Tag Archives: cosmology

Time travel isn’t possible

Better shelve that plan for a timehopping cultural tour of Renaissance Venice; physicists from the University of Hong Kong are convinced they’ve shown time travel to be impossible.

The possibility of time travel was raised 10 years ago when scientists discovered superluminal – or faster-than-light – propagation of optical pulses in some specific medium, the team said. It was later found to be a visual effect, but researchers thought it might still be possible for a single photon to exceed light speed. Du, however, believed Einstein was right and determined to end the debate by measuring the ultimate speed of a single photon, which had not been done before.

“The study, which showed that single photons also obey the speed limit c, confirms Einstein’s causality; that is, an effect cannot occur before its cause,” the university said.

I’ll give it a maximum of two years before someone has a counter-theory that makes it possible once again…

Cosmological constant not optimal after all

Interesting news from that weird and wonderful intellectual space where physics and theology trade slow, dignified blows; new research into the effects of varying the cosmological constant swings out like a haymaker from the atheist corner and knocks at least one God-of-the-gaps out of the ring [via SlashDot].

… although positive, the cosmological constant is tiny, some 122 orders of magnitude smaller than Planck’s constant, which itself is a small number.

So Page and others have examined the effects of changing this constant. It’s straightforward to show that if the the constant were any larger, matter would not form into galaxies and stars meaning that life could not form, at least not in the form we know it,.

So what value of the cosmological constant best encourages galaxy and star formation, and therefore the evolution of life? Page says that a slightly negative value of the constant would maximise this process. And since life is some small fraction of the amount of matter in galaxies, then this is the value that an omnipotent being would choose.

In fact, he says that any positive value of the constant would tend to decrease the fraction of matter that forms into galaxies, reducing the amount available for life.

Therefore the measured value of the cosmological constant, which is positive, is evidence against the idea that the constants have been fine-tuned for life.

I guess the obvious theist retort would be that God’s ineffable decision to use a sub-optimal value for the constant is a test of our faith… hi-ho, anthropic principle!

Is the universe a hologram?

Wrap your head around this on a Monday morning, if you can: what we think of as the three dimensional universe we inhabit may in fact just be a holographic projection, and Craig Hogan is trying to prove it [via BigThink and Ian Sales].

The idea that spacetime may not be entirely smooth – like a digital image that becomes increasingly pixelated as you zoom in – had been previously proposed by Stephen Hawking and others. Possible evidence for this model appeared last year in the unaccountable “noise” plaguing the GEO600 experiment in Germany, which searches for gravitational waves from black holes. To Hogan, the jitteriness suggested that the experiment had stumbled upon the lower limit of the spacetime pixels’ resolution.

Black hole physics, in which space and time become compressed, provides a basis for math showing that the third dimension may not exist at all. In this two-dimensional cartoon of a universe, what we perceive as a third dimension would actually be a projection of time intertwined with depth. If this is true, the illusion can only be maintained until equipment becomes sensitive enough to find its limits.

“You can’t perceive it because nothing ever travels faster than light,” says Hogan. “This holographic view is how the universe would look if you sat on a photon.”

Coming soon to a Greg Egan short story collection near you…

I just never get tired of these existential cosmology ideas. As I’ve said before, this is the sort of stuff that makes me wish I’d actually paid attention at university and gone on to study hardcore physics… though given how much I sucked at calculus, it’s probably best that I didn’t.

Have we found the first habitable exoplanet?

This story’s pretty much everywhere today, though the headlines would probably be more accurate (if more cumbersome and less exciting) if they said that researchers have located the first exoplanet confirmed to be in an orbit around its parent star that would permit the possibility of liquid-phase H2O on its surface, and to have sufficient mass to hold on to a “substantial” atmosphere.

(Kinda takes the magic out of it, this whole accuracy thing… but it’s still pretty awesome when you think about it. Or it is for me, anyway.)

The new planet is one of six orbiting the star Gliese 581, a red dwarf 20 light-years from Earth. Two of the planet’s siblings, dubbed planets C and D, have also been hailed as potentially habitable worlds. The two planets straddle the region around the star where liquid water could exist — 581c is too hot, and 581d is too cold. But 581g is just right. The discovery will be published in the Astrophysical Journal and online at arxiv.org.

The new planet is about three times the mass of Earth, which indicates it is probably rocky and has enough surface gravity to sustain a stable atmosphere. It orbits its star once every 36.6 Earth days at a distance of just 13 million miles.


Gravity dictates that such a close-in planet would keep the same side facing the star at all times, the same way the moon always shows the same face to Earth.

That means the planet has a blazing-hot daytime side, a frigid nighttime side, and a band of eternal sunrise or sunset where water — and perhaps life — could subsist comfortably. Any life on this exotic world would be confined to this perpetual twilight zone, Vogt says, but there’s room for a lot of diversity.

“You can get any temperature you want on this planet, you just have to move around on its surface,” Vogt said. “There’s a great range of eco-longitudes that will create a lot of different niches for different kinds of life to evolve stably.”

The inevitable disclaimer here is that although it looks like Gliese 581g could support some sort of life, whether it actually does is still something of a cosmological crapshoot with unknown odds. However, the relative ease with which we’ve found it suggests there may be many more planets in similar situations, which raises the chances of us humans not being the only gang in town after all. Those odds are still pretty long, of course… but hey, we can dream, right? Question is, will we find ’em and meet ’em before time runs out?

Time is running out

There’s never enough time in the day, is there? Well, turns out that there may be a finite limit on the number of days, too… though not a limit so hard that it’s going to impact our expected personal life-spans very much.

The prevalent theory among cosmologists and physics heads is that the universe can and should expand indefinitely, meaning that time is essentially infinite and unending, but an apostate little gang of researchers are now suggesting that there’s a 50% chance that the universe – and hence time – will run out in around 3.7 billion years. Given that our own Sun is supposed to last another 5 billion years, that’s something of a curtailment of scope…

Their argument against an infinite universe is simple. In an infinite universe, anything can happen and will happen, no matter how unlikely it is. When there’s an infinite amount of every possible observation occurring, then it becomes impossible to determine probabilities of events, making the laws of physics similarly impossible to determine.

Of course, this makes an important philosophical assumption. Do we need to be able to understand the laws of physics, rather than just observing that they work? If so, then the Universe has to have an end.

I love a bit of epic-scale philosophical wrangling (if only as a layman watching from the sidelines), but it’s a little early in the day for it. My head hurts; I think I’ll go and have a little lay down. If Greg Egan calls, please take a message and tell him I’ll get back to him as soon as I’m able.