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Why can’t I go faster than the speed of light?

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Re: Why can’t I go faster than the speed of light?

#141

Earlier quoted context omitted.

It does work out, in fact you could go arbitrarily far in space (and, mandatory, in external time) if you had infinite energy to spend. The acceleration and deceleration phase is almost negligible. Edit: the faster you go, the slower your own (inertial) time passes. That means the external time passes faster, and the factor grows to infinity the closer you get to C. In fact, subjectively there is no speed limit. As y…

But in your inertial reference frame the people on Earth are moving at (near) the speed of light. So they are the ones that should be staying young. Or similarly the planet you are traveling to is actually speeding towards you and you are staying still. This is why the twin paradox is a paradox, because of the reference frames.

You're right that the apparent symmetry is broken by acceleration(s!), and to show that I'd point to Michael Weiss's twin paradox equivalence principle analysis at https://www.desy.de/user/projects/Physics/Relativity/SR/Twin... rather than rewriting it.

There is a subtlety not explicitly raised in the writeup, mainly that in General Relativity metrics do not superpose cleanly, in the sense of getting another solution to the Einstein Field Equations. We do not worry about this in the ultrasimplified twin-paradox model where the spacetime is flat in the sense that the Riemann tensor vanishes everywhere. However, if we want to consider the behaviour of gravitational waves with amplitudes outside of the weak https://en.wikipedia.org/wiki/Linearized_gravity limit, we are in a world of calculational pain.

Physicalizing this subtlety, if our travelling twin is travelling in our neighbourhood of the galaxy, it is probably in for a bumpy ride due to gravitational waves from nearby binary stars https://news.berkeley.edu/2021/02/22/binary-stars-are-all-ar... . We cannot easily extract how bumpy by adding in the uniform pseudogravitational field proposed by Weiss. On the other hand, we probably cannot quantify the effects of gravitational waves at all by simple adapatation of the other strictly Special Relativity analyses at the related Weiss link, https://www.desy.de/user/projects/Physics/Relativity/SR/Twin... (which lists among other the resolution in the minutephysics youtube link you provided above).

You're also right that the problem is one of reference frames. We are not obliged to use that of one twin as the spatial origin. In principle any will do, but some choices have advantages driven by features deliberately excluded from the Special Relativity twin paradox.

Let's consider the "(s!)" tacked on at the end of acceleration. We have not only that of the travelling twin's spacecraft engine, but also that which drives the expansion of the universe.

From within our galaxy we observe a highly spatially homogeneous and isotropic arrangement of extragalactic luminous matter (and cosmic radiation, locally) without distortions in the shapes of distant spiral galaxies that imply a spatially non-flat universe. The metric expansion of this, retaining bulk isotropy, gives us a preferred foliation (Wald's 1984 textbook develops this pp 92-93, but alternatively we could use Weyl's principle). Each twin is free to use a "cosmic fluid" observable (like the dipole-free temperature of the cosmic microwave background, which expands adiabatically), even while accelerating, to determine the https://en.wikipedia.org/wiki/Scale_factor_%28cosmology%29 . For example, each twin could consider the dipole pattern dT/T = v/c where T in the twin's proper time. Each twin can thus determine whether it is the relativistic traveller or not, even if it only wakes up occasionally and only long enough to look at a snapshot of the CMB. The travelling twin thus sees a clear breaking of the Copernican principle along the direction of its travel. Or more precisely, with respect to the bulk flow of matter and radiation in the universe, the non-travelling twin can conclude that it is effectively a Eulerian or comoving observer, while the relativistically-travelling twin cannot.

Moreover, the twins (and any third party) can use "cosmic fluid" observables to determine the scale factor when the twins are together at the start of the travel, and when they (or at least one and the other's remains) are together again at the end.

In this approach there is no paradox at all, there is only the consequences of one twin with a worldline with sections where the proper time is at a higher tilt to the cosmic time than the other twin's. We also avoid the difficulties in attaching a pseudogravitational field to a spacetime where there are gravitational waves of reasonably large amplitude, or relativistic stars and other massive compact objects.

We head into the land of apparent paradox by stripping out evidence of an expanding universe. We must also eliminate evidence of the aging of galaxy clusters through gravitational collapse (including the rate of star formation and the change in abundance of heavy elements). Indeed, we have to arrive in a setting in which neither twin can determine that it has departed from a point at which some reasonable generalization of the Copernican principle applies.

Indeed, the usual formulation of the apparent paradox gets rid of everything but the twins, so that one cannot even use Rindler/Unruh-like observables in flat spacetime, and this really emphasizes the "Special" in Special Relativity.

In that setting, as I said above, relying on the equivalence of being in uniform acceleration (even if it's instantaneous) and being immersed in a uniform (pseudo)gravitational field, is a reasonable way to eliminate the apparent paradox.

Re: Why can’t I go faster than the speed of light?

#142

What would happen if you had a very large rotor of let's say 50.000km in diameter. And let it rotate at 1r/s. The inner part would only move at very low speed and the outer part at near lightspeed. What kind of time dilation effects would be seen? If you would sit on the tips for a few days and then stop the rotor, travel back to the center, you would see a very old one right?

I think veritasium discusses that as well. Essentially at relativistic speeds there are no chemical bonds that can hold and the device shreds itself.

Re: Why can’t I go faster than the speed of light?

#143
post #94

I have a probably very dumb question. If speed is relative, when we say something travels at... I don't know... 50% the speed of light, that speed is relative to what? how do you know it is 50% and not 53%? How do we know we're not already moving at 99% the speed of light (like our observable universe as a whole having that speed )? I love this stuff, but it is so counter intuitive for the average human.

No matter of fast you are moving, you will agree with a “stationary” observer about the speed of light. What you won’t agree on is the color of the light. Apparent frequency shifts to maintain the speed of light. The color shift is how we know how fast stars are moving relative to us.

Re: Why can’t I go faster than the speed of light?

#144
post #72

Earlier quoted context omitted.

If you were watching their lives out the window of your spaceship you’d see them in fast forward vv. they’d see you in slow motion. Everything is relative. The main ”paradox” found by experimention is that the speed of light is a constant, regardless of your velocity. The only way this could be true, if you do a thought experiment, is if time was dialating for you. As for the physical and mathematical “why is this ha…

> If you were watching their lives out the window of your spaceship you’d see them in fast forward vv. they’d see you in slow motion. Everything is relative. Both observers, looking at one another, would see the other moving near c. Neither would know who was ‘actually’ moving. Yet, you assume there would not be a symmetry in their respective views of the other’s passage of time. Explain why. In simpler terms, a twin…

The one who experienced the force of acceleration is the one who shifted their inertial frame w/re to the reference.

Re: Why can’t I go faster than the speed of light?

#145

Earlier quoted context omitted.

It's a consequence of the time dilation. You are not stationary towards the photon to an outside observer, so to them it looks like the photon moves away from them faster than it moves away from you. And from the obserer's perspective, your clock is ticking slower. That's the key. The amount that your clock is ticking slower is such that you, the one moving, would calculate the speed of the photon the same as the sta…

Thanks, that sounds like the explanations I've seen too, and that I hope to understand some day. Atm I'm at the point where I gladly accept that time may seem to dilate, but not that it actually does. Maybe I just need to dive deeper into it than I've been willing to do so far.

You may want to investigate how GPS works. They have to incorporate the effects of time dilation to get accurate positioning data. I don't know if this is the correct direction to get the answer for which you are searching but it is a concrete example with the maths worked out. http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....

Re: Why can’t I go faster than the speed of light?

#147
post #114
post #23

I think a lot of people assume that this means you couldn't go more than a few (~100) light-years in your lifetime... But this is not actually correct. Counter-intuitively you can theorically go any number of light-years (essentially) in your lifetime, as long as you are able to approach the speed of light because when you do so the distance is dilated and hence you're covering far more ground within your reference f…

Also, it doesn’t take too much acceleration to make that trip. Comfortable earthlike 1g (~ 10m/s/s) is enough to build up a decent speed in a very reasonable time. Energy is the issue though.

It's a fantasy as long as you are still living under the tyranny of the rocket equation.

Re: Why can’t I go faster than the speed of light?

#148

Light isn’t limited in the speed it can travel. Mass is. Light can’t exist without matter. The fundamental limit is mass versus all of the forces which act as drag.

Why are gravitational waves limited by the same speed limit? There is no mass traveling in that case, I suppose.

I wouldn’t dare to hazard a guess here, but I do think it’s worth remembering that matter is always in motion.

Re: Why can’t I go faster than the speed of light?

#149

Light isn’t limited in the speed it can travel. Mass is. Light can’t exist without matter. The fundamental limit is mass versus all of the forces which act as drag.

Why are gravitational waves limited by the same speed limit? There is no mass traveling in that case, I suppose.

What we think of as the speed of light could also be thought of as the rate at which change propagates throughout the universe. The rate at which cause and effect travel.
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