In search of hidden dimensions

Nature ^ | January 6, 2004 | Geoff Brumfiel

[snarks_when_bored]

Ping

[billorites]

[snarks_when_bored]

That's not a hidden dimension!

[Mikey]

"... no less than six cups of espresso during our hour-and-a-half interview..."

Wired while studying strings.

This is interesting. I've always thought that besides height, width and depth, time and gravity are also dimensional

[Moonman62]

The son of two Iranian physicists, Arkani-Hamed was born in Houston, Texas, and grew up in Boston. After the Iranian revolution of 1979, his family returned to their homeland, but as religious fundamentalists took over the government, his father was forced to go underground and the family eventually had to flee across the border to Turkey. By 1982, Nima was living in Toronto, Canada.

The free world benefits from oppression once again, though Canada barely qualifies.

[robertpaulsen]

E = MC⁶ ... not good.

E = MC⁵ ... no.

E = MC⁴ ... nope, not that.

E = MC³ ... nah, doesn't work.

Ah, screw it.

[snarks_when_bored]

You're right about height, width, depth and time being the standard four dimensions of spacetime. But gravity is not a dimension; it's one of the four fundamental forces, the other three being electromagnetism, the strong nuclear force and the weak nuclear force.

[snarks_when_bored]

I should have added that current versions of string theory require that there be additional spatial dimensions (up to 6 or even 7 more). Since these dimensions haven't been observed, it has been conjectured that they're quite tiny (curled up into up into any of a myriad of possible shapes). The present article discusses the possibility of getting experimental confirmation of these extra dimensions.

[snarks_when_bored]

Right on both counts.

[Beckwith]

You're right about height, width, depth and time being the standard four dimensions of spacetime. But gravity is not a dimension; it's one of the four fundamental forces, the other three being electromagnetism, the strong nuclear force and the weak nuclear force.

Gravity sucks . . .

[cake_crumb]

So in theory, this theory will be tested in 2008 when the first results of CERN come in? He needs more espresso. I'm only wired on regular coffee and cigars and feeling like an extra dimension is hovering just out of reach of my nose...

[snarks_when_bored]

This piece by Paul Steinhardt is not exactly on-topic, but Steinhardt does mention some drawbacks to string theory while disparaging the anthropic principle:

PAUL STEINHARDT
Albert Einstein Professor of Physics, Princeton University.

I believe that our universe is not accidental, but I cannot prove it.

Historically, most physicists have shared this point-of-view. For centuries, most of us have believed that the universe is governed by a simple set of physical laws that are the same everywhere and that these laws derive from a simple unified theory.

However, in the last few years, an increasing number of my most respected colleagues have become enamored with the anthropic principle—the idea that there is an enormous multiplicity of universes with widely different physical properties and the properties of our particular observable universe arise from pure accident. The only special feature of our universe is that its properties are compatible with the evolution of intelligent life. The change in attitude is motivated, in part, by the failure to date to find a unified theory that predicts our universe as the unique possibility. According to some recent calculations, the current best hope for a unified theory—superstring theory—allows an exponentially large number of different universes, most of which look nothing like our own. String theorists have turned to the anthropic principle for salvation.

Frankly, I view this as an act of desperation. I don't have much patience for the anthropic principle. I think the concept is, at heart, non-scientific. A proper scientific theory is based on testable assumptions and is judged by its predictive power. The anthropic principle makes an enormous number of assumptions—regarding the existence of multiple universes, a random creation process, probability distributions that determine the likelihood of different features, etc.—none of which are testable because they entail hypothetical regions of spacetime that are forever beyond the reach of observation. As for predictions, there are very few, if any. In the case of string theory, the principle is invoked only to explain known observations, not to predict new ones. (In other versions of the anthropic principle where predictions are made, the predictions have proven to be wrong. Some physicists cite the recent evidence for a cosmological constant as having anticipated by anthropic argument; however, the observed value does not agree with the anthropically predicted value.)

I find the desperation especially unwarranted since I see no evidence that our universe arose by a random process. Quite the contrary, recent observations and experiments suggest that our universe is extremely simple. The distribution of matter and energy is remarkably uniform. The hierarchy of complex structures ranging from galaxy clusters to subnuclear particles can all be described in terms of a few dozen elementary constituents and less than a handful of forces, all related by simple symmetries. A simple universe demands a simple explanation. Why do we need to postulate an infinite number of universes with all sorts of different properties just to explain our one?

Of course, my colleagues and I are anxious for further reductionism. But I view the current failure of string theory to find a unique universe simply as a sign that our understanding of string theory is still immature (or perhaps that string theory is wrong). Decades from now, I hope that physicists will be pursuing once again their dreams of a truly scientific "final theory" and will look back at the current anthropic craze as millennial madness.

[snarks_when_bored]

Maybe that's a floating, er, cake crumb??

[Ken H]

Let's say a star is formed a thousand light years from earth. How long before the gravity from the newly formed star reaches earth? IOW, how fast does gravity travel?

[snarks_when_bored]

Wow, that's not the way it looked in preview! Let me try it again:

---

PAUL STEINHARDT
Albert Einstein Professor of Physics, Princeton University.

I believe that our universe is not accidental, but I cannot prove it.

Historically, most physicists have shared this point-of-view. For centuries, most of us have believed that the universe is governed by a simple set of physical laws that are the same everywhere and that these laws derive from a simple unified theory.

However, in the last few years, an increasing number of my most respected colleagues have become enamored with the anthropic principle—the idea that there is an enormous multiplicity of universes with widely different physical properties and the properties of our particular observable universe arise from pure accident. The only special feature of our universe is that its properties are compatible with the evolution of intelligent life. The change in attitude is motivated, in part, by the failure to date to find a unified theory that predicts our universe as the unique possibility. According to some recent calculations, the current best hope for a unified theory—superstring theory—allows an exponentially large number of different universes, most of which look nothing like our own. String theorists have turned to the anthropic principle for salvation.

Frankly, I view this as an act of desperation. I don't have much patience for the anthropic principle. I think the concept is, at heart, non-scientific. A proper scientific theory is based on testable assumptions and is judged by its predictive power. The anthropic principle makes an enormous number of assumptions—regarding the existence of multiple universes, a random creation process, probability distributions that determine the likelihood of different features, etc.—none of which are testable because they entail hypothetical regions of spacetime that are forever beyond the reach of observation. As for predictions, there are very few, if any. In the case of string theory, the principle is invoked only to explain known observations, not to predict new ones. (In other versions of the anthropic principle where predictions are made, the predictions have proven to be wrong. Some physicists cite the recent evidence for a cosmological constant as having anticipated by anthropic argument; however, the observed value does not agree with the anthropically predicted value.)

I find the desperation especially unwarranted since I see no evidence that our universe arose by a random process. Quite the contrary, recent observations and experiments suggest that our universe is extremely simple. The distribution of matter and energy is remarkably uniform. The hierarchy of complex structures ranging from galaxy clusters to subnuclear particles can all be described in terms of a few dozen elementary constituents and less than a handful of forces, all related by simple symmetries. A simple universe demands a simple explanation. Why do we need to postulate an infinite number of universes with all sorts of different properties just to explain our one?

Of course, my colleagues and I are anxious for further reductionism. But I view the current failure of string theory to find a unique universe simply as a sign that our understanding of string theory is still immature (or perhaps that string theory is wrong). Decades from now, I hope that physicists will be pursuing once again their dreams of a truly scientific "final theory" and will look back at the current anthropic craze as millennial madness.

[snarks_when_bored]

Go figure. Now the original posting looks okay. Sorry...

[snarks_when_bored]

Gravity is currently thought to travel at the speed of light.

[Ken H]

Gravity is currently thought to travel at the speed of light.

Is it possible, in theory at least, to use gravity to transmit and receive information as is done with electricity and light?

[snarks_when_bored]

Is it possible, in theory at least, to use gravity to transmit and receive information as is done with electricity and light?

In theory, it's possible, but in practice it would be devilishly difficult (and would require an engineering expertise which is almost impossible to imagine). Almost the only events that produce gravitational disturbances that we could hope to detect are collapses of stars into neutron stars or black holes, or the orbiting of a neutron star around a black hole. So we'd have to figure out a way of using such phenomena to send gravitational signals.

I'm not holding my breath over here.