This conversation is closed.

## LIVE TED Conversation: Join TED Speaker Sean Carroll

LIVE conversation with cosmologist Sean Carroll, TEDxCaltech Speaker and author of From Eternity to Here: The Quest for the Ultimate Theory of Time.

The conversation will open at 4pm (Eastern Standard Time), May 19, 2011. Sean is on vacation in Las Vagas, and will join us from his hotel room there to discuss the cosmos, the nature of time, and the science of poker.

**Topics:**Cosmology Live Conversation physics time

## Closing Statement from Sean Carroll

Thanks for participating, everyone! I hope some light was shed. It's been a pleasure chatting with the TED community.

## Comment deleted

## Sean Carroll 50+

## Thomas Pisarchick 10+

## Will True 200+

## Oscar Pineda

Thanks!

## Sean Carroll 50+

Of course we want to know -- why is it like that? There we are still very unsure, but a lot of specific models for dark matter and dark energy (and the preference for matter over antimatter) are being carefully studied.

## Oscar Pineda

## Sean Carroll 50+

## Sebastien Zany

## Sean Carroll 50+

## Arhi Kuittinen

We can produce some kind of bastard copy versions of DNA-based programmed biomachines but they could not gain the independent consciousness to have self controlled and balanced emotions. 300 million year programming can not be copied. I think.

## Oscar Pineda

Also, I recently learned about a theoretical particle called a Tachyon, which is said to have the ability to travel faster than the speed of light. How is this even possible?

Thanks!

## Sean Carroll 50+

Likewise, we can imagine time travel, but it's hard to take seriously the possibility that it's for real. As I write about in my book, some of the conceptual problems with time travel come down to the arrow of time -- how can you remember the past but not the future if your past is in your future? That doesn't say it's impossible, but gives a hint of how hard it would be to make time travel real.

## Oscar Pineda

That's pretty much what I figured about tachyons, but wanted to hear from someone who actually knows what he's talking about.

## Rob Ross

A photon that just crosses the EH cannot escape, so why would a photon say 1 meter inside the EV ever be able to travel >= 1 meter from the EV?

And if that is the case, how can any photons inside the EH and just past the observer ever bounce back from anything inside the BH into the in-falling observers eyeballs?

Thanks!

## Sean Carroll 50+

## Rob Ross

If I cross the EH of a BH and I'm looking *opposite* my direction of travel (away from the singularity) I understand that what I would see would not look any different to me than before I crossed the EH. - just normal looking stars as before.

I still don't know what I would see if I look *toward* the singularity. If the singularity were at a point in space, I expect no photons to ever reach me from anywhere between my current location and the singularity, so I would not be able to "see" anything in that direction.

If the singularity is not in space but "in the future" as you say, then again, I would expect not to see anything from it since it hasn't happened yet.

I guess I can simplify by just asking you : If I cross the EH of a BH and look towards where I perceive is the "center", what would I see? As an example, say I fall into the BH with a friend who is 1 meter in front of me as we both approach the BH. I see him disappear as he cross the EH. Then I cross. I would think I should never be able to see him again. How do photons from his body bounce back into my eyeballs once we're both inside the EH?

Thanks again!

## Sean Carroll 50+

The point is: you don't see your friend disappear as he crosses the event horizon. You just see him emit light more and more slowly. But if you chase him in, it doesn't slow down infinitely, like it would if you remained outside.

## Alfred Essa

## Sean Carroll 50+

Physicists (including me) have thought long and hard about this phenomenon of "closed timelike curves," which is the sophisticated way to think about traveling backwards in time. The smart money says that they are purely hypothetical, and can't exist in the real world; but we don't know for sure.

## Alfred Essa

## Sean Carroll 50+

Unfortunately, we're talking about phenomena that are very, very far away from experimental accessibility. Unless you have a couple of black holes or cosmic strings in your back yard and know of some clever way of manipulating their gravitational fields.

## Sean Carroll 50+

## Papuna Bersenadze 10+

I'm wondering one thing tho... in the whole world of cosmology/physics, does the Fractal geometry play any role? is the whole Mandelbrot Set idea taken into consideration somewhere?

Cheers

## Sean Carroll 50+

These days people take very seriously the possibility that the *multiverse* is a fractal -- but that's obviously speculation, far outside what we know how to test.

## Papuna Bersenadze 10+

When it comes to our understanding of time(4th dimension, right?), in terms of physical equations do we assign a certain "speed" to time? we know it's direction, but what I'm trying to ask is, does our perception of the rate of it's progression affect the empirical/scientific approach to time?

## Sean Carroll 50+

Or, if you like: the speed of time is always one second per second.

## Paul van Zoggel

## phil halper

## Sean Carroll 50+

## Jean Millet

## phil halper

http://arxiv.org/abs/astro-ph/0612243

## Sebastien Zany

## Sean Carroll 50+

## phil halper

## Sean Carroll 50+

I'm not very optimistic about loop quantum cosmology or loop quantum gravity more generally, but I'm happy to be proven wrong.

## Oscar Pineda

Thanks!

## Sebastien Zany

## Sean Carroll 50+

http://www.amazon.com/Introduction-Modern-Cosmology-Andrew-Liddle/dp/0470848359/

http://www.amazon.com/Introduction-Cosmology-Barbara-Ryden/dp/0805389121/

## Lindsay Newland Bowker 50+

A second is an earth bound measurement..a solar system bound measurement. Why is it we think it is useful to understand cosmology..the age of the universe..its acceleration even the speed of light itself?

## Sean Carroll 50+

## Lindsay Newland Bowker 50+

## Comment deleted

## phil halper

## Sean Carroll 50+

## kamal koubaa

Is it possible to multiverse to be seen as many universes, each one includes another rather than separated ones, I think in such situation the accelerated expending can be explained by the impact of a bigger universe on the smaller one that is included in it?

## Sean Carroll 50+

## Oscar Pineda

Dr. Carroll: I wanted to ask you about the famous "Drake equation" to calculate the number of intelligent civilizations in the Universe. The number that results from the equation is very arbitrary because it depends on many unknown factors (at least at the time Drake presented it and Carl Sagan mentioned it in Cosmos). Do you know if there is any new data that can help us fine-tune the values that we give each of the variables so that they can be more accurate with reality?

Thanks!

## Sean Carroll 50+

http://blogs.discovermagazine.com/badastronomy/2011/05/18/the-galaxy-may-swarm-with-billions-of-wandering-planets/

However, we are still very very far away from nailing down all the terms in the Drake equation. It's the ones involving life and intelligence and civilization that are the most unknown, for obvious reasons. It will probably stay that way for a while.

## Oscar Pineda

## Tim Colgan 50+

## Sean Carroll 50+

## Tim Colgan 50+

## Sean Carroll 50+

http://arxiv.org/abs/hep-th/0508039

## Tim Colgan 50+

## Sean Carroll 50+

## Tim Colgan 50+

## Sami Kosonen

Thanks!

## Sean Carroll 50+

## Andrew Cross

Does this make life possible in the empty space era?

## Sean Carroll 50+

## Craig Counterman

## Sean Carroll 50+

I think about other Sean Carroll's, but they don't bother me. I hope they're having fun. (Statistically, some are and some are not.)

## Dana Shaw

## Sean Carroll 50+

## Dana Shaw

## Sean Carroll 50+

## Padriac Fowler

## Sean Carroll 50+

## Oscar Pineda

Thanks!

## Sean Carroll 50+

## Oscar Pineda

Thanks!

## Sean Carroll 50+

## Enrique Zepeda

Thanks!

## Sean Carroll 50+

## Enrique Zepeda

Thanks!

## Sean Carroll 50+

## Nicholas Pauley

## Sean Carroll 50+

Keep an open mind and always be learning new things.