Time travel doesn't exist because time travel wiped out the timelines where it did

[tomBitonti]

Haven't read either of these, but here is a public version of the second one: Velocities of distant objects in General Relativity revisited

Thanks!
TomB

 

[Paul Farquhar]

A lot of scientific research is simply bouncing around ideas. It’s not about pronouncing on Truth.

 

[Umbran]

However, this is an unsatisfying answer.

What, on this good green Earth, has led you to believe that the way things work should be satisfying?
The universe is under no obligation to make sense, much less satisfy, mere mortals like us.

What I have read is that local systems are affected, very slightly, and not in a way that result in continuous local expansion.

I have not seen any experimental measurements that indicate this effect occurring to a degree beyond experimental error, much less enough experiments to consider that to be the new consensus among those who do that science.

But, I'm not reading papers constantly these days, so, shrug.

 

[GMMichael]

So … this was an interesting read. (What parts that I followed.)

https://arxiv.org/pdf/0808.1081

(It seems) there is a lot more nuance than is being presented here.
Can some more practiced in the field say if this paper is legitimate?
TomB

I don't buy it. A Doppler effect requires a change in frequency measured as the amount of time/space between waves (and relative velocities). Light doesn't have that. A single photon has a frequency when it's detected. There's no need for a follow-up photon, or the next crest of the wave, to determine how far apart they are, i.e. their wavelength. No time between waves means no Doppler effect. So I guess I'm in the "gravitational shift" family.

But then, I'm not practiced in the field. I'm just someone who doesn't believe in time. Neither does quantum mechanics.

 

[briggart]

But then, I'm not practiced in the field. I'm just someone who doesn't believe in time. But then, neither does quantum mechanics.

Pretty sure QM doesn't believe in anything. It being a theory and not an actual intelligent entity. On the other hand, several things would make infinitely more sense if QM were an actual intelligent entity actively messing with us...

 

[MoonSong]

When we speak of observable universe, we usually refer to the farthest distance from which a signal could have reached us by now, which gives that 45 something billion years.

More like "the furthest distance a signal could ever reach us". Since the cosmos expands faster than the speed of light, by the time the light has travelled a fraction of the distance, the space has already expanded and added to the distance.

 

[Epic Meepo]

Sure, but there is a difference between "there is no local effect" and "the expected local effect is too small to be noticed". Uniform enlargement of space seems to imply local stretching. This seems to (obviously?) not occur. The usual answer to that is that space is not stretched in bound systems. However, this is an unsatisfying answer. What I have read is that local systems are affected, very slightly, and not in a way that result in continuous local expansion.
Thanks!
TomB

From what I understand, if the "expansion of space" is due to the cosmological constant term in Einstein's equations, the expansion of space can't happen in a gravitationally bound system. The equations describing a gravitationally bound system (one in which the distances between component parts aren't constantly expanding) already include a contribution from the cosmological constant, and (by definition) the system is gravitationally bound despite that contribution.

 

[briggart]

More like "the furthest distance a signal could ever reach us". Since the cosmos expands faster than the speed of light, by the time the light has travelled a fraction of the distance, the space has already expanded and added to the distance.

Sort of, but again this is mostly a coincidence. The ~46 billion light years Umbran quoted is the distance travelled by a CMB photon from its emission until it is detected by our instruments. That is the farthest we can see today .

If the universe is made up just by matter (relativistic or non relativistic) or radiation, that distance would keep increasing with time so that, wait long enough, we'd be able to see every part of the universe*, superluminal recession velocities notwithstanding because expansion would be decelerating.

However, it seems we are living in a universe which is mostly made up by dark energy, so in this case expansion is accelerating and it is this fact that implies that there is a finite limit to how far we can see, even with infinite time. Not expansion per se, or superluminal recession velocities.

As it stands, acceleration appears to have just started, so the radius of the observable universe will keep increasing for few billion years more, but won't become much larger than what we see today.

* At least in a spatially flat universe. On the top of my head, I can't remember if this is true in any possible open universe.