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Hi, Spaceland,

I'm Matt Kleban.

I'm an associate professor of physics at New York University

and I'm here to answer your questions.

First question is:

Is there a maximum size for a black hole?

So, it's a very good question.

Before I answer it, just quickly:

What do we mean by the "size of a black hole"?

The size of a black hole is the radius of the event horizon.

Black holes are regions that not even light can escape from.

There is a certain size to the region from which no light can get out.

The edge of that region is called the "event horizon"

and has a radius, it has a size.

That's what we mean by the size of a black hole.

Actually, the radius of the event horizon,

so the size of the black hole, is just proportional

to the amount of matter and energy that you throw into it.

In fact, if you had access to an infinite amount of matter and energy,

you could just keep feeding into the black hole,

and it could just keep growing bigger and bigger.

There's no limit in that sense.

But, of course, in the universe we live in,

we don't have access to an infinite amount

of matter and energy.

And even if we could somehow fly around

all of the universe and collect all the matter and energy in it,

even if we did that, we would still have a maximum size.

Because there's a finite amount of matter and energy

in the universe that we could collect.

That size would be a fraction of the size of the universe today.

So, it would be maybe billions of light year across,

very very big, but still a finite size.

The second question is:

Can matter exist without space?

That's a very good question and a very hard question.

The honest answer is: We don't know for sure.

But what we can say is that we have a theory that relates

matter to space, and to time as well.

Although we don't think that theory is an exact description

of the world, it's the best one we've got.

That theory is called "general relativity."

It's the theory that Albert Einstein, in 1916 or so, first discovered.

According to Einstein, there's an equation actually a set

of equations, called "Einstein's equations",

which relate matter to space and time.

It turns out it's possible to solve those equations

when there's no matter.

You can have zero matter and zero energy,

but still have space and time.

But it's not possible to solve those equations

when you have matter but without space and time.

In other words, you can have space and time without matter but,

at least according to Einstein's equations,

you can't have matter without space and time.

We don't think that that's the end of the story.

We don't think that the exact laws of physics.

It could be that whatever is the exact laws of physics could

somehow allow matter to exist without space.

But, I think, at least for the moment,

the state of the art is no.

As far as we know it's not possible.

The third question is:

If we were able to overcome the problem

of infinite energy for light travel

and travel through space at C [at the speed of light],

would time dilation mean that from our perspective

traveling anywhere would be instantaneous?

And would this then mean that we would actually

exist at every point in our vector at the same time?

That's another good question.

The reason for the question is that when you move very fast,

when you move very close to the speed of light,

there's a phenomenon called "time dilation",

which means that for you, if you're the one moving very

close to the speed of light, not very much time

is going to pass.

For instance, if you travel from the Earth to Alpha Centauri

on a spaceship that is moving

very close to the speed of light,

then very little time will pass for you.

You won't age very much at all.

Your clocks won't take off very much time.

The people on Earth and the people on Alpha Centauri will

watch you make this trip

and they'll say it took you a couple of years

at least to make that trip.

But for you, it could be a very short amount of time.

The interesting thing is this is a theory of relativity

and everything should be relative to the observer

so, you might ask:

How could it be that you could make that trip in such

a short period of time?

And the answer is if you go to your "reference frame",

if you're driving a car in the highway,

you see buildings and trees, and stuff like that

along the road, coming toward you at 60 m/h,

you at rest, the trees and the buildings

are coming towards you.

That's your reference frame.

In the spaceship, you are at rest,

and Alpha Centauri is coming towards you.

The speed is coming towards you is pretty close

to the speed of light.

It can never be greater.

It can never go faster than the speed of light.

But it will be coming toward you at nearly the speed of light.

The reason that you make the trip in such a short period of time,

is because there's another strange phenomenon

in relativity which is called "Lorenz contraction":

Distances get contracted.

Distances look shorter when objects are moving fast.

The distance that you would measure

between you and Alpha Centauri,

is much shorter than the distance that someone at rest

on Earth would measure.

That's how you are able to make the journey

in such a small period of time.

So, to answer the question about whether everything

would be instantaneous in the limit that

you're moving at the speed of light,

the answer is yes.

Because in the limit that you are moving

at the speed of light,

no time passes for you as you make this trip.

And from your point of view,

there's no distance actually between Earth and Alpha Centauri.

That's how you can cross it in just an instant.

So yes.

You would be everywhere between Earth and Alpha Centauri

at the same time in one instant.

Thanks for the great questions.

If you have more questions for the next expert,

please leave them in the comments below.