + reasoning. Is it right?
4. ### I How can an expanding Universe look homogeneous?

Observation shows that the Universe is homogeneous (and isotropic) at the large scale, while one expects to see inhomogeneity (increasing density at greater distances) on the past light cone due to expansion. This seems inconsistent. Am I misunderstanding something here?
5. ### A How to take Pz in quasiPDF to infinity to get Light-cone PDF

When I calculate light-cone PDF by taking pz in quasiPDF to infinity before one-loop integration, I will encounter all the integrations vanish. Such as this integral below: https://imgur.com/8DbDzsV This is from one of one-loop quasiPDF diagram, the sail diagram. The definition is above...
6. ### B Light cone and Cherenkov radiation

Hi All I read somewhere that at close to C the light emitting from a regular light globe ie diffuse light in all directions, will form a cone. what is the thinking behind this and does anyone have a link where I can read about it ?
7. ### I Can light near a black hole travel in -t in external coords?

Many diagrams show light cones tipping over when closer to a black hole singularity, such that emitted light can have a downwards (negative time) component in the distant observer coordinate frame. e.g this diagram: or this one: or this one: However, other diagrams show that the light...
8. ### B GR Curvature at Light Cone Surface: Smooth, Bent, Blocked?

For example, the curvature due to a mass; does that curvature continue passing from within to outside the mass's light cone? If so, is the mass subject to the external curvature? If not, does the curvature have a discontinuity at the light cone surface?
9. ### I Bell test where observers never were in a common light cone

Hi. I wonder if following thought experiment (which is most probably impossible to be put into practice) could have any implications concerning interpretations of QM. Consider five parties A, B, C, D and E, lined up in that order and with no relevant relative motion. No pair of them have ever...
10. ### Insights Relativity Variables: Velocity, Doppler-Bondi k, and Rapidity - Comments

robphy submitted a new PF Insights post Relativity Variables: Velocity, Doppler-Bondi k, and Rapidity Continue reading the Original PF Insights Post.
11. ### Insights Relativity on Rotated Graph Paper - Comments

robphy submitted a new PF Insights post Relativity on Rotated Graph Paper Continue reading the Original PF Insights Post.
12. ### Deriving Lorentz transformation by light cone coordinates

Homework Statement Derive the Lorentz Transformation using light cone coordinates defined by ##x^±=t±x## ##x^+ x^-~## is left invariant if we multiply ##~e^φ~## to ##~x^+~## and ##~e^{-φ}~## to ##~x^-~##, that is ##~x'^+ x'^-=x^+ x^-## Homework Equations ##t'^2 - x'^2 = t^2 - x^2...
13. ### Light Cone Analogue in Minkowski Space: Exploring Null Rays

in Minkowksi, the set of all possible null rays from a point defines a cone (light cone). Now imagine I change the signature of Minkowski from (-,+,+,+) to (-,-,+,+) i.e. a space with two timelike directions and a metric ##ds^2=-dx_1^2-dx_2^2+dx_3^2+dx_4^2##. What kind of surface would the set...
14. ### Light cone shape while speeding up

Hello I searched a lot but I am not sure if I understood correctly the change in the shape of light cone while speeding up. I am aware that the x and ct axis are getting closer to each other like scissors while you speed up as the graph below shows, both symmetricaly approaching the ct=x or v=c...
15. ### How Far Can We Exclude Alien EM Broadcasts in Our Light Cone?

out to what distance in our light cone can we exclude the existence of an alien civilization broadcasting EM signals at about our current levels?
16. ### Light Cone Coordinates Explained

Hi guys, I'm trying to understand light cone coordinates for which I uploaded this picture. The light cone coordinates are given by x^{+}= \frac{1}{\sqrt{2}} (x^{0}+x^{1}) x^{-}= \frac{1}{\sqrt{2}} (x^{0}-x^{1}) Now how should I think of this? I guess the space curves do only life in the space...
17. ### Why Do We Discuss Light Cones and Black Holes If There's No Light Present?

Why we talk and discuss about effects on light cones by black holes though we know there is no light left after a star dies and become a black hole? there should be no light and so no light cones...
18. ### Alcubierre / GR Warp drive and Entering/Exiting a Light Cone

So, this has really stirred my interest. To be clear, I'm not citing these as sources, simply linking for discussion; An article on the subject, And the abstract. Talking about this elsewhere I seem to find no shortage of objections. But to me it seems fundamentally pretty sound. One...
19. ### Lorentz Generators in Light Cone Coordinates

how can we write and interpret Lorentz generators in light cone coordinates?
20. ### Calculating EM field using only cross-section of past light cone?

If you want to know the value of the electromagnetic field at some point in space P at time t1, I assume that since EM is a relativistic theory, it should be possible to derive it using only the value of the field (along with charges, but let's say we are dealing with fields in free space) at an...
21. ### In holographic principle, the boundary is wrt which light cone?

It is stated that in the holographic principle (e.g., in http://en.wikipedia.org/wiki/Holographic_principle) that the the description of a volume of space is encoded on a light-like boundary to the region. But with respect to which position in the volume? In a black hole, it is clear, because...
22. ### Light Cone Distributions: Review by Eric Poisson, Ian Vega and Adam Pound

Hello, I have been reading the excellent review by Eric Poisson, Ian Vega and Adam Pound:http://relativity.livingreviews.org/Articles/lrr-2011-7/fulltext.html In section 12, Eq.12.15, there's something that I don't quite understand. They write...
23. ### Gravitational waves in elsewhere of light cone

Hello every one, This might sound as much stupid as it is confusing for me. Suppose the sun vanished right now (that would not happen practically, but I'm not concerned with that), then it will not be less than approximately a little more than eight minutes for us to know that the sun has gone...
24. ### Quantum tunnelling outside the light cone?

Zee's QFT in a Nutshell makes a short comment in the first chapter about the possibility of particles tunnelling outside their light cone - is there some probability that a particle could do this? I know the neutrino thing has been debunked as a faulty cable, but did not see this offered as an...
25. ### De Sitter Light Cone: Exploring Space-Time Diagrams

Hey there! I'm relatively new to this website and I have a quick question. If I have some function t(x), that is time for a given x, with a M (mass) constraint, namely... M^2 = 1 --> Null M^2 > 0 --> Time-like M^2 < 0 --> Space-like If I wanted to see what these space-time...
26. ### Is the Light Cone a Valid Representation of Light's Behavior?

As smatter in the subject therefore I have the following confused questions about the light-cone. As we all know that when a flash of light is released from source, light-rays spread out isotropically in space, tracing out a cone on a space-time diagram. As "light-cone" is expanded at the...
27. ### If Geodesic are light cone, how can be test particle trajectiories

I had this doubt studing GR, but let's consider SR for semplicity, where g_{\mu\nu}=\eta_{\mu\nu}the geodesics are 0=ds^2=dt^2-dr^2 we obtain the constraint we obtain the constraint r=(+/-)t So it is a well known light cone, but in SR we have that a (test?)particle can always move in a...
28. ### How do you calculate the light cone for the following line element?

Homework Statement Consider the two-dimensional spacetime spanned by coordinates (v,x) with the line element ds^2=-xdv^2 +2dvdx Calculate the light cone at a point (vx) The Attempt at a Solution I don't even know how the light cone for flat spacetime is calculated. So if that one's...
29. ### Tipler Cylinders: Solving for Light Cone Tilt Angle

I was looking for a paper or textbook that would provide "working equations" for Tipler Cylinders. In other words, is there an equation(s) that would provide the light cone tilt angle as a function of the cylinder's diameter, height, density, and angular velocity?
30. ### Are Light Cones and Black Hole Event Horizons the Same?

A forward light-cone is a surface that defines a region from which light cannot escape. Similarly, a backward light-cone defines a region that light cannot enter. What distinguishes these from event horizons?
31. ### Exploring the Fascinating Field of Light Cone Physics

I was trying to understand Light cone but everytime i stuck somewhere or the other.Wiki has good description of it but can't provide enough information.Its a interesting subject ,i suggest going through it and obviously please help me out.
32. ### What exactly is meant when people say that a Light Cone is tilting ?

What exactly is meant when people say that a Light Cone is "tilting"? I understand the general idea of a light cone when it comes to how it's used to represent light particles. However, I do not understand what is meant when one states that in Relativity, "Light cones cannot be tilted so that...
33. ### Our light cone was a black hole

My calculations show that until about 3.8 Gy ago (z ~ 1.7) the volume of space defined by our light cone had a high enough density to constitute a black hole, in the sense that 2GM > c2r. The further back in time, the more our light cone exceeded that threshold, because the density increases...
34. ### N=4 super yang mills on the light cone.

I'm currently trying to read a paper and it's not making much sense. Don't feel I expect anyone to read it in detail but it might give you an idea of the lack of understanding I am having. In all honesty I don't think it's terribly well written, coupled with the fact that I'm thick and only half...
35. ### Exploring the Shape of a Light Cone

Just wondering about the shape of a light cone. On the attachment below, if I were standing at A and pointed a flashlight in the positive time direction, it would form the cone show, correct? Is this cone a perfect cone? For instance, if I were standing on the north pole and pointed a...
36. ### Exploring the Light Cone in Big Bang Clock by Roger Penrose

Perhaps someone with a lot of patience would help me. I am watching Roger Penrose Clocks at the Big Bang 09/30/2008 on PIRSA http://streamer.perimeterinstitute.ca/mediasite/viewer/NoPopupRedirector.aspx?peid=940fba57-4cf9-4659-9c2d-1324d45cf4e4&shouldResize=False# At about 37 minutes...
37. ### QM of particles with no common past light cone

When discussing EPR experiments on this forum I made the claim that Bell's theorem does not prove classical determinism false because there is always the possibility that the correlations between distant measurements can be a result of the common past shared by particle source and the two...
38. ### 4D spacetime Light cone Twins paradox

I'm an "on-my-own-free-time" arm-chair student of physics. Lol. So if this question is way off the mark my apologies. Feel free to let me know where I’m off base. Anyway... For me, a great visual example of the twin paradox was found at this site...
39. ### Is FTL Travel Within a Light Cone Possible for Time Travel?

FTL travel in a light cone? I read in several places there there is one condition in GR, that will allow you to travel faster than light, and allow time travel to the past. Can someone explain this further, without getting too mathamatical (Ok, maybe a little math). Thanks in advance :smile:
40. ### How Does Light Refract When Emerging from Water into Air?

this is the entire question: at the shallow end of a swimming pool, the water is 70 cm deep. The diameter of the cone emerging from the water into the air above, emitted by a light source 10.0 cm in diameter at the bottom of the pool and measured by an observer on the edge of the pool 2.5...