If photons (particles of light) have zero mass, how can they exist?

It's very natural here on Earth to equate the word "mass" with the word "weight." In fact, in our everyday lives, we can usually get away with using the two terms interchangeably. A massive thing is also going to be heavy.

It's actually more accurate to define mass as "how much work it takes to push the thing down the road." Again, very similar to "weight," just a subtle distinction.

If I'm trying to push an elephant on roller skates down the road, and attain a speed of, say, 35 miles per hour, I have to do a lot of work.

I can either do this work all at once, maybe by firing the elephant out of a cannon, or I can spread the work out over time, pushing the elephant gradually faster and faster until the desired speed is attained.

Later, I can calculate how much work it took to get my elephant up to speed. (There are measurements for this stuff like joules and newtons.) In the end, I will find that, regardless of my method -- either shooting the elephant out of a cannon, or slowly pushing it -- the amount of work required to reach 35 miles per hour was exactly the same. It takes a certain, specific amount of work to get an object of a certain mass up to a certain speed.

With a massless object / particle like a photon, there's no work required at all. The photon does not have to "explode" out of its flashlight as though it was shot from a cannon, it does not have to gradually accelerate over time, it's already there. The "work," in a sense, has already been done. That is also the reason photons are always moving at 300,000 km/s. They never speed up or slow down, they pop into being already traveling at the speed of light.

What happens to an email once it is deleted?

If something is in your deleted box, from a computer perspective it has not been deleted at all. It's just been moved.

In fact, from a computer perspective, there is no such thing as deleting. What has been written cannot be unwritten, only overwritten. So when you delete an email fully or empty your recycling bin fully, you are really just making it so you can't see it. Eventually when new data comes along it will be overwritten.

However, even if it is fully deleted, people who specialize in recovery may be able to get it back even if it has been overwritten a little. It will have distortions though. There are instances of recoveries people have done of pictures and they are funky looking.

How do scientists know that a rock is from space and not of earthly origin?

Depends on what it is made of. Meteoric iron is unique because metal doesn't naturally make that kind of pattern on Earth.

Certain elements and compounds are reactive in earth conditions. Metallic aluminum famously doesn't exist naturally on Earth. If we found a hunk of it that wasn't man made, we'd know it wasn't on Earth however many thousands/millions of years ago, or it would have eroded away.

This also applies to certain isotopes. If something radioactive is too young, it can't be from Earth. Earth has a well known age and all the crap that was here has been aging all this time.

There are also more obvious signs. Something from space smacking into Earth has an impact. If a meteor slamsinto a desert, we're going to find a lot of natural glass around the impact site. Entire forests have been obliterated by meteors.

How does a computer unset a bit (set it from 1 to 0) in memory/storage?

It depends on the particular kind of storage.

In magnetic storage, like a spinning-disk hard drive, it magnetizes the region storing the bit in the opposite direction.  (In practice, hard drives write entire blocks at a time, but the technology could theoretically be managed a bit at a time.)

In typical flash storage, like in solid-state drives or external flash drives, it can't write a single bit at a time. In order to change a bit from a 1 to a 0, it erases an entire block of memory, then rewrites the new data into it.

In DRAM, which is the typical kind of RAM in a computer, it connects the capacitor holding the bit to a current drain, which allows it to discharge.  (Similarly to others, standard DRAM actually can only write a whole line at a time, so switching a single bit means writing the previous value into all the other bits.)

In SRAM, which is typically used for things like on-CPU caches, the bit line is set to 0 and then the write line is set to 1.  The transistors switch into the alternative configuration, and then the write line is set back to 0, which causes them to maintain their current configuration until written again.

How do motion detectors detect motion in the dark?

There are two common types of motion sensors - passive infrared (PIR) and Ultrasonic. The most common type is PIR which sees IR which is emitted by hot objects. There is a special lens in front of the sensor so a warm object moving through its field of view creates sudden transitions that are easy to detect. Since it is relying on heat emissions it works fine with or without light.

Ultrasonics send out a sonar burst and effectively echo locate like a bat. They check the sound they get back after each one and compare it to the last one, if it changed significantly then something in the space moved and they trigger.

How can a piece of paper folded 103 times be larger than the observable universe?

Exponential growth. Each fold doubles the thickness of the paper. If someone gave you 1 penny today, and promised to give you twice as much money tomorrow, and twice as much the day after, and so on and so on, on day 30 they would have given you over $5 million.

You can do the math on this. Open the calculator on your computer, and type in 0.0039 (the thickness of a piece of paper).  Press the Times button, and then the number 2, and press enter. Now press enter 102 more times. By the mid-50's your calculator will actually reach a point where it has to start using exponents, and by 60 you'll have surpassed the what the calculator is capable of.

How is it possible for ISP's to know we are doing online? Isn't HTTPS supposed encrypt content so it can't be read?

The ISP (Internet Service Provider) is your mailman. They need to get packages to where they need to go. HereBeAnswers, for example, is sending you a package containing this answer. You pay the mailman monthly for a rate at which they send packages from you and to you.

HTTPS encrypts the package's contents, however the ISP's responsibility is still to move the package from A to B, and therefore needs to know what these A and B are. Therefore the postage address cannot be encrypted, and your ISP can track who you are exchanging packages with, be it HBA or YouTube or Netflix.

So your ISP can't actually see what you are viewing on HBA, YouTube, or Netflix, but they can see which sites you are accessing.

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