# How did Einstein prove nothing's faster than light?
**URL:** https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343
**Category:** Factual Questions
**Created:** [July 21, 2000, 1:50pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343 "2000-07-21T13:50:34Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@cmkeller](https://boards.straightdope.com/u/cmkeller)
#### Post date: [July 21, 2000, 1:50pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/1 "2000-07-21T13:50:34Z")
The discussions of the “Faster than Light light-pulses” made me wonder:
I understand how certain parts of the theory of relativity were proven, e.g., that energy is just accelerated mass was proven by the way light bends around a large-massed body during an eclipse, and that time is relative to acceleration is proven by comparing clocks under the influence of different levels of gravity.
But what proved that light was the fastest thing in the universe? Surely it wasn’t merely the fact that it was the fastest thing we humans have managed to observe. What makes scientists so sure that there’s nothing out there that could possibly be faster?
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### Author:  [@Myrr21](https://boards.straightdope.com/u/Myrr21)
#### Post date: [July 21, 2000, 1:53pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/2 "2000-07-21T13:53:01Z")
Well, I don’t remember the proof exactly, but it was done with one of his “thought experiments” and involved shining a beam of light inside a moving train. I remember vaguely that the most difficult math involved is just the pythagorean theorem. Course, I could be way off-base, but…
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### Author:  [@Dinsdale](https://boards.straightdope.com/u/Dinsdale)
#### Post date: [July 21, 2000, 2:11pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/3 "2000-07-21T14:11:49Z")
An exhaustive series of time trials.
Light won consistently under all track conditions.
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### Author:  [@manhattan](https://boards.straightdope.com/u/manhattan)
#### Post date: [July 21, 2000, 2:14pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/4 "2000-07-21T14:14:42Z")
> [@](#):
>
> \*Originally posted by Dinsdale \*
> \*\*
> Light won consistently under all track conditions. \*\*
Ha! Wait till next year when they introduce night racing. We’ll kick them photon’s ass then. Why, I’ll bet the don’t even qualify.
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### Author:  [@Phobos](https://boards.straightdope.com/u/Phobos)
#### Post date: [July 21, 2000, 2:15pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/5 "2000-07-21T14:15:40Z")
My impression is that it comes from the mathematics & the description of spacetime.
good question though. Chronos?
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### Author:  [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)
#### Post date: [July 21, 2000, 2:18pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/6 "2000-07-21T14:18:49Z")
It’s all in the math. While I don’t know the calculations (I’m sure Chronos or a few other can do them) it works out like this:
As you work the mathematics and increase your speed you get smaller and smaller numbers for the passage of time. If you plug in the speed of light you get an infinity…i.e. you’ll never get there…time stops. You can go 99.99999999(ad infinitum)% the speed of light but you’ll never cross that line.
Doing other calculations you run into other road blocks. How much energy does it take to go light speed? Do the math and it comes out infinite…there’s not enough energy in the entire Universe to get you to that speed. The reason? As you increase speed you increase your mass. Reach light speed and you have an infinite mass (which would take infinite energy to move).
That last bit also has other implications that show light speed as a barrier. Time stops at light speed and you have infinite mass. You are, in effect, everywhere in the Universe at once.
I was never big on math in high school and college but it is kinda cool when you see things like the above pop out of equations. It all paints a picture if you know how to look at it.
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### Author:  [@Dinsdale](https://boards.straightdope.com/u/Dinsdale)
#### Post date: [July 21, 2000, 2:27pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/7 "2000-07-21T14:27:56Z")
Sweet, manny!
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### Author:  [@mrblue92](https://boards.straightdope.com/u/mrblue92)
#### Post date: [July 21, 2000, 2:34pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/8 "2000-07-21T14:34:15Z")
Geez, I used to be able to do this (I minored in math and physics), but now four years out of college I’d probably have to look it up. IIRC, it can be done in a couple different ways, and the one that seems to come to mind used basic Newtonian mechanics combined with shifting frames of reference. The math **is** amazingly simple, at least for the Special theory…
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### Author:  [@RussellM](https://boards.straightdope.com/u/RussellM)
#### Post date: [July 21, 2000, 2:36pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/9 "2000-07-21T14:36:02Z")
This is then end of a long discussion in **Elementary Physics, Weidner & Sells** which was the main standard text og my Physics degree.
> [@](#):
>
> T = T’ / (sqrt(1 - v/c)^2)
>
> This is the fundamental _time-dliation_ equation. It is crucial that the meaning of the terms appearing in it be clearly understood. T’ is the time interval elapsing between two events occurring at the _same location_ and measured on the clock of an observer at rest at this location; T’ is termed the _proper time_ or rest time. On the other hand, T is the time interval between the _same two events_ as registered on the clock of an observer travelling at speed _v_ relative to the location at which the two events take place (and who, therefore, sees the two events taking place at _differeny_ loactions relative to his rest frame). The clocks of the two observers when at rest give _identical_ readings. This equation shows that in general T \> T’; relative to a moving observer time intervals are increased, or dilated.
From this equation, we can see that as _v_ approaches _c_, time intervals approach infinity - time slows down.
Experiments have shown this effect with short lived, high speed particles. From the same book
> [@](#):
>
> A muon is created when another unstable particle (a pion) decays; a muon decays in turn into an electron (together with two uncharged, massless particles called neutrinos). One cannot specify when any one muon will be created or decay, but the _average_ lifetime of a _large_ number of muons can be given quite precisely. The half life of a muon is 1.52x10^-6 seconds…if a large number of muons are in motion at speed _v_=0.98_c_, then the half life is 7.60x10^-6 seconds. Muons in flight at high speed live significantly longer than muons at rest.
Russell
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### Author:  [@douglips](https://boards.straightdope.com/u/douglips)
#### Post date: [July 21, 2000, 2:48pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/10 "2000-07-21T14:48:37Z")
The basic idea is that as you approach the speed of light it would take an infinite amount of energy to get you there, and your clock slows to the point where you can’t do anything anyway.
I wouldn’t say this ‘proves’ you can’t go faster than the speed of light - this is a predicted limit from relativity. It is possible, though not likely, that relativity has flaws.
Finally, it is interesting to note that it is equally sound in the theory to discuss tachyons, particles whose speed can never be _slower_ than the speed of light. It takes infinite energy to slow them down to the speed of light, and they go infinitely fast at zero energy. None have been observed, and they may not exist, but it is cool.
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### Author:  [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)
#### Post date: [July 21, 2000, 3:03pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/11 "2000-07-21T15:03:10Z")
NOTE: The Speed of light as a speed limit only applies to ‘things’ that have mass. Obviously light moves at light speed along with a bunch of other massless particles. Tachyons, if they exist, have to be massless (or have negative mass which IIRC is what is required for FTL movement…needless to say none has ever been found and is hard to even conceive of as being realistic).
As an aside here’s something I picked up from a semi-old sitcom with Dan Akroyd playing a single father/minister that helps you get your head around just how BIG infinity really is. In the show he was explaining to one of his sons how long ‘forever’ (basically infinity) is as a way to show him just how long burning in hell for his misdeeds would really take.
> [@](#):
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> Imagine a giant, solid brass ball the size of the earth. Now imagine that once every hundred years a dove flies by and _just_ brushes the brass sphere with one of its wings. When that ball is ground to dust forever will have barely begun.
Maybe it’s just me but that’s the best explanation I’ve ever had for getting a feel for how BIG infinity really is.
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### Author:  [@Arjuna34](https://boards.straightdope.com/u/Arjuna34)
#### Post date: [July 21, 2000, 3:09pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/12 "2000-07-21T15:09:55Z")
As douglips mentioned, technically it’s still not PROVEN, as nothing in science ever gets proven (unless make certain assumptions which reduces the question to a mathematical proof).
Countless experiements suggest that relativity is an accurate description of the world, but it could all come crashing down tomorrow with a new observation.
I wouldn’t hold my breath though 🙂
Arjuna34
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### Author:  [@handy](https://boards.straightdope.com/u/handy)
#### Post date: [July 21, 2000, 3:28pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/13 "2000-07-21T15:28:27Z")
cm, you need to understand what a ‘theory’ is. A theory is just that, it’s NOT a truth.
As for light, surely anything that light is made up of must be faster than light itself, ja?
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### Author:  [@KeithB](https://boards.straightdope.com/u/KeithB)
#### Post date: [July 21, 2000, 3:55pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/14 "2000-07-21T15:55:33Z")
As was pointed out, Einstein did not _prove_ that the speed of light was the fastest thing in the universe. He axtually took that as one of the _postulates_ for relativity. Though it is better stated that the speed of light is the same for all observers.
It was Michaelson and Morley that showed that light had the same speed for all observers. They showed that light coming directly towards the earth had the same speed as light coming from behind the earth, where “forward” and “behind” are relative to its orbital motion. This was a big surprise at the time.
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### Author:  [@Phobos](https://boards.straightdope.com/u/Phobos)
#### Post date: [July 21, 2000, 3:55pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/15 "2000-07-21T15:55:45Z")
> [@](#):
>
> \*Originally posted by handy \*
> \*\*As for light, surely anything that light is made up of must be faster than light itself, ja? \*\*
aren’t photons fundamental particles? or are they non-baryonic?
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### Author:  [@JonF](https://boards.straightdope.com/u/JonF)
#### Post date: [July 21, 2000, 4:11pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/16 "2000-07-21T16:11:46Z")
As **KeithB** said, the principle that nothing can travel faster than light is essentially one of the postulates of special relativity, or it can be directly derived from alternate statements of the second postulate. That is, if things can travel faster than light then _all_ the predictions of relativity are wrong to some degree, and many of the are _way_ wrong. We know that the predictions of relativity are pretty darned good under all but the most pathological circumstances, therefore it is essentially impossible that information or known particles can travel faster than the speed of light. [Foundations of Modern Cosmology - Chapter 7: The Special Theory of Relativity](http://astsun.astro.virginia.edu/~jh8h/Foundations/chapter7.html).
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### Author:  [@Konrad](https://boards.straightdope.com/u/Konrad)
#### Post date: [July 21, 2000, 4:15pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/17 "2000-07-21T16:15:33Z")
I think KeithB has got it right. Einstein didn’t prove that nothing can go faster than light. That was an idea that was being thrown around already. He just figured out what would happen IF it was true.
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### Author:  [@Achernar](https://boards.straightdope.com/u/Achernar)
#### Post date: [July 21, 2000, 5:27pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/18 "2000-07-21T17:27:46Z")
Quoth **Phobos** : “aren’t photons fundamental particles? or are they non-baryonic?”
Photons _are_ fundamental particles, so as far as we know, **handy** , they’re not made up of anything smaller. (Or did you mean that photons travel faster than light?) However, even if they were, I don’t think that it necessarily follows that these sub-photons would go FTL. They’re also not baryons, although I don’t see what that has to do with it.
And I believe everyone else explained pretty well Einstein proving the Universal Speed Limit - Einstein didn’t prove jack, or even try. He just sat around all day with his dusty old blackboard and his Nobel Prize, thinking stuff up, and made everyone else do the dirty work.
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### Author:  [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)
#### Post date: [July 21, 2000, 5:44pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/19 "2000-07-21T17:44:40Z")
I can see how “The speed of light the maximum possible speed” implies “The speed of light is the same for all observers”. I don’t see how the second statement implies the first. The second does imply that nothing can accelerate past the speed of light, but I fail to see how it implies nothing can go faster than light.
I’m not saying the two statements are not equivalent. I just don’t see it.
> [@](#):
>
> \*Originally posted by **Achernar** \*
> Einstein didn’t prove jack, or even try. He just sat around all day with his dusty old blackboard and his Nobel Prize, thinking stuff up, and made everyone else do the dirty work.
And that is why the man was a genius!
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### Author:  [@Achernar](https://boards.straightdope.com/u/Achernar)
#### Post date: [July 21, 2000, 5:55pm UTC](https://boards.straightdope.com/t/how-did-einstein-prove-nothings-faster-than-light/26343/20 "2000-07-21T17:55:08Z")
**DrMatrix** , just when I was about to go through a big elaborate Mathematical proof showing you your shortcoming, I realized you were right. Special Relativity, as you noted, does not state that “The speed of light the maximum possible speed”. Rather, it states, as you said, “that nothing can accelerate past the speed of light”. In fact, particles travelling faster than light, like the tachyons mentioned by **douglips** , are consistent with the Theory. Of course, tachyons, while a cool idea, are not known to exist. Perhaps another way of stating this aspect of the Theory would be:
“As far as mundane objects, like atoms (and as far as we know, everything in the entire Universe), are concerned, nothing can go faster than c.”
“And that is why the man was a genius!” Yeah, I know. I’m just jealous. 🙂 I’ve applied for the position of genius many times, but I keep getting turned down!
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