The UniverseAnswer
Why do physicists still call the matter-antimatter imbalance unsolved?
Why the matter surplus remains unexplained, what Sakharov's conditions require, and how sphalerons and proposed neutrino mechanisms might connect.
THE QUESTION LIBRARY THE UNIVERSE
What would happen if you could send a message faster than light?
Light cones distinguish ordinary causal connections from spacelike-separated events.
THE SHORT ANSWER
Hypothetical controllable faster-than-light replies in moving senders’ own frames could precede the original send. No device is demonstrated. A quickly moving pattern alone is not such a message.
- Observers in relative motion can disagree about distant simultaneity.
- Both signal legs must be permitted.
- Even a tiny speed excess allows a loop with sufficient relative motion in this setup.
- Fast-moving patterns need not carry chosen information.
On this page
WHERE THE EVIDENCE STANDS
How sure are we?
- One reversed distant event order proves a messaging loop.The antitelephone also needs an available, controllable return leg.
- A tiny speed excess avoids the puzzle.Under the reciprocal setup, sufficient relative motion can still yield a loop.
- A sweeping light spot transmits messages between successive positions.A moving illumination pattern need not carry chosen information along its apparent path.
WHAT WE DON’T KNOW YET
Whether these reciprocal signaling permissions are physically available remains open.
WHAT WOULD CHANGE THIS ANSWER
A claimed communication channel needs evidence that chosen information is transmitted, not merely a fast-moving pattern.
Einstein’s special relativity relates the clock coordinates of observers moving relative to one another. Distant events can have different time orderings; this is about synchronized clocks, not delayed images. Could a reply return before transmission?
Here message means information a sender can choose. A rapidly sweeping light spot or shadow does not pass information between its successive positions, as Gibbs’s FAQ explains. Apparent speed is not enough.
THE LONG ANSWER
Free long answer 1 of 3 this month.
Assume two usable channels
Alice and Bob meet, then move inertially apart. Each can aim a signal at twice light speed in their own rest frame; Bob replies immediately. No preferred-frame restriction blocks either leg. These are hypothetical permissions, not capabilities relativity supplies.
Follow the two messages
Set their meeting at zero. Bob moves at 0.9 times light speed relative to Alice. Alice sends at her clock reading 1, using an arbitrary time unit. The Lorentz transformations give:
- Bob receives at about 1.818 in Alice’s coordinates.
- Bob immediately sends back at twice light speed in his own frame.
- The reply reaches Alice at about 0.628 on her clock, before her original send at 1.
Both arrival and transmission occur at Alice herself. The reply arrives before the send that prompted it. Moving clock rates, discussed in time travel, are a different issue.
Would a tiny speed excess help?
In this same reciprocal setup, any signal speed above light permits a loop at sufficiently large relative speed below light. As the excess shrinks, the needed motion approaches light speed. Reversing one leg’s coordinate order has a different, less restrictive threshold.
Which assumption could change?
Liberati, Sonego and Visser distinguish this exchange from restricted signaling. Requiring every permitted signal to advance in a common preferred time ordering blocks the construction. Physical conditions can select an ordering while the underlying laws still treat uniformly moving observers equally.
The light-cone picture helps classify ordinary causal connections. Read the speed-limit answer for background and spacetime shortcuts for a separate question.
THREE THINGS TO REMEMBER
- Distant simultaneity depends on relative motion.
- Check permission for both signal legs.
- A calculation is not a communication device.
Keep asking
Why is the speed of light the universe's speed limit?
Because space and time obey rules that give every local observer the same speed for light in a vacuum: exactly 299,792,458 meters per second.
New here?
God Plays Dice answers big questions in plain words, with sources and one of six honest labels.
Words worth knowing (4)
- Antitelephone
- A hypothetical two-signal exchange returning information before the first transmission.
- Inertial frame
- Coordinates for an observer moving without acceleration.
- Light cone
- The boundary between ordinary light-accessible futures or pasts and spacelike-separated events.
- Controllable message
- Information whose content a sender can choose, rather than a moving pattern alone.
Sources & further reading
(4) · Checked Oct 2, 2026
- Relativity (Kinematics), Chapter 11 ↗David Morin (2007 draft)Lorentz clock transformations; conditional worked exchange derived from them.
- Faster-than-c signals, special relativity, and causality ↗Stefano Liberati, Sebastiano Sonego and Matt Visser (2002)Reciprocal loop permissions and restricted signaling.
- Spacetime ↗Einstein Online, Max Planck Institute for Gravitational Physics (undated)Light cones and coordinate relations, distinct from a returned reply.
- Is Faster-Than-Light Travel or Communication Possible? ↗Philip Gibbs (1997–1998); UC Riverside hostingSelected message-versus-pattern definitions only.
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