# Why is light speed the limit? **URL:** \ **Category:** Factual Questions\ **Created:** [December 30, 2003, 10:48am UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742 "2003-12-30T10:48:52Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![muttrox](https://avatars.discourse-cdn.com/v4/letter/m/a8b319/32.png) [@muttrox](https://boards.straightdope.com/u/muttrox)\ **Post date:** [December 31, 2003, 12:09pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/21 "2003-12-31T12:09:45Z")
Meaneko, Your assumption is that if there is a possibility that causality can be violated, than the science is wrong. That assumption is not supported. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 31, 2003, 12:23pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/22 "2003-12-31T12:23:33Z")
> [@](#): > > \*Originally posted by Shade \* > **so shoot me anyway.** Never. 🙂 ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [December 31, 2003, 12:29pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/23 "2003-12-31T12:29:30Z")
Ok, so why does the speed of light have the value it does? Why not faster or slower? Or is it a case of “the speed of light is the speed of light” and it’s everything else that’s derived from it? ---
**Author:** ![netscape\_6](https://avatars.discourse-cdn.com/v4/letter/n/8c91f0/32.png) [@netscape\_6](https://boards.straightdope.com/u/netscape_6)\ **Post date:** [December 31, 2003, 1:08pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/24 "2003-12-31T13:08:33Z")
I remember reading that in big bang conditions the speed of light is greater. Is this true? Also what happens if 2 bodies pass each other at .7 of c? wouldn’t that put each of them at 1.4 of c in the others frame? ---
**Author:** ![netscape\_6](https://avatars.discourse-cdn.com/v4/letter/n/8c91f0/32.png) [@netscape\_6](https://boards.straightdope.com/u/netscape_6)\ **Post date:** [December 31, 2003, 1:26pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/25 "2003-12-31T13:26:09Z")
> [@](#): > > \*Originally posted by Meganeko \* > \*\*I’ve heard it said that travelling faster than light, whether by wormholes or warps or whatever, is completely ruled out because it violates causality. Here’s [an explanation](http://www.physicsguy.com/ftl/html/FTL_part4.html#sec:howwegetup) which doesn’t rely on mindboggling mathematics. \*\* if I understand it correctly,which I very well might not, it leaves open the possibility that spacial distortion could allow for FTL. under [diagram 9-1](http://www.physicsguy.com/ftl/html/FTL_part4.html#di:abcd) it says: > [@](#): > > # (a) Op sends an FTL bullet from A to B (using space-time manipulation) as the “passing event” occurs > > # (b) The bullet strikes and kills a victim at B (event “\*” in Diagram 8-1 ). > > # © The third observer witnesses the death. However, now (because the FTL travel of the bullet may have changed the space-time between A and B, we can no longer assume that our space-time diagram of the situation is correct.\*\* It may be that with the changed space-time, this third observer’s frame of reference no longer has the victim’s death occurring before the passing event.\*\* However, we can continue as follows: > > # (d) The third observer sends a signal over to C using ordinary (slower-than-light) means. > > # (e) An observer at C sends an FTL signal to D. Since the space-time between C and D need not be effected by the bullet’s FTL travel, our space-time diagrams can be applied. > > # (f) An observer at D receives the signal before event (a) (and thus before the bullet effected any space-time). > > # (g) The observer at D can now send a signal over to O, and O can receive it before (a) occurs. bolding mine. If I understand correctly the bolded part means that spacial distortion could cause the victim’s death to happen after the bullet was fired removing causality violations. The rest of the scenario assumes the bullet did go back in time. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 31, 2003, 2:22pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/26 "2003-12-31T14:22:04Z")
> [@](#): > > \*Originally posted by netscape 6 \* > \*\*I remember reading that in big bang conditions the speed of light is greater. Is this true? > > Also what happens if 2 bodies pass each other at .7 of c? wouldn’t that put each of them at 1.4 of c in the others frame? \*\* No, it doesn’t - the apparent relative velocity is still less than c, but I can’t quite remember how to calculate it or the exact phenomenon that makes it happen. ---
**Author:** ![Meganeko](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Meganeko](https://boards.straightdope.com/u/Meganeko)\ **Post date:** [December 31, 2003, 2:37pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/27 "2003-12-31T14:37:42Z")
Here’s an example of FTL’s potential to cause paradoxes which I found a little easier to follow than the link I posted earlier in the thread: > [@](#): > > Picture a starship haring away from Earth at close to the speed of light, for the purposes of this example at such a speed that the ship’s crew experiences a 2:1 time dilation, i.e. for every six months that pass on Earth, only three months pass on the ship. The ship’s reference frame is just as valid as Earth’s, so to the ship’s crew it appears as though Earth were receding from them at a large percentage of the speed of light, and it’s Earth that’s running through three months of time for every six months of shipboard time. > > Let’s say the spaceship left Earth on Jan 1st, 2100. Mission Control on Earth waits until a year has passed and on Jan 1st 2101 fires up their FTL communicator (which we’ll refer to as an “ansible” for the sake of brevity) and sends an instantaneous message to the spaceship: “Happy New Year!” > > On the ship, due to time dilation, only six months have passed and it’s June 2100. The crew use their ansible to send a reply saying “Yes, very funny, by the way the Captain just died in an unfortunate accident, details enclosed.” > > And here’s the causality violation: according to ship time, only three months have passed on Earth, so the ship’s reply arrives on Earth in March 2100, nine months before Mission Control sent the original FTL transmission. So what if someone immediately transmits to them regarding the Captain’s impending demise including details of the circumstances? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 31, 2003, 2:43pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/28 "2003-12-31T14:43:59Z")
That doesn’t sound quite right **Meganeko** - lets make the instantaneous communication possible by imagining that the journey is a circular one that brings them back to Earth - when they land, they have only experienced three months of voyage time, but to people on Earth, they have been gone for six months; there is no violation of causality, just different experiences of the amount of time that seems to have passed. ---
**Author:** ![Sergio](https://avatars.discourse-cdn.com/v4/letter/s/b77776/32.png) [@Sergio](https://boards.straightdope.com/u/Sergio)\ **Post date:** [December 31, 2003, 3:38pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/29 "2003-12-31T15:38:28Z")
The round trip involves no paradox, since the speed of light has not been exceded. Only the instantaneous communication causes paradox. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [December 31, 2003, 5:00pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/30 "2003-12-31T17:00:14Z")
> [@](#): > > \*Originally posted by Mangetout \* > \*\*No, it doesn’t - the apparent relative velocity is still less than c, but I can’t quite remember how to calculate it or the exact phenomenon that makes it happen. \*\* The formula you want is w = (u + v)/(1 + uv/c[sup]2[/sup]) Where u and v are the two velocities in our refernce frame and w (by symmetry) is the velocity that either ship will observe the other travelling. In this example u and v are both 0.7c which menas that w = (1.4/1.59)c which is about 0.88c. I you want you can see this as a result of lenght contraction/time dialtion. ---
**Author:** ![RevCo](https://avatars.discourse-cdn.com/v4/letter/r/9fc348/32.png) [@RevCo](https://boards.straightdope.com/u/RevCo)\ **Post date:** [December 31, 2003, 6:33pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/31 "2003-12-31T18:33:44Z")
> [@](#): > > \*Originally posted by SĂ©rgio \* > \*\*I think a more understandable way to answer the OP is that, according to the Theory of Relativity, the mass of an object increases with its velocity. When the speed equals that of the light, the mass becames infinite. There is no force capable to accelerate an infinite mass, so the speed of light is a limit. \*\* This is a terribly interesting thread, and I’m not even a science guy - very informative. Just a quick question, tho: Are there any examples of something that increases in size as it accelerates for the layman? I don’t mean everything, I mean something specific that has been used to demonstrate the above is true? And when they say that mass becomes infinite - what do you mean? The atoms agitate and take up more space or something? So as something gets closer to the speed of light it disintegrates - or could I accelerate gold bars and make a fortune after they slow down (or would I have to show rubes my gigantic gold bar in my supercolider, sell them a gigantic gold bar, and when it stops spinning, they have a tiny bar and I’m off like a thief in the night?) ---
**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\ **Post date:** [December 31, 2003, 7:02pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/32 "2003-12-31T19:02:44Z")
> [@](#): > > \*Originally posted by Achernar \* > \*\*Light-speed being the limit that it is is a result of Special Relativity, \*\* No, that’s backwards. Special relativity is the result of light speed being the limit! If the universe happened to have entirely different physics, then SR might not describe things. Special relativity is a tool, a description of the universe, but the laws of the universe are not “caused” by special relativity mathematics. A better question: why does c equal a finite number? Why doesn’t c equal infinity? Also, WHY does light speed have that value and no other? Why are photons associated with only electric fields and magnetic fields and not others as well (such as boobatism and frempth!) And why do we live in three spatial dimensions, why does time exist, why are bare neutrons unstable, why is 1/137 an important number, etc., etc. Because! 🙂 Or another explanation: if light speed was not the maximum, if this universe lacked the Special Relativity rules, then for many reasons perhaps particles could not exist in this universe, so there would be no atoms or stars or life forms
 so if you are asking the question, it must be that you live in an SR universe. Anthropic principle. But then what about the OTHER universes? In a recent book (by Kip Thorne, I think) there was the speculation that not only is the quantum foam constantly creating universe-bubbles having various random laws, but also if the laws are right, the bubble expands and has long duration
 and its quantum foam then has characteristics of that universe, but with changes made when new universes spontaneously bubble up. So
 this would be a “Darwinian Natural Selection” explanation for why the universe is the way it is. “Good” universes get promoted, and their characteristics are passed on to any altered progeny universes which bud off from them. Universes which almost instantly vanish again after being born will not bud off a lot of progeny. Then we must ask, what keeps the speed of light constant? Why doesn’t it become larger or shrink? There must be some “meta laws” which determine the laws of physics. And there must be some meta^2 laws which determine the metalaws. And the meta^n laws (where n=infinity) are just in the mind of god, and god sits on an elephant riding on a turtle, which sits on another turtle, which sits on another. And, of course, it’s turtles all the way down. 🙂 For a good book NOT about the above, see the following book by the physicist who generalized quantum electrodynamics for universes having infinite dimensions. It’s a bunch of Feynman anecdotes, and the author’s emotional upheavals as he was deciding to give up physics and become a writer. Feynman’s Rainbow, L. Mlodinow [http://www.amazon.com/exec/obidos/ASIN/044653045X](http://www.amazon.com/exec/obidos/ASIN/044653045X) ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [December 31, 2003, 7:13pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/33 "2003-12-31T19:13:10Z")
No the speed of light being a speed limit follows on from the equivalence principle (of special relativity) and electromagnetism. This limit is a result not an axiom of special relativity. You could argue that the value of c is a property of free space. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 31, 2003, 7:24pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/34 "2003-12-31T19:24:08Z")
> [@](#): > > \*Originally posted by SĂ©rgio \* > \*\*The round trip involves no paradox, since the speed of light has not been exceded. > Only the instantaneous communication causes paradox. \*\* Save for the fact that the instantaneous communication isn’t possible at a distance, there is no difference between the two scenarios; in yours, the travellers experience three months of time passing, but ground control experiences six months - this is the same in mine - for both crews, ‘now’ is ‘now’ and talking to someone back at home, whether it takes place face to face for the circle trippers or by the (technically impossible) ansible, still is a conversation between someone at their ‘now moment’ and someone else in their ‘now moment’. Or in other words, the folks taking the circle trip return home in three months of their own time, but their arrival date does not travel three months into the past for the welcoming party - all that happens is that their calendars don’t agree. ---
**Author:** ![robo99](https://avatars.discourse-cdn.com/v4/letter/r/a8b319/32.png) [@robo99](https://boards.straightdope.com/u/robo99)\ **Post date:** [December 31, 2003, 9:25pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/35 "2003-12-31T21:25:23Z")
_The speed of light is the maximum speed at which the universe can transfer energy or information from one place to another_ Quantum Teleportation allows FTL transfer of information. [http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=121010&highlight=quantum+teleportation](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=121010&highlight=quantum+teleportation) ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [December 31, 2003, 11:11pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/36 "2003-12-31T23:11:29Z")
> [@](#): > > \*Originally posted by Desmostylus \* > \*\*3) It’s not something that can be “proved” in the mathematical sense. 🙂 \*\* Another nitpick: you can’t prove the axioms of relativity, but you can prove that the axioms of relativity imply that c is the speed limit. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [December 31, 2003, 11:29pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/37 "2003-12-31T23:29:58Z")
> [@](#): > > \*Originally posted by robo99 \* > \*\*_The speed of light is the maximum speed at which the universe can transfer energy or information from one place to another_ > > Quantum Teleportation allows FTL transfer of information. > > [http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=121010&highlight=quantum+teleportation](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=121010&highlight=quantum+teleportation) \*\* No it doesn’t, quantum teleportation requires a classical communication channel for any information to be transferred so this places a definte limit of c on the trnasfer of information by quantum teleportation. ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [January 1, 2004, 3:58am UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/38 "2004-01-01T03:58:11Z")
I thought of an exception ot the speed of light = max for anything, ever theory today. Imagine a mirror. Move that mirror towards a point-source of light. Shouldn’t the velocity of the resultant beam be \>C? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [January 1, 2004, 5:33am UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/39 "2004-01-01T05:33:18Z")
Mass does not increase with velocity. 1. If it did an object would possess both a longitudinal mass and a transverse mass. 2. All the massive objects in the universe would collapse to black holes. 3. You can’t even approach the speed of light because no matter what your velocity light will always travel at c with respect to your frame of reference. If the speed of light were not the ultimate speed limit an electromagnetic wave would appear, as you traveled at c, as a spatially stationary waveform with no time varying electric and magnetic fields. But light _is_ a time varying electric and magnetic field. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [January 1, 2004, 3:53pm UTC](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742/40 "2004-01-01T15:53:38Z")
In our imaginary SF universe wormholes are permitted, as they may be in the real universe; however if the configuration of the wormholes ever approach the state where time travel and closed timelike curves are possible, one or both of the wormholes collapse. This does not occur as the wormholes arecarefully controlled. FTL travel is therefore enabled by geometry. [http://www.orionsarm.com/main](http://www.orionsarm.com/main) [Previous page](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742.md?page=1) [Next page](https://boards.straightdope.com/t/why-is-light-speed-the-limit/221742.md?page=3)