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# How Do Scientists Measure the Speed of Light?

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How do scientists measure the speed of light?

Jack Fraser:

We don't.

No, seriously, we don't measure the speed of light (which always refers to the speed in a vacuum). We know exactly what the speed of light is.

It is: c = 299,792,458 meters per second.

And that is absolutely 100 percent accurate, with no measurement errors. The reason we know that that's exactly the speed of light is that we defined it to be that number.

We then take our definition of a second (the length of time for a certain number of periods of the radiation emitted in hyperfine transitions in caesium-133), and from that we define a meter. So the thing we would be measuring is what a meter is.

We use the speed of light as a fixed velocity, from which all observers can define their own length scale.

To measure the speed of light would require an external definition of what a meter is—and since about the 1970s, we don't have one.

And if you did want to measure the speed of light using this external distance reference, it's easy to test: You just release a light pulse at t=0 toward a mirror and then time how long it takes to get back to you. This is the exact principle that radar and sonar work on (although again, they measure the distance knowing the speed—but it works either way round).

Some background:

The meter was originally defined after the French Revolution, in about 1799. It was defined as 1⁄10,000,000 the distance between the equator and the pole.

The “meter” was formally defined from 1889 as the length of a platinum rod, held in a vault in Paris.

From this definition of a meter (and an old definition of a second), we measured (using the mirror-timing method, or based on astronomical observations) the speed of light to be about 299792458, plus a non-integer bit, and error bars from the measurement errors.

Eventually, we realized that having a meter defined by something there was only one of was a bit annoying. So, we attempted to define it in a way that anyone could replicate—without having to refer to a "standard object."

Therefore, we redefined the meter—using the speed of light.

The official definition of a meter today is: 1⁄299792458 of the distance traveled by light in a vacuum in one second, using the caesium definition of a second.

Therefore, this was exactly equivalent to defining the speed of light to be the number given above.

We chose that number (and not a more convenient number like 300,000,000), because that number changed the definition of a meter by only a fraction of a fraction of a percent—but made everything all nice and integer-y.

A consequence of using this definition is that any attempt to measure the speed of light is cyclicalyou must use a “meter” to measure it at some point—which relies on the speed of light.

Therefore what you actually do now, when you “measure” the speed of light (in a vacuum), is actually “measure how accurate your measuring instruments are."

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Big Questions
Why Do We Dive With Sharks But Not Crocodiles?
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Why do we dive with sharks but not crocodiles?

Eli Rosenberg:

The issue is the assumption that sharks' instincts are stronger and more basic.

There are a couple of reasons swimming with sharks is safer:

1. Most sharks do not like the way people taste. They expect their prey to taste a certain way, like fish/seal, and we do not taste like that. Sharks also do not like the sensation of eating people. Bigger sharks like great whites enjoy prey with a high fat-bone ratio like seals. Smaller sharks enjoy eating fish, which they can gobble in one bite. So, while they might bite us, they pretty quickly decide “That’s not for me” and swim away. There is only one shark that doesn’t really care about humans tasting icky: that shark is our good friend the tiger shark. He is one of the most dangerous species because of his nondiscriminatory taste (he’s called the garbage can of the sea)!

2. Sharks are not animals that enjoy a fight. Our big friend the great white enjoys ambushing seals. This sneak attack is why it sometimes mistakes people for seals or sea turtles. Sharks do not need to fight for food. The vast majority of sharks species are not territorial (some are, like the blacktip and bull). The ones that are territorial tend to be the more aggressive species that are more dangerous to dive with.

3. Sharks attacked about 81 people in 2016, according to the University of Florida. Only four were fatal. Most were surfers.

4. Meanwhile, this is the saltwater crocodile. The saltwater crocodile is not a big, fishy friend, like the shark. He is an opportunistic, aggressive, giant beast.

5. Crocodiles attack hundreds to thousands of people every single year. Depending on the species, one-third to one-half are fatal. You have a better chance of survival if you played Russian roulette.

6. The Death Roll. When a crocodile wants to kill something big, the crocodile grabs it and rolls. This drowns and disorients the victim (you). Here is a PG video of the death roll. (There is also a video on YouTube in which a man stuck his arm into an alligator’s mouth and he death rolled. You don’t want to see what happened.)

7. Remember how the shark doesn’t want to eat you or fight you? This primordial beast will eat you and enjoy it. There is a crocodile dubbed Gustave, who has allegedly killed around 300 people. (I personally believe 300 is a hyped number and the true number might be around 100, but yikes, that’s a lot). Gustave has reportedly killed people for funsies. He’s killed them and gone back to his business. So maybe they won’t even eat you.

8. Sharks are mostly predictable. Crocodiles are completely unpredictable.

9. Are you in the water or by the edge of the water? You are fair game to a crocodile.

10. Crocodiles have been known to hang out together. The friend group that murders together eats together. Basks of crocodiles have even murdered hippopotamuses, the murder river horse. Do you think you don't look like an appetizer?

11. Wow, look at this. This blacktip swims among the beautiful coral, surrounded by crystal clear waters and staggering biodiversity. I want to swim there!

Oh wow, such mud. I can’t say I feel the urge to take a dip. (Thanks to all who pointed this out!)

12. This is not swimming with the crocodiles. More like a 3D aquarium.

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Big Questions
Can You Expel a Sitting Senator?
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In light of recent allegations, Republican Senator Cory Gardner of Colorado this week said that if Alabama Senate candidate Roy Moore “refuses to withdraw and wins, the Senate should vote to expel him, because he does not meet the ethical and moral requirements of the United States Senate.” Meanwhile, Senator Bob Menendez, Democrat of New Jersey, has been involved in a high profile corruption trial, with calls that he should resign or be expelled if convicted. Has anything this drastic ever happened before?

Yes, but not for a very long time. Once you’ve been voted into the Senate, it’s difficult to get you out.

#### REFUSING TO SEAT

Refusing to even seat a senator is very rare, but one example from over 100 years ago also involved Alabama.

In 1913, Alabama Senator Joseph F. Johnston died just a few months after the ratification of the 17th Amendment to the Constitution. The Amendment allowed for direct election of senators, as well as clarifying the role of the state in calling special elections. Alabama’s governor put up Representative Henry Clayton, but he soon resigned the appointment. This was followed by Frank Glass, a local newspaper editor. As Glass was about to be seated, senators worried that his appointment was illegitimate (similar fears had surrounded Clayton). As one senator said at the time, “I believe that the [17th] Amendment means exactly what it says. It is perfectly plain and unambiguous. It simply means from this time forward every senator of the United States must be elected by the people, unless the legislature of a state by express terms empowers the executive to make temporary appointments to fill vacancies. The legislature of the state of Alabama has not given such power to the executive.”

By a vote of 32-31, the rest of the Senate agreed and refused to seat Glass, leading to a special election in 1914 that brought in a new senator.

Since then there have been multiple attempts to not seat a senator—most famously Roland Burris in 2009, who was appointed by Illinois governor Rod Blagojevich under the cloud of corruption charges (though he was ultimately let in). But in reality a refusal to seat a senator is unlikely to succeed.

In 1969, the Supreme Court ruled in Powell v. McCormack that as long as a duly elected representative met the age, citizenship, and residence requirements of the Constitution, they could not be excluded from the House. They could be expelled after taking their seat, but not excluded. Since it’s generally felt that this ruling extends to the Senate, it would likely not be possible to exclude an elected senator from their seat. But once that seat is taken, expulsion becomes a possibility.

#### EXPULSION

The United States Constitution states that, “Each House may determine the Rules of its proceedings, punish its members for disorderly behavior, and, with the concurrence of two-thirds, expel a member.” However, this is exceedingly rare.

The first time it happened was in the 1797 case of William Blount, one of the first two senators from Tennessee. According to the Senate, Blount had worked on a plan to take control of Spanish Florida and Louisiana and transfer them to the British with the help of Native Americans and frontiersmen. This plot was discovered and Blount was expelled, but not until he was impeached by the House of Representatives (the House has the sole power of impeachment, and it falls to the Senate to try the impeachment). The Senate ultimately decided not to try the impeachment, although whether that’s because senators believed that they themselves are unimpeachable or because Blount was unimpeachable because he had already been expelled and thus ceased being a senator is up for debate.

The next attempt at expulsion was in 1808, when Ohio’s John Smith was caught up in the Aaron Burr controversies. When it came to vote, the tally was 19 yeas for expulsion and 10 nays. Since the Constitution requires a two-thirds majority, Smith was saved from expulsion by one vote, although he would resign soon after.

The largest crop of expulsions was in 1861 and 1862, in regards to senators from southern states. As some senators were still officially members of the Senate, despite representing seceding states, it was felt that their status should be clarified by expulsion. As a result, 10 senators were expelled on July 11, 1861 (the expulsion order of one of the senators, William K. Sebastian of Arkansas, was later posthumously revoked after it was determined the charges “were as regards Sebastian merely a matter of suspicion and inference and wholly unfounded as to fact” and he didn’t commit conspiracy against the government). Later, a few more senators were expelled on the charge of supporting the rebellion. Including Sebastian, a grand total of 14 senators would be expelled during the Civil War. Since then, no senator has been expelled.

That’s not to say there haven’t been attempts. Cases since the Civil War have ended in either an exoneration or the senator leaving office before the vote. The most recent near-expulsion was Nevada Senator John Ensign in 2011 under accusations that he broke federal laws while attempting to cover up an affair. At the time, Senator Barbara Boxer of California said the case was “substantial enough to warrant the consideration of expulsion.” Ultimately, Ensign resigned.

It has been 155 years since the last senator was expelled. Whether—or when—that fact will change only time will tell.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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