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Comment by Pxtl | Hacker News Reader

Just making sure, the "fire" of the firewall is the Hawking radiation, right? That is, the event horizon divides matter from antimatter in the energy of space and spews it outwards. Or are they distinct concepts?

I never understand how Einstein's concept of relativity and his discovery of an arbitrary maximum speed of the universe can coexist. I mean, how can everything be relative when there's an absolute maximum velocity? The event horizon and Hawking radiation/the firewall naturally come out of the speed of light.

Physics confuses me ever so much.

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The short version of maximum speeds is that velocities don't add how you'd expect at relativistic scales. If a ship is moving at 0.5c relative to an observer and it launches a missile forward at 0.5c relative to itself, the missile is moving at less than 1.0c relative to the same observer.

In fact, the missile could launch a smaller missile forward at 0.5c, and that missile would still be moving under the speed of light relative to the observer. If you just added speed up classically, it'd be going 1.5c.

Really it always works like this, but at speeds we consider normal the error from simplifying to v1+v2=v3 is incomprehensibly small.

I assume time-dilation accounts for that, since the ship perceives that it's launching something at 0.5C but its perception is skewed by time dilation... but that implies the reverse, since time dilation means that the ship perceives time traveling slower than the outside universe and thus the outside universe is faster and so the 0.5C missile should actually be moving faster than the sum of the accelerations... but relativity says that both sides perceive their counterpart as slower, and then you get the to the twin paradox.

ow, my head.

It's not just time that get's distorted, but also distance.

More importantly, there is no privileged viewpoint. As far as a spaceship is concerned it's everything else that's moving and it's standing still and the same physics predictions work just fine it's only the measurements that seem different.

Oh, right, I forgot the flattening effect. That makes me feel a bit better.

You're doing fine so far. The thing that breaks symmetry in the twin paradox is that one twin accelerates (and observes, in effect, time passing very fast in the outside universe as they do so), and the other does not.

fennecfoxen, you were marked dead for that comment. No idea why, but I hear emailing pg is the thing to do about it?

I think the way we try and explain physics with analogies and simplifications is often more confusing than just showing the equations.

Pre-Einstein we assumed "Galilean relativity" almost by default: the idea that if you're moving at a constant velocity, the results of any experiment you can perform are the same as if you were standing still.

Maxwell's electrodynamics is a wonderful, beautiful thing and well worth learning, but the relevant part here is that it implies that light always propagates at a fixed speed. And we can perform experiments that verify this, and we can - more or less - perform those experiments while moving at speed (or we can find clever ways to test the theory using the motion of the Earth, as with Michaelson-Morely), and we observe that they still hold.

So then just think about objects moving at constant velocities, and how to reconcile their observations with each other - for simplicity, you probably want to start with 1D space. Plug the observations in, write some numbers, and special relativity more-or-less drops out as the only way things could possibly work. It really is a lot less confusing than it sounds.

The "firewalls" idea is rather different than Hawking radiation; it's a new notion that's been a subject of intense debate and interest in the relativity/high energy physics community for the past couple of years.

Your confusion about how a "maximum speed" can coexist with "every reference frame is equally valid" (i.e. relativity) is completely understandable: it's really weird. One way of framing the basic issue is that the equations for how relative velocity works aren't as simple as we intuitively think they should be. "If I'm going 55mph and someone passes me 15mph faster than that, they must be going 70mph" is an awfully good approximation at low speeds, but it just turns out that the universe doesn't quite work that way when the speeds get really high. It's tough to wrap your brain around it, absolutely. (I like using "space-time diagrams" to visualize this. Here's a handout from one of my classes a few years back that probably isn't self-contained enough to understand from scratch but might convey a bit of the gist of it: http://www.slimy.com/~steuard/teaching/classes/spacetime.pdf )

> ... how can everything be relative when there's an absolute maximum velocity?

The observations leading to relativity theory put this question differently: Apparently the speed of light is constant, no matter how fast it's origin moves [1] : so we have to interpret our measurements relative to the speed of the observer. I still find Einstein's explanation the most readable [2]. He stays close to the geometrical description of reference frames and avoids the pictorial freedom known from monad tutorials...

[1] http://en.wikipedia.org/wiki/Michelson%E2%80 %93Morley_experiment

[2] http://www.bartleby.com/173/5.html

There is one thing not relative in Einstein's universe: The universe centre of mass.

How is that not relative to the motions of all of the other masses in the universe?

Good question. Say you spin like a ballerina in a circle, that force that pulls up your arms (angular momentum / centrifugal force). But why should your arms go up? If you take the person's face's point of view, the arms are still and some mysterious force is pulling them up.

This is because it is relative to the CoM and spin of the universe itself (although things get a bit wonky when we look at this even closer).

However, in the face of inflation that can still end up ill-defined, requiring measurements that can not be taken from the inside. And recall that while the specific inflation that our universe actually experienced may not fit directly into Einstein's original equations, a "cosmological constant" that permits a certain type of inflation does exist.

The universe doesn't have a center of mass.

There's a maximum velocity, but no maximum energy. Photons get higher frequencies when they have more relative energy, and lower frequencies as they have relatively less energy.

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