转自知乎
作者：Gunter Schoech
链接：https://www.zhihu.com/question/44049485/answer/646171642
来源：知乎
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As a German living in France, I wrote in English (included), but thank JY for the kind translation
坐标法国的德国人，用英文作答，翻译内容由热心知友JY huang提供：
The short answer: Velocity and psychology.
简而言之，原因就是两个：货币流通速度以及人民的心理
"Boosting the economy" means increasing GDP through increased spending (consumers, companies, government), and the needed supply of the goods and services being consumed.
促进经济增长，意味着通过加大个人、公司以及政府的消费，提高他们对各种商品以及服务的需求，从而拉动GDP增长。
QE tries to reach this by central banks buying up bonds (debt) from the government, or other assets (corporate bonds or even equity). They can do this, because they can create money from nothing, and pay with it. This extra money is thus injected into the economy, and should ideally jump-start the economy, leading to higher spending and somewhat higher inflation as a buffer against a deflationary situation.
量化宽松政策试图通过央行从政府购买债券（债务）或其他资产（公司债券甚至股权）来实现这一目标。央行可以这样操作是因为央行有印制和发行货币的职能，它能用印制出来的钱进行支付。在理想的情况下，通过将这笔额外的资金注入，整个经济应受到积极的推动，促进各类支出以及实现一定程度的通货膨胀，缓解通货紧缩的局面。
The equation for GDP is: GDP=Money Supply x Velocity of Money
GDP是这样计算的：GDP=货币供应量×货币流通速度
Velocity describes, how many times a unit of currency is used to make a transaction.
货币流通速度表达了一个货币单位在一定时间内用于交易的次数
E.g. if you earn 100 RMB, and you immediately take a taxi home, the taxi driver goes to a bar and has a drink, tipping the waiter, and the waiter buys a sandwich, and the sandwich vendor buys fresh vegetables, all in the same day, the velocity is 5x higher than if you take your 100 RMB and put them in your savings account and leave them there.
例如在一天内，你将刚获得的100人民币收入拿去打车回家，出租车司机用这笔收入去了酒吧喝酒，酒吧又将100元当小费给了服务员，服务员用这笔小费买了个三文治，三文治的摊主将这笔收入拿去采购蔬菜作为原材料。在这样的过程里，货币流通速度就相当于你仅仅将它们存到银行的5倍。
The Quantiative Easing (QE) can only influence the money supply, but it can't influence the velocity.
量化宽松只能影响货币供应量，却不能影响货币流通速度。
In fact, it can even only influence the so called base-money, which is the money created by the governments. But in most countries, this money makes up only ~10% of the total money, the 90% remainder are created by commercial banks when they grant credits. But if nobody wants to borrow money, or worse, people prefer to de-leverage = pay back their debts instead of using free cash flow for consumption, the total money in the system can very well contract, while the government "prints" additional money.
事实上，它甚至只能影响所谓的基础货币，即政府创造的货币。但在大多数国家，这笔钱仅占货币总量的10％左右。另外90％的货币存在于银行信贷中。如果没有人想借贷，或者更糟一点，人们更希望去杠杆=偿还债务，而不是使用自由的现金流进行消费，则货币总量会发生重大缩减，即使政府正在“印”钱。
That's why central banks try to indirectly control the credit volume too, through higher or lower interest rates.
这就是为什么央行也会试图通过调整利率来控制信贷总量。
For a long time, economics assumed velocity to be +/- constant, and thus the growth stimulus (adding money supply) and curbing inflation (increasing interest rates) could directly be influenced over the money supply.
长期以来，经济学假设货币流通速度是恒定的，因此增长刺激（增加货币供应量）和抑制通货膨胀（增加利率）可能直接影响货币供应量。
But velocity is far from constant. In the US, it dropped dramatically when the QEs really took of as you can see below.
但实际上，货币流通速度远非恒定的。如下图所示，在美国，当量化宽松政策上台之后，货币流通速度大幅下降。
In fact, if a population is pessimistic about the future, they will rather save money than consume. If the environment, as is the case in Japan, is even rather deflationary, they will wait with consumption forever, waiting for ever lower prices. This happened in Germany after WW 1, when people would simply not "squander" money after all the losses in WW 1.
事实上，如果人们对经济前景感到悲观时，他们宁愿储蓄而不是消费。如果像日本那样，通货紧缩持续，则人们会停止消费，一直等待更低的价格。这样的情况也曾经在一战后的德国发生过，在巨大的损失下，人们根本不愿意去“挥霍”手里的钱。
The government printed stimulus money to kick start the economy, but it didn't work, although the money was there.
政府发行更多货币以期刺激经济，但即使钱已经在那里了，政策却依然不凑效。
Where does it go? If it is not used in every day transactions or productive investments like machines, it goes into assets like stocks and real estate. We can see that very well in most Western countries.
那么这些钱到底去哪里了？如果这些钱不是用于日常的流动交易或者是如购买机器这样的生产性投资的话，则它们会进入股票和房地产市场中。我们在很多西方国家里都能观察到这点。
As a result, the US FED now needs to take on 4$ of new debt instead of 1.5$ in the 70's to create one more $ GDP. Obviously, this can only lead to debt rising faster than GDP, which we already see today in the US, and which is not sustainable.
因此，今天美国每增加1美元GDP，美联储就要为此负担4美元的新债务，而不是70年代时的1.5美元。很显然，这会导致债务上升快于GDP，我们在现在的美国看到这种状况了，而这种状况不可能永远维持。
We live in world with many deflationary forces:
现实世界里，有很多导致通货紧缩的因素
The demographics, automation, saturation in many areas, pre-existing debt burden taking liquidity away from consumption etc. are just some of them.
人口结构的改变，自动化的普及，各个（商业、工业）领域的饱和，既存债务的负担等等，都降低了消费所需要的资金流动性，
As long as that is dominating the minds of people, increased base money won't turn that around.
只要这些因素主导了人们的思想，再怎么增加基础货币也无济于事。
Sure, if the stock market and real estate are in all-time high territory, people feel rich, and might consume more, although they don't actually have more liquidity. But it's all about the psychology. That is what we see somewhat in the US right now.
当然，如果股市和房市处于历史高位，人们会觉得富裕，并可能做更多的消费，尽管实际他们并没有更多的现金流。然而这完全是一种心理状态，我们在美国看到的繁荣正是因为这样而产生的。
But don't forget: The US deficit is growing $ 1-1.3 TRILLION per year, and has passed $ 22 trillion in total. Compare that to 350 billion for Greece in total (adding 4 or 5 times Greece PER YEAR).
但不要忘记：美国的赤字每年增长1-1.3万亿美元，总计超过22万亿美元。当年希腊才总计3500亿债务，而且现在美国每年债务增长更是希腊的4到5倍。
And don't confuse "good economy" with "good stock market". Right now, the US stocks are high, but that is fueled by stock buy backs by companies which received Trump's tax reform. Instead of creating jobs by investing, they rather buy back their own stock and "destroy" it, thus driven the price of the remaining shares higher. Good for a manager with a stock option plan, or if you already earn the share. But not creating any value. P/E keeps going up as earnings don't keep up.
同时，请不要把“经济好”与“牛市”混为一谈。目前，美国股市走高，但这实际是受到特朗普税收改革的推动，公司对自己股票进行回购所造成的。企业在这期间并不是通过投资创造就业机会，而是通过回购自己的股票并“摧毁”它（原有的价格和人们既定的预期），从而推动剩余股票的价格走高。这对于已经拥有股票期权的金融市场经理人，或者是已经赚得股权的人来说（上市企业支付股票作为员工福利），是件好事。但这实际没有创造任何价值。而由于盈利没有提升，则市盈率继续上涨。
What you see here, is the same hesitation as for the consumers: The companies have more liquidity than worthwhile investment ideas. They do M&A or buy backs, but neither drives up GDP. They don't see good opportunities, and they don't want to give back the money to shareholders either (as dividend).
我们可以从中可以发现，企业与普通消费者有着一样的犹豫：公司有了更多的现金流，却没有有价值的投资方向。企业进行并购或回购，但这些都没法推高GDP。企业看不到好的发展机会，却也不想把钱还给股东（作为股息）。
So overall, we see that the stimulations don't really work in the long run. In the very short term they might work, and ideally, that can be enough to reverse the expectations of consumers and companies, and thus led to self-sustained growth.
总的来说，我们发现这些刺激举措长期来看并没有发挥效用。或者在理想状态下，短期内可能可以扭转消费者和企业的期望，从而实现自发的持续增长。
Not any more. The FED has as 4 trillion balance sheet they can't reduce without causing the next recession. At the same time, they are in no position to fight a recession when it happens, with an already bloated balance sheet and very low interest rates.
而实际上已经不再如此。在不引起经济衰退的情况下，美联储所持有的4万亿负债是不可能削减的。同时，在负债累积至此而利率还非常低的情况下，美联储也无法应对经济衰退。
Trump couldn't resist the temptation to fire the last bullet towards the end of a very long, already artificial, bull market by doing his tax reform. He can't pull that rabbit out of the hat another time.
特朗普没有办法拒绝税制改革所带来的诱惑，因为那是这个人为牛市最后一次的推波助澜。但一旦这招过后，他恐怕只能黔驴技穷了。
The European Central Bank has equally amassed € ~2.5 Trillion in assets, esp. by keeping countries like Italy afloat by buying up their bonds.
Interest rates are still at 0, no possibility for a raise in sight.
Only the debts of the different governments is much lower than in US (let alone Japan), and the annual incremental debt as well (maximum a EU country is allowed is 3% of GDP, the US is at 5 or 6% already). Germany even had small surplus in the last couple of years, but to be fair, that was also only possible due to the quasi 0 interest rates.
同样地，欧洲央行累积了约2.5万亿欧元的资产，尤其是通过购买债券来保持像意大利这样国家的经济稳定。现时利率仍为0，且看不到加息的可能性。唯一的区别是不同政府间，债务的持有远低于美国（更不用说日本了），而且债务的年增加量也低于美国（欧盟国家最高限制在GDP的3%，美国已经是5%或6%了）。德国在过去几年财政甚至实现小幅盈余，但这很可能只是因为0准利率带来的好处。
Bottom line: If stimulus were sure to work, we would all do it, and all live in economies without problems. But the effects are short lived. It's like a drug. The world got used to it, and can't get off it anymore. If it tries (like the FED in Q4 2018), the "withdrawal effects" come immediately. If it keeps taking the drug, it works less and less, or you need more an more.
总而言之，如果刺激措施一定有效的话，所有人都会这样做，所有人都能生活在没有问题的经济环境里。但这些措施的效果往往是短暂的。它们就像兴奋剂一样，而世界已经对它们产生“耐药性”，但同时也产生“成瘾性”。如果我们尝试停止这些刺激（像18年第四季度的美联储），则马上出现了“戒断反应”。如果继续这样服用，则效果越来越小，“剂量”却需要越来越多。
And we all know, where that ends.
我们都清楚，这最终的结局会是如何。
