Why can't I go faster than light?
November 4, 2017 2:57 PM Subscribe
Why can't you go faster than the speed of light? Here it is, explained just about as simply as possible. From Fermilab.
186,000 miles a second. It's not just a good idea; it's the law.
posted by hippybear at 3:14 PM on November 4, 2017 [5 favorites]
posted by hippybear at 3:14 PM on November 4, 2017 [5 favorites]
Know your limits. I stopped at first year physics.
posted by Jessica Savitch's Coke Spoon at 3:19 PM on November 4, 2017 [2 favorites]
posted by Jessica Savitch's Coke Spoon at 3:19 PM on November 4, 2017 [2 favorites]
I always liked this explanation, which phrases it in terms of causality.
posted by heathkit at 3:27 PM on November 4, 2017 [2 favorites]
posted by heathkit at 3:27 PM on November 4, 2017 [2 favorites]
So how did Einstein know 1. That we're moving through spacetime at a rate that doesn't vary and 2. That the speed of light is that rate?
posted by straight at 3:43 PM on November 4, 2017 [1 favorite]
posted by straight at 3:43 PM on November 4, 2017 [1 favorite]
He did the math.
posted by hippybear at 3:46 PM on November 4, 2017 [7 favorites]
posted by hippybear at 3:46 PM on November 4, 2017 [7 favorites]
So how did Einstein know 1. That we're moving through spacetime at a rate that doesn't vary and 2. That the speed of light is that rate?
Because the only other option is the speed of light isn't invariant and the question you have to ask is "what is the universe's preferred reference frame?".
posted by Talez at 3:48 PM on November 4, 2017 [9 favorites]
Because the only other option is the speed of light isn't invariant and the question you have to ask is "what is the universe's preferred reference frame?".
posted by Talez at 3:48 PM on November 4, 2017 [9 favorites]
And that was such a ridiculous question, it seemed more likely that time is relative for the speed of light to stay invariant. Also, the geometry fits (see: photon clock)
posted by Talez at 3:50 PM on November 4, 2017
posted by Talez at 3:50 PM on November 4, 2017
Because the only other option is the speed of light isn't invariant and the question you have to ask is "what is the universe's preferred reference frame?"
Is that the part that requires math to understand? Why would light having a speed that varied imply (require?) the existence of a preferred reference frame?
posted by straight at 3:56 PM on November 4, 2017
Is that the part that requires math to understand? Why would light having a speed that varied imply (require?) the existence of a preferred reference frame?
posted by straight at 3:56 PM on November 4, 2017
This video sent me down a YouTube hole of related videos, so I now also know what things would look like out a spaceship window if I were traveling at light speed, what the platinum atom looks like, and why wave functions apply to particles. So thanks for that. I forgot there's more to YouTube than ear-splitting LP vids.
posted by Aya Hirano on the Astral Plane at 3:59 PM on November 4, 2017 [4 favorites]
posted by Aya Hirano on the Astral Plane at 3:59 PM on November 4, 2017 [4 favorites]
Why would light having a speed that varied imply (require?) the existence of a preferred reference frame?
Because the speed of light would have a maximum based on your particular circumstances in the universe. You would need to know the universe's preferred reference frame to work out the ultimate speed of causality and everything that goes along with it (i.e. mass-energy equivalence).
posted by Talez at 4:06 PM on November 4, 2017
Because the speed of light would have a maximum based on your particular circumstances in the universe. You would need to know the universe's preferred reference frame to work out the ultimate speed of causality and everything that goes along with it (i.e. mass-energy equivalence).
posted by Talez at 4:06 PM on November 4, 2017
I have only taken high school physics (which I found baffling and terrifying) so I'm working with a limited capacity here but...I get the moving through time but not space part but when it moves towards the moving through space but NOT time thing...I just don't get it? how I can be moving through space without any time passing? is there a simple way to explain this in words (not math) that a non-scientist might understand?
posted by supermedusa at 4:06 PM on November 4, 2017 [1 favorite]
posted by supermedusa at 4:06 PM on November 4, 2017 [1 favorite]
YOU MATTER.
Unless you multiply yourself by the speed of light
then YOU ENERGY.
posted by The otter lady at 4:09 PM on November 4, 2017 [22 favorites]
Unless you multiply yourself by the speed of light
then YOU ENERGY.
posted by The otter lady at 4:09 PM on November 4, 2017 [22 favorites]
how I can be moving through space without any time passing? is there a simple way to explain this in words (not math) that a non-scientist might understand?
Time still passes normally for you. One second will still feel like one second. It's just that for instance, if you were travelling towards a planet at .86c and the time dilation factor is two. Let's say you have a telescope pointed at a clock on the planet. You look at it for an hour of your time. Now your spaceship is catching up to the light rays being emitted by the clock so you see the clock moving twice as fast. So in that hour of your time, the clock on the planet moves two hours.
Always keep in mind it's not absolutivity, it's relativity. It's about how you look to the rest of the universe and how the rest of the universe looks to you. It will always feel the same for you. One second will feel like one second.
posted by Talez at 4:11 PM on November 4, 2017 [6 favorites]
Time still passes normally for you. One second will still feel like one second. It's just that for instance, if you were travelling towards a planet at .86c and the time dilation factor is two. Let's say you have a telescope pointed at a clock on the planet. You look at it for an hour of your time. Now your spaceship is catching up to the light rays being emitted by the clock so you see the clock moving twice as fast. So in that hour of your time, the clock on the planet moves two hours.
Always keep in mind it's not absolutivity, it's relativity. It's about how you look to the rest of the universe and how the rest of the universe looks to you. It will always feel the same for you. One second will feel like one second.
posted by Talez at 4:11 PM on November 4, 2017 [6 favorites]
i still don't understand something - if you're moving at the speed of light and you sneeze what happens? - does the back of your head blow up because the sneeze can't move forward? - if you take contac, does the little hourglass last forever?
what happens when you fart?
posted by pyramid termite at 4:17 PM on November 4, 2017 [3 favorites]
what happens when you fart?
posted by pyramid termite at 4:17 PM on November 4, 2017 [3 favorites]
if you're moving at the speed of light and you sneeze what happens? - does the back of your head blow up because the sneeze can't move forward? - if you take contac, does the little hourglass last forever?
You can't move at the speed of light because you have mass.
posted by Talez at 4:20 PM on November 4, 2017 [2 favorites]
You can't move at the speed of light because you have mass.
posted by Talez at 4:20 PM on November 4, 2017 [2 favorites]
So go ahead and sneeze.
posted by Stonestock Relentless at 4:27 PM on November 4, 2017 [4 favorites]
posted by Stonestock Relentless at 4:27 PM on November 4, 2017 [4 favorites]
What if you're divorced and excommunicated, then can you move at the speed of light?
posted by sfenders at 4:27 PM on November 4, 2017 [9 favorites]
posted by sfenders at 4:27 PM on November 4, 2017 [9 favorites]
For the adventurous folk who are curious about a universe in which this is not the case, please read Greg Egan's Orthogonal trilogy (previously)
And really, for any physics/transhumanism "what if?" Egan has probably written a book about it.
posted by curious nu at 4:29 PM on November 4, 2017 [5 favorites]
And really, for any physics/transhumanism "what if?" Egan has probably written a book about it.
posted by curious nu at 4:29 PM on November 4, 2017 [5 favorites]
So if an airplane is traveling at the speed of light on a treadmill...
posted by Aya Hirano on the Astral Plane at 4:29 PM on November 4, 2017 [4 favorites]
posted by Aya Hirano on the Astral Plane at 4:29 PM on November 4, 2017 [4 favorites]
The highest energy proton we've ever seen, the "Oh my god" particle was a single proton with 50J of energy. A proton normally weighs 1.6726 x 10-27 kg. It got to 100−9.88∗10^-18 % (i.e. a few ATTOMETERS short of the speed of light) So when you think about how a typical human weighs 80kg, you need 1.8 x 10^28 joules of energy to get that close to the speed of light.
The output of the sun in a second is 400 x 10^24 joules of energy. So if you took the entire energy output of the sun over about half a minute and applied it all in a single direction then you too could get that fast.
posted by Talez at 4:30 PM on November 4, 2017 [5 favorites]
The output of the sun in a second is 400 x 10^24 joules of energy. So if you took the entire energy output of the sun over about half a minute and applied it all in a single direction then you too could get that fast.
posted by Talez at 4:30 PM on November 4, 2017 [5 favorites]
Funnily enough, it's in that Yahoo Serious movie Young Einstein:
Albert Einstein: That's it! That's the theory of relativity! Light travels to us from the hands of the clock, to tell us the time. But, if we were to travel away from the clock at the speed of light...posted by Talez at 4:33 PM on November 4, 2017 [8 favorites]
Marie Curie: The hands of the clock would appear to have stopped!
Albert Einstein: Time would stand still! This moment *would* last forever.
If it were possible to move faster than the Speed of Light, then Light would already be doing it, because Light is an asshole.
Sneezing at the Speed of Light is an interesting test case... I think physicists have a term for it: "The Speed of Snot'.
I don't know about joules in general, but as a longtime lover of cartoons and graphic novels, I believe the most powerful is Jules Feiffer.
Thank you, I'll be here all Light Year.
posted by oneswellfoop at 4:37 PM on November 4, 2017 [1 favorite]
Sneezing at the Speed of Light is an interesting test case... I think physicists have a term for it: "The Speed of Snot'.
I don't know about joules in general, but as a longtime lover of cartoons and graphic novels, I believe the most powerful is Jules Feiffer.
Thank you, I'll be here all Light Year.
posted by oneswellfoop at 4:37 PM on November 4, 2017 [1 favorite]
Thank you, I'll be here all Light Year.
Did you also do the Kessel Run in less than 12 parsecs?
posted by Talez at 4:39 PM on November 4, 2017 [3 favorites]
Did you also do the Kessel Run in less than 12 parsecs?
posted by Talez at 4:39 PM on November 4, 2017 [3 favorites]
So, unpacking the video....
You can't go faster than light because everything already is traveling at the speed of light, just in spacetime, not space alone. (We don't know why.) All you can do is change your velocity relative the direction of time.
But we also know that there is no absolute reference frame by which to measure velocity. Speed only exists relative to other things.
I wonder how these two facts are reconciled?
posted by JHarris at 4:40 PM on November 4, 2017 [1 favorite]
You can't go faster than light because everything already is traveling at the speed of light, just in spacetime, not space alone. (We don't know why.) All you can do is change your velocity relative the direction of time.
But we also know that there is no absolute reference frame by which to measure velocity. Speed only exists relative to other things.
I wonder how these two facts are reconciled?
posted by JHarris at 4:40 PM on November 4, 2017 [1 favorite]
I wonder how these two facts are reconciled?
Time dilation is literally how those two things are reconciled.
posted by Talez at 4:45 PM on November 4, 2017 [7 favorites]
Time dilation is literally how those two things are reconciled.
posted by Talez at 4:45 PM on November 4, 2017 [7 favorites]
Back in my school days, "unpacking the video" meant wheeling the cart with the tv and VCR on it into the classroom and finding the right plug where the whole class could see.
posted by hippybear at 4:53 PM on November 4, 2017 [5 favorites]
posted by hippybear at 4:53 PM on November 4, 2017 [5 favorites]
Did you also do the Kessel Run in less than 12 parsecs?
Yes, but my ship is the Millennium Condor.
And herding nerfs IS harder than herding cats. In most dimensions.
posted by oneswellfoop at 4:56 PM on November 4, 2017 [1 favorite]
Yes, but my ship is the Millennium Condor.
And herding nerfs IS harder than herding cats. In most dimensions.
posted by oneswellfoop at 4:56 PM on November 4, 2017 [1 favorite]
Yikes, that video doesn't really make any sense, nor does it seem very helpful to someone who hasn't studied relativity.
Actually, the question is wrong. Einstein's axiom is
As others noted above, no matter how fast you travel relative to others, your clock seems to tick on as usual. It's only when you compare your clock to other people's clocks that things get interesting. And all of that occurs precisely because the speed of light is the same in every observer's frame.
Asking why you can't go faster than the speed of light is like asking why the force of object A on object B is the opposite of the force of object B on object A (in Newtonian mechanics). There is no explanation; it is one of the axioms about Nature.
And in relativity, the axiom seem to hold up, based on all measurements we've made.
posted by pjenks at 5:06 PM on November 4, 2017 [4 favorites]
Actually, the question is wrong. Einstein's axiom is
So, no matter how fast you are going (relative to some other observer) you will always measure the speed of light to be the same. And of course you will never "go" as fast as light because you always measure it to be 300,000,000 m/s, relative to you.
The speed of light is the same for all observers.
As others noted above, no matter how fast you travel relative to others, your clock seems to tick on as usual. It's only when you compare your clock to other people's clocks that things get interesting. And all of that occurs precisely because the speed of light is the same in every observer's frame.
Asking why you can't go faster than the speed of light is like asking why the force of object A on object B is the opposite of the force of object B on object A (in Newtonian mechanics). There is no explanation; it is one of the axioms about Nature.
And in relativity, the axiom seem to hold up, based on all measurements we've made.
posted by pjenks at 5:06 PM on November 4, 2017 [4 favorites]
I dunno, I'm a high school grad and found the video very helpful, especially the North-East comparison (which the host made a point to emphasise was not a perfect analogy).
posted by Aya Hirano on the Astral Plane at 5:10 PM on November 4, 2017
posted by Aya Hirano on the Astral Plane at 5:10 PM on November 4, 2017
You're right, that was crass of me.
posted by Aya Hirano on the Astral Plane at 5:17 PM on November 4, 2017 [1 favorite]
posted by Aya Hirano on the Astral Plane at 5:17 PM on November 4, 2017 [1 favorite]
He did the math.He did the monster math.
posted by rlk at 5:29 PM on November 4, 2017 [54 favorites]
This is why you need to stop trying to travel from point A to point B and instead try to tunnel from point A to point B. If you fold the two locations together, there is a lot less distance you need to travel. In this case, rather than fold them in just 3 or 4 dimensions, see if you can create an n dimensional point such that the distance between the 3 or 4 dimensions you care about is shorter than it is from your initial points of reference.
The other part of this is that to move those points closer to eachother like that probably creates some other very neat harmonics, meaning once you figure out how to solve for that location, you likely get some other fun bonus locations, hopefully you pick the right one... because... well, trans-dimensional travel can be tricky to predict... the last thing you want to do is accidentally appear in two different continuums - if for no other reason than to prevent yourself from using up your one AskMe question a week.
posted by Nanukthedog at 5:40 PM on November 4, 2017
The other part of this is that to move those points closer to eachother like that probably creates some other very neat harmonics, meaning once you figure out how to solve for that location, you likely get some other fun bonus locations, hopefully you pick the right one... because... well, trans-dimensional travel can be tricky to predict... the last thing you want to do is accidentally appear in two different continuums - if for no other reason than to prevent yourself from using up your one AskMe question a week.
posted by Nanukthedog at 5:40 PM on November 4, 2017
if for no other reason than to prevent yourself from using up your one AskMe question a week.
But.. it's two-a-week now!
W-where are we? Or.. when...
posted by curious nu at 5:50 PM on November 4, 2017 [1 favorite]
But.. it's two-a-week now!
W-where are we? Or.. when...
posted by curious nu at 5:50 PM on November 4, 2017 [1 favorite]
@curious nu... you thought that this was simply a post count increment implemented in the codebase? No no no... This was research on Quantum retrocausality and entanglement. Cortex is/was/fish now an inter-dimensional being...
posted by Nanukthedog at 5:55 PM on November 4, 2017 [1 favorite]
posted by Nanukthedog at 5:55 PM on November 4, 2017 [1 favorite]
Thankfully, if this one doesn't do it for you, there are probably a thousand other videos on YouTube that explain the basic concepts of SR and GR.
E²=m²c⁴+p²c² explains perfectly well why you can't reach the speed of light if you have mass and why if you don't you're stuck there. Well, it explains it as much as science can. No science can tell us why the physical laws are what they are, though it can tell us why they necessarily relate to each other in certain ways.
I suppose you do also need some notion of what energy scales are physically possible to really internalize it.
That said, everything above is in some sense wrong, since quantum mechanics and more to the point, quantum entanglement is a thing. A thing that has been proven to have effects that propagate faster than the speed of light. Instantly, as near as we can measure. Luckily, that in no way challenges the correctness of relativity. If it were wrong, we wouldn't have the (apparent) mass we do, a very large fraction of which is actually the energy of quarks flying about inside our nucleons at very near the speed of light.
posted by wierdo at 6:21 PM on November 4, 2017
E²=m²c⁴+p²c² explains perfectly well why you can't reach the speed of light if you have mass and why if you don't you're stuck there. Well, it explains it as much as science can. No science can tell us why the physical laws are what they are, though it can tell us why they necessarily relate to each other in certain ways.
I suppose you do also need some notion of what energy scales are physically possible to really internalize it.
That said, everything above is in some sense wrong, since quantum mechanics and more to the point, quantum entanglement is a thing. A thing that has been proven to have effects that propagate faster than the speed of light. Instantly, as near as we can measure. Luckily, that in no way challenges the correctness of relativity. If it were wrong, we wouldn't have the (apparent) mass we do, a very large fraction of which is actually the energy of quarks flying about inside our nucleons at very near the speed of light.
posted by wierdo at 6:21 PM on November 4, 2017
Quantum entanglement doesn't let you send information faster than light though. The universe seems to have no problem so long as it doesn't violate causality.
posted by Talez at 6:32 PM on November 4, 2017 [3 favorites]
posted by Talez at 6:32 PM on November 4, 2017 [3 favorites]
What if you're divorced and excommunicated, then can you move at the speed of light?
We know the speed of light.
We know the Light of the World.
What is the speed of the Light of the World?
posted by GenjiandProust at 6:47 PM on November 4, 2017 [1 favorite]
We know the speed of light.
We know the Light of the World.
What is the speed of the Light of the World?
posted by GenjiandProust at 6:47 PM on November 4, 2017 [1 favorite]
Whether information can be transmitted or not (and the physical proof of that impossibility does not yet exist), it makes clear that while the speed of light is our speed limit, it is not necessarily universal, as was once thought.
Getting back to relativity, another way to gain intuition about it is to rearrange the terms a bit and use the Pythagorean theorem to think about mass, energy, and momentum as legs of a right triangle. You'll quickly see that you are doomed to move forward in time so long as negative mass fails to be physically real.
posted by wierdo at 6:48 PM on November 4, 2017 [1 favorite]
Getting back to relativity, another way to gain intuition about it is to rearrange the terms a bit and use the Pythagorean theorem to think about mass, energy, and momentum as legs of a right triangle. You'll quickly see that you are doomed to move forward in time so long as negative mass fails to be physically real.
posted by wierdo at 6:48 PM on November 4, 2017 [1 favorite]
But why male models?
posted by Literaryhero at 6:52 PM on November 4, 2017 [1 favorite]
posted by Literaryhero at 6:52 PM on November 4, 2017 [1 favorite]
Y'all are freaking me out!
posted by hippybear at 6:52 PM on November 4, 2017 [1 favorite]
posted by hippybear at 6:52 PM on November 4, 2017 [1 favorite]
how I can be moving through space without any time passing?
Time dilation goes hand-in-hand with length (Lorentz) contraction. From the perspective of a photon, no time passes between its departure and its arrival because the distance between the two points (from its perspective) has contracted to nothing. It is emitted and absorbed at the same moment and seems to travel no distance.
posted by dephlogisticated at 7:04 PM on November 4, 2017 [3 favorites]
Time dilation goes hand-in-hand with length (Lorentz) contraction. From the perspective of a photon, no time passes between its departure and its arrival because the distance between the two points (from its perspective) has contracted to nothing. It is emitted and absorbed at the same moment and seems to travel no distance.
posted by dephlogisticated at 7:04 PM on November 4, 2017 [3 favorites]
Which is why it's meaningless to ask what would happen if, while going the speed of light, you throw a baseball forward. Space is infinitely contracted and time has slowed to zero, so there's no distance to throw it, and no timeframe over which that could occur.
posted by dephlogisticated at 7:15 PM on November 4, 2017 [7 favorites]
posted by dephlogisticated at 7:15 PM on November 4, 2017 [7 favorites]
I have loved this little tidbit since reading “A Briefer History Of Time” by Hawkins. Having a down day? Ueah, but I’m fucking traveling at the speed of light (just in 4 directions). Okay; that’s cool, guess life is pretty okay.
Also a great book for people who like science but hate math. The concepts are explained well.
(I did take issue with the idea that if we’re not actively moving, we’re not traveling through space. Sure we are. The earth is hurling around the sun at a blinding 67,000 miles per hour, in a solar system moving 514,000 miles per hour, in a galaxy moving 1.3 million miles per hour.
But I get why he framed that way. Still...)
posted by [insert clever name here] at 7:34 PM on November 4, 2017 [2 favorites]
Also a great book for people who like science but hate math. The concepts are explained well.
(I did take issue with the idea that if we’re not actively moving, we’re not traveling through space. Sure we are. The earth is hurling around the sun at a blinding 67,000 miles per hour, in a solar system moving 514,000 miles per hour, in a galaxy moving 1.3 million miles per hour.
But I get why he framed that way. Still...)
posted by [insert clever name here] at 7:34 PM on November 4, 2017 [2 favorites]
I hate Lorentz contraction. The fact that to the entity moving everything else contracts along its direction of motion while stationary observers see the entity as having contracted and both being correct because time is passing at a different rate for each and therefore events happen in a different order from the perspective of each breaks my damn brain.
posted by wierdo at 7:43 PM on November 4, 2017 [4 favorites]
posted by wierdo at 7:43 PM on November 4, 2017 [4 favorites]
Whether information can be transmitted or not (and the physical proof of that impossibility does not yet exist), it makes clear that while the speed of light is our speed limit, it is not necessarily universal, as was once thought.
There's plenty of things that can go faster than light. You can project a shadow that goes faster than light, space can expand faster than light, quantum tunneling, phase velocities in certain materials. It's a limit to causality and since our reality is causally connected, well, yeah.
posted by Talez at 8:01 PM on November 4, 2017 [1 favorite]
There's plenty of things that can go faster than light. You can project a shadow that goes faster than light, space can expand faster than light, quantum tunneling, phase velocities in certain materials. It's a limit to causality and since our reality is causally connected, well, yeah.
posted by Talez at 8:01 PM on November 4, 2017 [1 favorite]
This is why you need to stop trying to travel from point A to point B and instead try to tunnel from point A to point B. If you fold the two locations together, there is a lot less distance you need to travel.
Unfortunately, that doesn't change anything. If it can appear to an observer that you've traveled from Point A to Point B faster than light, then you've violated causality. It doesn't matter how you do it.
I mean the real question is whether your tunneling scheme can allow for time travel. And yes, it does.
posted by happyroach at 8:45 PM on November 4, 2017 [1 favorite]
Unfortunately, that doesn't change anything. If it can appear to an observer that you've traveled from Point A to Point B faster than light, then you've violated causality. It doesn't matter how you do it.
I mean the real question is whether your tunneling scheme can allow for time travel. And yes, it does.
posted by happyroach at 8:45 PM on November 4, 2017 [1 favorite]
If it can appear to an observer that you've traveled from Point A to Point B faster than light, then you've violated causality.
That’s only because of time dilation. If you can warp space so that Earth and Proxima Centauri b were next to each other, jump from Earth to it, and then warp space back it wouldn’t violate causality because you never travel FTL and the universe is quite happy for spacetime to go FTL.
posted by Talez at 8:54 PM on November 4, 2017
That’s only because of time dilation. If you can warp space so that Earth and Proxima Centauri b were next to each other, jump from Earth to it, and then warp space back it wouldn’t violate causality because you never travel FTL and the universe is quite happy for spacetime to go FTL.
posted by Talez at 8:54 PM on November 4, 2017
This is why the spice must flow!
posted by hippybear at 8:58 PM on November 4, 2017 [2 favorites]
posted by hippybear at 8:58 PM on November 4, 2017 [2 favorites]
A student recognizes Einstein in a train, “Excuse me, professor, but does New York stop by this train?”
posted by storybored at 8:58 PM on November 4, 2017 [12 favorites]
posted by storybored at 8:58 PM on November 4, 2017 [12 favorites]
So how did Einstein know 1. That we're moving through spacetime at a rate that doesn't vary and 2. That the speed of light is that rate?
I think he realized that since light moves at c for all observers, that means that the Maxwell equations must be the same for all observers, and then those things come out as consequences.
posted by thelonius at 10:45 PM on November 4, 2017 [1 favorite]
I think he realized that since light moves at c for all observers, that means that the Maxwell equations must be the same for all observers, and then those things come out as consequences.
posted by thelonius at 10:45 PM on November 4, 2017 [1 favorite]
@straight: Maxwell's EM laws form a wave equation with the speed of light as the constant in Maxwell's wave equations. Maxwell's second derivative wave equations had no reference frame, so they were fundamentally true in all reference frames, which meant that the speed of light was constant in all reference frames. Einstein likely was the first to understand the implications of Maxwell's wave equation and was able to build up his theory of relativity from it, while most other physicists were still stuck on Newtonian mechanics.
posted by DetriusXii at 10:58 PM on November 4, 2017 [5 favorites]
posted by DetriusXii at 10:58 PM on November 4, 2017 [5 favorites]
Seems to me the trick has got to be to build a machine to slow down time artificially so that you have no other choice but to go really fast. And then put racing stripes on it.
posted by darksasami at 11:14 PM on November 4, 2017
posted by darksasami at 11:14 PM on November 4, 2017
I refute it thus:
posted by kirkaracha at 11:19 PM on November 4, 2017 [1 favorite]
There was a demon that lived in the air. They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, seven hundred and fifty miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it the sound barrier.Whenever someone says we can't do something I always add "as far as we know."
posted by kirkaracha at 11:19 PM on November 4, 2017 [1 favorite]
Maxwell's EM laws form a wave equation with the speed of light as the constant in Maxwell's wave equations.
Was that based on an observation that, hey, light seems to always go the same speed what if we put that in as a constant and see if the math works? Or some other observation about EM? Or was more that the math suggested the invariant speed and then stuff like Michelson–Morley confirmed that prediction?
posted by straight at 11:32 PM on November 4, 2017
Was that based on an observation that, hey, light seems to always go the same speed what if we put that in as a constant and see if the math works? Or some other observation about EM? Or was more that the math suggested the invariant speed and then stuff like Michelson–Morley confirmed that prediction?
posted by straight at 11:32 PM on November 4, 2017
Any physicists out there? Here's a question I've always wondered about, especially if we ever want to do a one-way trip to a far star.
Let's say there's someone on a track team. He doesn't trust his coach's watch so he always carries his own. At the end of the run he stops his stopwatch, and divides the distance he ran by the elapsed time to determine how fast he thinks he ran.
Now, years later, he's an astronaut. We discover a very Earth-like planet 50 light years away and he's being sent on a one way trip to analyze it. This is in the future, so there's a rocket that can get close to the speed of light pretty quickly, let's say within a year (as measured on Earth).
As when he was young, he doesn't trust NASA, so he has his own stopwatch, which he starts when he takes off and stops when he gets there. He's been told the planet is 50 light years away, (about 3x10^14 miles), so he divides this distance with the time he has measured. How fast does he think he has traveled?
(Note that I very specifically did not ask, "How fast did he travel?" which is the question physicists insist on answering).
This is a typical clueless non-physicist question, but I think the answer is important if we ever plan on visiting other stars.
posted by eye of newt at 11:41 PM on November 4, 2017 [1 favorite]
Let's say there's someone on a track team. He doesn't trust his coach's watch so he always carries his own. At the end of the run he stops his stopwatch, and divides the distance he ran by the elapsed time to determine how fast he thinks he ran.
Now, years later, he's an astronaut. We discover a very Earth-like planet 50 light years away and he's being sent on a one way trip to analyze it. This is in the future, so there's a rocket that can get close to the speed of light pretty quickly, let's say within a year (as measured on Earth).
As when he was young, he doesn't trust NASA, so he has his own stopwatch, which he starts when he takes off and stops when he gets there. He's been told the planet is 50 light years away, (about 3x10^14 miles), so he divides this distance with the time he has measured. How fast does he think he has traveled?
(Note that I very specifically did not ask, "How fast did he travel?" which is the question physicists insist on answering).
This is a typical clueless non-physicist question, but I think the answer is important if we ever plan on visiting other stars.
posted by eye of newt at 11:41 PM on November 4, 2017 [1 favorite]
@straight: I was always taught, through multiple classes in relativity, that the Michelson-Morley experiments were trying to measure Newtonian mechanics velocity shifting in the speed of light. They weren't trying to confirm that the speed of light was constant, they were trying to measure the phase difference between light reflected off different mirrors and use the phase difference to show that the speed of light had changed. That didn't happen.
Maxwell's EM equations were already known. The constant in Maxwell's EM equations wasn't a mystery and the physics community had already understood that visible light, UV light, and infrared light were all the same thing by this point in time.
I'm going go with the latter. The math had already suggested that the speed of light was invariant in all reference frames.
posted by DetriusXii at 12:15 AM on November 5, 2017 [2 favorites]
Maxwell's EM equations were already known. The constant in Maxwell's EM equations wasn't a mystery and the physics community had already understood that visible light, UV light, and infrared light were all the same thing by this point in time.
I'm going go with the latter. The math had already suggested that the speed of light was invariant in all reference frames.
posted by DetriusXii at 12:15 AM on November 5, 2017 [2 favorites]
You can't go faster than light because everything already is traveling at the speed of light, just in spacetime, not space alone. (We don't know why.) All you can do is change your velocity relative the direction of time.
But we also know that there is no absolute reference frame by which to measure velocity. Speed only exists relative to other things.
I wonder how these two facts are reconciled?
By means of directly observing what is directly observable, and calculating what isn't.
We wish to find out whether we inhabit anything like a shared, objective reality. To test this, we agree on a set of events that both of us can observe, measure where they are and what order they occur in, cross-check each other's measurements and agree (or not) that each of us has indeed observed the same set of events.
So I take a set of measurements of those events relative to me; you take a set as well, relative to you.
Because I do not occupy the same physical location as you, our measurements will not in general be the same either. I will be closer to some of those events than you are and vice versa, and we will in general not perceive them as being in the same direction.
However, if you and I are stationary with respect to one another, then I can apply a very simple formula or transformation to your measurements in order to to cross-check them against mine. All I need to do is multiply each of your measurements by the ratio between the units you measured in and those I measured in, then add to each of them a measurement of your position relative to mine. If we are indeed measuring the same set of events, this transformation will yield results that agree with each of my measurements, within the limits of experimental accuracy.
Further turns out that for almost all of human history, pairs of observers attempting to establish a consensus ("objective") view of their surroundings have indeed been stationary with respect to one another, or at least moving with negligible relative velocity.
Since it's inconvenient to have to keep doing this scaling and offsetting step every time we want to cross-check a set of measurements, people came up with the idea of taking those measurements not in arbitrary units with respect to themselves, but in pre-agreed units with respect to a pre-agreed base position. In effect, this builds the scaling and offsetting step into the measurements themselves, allowing them to be cross-checked without further manipulation.
What this convenience does, though, is establish a completely imaginary reference observer whose point of view seems somehow necessary for the existence of an objective reality; and once having done and cross-checked a whole bunch of measurements (which we all do, informally, just by being alive and talking to each other) there's an almost irresistible tendency to take that imaginary observer's imagined observational results as somehow more real than either of ours.
But if you and I are moving with respect to one another at sufficiently high velocities - if we're on different planets, say - things get more complicated. Turns out there's not really anywhere natural to put our imaginary pre-agreed point of reference; and even worse than that, it turns out that there's no reliable way even to pre-define a shared set of measurement units. Which means that the only way to deal with the work of cross-checking observations is to go back to doing it the slow way, by making all measurements self-relative and then cross-checking them using a formula or transformation that's testable and works.
And it turns out that for observers whose relative acceleration is zero, that's the Lorentz transformation made famous by Special Relativity; for nonzero relative acceleration (including being subject to different strengths and directions of gravity), it's the equations of General Relativity.
Einstein's huge insight boils down to noticing that our customary, imaginary reference observer is indeed a mere convenience and does not deserve a privileged conceptual position.
Does that help?
posted by flabdablet at 12:54 AM on November 5, 2017 [5 favorites]
But we also know that there is no absolute reference frame by which to measure velocity. Speed only exists relative to other things.
I wonder how these two facts are reconciled?
By means of directly observing what is directly observable, and calculating what isn't.
We wish to find out whether we inhabit anything like a shared, objective reality. To test this, we agree on a set of events that both of us can observe, measure where they are and what order they occur in, cross-check each other's measurements and agree (or not) that each of us has indeed observed the same set of events.
So I take a set of measurements of those events relative to me; you take a set as well, relative to you.
Because I do not occupy the same physical location as you, our measurements will not in general be the same either. I will be closer to some of those events than you are and vice versa, and we will in general not perceive them as being in the same direction.
However, if you and I are stationary with respect to one another, then I can apply a very simple formula or transformation to your measurements in order to to cross-check them against mine. All I need to do is multiply each of your measurements by the ratio between the units you measured in and those I measured in, then add to each of them a measurement of your position relative to mine. If we are indeed measuring the same set of events, this transformation will yield results that agree with each of my measurements, within the limits of experimental accuracy.
Further turns out that for almost all of human history, pairs of observers attempting to establish a consensus ("objective") view of their surroundings have indeed been stationary with respect to one another, or at least moving with negligible relative velocity.
Since it's inconvenient to have to keep doing this scaling and offsetting step every time we want to cross-check a set of measurements, people came up with the idea of taking those measurements not in arbitrary units with respect to themselves, but in pre-agreed units with respect to a pre-agreed base position. In effect, this builds the scaling and offsetting step into the measurements themselves, allowing them to be cross-checked without further manipulation.
What this convenience does, though, is establish a completely imaginary reference observer whose point of view seems somehow necessary for the existence of an objective reality; and once having done and cross-checked a whole bunch of measurements (which we all do, informally, just by being alive and talking to each other) there's an almost irresistible tendency to take that imaginary observer's imagined observational results as somehow more real than either of ours.
But if you and I are moving with respect to one another at sufficiently high velocities - if we're on different planets, say - things get more complicated. Turns out there's not really anywhere natural to put our imaginary pre-agreed point of reference; and even worse than that, it turns out that there's no reliable way even to pre-define a shared set of measurement units. Which means that the only way to deal with the work of cross-checking observations is to go back to doing it the slow way, by making all measurements self-relative and then cross-checking them using a formula or transformation that's testable and works.
And it turns out that for observers whose relative acceleration is zero, that's the Lorentz transformation made famous by Special Relativity; for nonzero relative acceleration (including being subject to different strengths and directions of gravity), it's the equations of General Relativity.
Einstein's huge insight boils down to noticing that our customary, imaginary reference observer is indeed a mere convenience and does not deserve a privileged conceptual position.
Does that help?
posted by flabdablet at 12:54 AM on November 5, 2017 [5 favorites]
He's been told the planet is 50 light years away, (about 3x10^14 miles), so he divides this distance with the time he has measured. How fast does he think he has traveled?
At 95% of the speed of light, the distance he is traveling will shorten to about 16 light years (50 light years times the square root of 1 - 0.952). So he'll be traveling around "3 times the speed of light" if you divide by the original distance—but that's not the distance that he actually travels.
The truth is, there's no speed limit in this sense: you can get somewhere (from your perspective) as fast as you can accelerate. It just might be 50 years in the future for everyone else.
posted by panic at 12:58 AM on November 5, 2017 [2 favorites]
At 95% of the speed of light, the distance he is traveling will shorten to about 16 light years (50 light years times the square root of 1 - 0.952). So he'll be traveling around "3 times the speed of light" if you divide by the original distance—but that's not the distance that he actually travels.
The truth is, there's no speed limit in this sense: you can get somewhere (from your perspective) as fast as you can accelerate. It just might be 50 years in the future for everyone else.
posted by panic at 12:58 AM on November 5, 2017 [2 favorites]
At 95% of the speed of light, the distance he is traveling will shorten to about 16 light years (50 light years times the square root of 1 - 0.952). So he'll be traveling around "3 times the speed of light" if you divide by the original distance—but that's not the distance that he actually travels.
And if he finds himself traveling at 3 times the speed of light, this will be because he's trusting NASA's map to the extent of pulling distance numbers off it, while refusing to trust their stopwatches as well, which seems inconsistent and is in fact untenable.
If he had any way of measuring the distance for himself while in flight, he would find that from his point of view, objects along his line of travel (which includes Earth and his destination planet) really are closer together than they would be when measured from Earth.
This idea - that the spatial relationships between objects external to some observer are not absolute, but can only be defined relative to that observer - is wildly unintuitive to observers like us, who have spent our entire lives in proximity to other observers whose velocity with respect to our own is a completely negligible fraction of lightspeed.
It seems as if the very idea of an objective reality is undermined if you can't get a single definitive answer to the question of where things are really located with respect to one another.
But it turns out that you kind of can, as long as you understand that the things concerned are events rather than objects, and that specifying the "position" of those events with respect to one another requires a four-dimensional map, not a three-dimensional one.
posted by flabdablet at 1:02 AM on November 5, 2017 [2 favorites]
And if he finds himself traveling at 3 times the speed of light, this will be because he's trusting NASA's map to the extent of pulling distance numbers off it, while refusing to trust their stopwatches as well, which seems inconsistent and is in fact untenable.
If he had any way of measuring the distance for himself while in flight, he would find that from his point of view, objects along his line of travel (which includes Earth and his destination planet) really are closer together than they would be when measured from Earth.
This idea - that the spatial relationships between objects external to some observer are not absolute, but can only be defined relative to that observer - is wildly unintuitive to observers like us, who have spent our entire lives in proximity to other observers whose velocity with respect to our own is a completely negligible fraction of lightspeed.
It seems as if the very idea of an objective reality is undermined if you can't get a single definitive answer to the question of where things are really located with respect to one another.
But it turns out that you kind of can, as long as you understand that the things concerned are events rather than objects, and that specifying the "position" of those events with respect to one another requires a four-dimensional map, not a three-dimensional one.
posted by flabdablet at 1:02 AM on November 5, 2017 [2 favorites]
kirkaracha, that's a tempting analogy, but not really an accurate one.
In 1947, we knew it was perfectly possible to travel faster than sound in air. Every rifle bullet and artillery shell did so. Indeed, Bell designed the X-1 fuselage with the profile of a 50-calibre bullet because those were known to be stable in supersonic flight. What was difficult (and some thought maybe impossible) was getting from stable subsonic flight to stable supersonic flight. The 'sound barrier' was a series of phenomena associated with transonic flight; shapes that are reasonably aerodynamic at lower speeds can become very 'draggy' approaching Mach 1, and parts of the airflow around an aircraft can go supersonic before the aircraft as a whole does, causing all sorts of problems for how well the control surfaces work. Better aerodynamic design and engines with more thrust solved these problems.
By contrast, we do not know of anything that travels faster than light, and some pretty fundamental theories of how the universe works predict that you can't. Those theories have practical application in a lot of technology, so when I come across people who don't believe in the theory of relativity (and I have) I ask them if they trust their satnav, because GPS relies for its accuracy on relativistic corrections.
posted by Major Clanger at 1:08 AM on November 5, 2017 [8 favorites]
In 1947, we knew it was perfectly possible to travel faster than sound in air. Every rifle bullet and artillery shell did so. Indeed, Bell designed the X-1 fuselage with the profile of a 50-calibre bullet because those were known to be stable in supersonic flight. What was difficult (and some thought maybe impossible) was getting from stable subsonic flight to stable supersonic flight. The 'sound barrier' was a series of phenomena associated with transonic flight; shapes that are reasonably aerodynamic at lower speeds can become very 'draggy' approaching Mach 1, and parts of the airflow around an aircraft can go supersonic before the aircraft as a whole does, causing all sorts of problems for how well the control surfaces work. Better aerodynamic design and engines with more thrust solved these problems.
By contrast, we do not know of anything that travels faster than light, and some pretty fundamental theories of how the universe works predict that you can't. Those theories have practical application in a lot of technology, so when I come across people who don't believe in the theory of relativity (and I have) I ask them if they trust their satnav, because GPS relies for its accuracy on relativistic corrections.
posted by Major Clanger at 1:08 AM on November 5, 2017 [8 favorites]
If it can appear to an observer that you've traveled from Point A to Point B faster than light, then you've violated causality. It doesn't matter how you do it.
Feh. Causality is overrated anyway.
posted by flabdablet at 4:06 AM on November 5, 2017 [1 favorite]
Feh. Causality is overrated anyway.
posted by flabdablet at 4:06 AM on November 5, 2017 [1 favorite]
If you could think at the speed of light, everything would make sense.
posted by Obscure Reference at 5:51 AM on November 5, 2017
posted by Obscure Reference at 5:51 AM on November 5, 2017
Time still passes normally for you. One second will still feel like one second. It's just that for instance, if you were travelling towards a planet at .86c and the time dilation factor is two. Let's say you have a telescope pointed at a clock on the planet. You look at it for an hour of your time. Now your spaceship is catching up to the light rays being emitted by the clock so you see the clock moving twice as fast. So in that hour of your time, the clock on the planet moves two hours.
But if you turn that telescope around and look at a clock on Earth, wouldn't you also observe it moving twice as fast? Observed distance contraction applies to both directions along the line of travel. It's not about catching up to light rays.
posted by rocket88 at 6:12 AM on November 5, 2017 [1 favorite]
But if you turn that telescope around and look at a clock on Earth, wouldn't you also observe it moving twice as fast? Observed distance contraction applies to both directions along the line of travel. It's not about catching up to light rays.
posted by rocket88 at 6:12 AM on November 5, 2017 [1 favorite]
It's just that for instance, if you were travelling towards a planet at .86c and the time dilation factor is two. Let's say you have a telescope pointed at a clock on the planet. You look at it for an hour of your time.
...and die, because you're focusing an intense stream of hard radiation right into your noggin?
posted by GCU Sweet and Full of Grace at 6:30 AM on November 5, 2017 [4 favorites]
...and die, because you're focusing an intense stream of hard radiation right into your noggin?
posted by GCU Sweet and Full of Grace at 6:30 AM on November 5, 2017 [4 favorites]
I find, this morning, that the idea that I am moving through spacetime at the speed of light is giving me a deep sense of peace. Also it proves I'm not lazy, no matter what Mrs. Kahn in 5th grade said.
posted by thelonius at 7:53 AM on November 5, 2017 [1 favorite]
posted by thelonius at 7:53 AM on November 5, 2017 [1 favorite]
if you turn that telescope around and look at a clock on Earth, wouldn't you also observe it moving twice as fast?
No. Although Planet D's speed with respect to you is the same as Earth's (given the simplifying assumption that the relative velocity of Planet D with respect to Earth is zero), its velocity with respect to you has the opposite sign; so you're not looking at symmetrical measurements and you shouldn't expect symmetrical results.
If you want weird nonintuitive results based on symmetry, consider comparing the results of having an Earth-based observer recording the time between ticks on the ship's clock with the results of having a ship-based observer recording the time between ticks on an identical Earth clock. For a ship moving away from the Earth, each of these observers will perceive that the other's clock is running slow!
Which appears to be paradoxical, until you realize that it's only paradoxical because it's intuitive to assume that the clocks' identical construction implies that the second tick on the Earth clock must occur simultaneously with the second tick on the ship's clock; but this assumption is wrong! The relative velocity of the two clocks breaks the intuitive notion of simultaneity.
The only vantage point from which both ship and Earth clocks do tick in sync is exactly half as far from Earth as the ship and receding from Earth at exactly half the speed. But they don't tick in sync with a clock at that vantage point - they both run slow as measured from there, just not as slow as they run with when measured from each other.
posted by flabdablet at 10:21 AM on November 5, 2017 [2 favorites]
No. Although Planet D's speed with respect to you is the same as Earth's (given the simplifying assumption that the relative velocity of Planet D with respect to Earth is zero), its velocity with respect to you has the opposite sign; so you're not looking at symmetrical measurements and you shouldn't expect symmetrical results.
If you want weird nonintuitive results based on symmetry, consider comparing the results of having an Earth-based observer recording the time between ticks on the ship's clock with the results of having a ship-based observer recording the time between ticks on an identical Earth clock. For a ship moving away from the Earth, each of these observers will perceive that the other's clock is running slow!
Which appears to be paradoxical, until you realize that it's only paradoxical because it's intuitive to assume that the clocks' identical construction implies that the second tick on the Earth clock must occur simultaneously with the second tick on the ship's clock; but this assumption is wrong! The relative velocity of the two clocks breaks the intuitive notion of simultaneity.
The only vantage point from which both ship and Earth clocks do tick in sync is exactly half as far from Earth as the ship and receding from Earth at exactly half the speed. But they don't tick in sync with a clock at that vantage point - they both run slow as measured from there, just not as slow as they run with when measured from each other.
posted by flabdablet at 10:21 AM on November 5, 2017 [2 favorites]
it proves I'm not lazy, no matter what Mrs. Kahn in 5th grade said.
My favourite slogan shirt ever
posted by flabdablet at 10:26 AM on November 5, 2017 [1 favorite]
My favourite slogan shirt ever
posted by flabdablet at 10:26 AM on November 5, 2017 [1 favorite]
Oh, and if you really must have some quantum "exploit," get your hands on some negative mass. Since it has negative inertia, it behaves bizarrely. If you push on it, it moves toward you, but you move away at the same rate, so you do not get closer.
Think through the implications of that! (And also realize that it's all OK thanks to the minus sign, even if negative mass objects are indeed physical)
Lately, I've become convinced (without sufficient evidence) that the solution to the matter/antimatter problem is that the universe did begin with (for some values of "begin" and still has) equal amounts of matter and antimatter, it's just that we can't see it because it's on the "other side" of the Big Bang. Since antimatter can be equally well described as time reversed matter, why not take it seriously? Obviously, both "halves" still see antimatter as existing from their perspective, it's just all annihilated before making it through the proverbial looking glass.
posted by wierdo at 1:37 PM on November 5, 2017 [1 favorite]
Think through the implications of that! (And also realize that it's all OK thanks to the minus sign, even if negative mass objects are indeed physical)
Lately, I've become convinced (without sufficient evidence) that the solution to the matter/antimatter problem is that the universe did begin with (for some values of "begin" and still has) equal amounts of matter and antimatter, it's just that we can't see it because it's on the "other side" of the Big Bang. Since antimatter can be equally well described as time reversed matter, why not take it seriously? Obviously, both "halves" still see antimatter as existing from their perspective, it's just all annihilated before making it through the proverbial looking glass.
posted by wierdo at 1:37 PM on November 5, 2017 [1 favorite]
However, if you and I are stationary with respect to one another, then I can apply a very simple formula or transformation to your measurements in order to to cross-check them against mine. All I need to do is multiply each of your measurements by the ratio between the units you measured in and those I measured in, then add to each of them a measurement of your position relative to mine. If we are indeed measuring the same set of events, this transformation will yield results that agree with each of my measurements, within the limits of experimental accuracy.
...
But if you and I are moving with respect to one another at sufficiently high velocities - if we're on different planets, say - things get more complicated. Turns out there's not really anywhere natural to put our imaginary pre-agreed point of reference; and even worse than that, it turns out that there's no reliable way even to pre-define a shared set of measurement units. Which means that the only way to deal with the work of cross-checking observations is to go back to doing it the slow way, by making all measurements self-relative and then cross-checking them using a formula or transformation that's testable and works.
THANK YOU flabdablet!! That's the part where all the explanations I've seen before went into math and equations (and I got lost) rather than explaining what the equations were trying to accomplish.
I think the only part I'm still missing from your excellent explanation is how/why c comes into the attempts to compare the measurements of observers in different places. If I'm devising equations to compare measurements, why do I stick c in there? And from what you said I don't see where the impossibility of c differing based on the observer's location/motion becomes evident. (From what other people have said it sounds like c being a constant is not merely an empirical observation.)
posted by straight at 1:41 PM on November 5, 2017 [1 favorite]
...
But if you and I are moving with respect to one another at sufficiently high velocities - if we're on different planets, say - things get more complicated. Turns out there's not really anywhere natural to put our imaginary pre-agreed point of reference; and even worse than that, it turns out that there's no reliable way even to pre-define a shared set of measurement units. Which means that the only way to deal with the work of cross-checking observations is to go back to doing it the slow way, by making all measurements self-relative and then cross-checking them using a formula or transformation that's testable and works.
THANK YOU flabdablet!! That's the part where all the explanations I've seen before went into math and equations (and I got lost) rather than explaining what the equations were trying to accomplish.
I think the only part I'm still missing from your excellent explanation is how/why c comes into the attempts to compare the measurements of observers in different places. If I'm devising equations to compare measurements, why do I stick c in there? And from what you said I don't see where the impossibility of c differing based on the observer's location/motion becomes evident. (From what other people have said it sounds like c being a constant is not merely an empirical observation.)
posted by straight at 1:41 PM on November 5, 2017 [1 favorite]
c is the only invariant (there are other relevant constants, but they are defined in terms of c), so you need it to define any transformations between coordinate systems, as is needed to translate between different reference frames.
The fact of it being invariant across reference frames is implied by Maxwell's equations describing electromagnetism and has been proven to be the case experimentally and observationally, at least to the limits of our current experimental capability.
posted by wierdo at 2:50 PM on November 5, 2017
The fact of it being invariant across reference frames is implied by Maxwell's equations describing electromagnetism and has been proven to be the case experimentally and observationally, at least to the limits of our current experimental capability.
posted by wierdo at 2:50 PM on November 5, 2017
At 95% of the speed of light, the distance he is traveling will shorten to about 16 light years (50 light years times the square root of 1 - 0.952). So he'll be traveling around "3 times the speed of light" if you divide by the original distance—but that's not the distance that he actually travels.
The truth is, there's no speed limit in this sense: you can get somewhere (from your perspective) as fast as you can accelerate. It just might be 50 years in the future for everyone else.
Our intrepid traveler can arrive at his 50 LY destination in much less than 50 years. Suppose at the time of his departure, a massive laser was fired from Earth at his destination. Our traveler might think that some years after his arrival, he could look towards earth and see a flash of light. In fact, the laser flash arrived at the destination well before him.
posted by HiroProtagonist at 5:44 PM on November 5, 2017
The truth is, there's no speed limit in this sense: you can get somewhere (from your perspective) as fast as you can accelerate. It just might be 50 years in the future for everyone else.
Our intrepid traveler can arrive at his 50 LY destination in much less than 50 years. Suppose at the time of his departure, a massive laser was fired from Earth at his destination. Our traveler might think that some years after his arrival, he could look towards earth and see a flash of light. In fact, the laser flash arrived at the destination well before him.
posted by HiroProtagonist at 5:44 PM on November 5, 2017
And if he finds himself traveling at 3 times the speed of light, this will be because he's trusting NASA's map to the extent of pulling distance numbers off it, while refusing to trust their stopwatches as well, which seems inconsistent and is in fact untenable.--flabdablet
Hey wait a minute now. He's my hypothetical astronaut. Don't go changing his personality to suit your narrative. It just so happens that his track coach had a stopwatch that ran very fast, so during tryouts he didn't make the team. despite a quick run. This was very traumatic to him. He never got over it, which is why from then on he carried a highly calibrated timekeeping device with him at all times. He never had an issue with the length of the track, and he had no trouble trusting NASA and the astronomers with the distance to the Earthlike planet.
(He also didn't trust his relatives who told him that he'd age at least 50 years travelling to the planet 50 light years away.)
posted by eye of newt at 5:58 PM on November 5, 2017 [2 favorites]
Hey wait a minute now. He's my hypothetical astronaut. Don't go changing his personality to suit your narrative. It just so happens that his track coach had a stopwatch that ran very fast, so during tryouts he didn't make the team. despite a quick run. This was very traumatic to him. He never got over it, which is why from then on he carried a highly calibrated timekeeping device with him at all times. He never had an issue with the length of the track, and he had no trouble trusting NASA and the astronomers with the distance to the Earthlike planet.
(He also didn't trust his relatives who told him that he'd age at least 50 years travelling to the planet 50 light years away.)
posted by eye of newt at 5:58 PM on November 5, 2017 [2 favorites]
MetaFilter: my hypothetical astronaut
posted by hippybear at 6:06 PM on November 5, 2017 [1 favorite]
posted by hippybear at 6:06 PM on November 5, 2017 [1 favorite]
And from what you said I don't see where the impossibility of c differing based on the observer's location/motion becomes evident. (From what other people have said it sounds like c being a constant is not merely an empirical observation.)
James Clerk Maxwell, dissatisfied with his inability to explain fucking magnets, figured out how they work and published the results as a set of equations.
Don't worry too much about digging into the mathematics; the equation to focus on is the rather simple c = 1/√(μ0ε0), which relates the speed of light in a vacuum c to the permeability of the vacuum μ0 and the permittivity of the vacuum ε0.
It's also not particularly important to understand what permeability and permittivity are, just that both of them are derived from quantities directly measurable by experimenting with electric charges and magnets. The other thing to know is that Maxwell's equations worked. By the time Einstein turned up, they were settled science.
But Einstein was troubled by this expression for a speed derivable from two measurable physical quantities with no obvious connection to motion. That simple little equation says nothing about what the speed in question is relative to, and Einstein could also think of no plausible reason why measuring permeability and permittivity or successful application of Maxwell's equations ought to depend on how fast you were going.
So either the speed of light had to be absolute in the sense of being the same for all observers, or there were hidden assumptions in Maxwell's equations that meant they were valid only with respect to some absolutely motionless frame of reference.
The Michelson-Morley experiment had already shown that the speed of light does not depend on the direction one measures it in. So if Maxwell's equations really did work properly only in motionless reference frames, then the fact that they were known to work properly in the laboratory, combined with the lack of any observable correction being required to account for the laboratory's own motion, meant that the laboratory itself had to be absolutely motionless.
This might be an acceptable conclusion for a flat earther but it didn't sit well with Einstein, who was left with no option but to take seriously the idea that c is indeed absolute in the sense of being the same for all possible observers.
posted by flabdablet at 7:59 PM on November 5, 2017 [4 favorites]
James Clerk Maxwell, dissatisfied with his inability to explain fucking magnets, figured out how they work and published the results as a set of equations.
Don't worry too much about digging into the mathematics; the equation to focus on is the rather simple c = 1/√(μ0ε0), which relates the speed of light in a vacuum c to the permeability of the vacuum μ0 and the permittivity of the vacuum ε0.
It's also not particularly important to understand what permeability and permittivity are, just that both of them are derived from quantities directly measurable by experimenting with electric charges and magnets. The other thing to know is that Maxwell's equations worked. By the time Einstein turned up, they were settled science.
But Einstein was troubled by this expression for a speed derivable from two measurable physical quantities with no obvious connection to motion. That simple little equation says nothing about what the speed in question is relative to, and Einstein could also think of no plausible reason why measuring permeability and permittivity or successful application of Maxwell's equations ought to depend on how fast you were going.
So either the speed of light had to be absolute in the sense of being the same for all observers, or there were hidden assumptions in Maxwell's equations that meant they were valid only with respect to some absolutely motionless frame of reference.
The Michelson-Morley experiment had already shown that the speed of light does not depend on the direction one measures it in. So if Maxwell's equations really did work properly only in motionless reference frames, then the fact that they were known to work properly in the laboratory, combined with the lack of any observable correction being required to account for the laboratory's own motion, meant that the laboratory itself had to be absolutely motionless.
This might be an acceptable conclusion for a flat earther but it didn't sit well with Einstein, who was left with no option but to take seriously the idea that c is indeed absolute in the sense of being the same for all possible observers.
posted by flabdablet at 7:59 PM on November 5, 2017 [4 favorites]
That’s only because of time dilation. If you can warp space so that Earth and Proxima Centauri b were next to each other, jump from Earth to it, and then warp space back it wouldn’t violate causality because you never travel FTL and the universe is quite happy for spacetime to go FTL.
Time dilation compared to what? You're assuming a universal reference frame, when there isn't one. To cosmic rays for example, we are undergoing time dilation. Your space warping concept can still result in a situation where I can transmit the information of an event before it happened. It doesn't matter how it's done.
Note that in Physics Matt's explanation, he doesn't state how the FTL works, because he doesn't need to. The method is irrelevant. Also, chapter 9.3 of the Relativity and FTL travel FAQ addresses your concept of source folding, and shows it doesn't avoid the Causality problem.
posted by happyroach at 8:50 PM on November 5, 2017
Time dilation compared to what? You're assuming a universal reference frame, when there isn't one. To cosmic rays for example, we are undergoing time dilation. Your space warping concept can still result in a situation where I can transmit the information of an event before it happened. It doesn't matter how it's done.
Note that in Physics Matt's explanation, he doesn't state how the FTL works, because he doesn't need to. The method is irrelevant. Also, chapter 9.3 of the Relativity and FTL travel FAQ addresses your concept of source folding, and shows it doesn't avoid the Causality problem.
posted by happyroach at 8:50 PM on November 5, 2017
c = 1/√(μ0ε0), which relates the speed of light in a vacuum c to the permeability of the vacuum μ0 and the permittivity of the vacuum ε0.
So I guess the units you measure permeability and permittivity in somehow cancel out to speed (or the reciprocal of speed)? Did Maxwell take empirical measurements of permeability and permittivity and notice that they happened to relate to the speed of light in that way? Or is it just that the nature of those concepts by definition entails that they would be proportional to how fast light was going?
posted by straight at 12:46 AM on November 6, 2017
So I guess the units you measure permeability and permittivity in somehow cancel out to speed (or the reciprocal of speed)? Did Maxwell take empirical measurements of permeability and permittivity and notice that they happened to relate to the speed of light in that way? Or is it just that the nature of those concepts by definition entails that they would be proportional to how fast light was going?
posted by straight at 12:46 AM on November 6, 2017
Hey, I just discovered that the Introduction to Maxwell's Dynamical Theory of the Electromagnetic Field pretty much explains everything I was asking about without using math or jargon that I couldn't follow.
posted by straight at 1:11 AM on November 6, 2017 [3 favorites]
posted by straight at 1:11 AM on November 6, 2017 [3 favorites]
Did Maxwell take empirical measurements of permeability and permittivity and notice that they happened to relate to the speed of light in that way?
My own level of understanding about magnets is far closer to Shaggy 2 Dope's than Maxwell's, so I'm sorry but I don't know how he went about deriving his equations.
posted by flabdablet at 5:31 AM on November 6, 2017 [1 favorite]
My own level of understanding about magnets is far closer to Shaggy 2 Dope's than Maxwell's, so I'm sorry but I don't know how he went about deriving his equations.
posted by flabdablet at 5:31 AM on November 6, 2017 [1 favorite]
I guess the units you measure permeability and permittivity in somehow cancel out to speed (or the reciprocal of speed)?
μ0 is in henries (units of inductance) per metre. Wikipedia tells me that a henry, expressed in SI base units, is a kg⋅m2⋅s−2⋅A−2. So that puts μ0 in kg⋅m⋅s−2⋅A−2.
ε0 is in farads (units of capacitance) per metre. In SI base units, a farad is a s4⋅A2⋅m−2⋅kg−1, so ε0 is in s4⋅A2⋅m−3⋅kg−1.
So the product μ0ε0 makes the kilograms, the amps and some of the seconds and metres cancel out and ends up in s2⋅m−2. The square root of that will be in s⋅m−1, and the reciprocal of that in m⋅s−1 which is metres per second, the unit of speed.
posted by flabdablet at 5:55 AM on November 6, 2017 [1 favorite]
μ0 is in henries (units of inductance) per metre. Wikipedia tells me that a henry, expressed in SI base units, is a kg⋅m2⋅s−2⋅A−2. So that puts μ0 in kg⋅m⋅s−2⋅A−2.
ε0 is in farads (units of capacitance) per metre. In SI base units, a farad is a s4⋅A2⋅m−2⋅kg−1, so ε0 is in s4⋅A2⋅m−3⋅kg−1.
So the product μ0ε0 makes the kilograms, the amps and some of the seconds and metres cancel out and ends up in s2⋅m−2. The square root of that will be in s⋅m−1, and the reciprocal of that in m⋅s−1 which is metres per second, the unit of speed.
posted by flabdablet at 5:55 AM on November 6, 2017 [1 favorite]
Maxwell says:
Thanks everybody, you've helped me clear up some bits about relativity I was really curious about but hadn't found an explanation I could understand before. This here, plus flabdablet's comment
I had forgotten the fact that primary sources are often not as incomprehensible as you might think (and often explain their assumptions better than secondary summaries of them). Makes me want to try reading Einstein's papers now.
posted by straight at 7:13 AM on November 6, 2017
(20) The general equations are next applied to the case of a magnetic disturbance propagated through a non-conductive field, and it is shown that the only disturbances which can be so propagated are those which are transverse to the direction of propagation, and that the velocity of propagation is the velocity v, found from experiments such as those of Weber, which expresses the number of electrostatic units of electricity which are contained in one electromagnetic unit.So it looks like putting c in those equations was based on empirical measurements. Nifty.
This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.
Thanks everybody, you've helped me clear up some bits about relativity I was really curious about but hadn't found an explanation I could understand before. This here, plus flabdablet's comment
But Einstein was troubled by this expression for a speed derivable from two measurable physical quantities with no obvious connection to motion. That simple little equation says nothing about what the speed in question is relative to, and Einstein could also think of no plausible reason why measuring permeability and permittivity or successful application of Maxwell's equations ought to depend on how fast you were going.finally helps me get why Einstein thought c would be invariant. (My naïve understanding of generation of electrical current involving motion in a magnetic field makes me wonder why permeability and permittivity have "no obvious connection to motion," but I can accept that as being clear to someone who understands exactly what those concepts mean.)
I had forgotten the fact that primary sources are often not as incomprehensible as you might think (and often explain their assumptions better than secondary summaries of them). Makes me want to try reading Einstein's papers now.
posted by straight at 7:13 AM on November 6, 2017
late to this and likely way out of my depth (or is it velocity?), but I'm reminded of this fragment found in the midst of some weird art pamphlet years ago, and jotted down because I liked the sound of it ...
"Culture and consciousness are rapidly disappearing. Soon the struggle of life will no longer trouble us. In the near future, speed itself will disappear in a blinding flash, we will pull up alongside our maker. Structures, limitations, definitions, parameters will all merge in simultaneity."
posted by philip-random at 8:16 AM on November 6, 2017
"Culture and consciousness are rapidly disappearing. Soon the struggle of life will no longer trouble us. In the near future, speed itself will disappear in a blinding flash, we will pull up alongside our maker. Structures, limitations, definitions, parameters will all merge in simultaneity."
posted by philip-random at 8:16 AM on November 6, 2017
Makes me want to try reading Einstein's papers now.
I got most of what I understand about Special Relativity from reading the man himself. He's quite approachable.
posted by flabdablet at 8:50 AM on November 6, 2017 [1 favorite]
I got most of what I understand about Special Relativity from reading the man himself. He's quite approachable.
posted by flabdablet at 8:50 AM on November 6, 2017 [1 favorite]
One catch is that lorentz contraction and time dialation aren't descriptions of images you would see through a telescope, but descriptions of events you would reconstruct after the fact once you take the finite speed of light into account. If the earth is moving toward you at .5 c, you're going to see earthlings walking faster, not slower, in your telescope. That's because the time delay for the light to reach you is rapidly decreasing. Once you correct for the time delay, then you'll notice the time dilation.
posted by mscibing at 9:54 AM on November 6, 2017
posted by mscibing at 9:54 AM on November 6, 2017
I'd seen several other explanations, and internalized them, but that Space Axis/Time Axis/you-can-put-all-your-c-into-one-or-the-other-or-some-combination-of-the-two was an elegant one I hadn't seen before.
posted by Guy Smiley at 7:49 PM on November 6, 2017 [1 favorite]
posted by Guy Smiley at 7:49 PM on November 6, 2017 [1 favorite]
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