One possibility is that they are not there, and that Dr Arkani-Hamed is wrong.
"Because we don't see them doesn't necessarily mean they are not there, " he said.
They are not there, says Mr Mugabe, to escalate the war, but to bring peace and stability to central Africa.
You may look under your bed at night for spies and not see them, but that does not mean they are not there.
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Experts have long been divided on whether North Korea has made much headway on clearing that hurdle, though the general consensus is they are not there yet.
Patients said doctors talked about them as if they are not there, did not tell them what is going on and did not consult them about their treatment.
"They are not there to support Leo or me, " HP Chairman Ray Lane said in an interview with Bloomberg News on Jan. 26 at the World Economic Forum, in Davos.
As far as the defeat of the Taliban is concerned, they are defeated, they are gone - as a movement, as a government, as a structure, a political structure, a religious structure - they are not there.
"When we get on the baseball field with whomever the opponent is, they are not sitting there saying: 'Well, their players make more money than us so therefore you're deemed a winner and we're deemed a loser, '" Astros manager Bo Porter said Thursday.
But it's not just whether they are there or not, and whether they are an enemy combatant or not.
There are different stuff - they're are not yours, but there are a lot of different ways you can make a planter out of them.
American voters are not there yet, and they may not get there in time to avoid a crash.
These are most of what they are using while they are there, and they are not using much electricity, water, or internet back in their home apartment.
So the ground operations will go in and go out, they're are not there for a long term basis.
If development programs are curtailed their absence will not be noticed on the battlefield because the weapons they would have yielded are not yet there, and the programs usually have a small domestic political footprint due to the absence of a production line and articulated supplier base.
"I have had assurances that serious sex offenders are not there, they have been assessed as not being a danger, " Mr Jones said.
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Graphics are obviously up there, but they are not the be-all end-all.
Although some are registered with the authorities, many are not - and even if they are, there is a way round it.
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It is no good having ambulances coming from 40 miles away because they are not going to get there in enough time.
First, I want those who are addicted and those who love them to know they are not alone and that there is hope.
But if Obama and Congress are serious about balancing the budget, they are not going to get there by squeezing tax revenues from just a small sliver of households.
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The Los Angeles public school system is backing off its plan to end so-called social promotion, whereby kids are sent to the next grade even though they are not qualified to be there.
With many small business owners saying sales are down in a recent survey by the National Small Business Association, strategic use of social networking sites seems like a good way for many to pull in new business from customers they may not know are out there.
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"Many Muslims are not buying processed food now because they are concerned there might be contamination, " he said.
"We are committed to not sending troops over there until they are fully trained and equipped for the mission, " said Gen.
The Home Secretary, Theresa May, has written to the police to say she believes that people who are arrested should not normally be named until they are charged unless there are "extremely unusual circumstances".
WMAP's picture is detailed enough to discern the relevant blips (assuming they are there) is not yet clear.
And research shows that many such people are not aware there is a problem until they are fired, demoted or otherwise derailed.
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